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Brian Paul Wiegand, B.M.E., P.E.
1
AUTOMOTIVE DYNAMICS and DESIGN
AUTOMOTIVE DYNAMICS,LATERAL
There are a number of automotive performance aspects
which are associated with motions in the lateral
direction: maneuver ( & ),
, and . With regard to maneuver,
the which can be attained
in turning is an important index of
performance and safety; the lateral acceleration point
at which roll-over can occur is generally at a level
significantly greater than this maximum lateral
acceleration. However, the level at which rollover could
occur is still an important . Before
attaining a steady-state condition, a turning maneuver
must first go through a phase, which is also a
matter of some significance. Lastly, there is the matter
of , which has to do with the lateral
tire traction force balance front-to-rear, and the
of the vehicle tires due to
those forces.
AUTOMOTIVE DYNAMICS and DESIGN 2
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 3
An automobile undergoing directional change is in general
plane motion: translation + rotation. Here the radius of the
turn is considered large enough that the forces producing
acceleration are to be considered purely lateral in
orientation:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 4
For this case, the principle of “dynamic equilibrium”
requires the following relationships between forces,
moments, and the accelerations produced thereby:
TRANSLATIONAL:
ROTATIONAL:
Consider for the moment only the steady-state
condition of constant angular velocity (“ ”);
in this state the equations become…
In the lateral force equation substitute “ ” for “m”
and “ ” for “ ”:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 5
In the moments about the CG equation substitute “ ” for
“ ”:
Substitute this expression for “ ” in the force
equation and do a little manipulation:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 6
Now solve for “ ”, and “ ”, then substitute “ ” for
“ ”, “ ” for “ ”, and “ ” for “ ”:
So, for steady-state turning the lateral tire traction
force(s) at each axle need only equal the weight load at
each axle times the lateral acceleration in g’s. However,
the generation of those tire forces is a bit more complex
than might first be supposed; the matter is not simply a
case of applying Coulomb’s friction law…
AUTOMOTIVE DYNAMICS,LATERAL
Automobiles produce all primary direction
controlling forces at the tire/road interface. This
force generation is not in accord with Coulomb’s
Friction Law: “ ”. Because of the nature of
rubber pneumatic tires, the traction force (“ ”) and
normal load (“ ”) relation is nonlinear. Empirical
studies show that for a tire the lateral traction
coefficient “ ” is itself a function of the normal
load:
AUTOMOTIVE DYNAMICS and DESIGN 7
The coefficients “ ” and “ ” are particular to the
type of tire concerned.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 8
The “ ” coefficient is the
basic coefficient of
traction; it is dependent
upon the type of tire
material / road surface,
and directly proportional
to the magnitude of the
contact area. The “ ”
coefficient is a measure
of the decrease of
contact area due to tire
distortion under lateral
load:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 9
Combining the lateral traction
coefficient and Coulomb’s
law by substitution for “ ”
results in the normal
load/potential lateral force
relationship
Graphically, this function may
be depicted for a “typical” set
of coefficient values as
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 10
A lateral
traction
force at the
tire / road
contact area
not only
causes
“curl-up”
distortion of
the tire in
the
plane, but
also…
…causes a
distortion in
the
plane
resulting in
what is
erroneously
called the
“ ”,
but here is
called the
“
”…
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 11
The
“ ”
relation, for a
particular
normal load/
tire/inflation
pressure,
looks as per
the depiction
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 12
The lateral force generation potential for an axle is
essentially only a matter of adding the lateral force
generation potentials of the tires:
In the static case the normal loads would be equal, “
”. However, in a turning situation a “weight
transfer” occurs which alters the lateral force
generation potential by decreasing the normal load on
the tire closest to the turn center and increasing, by
an equivalent amount, the normal load on the tire
furthest from the turn center.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 13
This situation may be depicted as follows:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 14
From the information given in the figure expressions
for the normal loads at the inner and outer tires may be
determined:
The quantity “ ” is the “weight transferred”.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 15
Substituting “ ” and “ ” into the “ ” equation
produces the axle lateral traction force potential taking
“weight transfer” into account:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 16
From that equation, further equations for the maximum
axle lateral acceleration limits of slide and overturn can
be determined:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 17
A plot of traction potential vs. lateral load illustrates the
situation:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 18
The “ ” and “ ”
points are given
directly by the means
of the appropriate
equations. Observe
what happens when
these equations are
plotted vs. increasing
axle weight load (with
no change in vertical
c.g.):
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 19
Plotting those same
equations vs. the
center of gravity
height produces:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 20
A conventional vehicle has two axles in tandem. The
static portion of vehicle weight that can be assigned to
each axle equation is determined from the total vehicle
weight and the longitudinal center of gravity:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 21
Using the same
values for axle
weight (2000 lb),
track, and tire
coefficients the
effect of varying
the longitudinal
on the
maximum lateral
acceleration
(slide) may be
visualized:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 22
For a conventional automotive configuration
(rear drive, same type & size tires/wheels all
around) obtaining a maximum lateral
acceleration level favors an even longitudinal
weight distribution. However, directional
stability favors a forward bias, while
acceleration and braking favor a rear bias.
The longitudinal weight distribution of a
particular vehicle depends upon the vehicle’s
design intent (family car, race car, sporty
road car, etc.)
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 23
2o14 Chevrolet Corvette Stingray
Weight Distribution Frt/Rr: 49.5%/50.5%
Tires Frt/Rr: 245/35R19 / 285/30R20
2013 F1 (typical, per FIA regulations)
Wt Distribution Frt/Rr: 45.5-46.7% / 54.5-53.3%
Tires Frt/Rr: 245/660R13 / 325/660R13
1980 Ford Fiesta S
Weight Distribution Frt/Rr: 63.0%/37.0%
Tires Frt/Rr: 155/80R12 / 155/80R12
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 24
The two-axle model for static “weight transfer”
between axles as used so far was highly simplified;
there was no consideration of the effects resulting
from the presence of a suspension in a dynamic
situation. By design, a suspension allows for
deflection under load, and consequently there is
some deflection under longitudinal loads (dive,
squat) and some deflection under lateral loads
(roll). The deflection under lateral loads modifies
the “weight transfer” results from that obtained
using the previous simple moment balance
equations.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 25
The suspension type and dimensions determines the
location of the roll center at either front or rear axle
line. The line delineated by connecting the two roll
centers constitutes the “ ” about which the
sprung mass will rotate under lateral load:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 26
To illustrate the matter, consider this depiction of the
roll axis and c.g. situation of a 1980 Ford Fiesta S (1.1
liter, European version) in a “4 up” condition:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 27
Some simple weight accounting was used to establish
the sprung weight and its c.g. coordinates:
Note the unsprung mass c.g. is assumed to be at mid-
wheelbase longitudinally, at centerline laterally, and at
the rolling radius height of the 155SR12 tires.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 28
Looking at the vehicle cross-section through the
sprung c.g., the situation is as depicted:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 29
The roll angle “ ” of the sprung mass under a 0.5 g
lateral acceleration “ ” can be determined from the
fact that the roll resistance (roll angle “ ” times the
roll stiffness “ ”) has to equal the roll moment
(lateral force “ ” times the roll moment arm “ ”) at
equilibrium:
Into this simple equation plug all the necessary
parameters and solve for “ ”:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 30
However, the roll angle determination not quite so simple. The roll
height “ ” decreases by an amount “ ” during the rolling action,
which would tend to make the roll angle “ ” less than 3.9 degrees,
but the rolling also moves the sprung mass laterally by an
amount “ ”, which would tend to make the roll angle “ ” more than
the 3.9 degrees:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 31
There is further complication in that the unsprung
masses at the front and rear axles also make their
contributions to the sprung roll moment as transmitted
through their linkages (if the suspension is of an
independent type, otherwise the unsprung mass
moments are absorbed internally). From this fact, a more
complex reality may be expressed by the equation:
The symbolism in this equation is as follows…
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 32
= The sprung mass roll angle, degrees.
= The total vehicle roll resistance, lb-ft/deg.
= The weight of the sprung mass, lb.
= The lateral acceleration, g’s.
= The sprung mass roll moment arm, ft.
= The front axle unsprung mass weight, lb.
= The front axle unsprung mass vertical c.g.
(approx. the rolling radius), ft.
= The rear axle unsprung mass weight, lb.
= The rear axle unsprung mass vertical c.g.
(approx. the rolling radius), ft.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 33
Even with all this complication, the resulting value
for the roll angle will still just be approximate as
matters such as free surface effect of liquids,
lateral shift of the unsprung mass c.g.(s), and
various secondary deflections (including shift of
the RC’s in roll) are still unaccounted for.
However, the roll angle value as determined may
have all the accuracy that is needed for early
design studies. A simple iterative spreadsheet
solution yields , which is reasonable,
but probably a bit low considering certain effects
still were not taken into account.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 34
Now that the roll angle “ ” has been estimated, the
normal loads “ ” and “ ”, can be determined at the front
and rear axles. First, consider the IFS which is a
MacPherson strut type:
Rolled sprung mass
normal load equations:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 35
Plugging the appropriate values into the new sprung
equations produces the following results:
If the previous simple equations were used, i.e. if roll
stiffness was not considered, the results would have
been:
Nif = (1241.7/2)–(12(4.3)218.3)/52.52)–2138.4(0.5)7.28(48.8)/(90×52.2)= 326.0 lb
Nof = (1241.7/2)+(12(4.3)218.3/52.52)+2138.4(0.5)7.28(48.8)/(90×52.2)= 915.7 lb
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 36
Now that the front axle has been considered, let’s turn our
attention to the rear axle, which is non-independent of the
dead beam type:
Rolled sprung mass
normal load equations:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 37
Plugging the appropriate values into the new sprung
equations produces the following results:
If the previous simple equations were used, i.e. if roll
stiffness was not considered, the results would have
been:
Nir = (1057.7/2)-(12(4.3)154.9/52.01)-2138.4(0.5)7.52(41.2)/(90×52.01) = 304.4 lb
Nor = (1057.7/2)+(12(4.3)154.9/52.01)+2138.4(0.5)7.52(41.2)/(90×52.01) = 753.3 lb
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 38
Note that without roll, the little Fiesta’s normal loads
in a 0.5g maneuver would have been much more
even front-to-rear on the heavily loaded outside
wheels. The effect of the Fiesta’s relative roll
stiffnesses, which is the result of the spring rates
and moment arms, is to have much more of the roll
moment resisted by the front suspension than by the
rear. Consequently, when roll is taken into account
the Fiesta’s normal loads are much more skewed
toward the front outer wheel. This is by design; the
suspension roll centers (roll axis only slightly down
towards front), spring rates (greater roll stiffness at
front), and tracks were chosen for this effect.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 39
Speaking of lateral
performance in the
current context of
roll is a natural
lead-in to the
subject of “roll
gain”. Roll gain is
the steady state
equilibrium amount
of roll, usually in
degrees, per lateral
acceleration, in g’s,
as illustrated:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 40
The subject of roll gain is closely inter-related with the
topic of transient response in maneuver; there are a
number of aspects to the subject of transient response of a
vehicle in maneuver, and roll gain is just one such aspect.
There is also an angular acceleration “ ” associated with
the transient condition at the initiation or termination of a
turn (or with the application of the accelerator or brakes in
a turn, or with a turn of varying radius). The yaw inertia of
the vehicle “ ” times this angular acceleration represents
an inertial moment which must be equaled by moments
generated by forces at the tires in order for the transition
from straight ahead to turning to occur…
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 41
In the above equation, “ ” is the case when the
vehicle is initiating a turning maneuver. When the
vehicle comes out of the turning maneuver there will
be a less intense shorter lived reversal of this
situation, i.e., “ ”. Then, as is often the case in
slaloms, chicanes, or lane changing maneuvers, a
turning action in the opposite direction may
commence causing a situation of “ ” once
again, only now the forces will be pointing in the
direction opposite from before!
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 42
With all this fluxing of forces and moments
inflicted upon a damped spring-mass system
it should not be surprising that there would
be some oscillatory behavior observable. A
plot of vehicle yaw velocity “ ” versus time
“ ” for the transient phase at the
commencement of a turn leading up to a
steady state condition illustrates such yaw
oscillation behavior and its two phases, “rise”
and “decay”, which sum to the total vehicle
“response time”:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 43
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 44
For the same specific vehicles as noted in the previous
figure, the over-plot of transient responses is illustrated
by:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 45
Just as the or “ ” is an important factor
in the pitch motion of the sprung mass, the or
“ ” (a.k.a. “ ”) is an important factor in the
yaw motion of the entire vehicle as it determines an
“oscillation center” ( ) about which the vehicle will
initially tend to pivot. The value of this important
yaw motion factor has three possibilities:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 46
Each of these possibilities corresponds to a certain
physical reality with a particular location for the
oscillation center:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 47
(1) As soon as the front wheels begin to steer the tendency
to pivot about the will generate lateral reaction forces
at the rear tires in the same direction as when the steady
state condition is reached;
(2) The tendency to pivot about the rear axle means that
steering angle input at the front wheels will not
immediately generate lateral reaction forces at the rear
tires; .
(3) As the front wheels begin to steer the pivot about the
will generate reaction forces at the rear tires in opposite
direction from steady state condition; there will be a
reversal of forces from transient to steady state.
.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 48
The transient condition has terminated and the
steady state has begun when the sprung mass
has attained some constant roll angle, the
vehicle has reached some constant yaw
velocity (no yaw acceleration), and the
associate fluctuation of traction forces at the
tires has ceased. This is the condition
generally sought while circling the
circumference of that test course known as
the “skidpad”.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 49
A
common
skidpad
in plan
view
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 50
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 51
As noted earlier, rollover seldom occurs for modern
passenger cars as the slide point is generally reached
first. However, this does not mean that rollover is not a
concern. Although rollover was involved in less than 3%
of passenger vehicle accidents in the US for 2009,
rollover was involved in about 35% of all fatalities
(23,437 total fatalities, 8,296 roll-over fatalities). Of the
8,296 fatalities about 66% failed to wear seatbelts, with
many resultant ejections from the vehicle. However, that
leaves 34% who were properly belted in, yet who fail to
survive anyway, which still represents a disconcertingly
large proportion of all fatalities at about 12%.
(Highway Loss Data Institute, Insurance Institute for Highway Safety, “Rollover and
Roof Strength”, ‘www.iihs.org’, March 2011, pg. 1).
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 52
Therefore, even though rollover is unlikely, it still merits serious
concern. The NHTSA used to (NCAP 2001-2003) rate vehicles for
rollover resistance based solely on a mathematically derived figure
of merit called the “Static Stability Factor” ( ). The is
exactly the same as “ ” as calculated by the previous rigid
model equation:
Where:
= A figure numerically equal to the lateral
acceleration for overturn (in “g’s”).
= The average vehicle track width, front plus rear
divided by two.
= The vehicle center of gravity height above the
ground plane.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 53
The can’t be a totally accurate means of comparison
between vehicles for reasons touched on earlier: under lateral
inertial load a rolling movement of the sprung mass will occur
through some angle “ which will reduce the “ ” by some
amount “ ” and cause the sprung weight to shift laterally by
some amount “ ”, all of which is among a number of
suspension dependent changes which will render the real
overturn point somewhat less than “ ”. Still, while the
difference between the actual “ ” and the is
significant, the is simple to determine and exhibits a
strong statistical correlation with the incidence of roll-over
accidents, as determined by NHTSA statistical analysis. Since
2004 the NHTSA has been using the combined results of the
calculation plus the “Fishhook” test for new vehicle
rollover resistance ratings.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 54
The “Fishhook” name comes from the shape of the
path taken by the vehicle during the test. The
Fishhook invokes the rollover tendency of a vehicle
by approaching as close as possible to actual
rollover through a rather harsh maneuver. The
Fishhook uses steering inputs that approximate the
steering a panicked driver might use to regain lane
position after dropping two wheels off the roadway
onto the shoulder, but is performed on a level
pavement with a rapid initial steering input followed
by an over correction. The original version of this
test was developed by Toyota, and variations of it
were adopted by Nissan and Honda.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 55
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 56
NHTSA’s test version includes roll rate measurement in order to
time the counter-steer to coincide with the maximum roll angle each
vehicle takes in response to the initial steering input. The test
utilizes an automated steering system programmed with inputs
intended to compensate for differences in vehicle steering gear
ratio, wheelbase, and stability properties. To begin, the vehicle is
driver controlled in a straight line. The driver releases the throttle,
coasts to the target speed, which starts around 35 mph (56 kph) and
is increased in 5 mph (8 kph) increments for each run (until
“termination” is achieved), and then activates the auto-pilot which
commences the maneuver. The test runs conclude when a
“termination” condition is achieved involving two inch or greater lift
of the vehicle’s “inside” tires (fail), or if the vehicle completes the
final run at maximum speed of 50 mph/80 kph without lift (pass). If
needed, further testing is undertaken to confirm the exact speed at
which lift occurs, and that the lift point is repeatable.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 57
Vehicle directional stability is a subject that was
initially developed for, and by, the aircraft industry
starting from that industry’s earliest days. Although
the advent of the automobile preceded the airplane, it
would be more than 35 years before the science of
stability would even start to be applied to ground
vehicles. Stability as it pertains to a vehicle can have
many aspects, mainly yaw (directional) stability, roll
stability, and pitch stability. The emphasis of this
course segment on lateral acceleration narrows the
focus to just directional stability and roll stability, and
to a limited extent the latter has already been dealt
with.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 58
What held back the development of directional
stability theory as applied to automobiles was the
lack of information regarding tire behavior. The
behavior of an airplane in flight is determined by
aerodynamic forces, and the effort to understand
such forces was present at the very dawn of the
airplane; Orville and Wilber Wright built a wind
tunnel to study aero effects long before their first
airplane ever left the ground. The behavior of
automobiles is predominantly determined by tire
behavior, and the first tire testing machine was
possibly the rotating steel drum tire tester of
Becker, Fromm, and Maruhn circa 1930.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 59
Those German researchers generated tire data as a
prerequisite for their investigation of the great
automotive problem of the time: steering “shimmy”.
In this they were following up on the French
researcher George Broulheit who had identified the
tire characteristic of “slip angle” in his investigation
relating to shimmy. A notable follower in the
footsteps of these Europeans was R.D. Evans of the
Goodyear Tire and Rubber Company who continued
the investigation of the physical properties of tires
via another drum tire tester.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 60
The war years of 1939-1945 brought most research into tire
and automotive behavior to a halt. However, in Germany at
Junkers Aircraft the researchers Von Schlippe and Dietrich
developed a simple structural model of the pneumatic tire
which after the war would tie in with the research of Dr. A.W.
Bull and (later) S. A. Lippmann at US Rubber. Essentially
this had to do with the lag time of drift angle formation after
application of a side force being a function of the distance
rolled, and hence of velocity and time. They found that, for
highway speeds and small drift angles, the transient phase
of the tires adjusting to the lateral forces could be neglected
and the modeling of vehicle behavior for just the steady
state condition would still have broad validity. This made
possible all the subsequent steady state modeling
investigations into vehicle directional stability.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 61
One of the first to attempt a mathematical analysis of
automotive stability was Prof. Yves Rocard of the
Sorbonne in 1954. Prof. Rocard’s model was 4-wheeled
but rigid; no weight transfer or roll effects were
included. In 1955 M.A. Julien and G.J. Arnet presented
a paper based on an analysis of a less simplified model
in that it separated the vehicle into a sprung and an
unsprung mass. Maurice Olley at General Motors had
begun research into the “shimmy” problem in the early
1930’s, and by the 1950’s he and his GM compatriots
had progressed to major revelations regarding
automotive stability, benefiting greatly from the tire
work of Evans, Bull, Gough, and others.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 62
Also around this 50’s period William F. Milliken, David
W. Whitcomb, and Leonard Segel of the Cornell
Aeronautical Laboratory would do a great deal to
advance the understanding of tire behavior and
automotive directional stability, including the
construction of the AF-CAL on-road tire tester. The
stability model utilized by the CAL investigators was a
four wheel model of sprung and unsprung masses, so
the effects of lateral “weight transfer” and roll were
accounted for, and steady-state conditions prevailed.
Tire drift angle and other functions were linearized
such that the stability analysis results were valid only
up to about 0.3 to 0.4 g’s.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 63
Since the heady period of the 1950’s many new approaches to
understanding automotive directional stability have been taken.
Mathematical vehicle modeling and empirical tire research, utilizing
newer and ever more realistic test machines, would continue, but
computer simulations would come to play an ever more prominent role.
At first the computer simulations were largely through the use of analog
devices intended to serve in validation of theory, and vice versa.
However, analog computers were limited in capability, and were to be
replaced by more versatile digital machines. Since early digital machines
were typically slower than analogs, for a while in the early 1970’s hybrid
computers found favor. As the computational speed and other
characteristics of digital computers rapidly improved they came to
dominate the simulation world, initially as just mainframes but ultimately
also as “personal computers”. The change in platforms also coincided
with corresponding changes in simulation software; specific purpose
built software was to a large extent supplanted by general multipurpose
dynamic simulation and finite element analysis (FEA) codes.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 64
The result has been a modern plethora of highly
detailed and complex modeling attempts. A full
understanding would involve a long exposition
involving a good deal of higher math and complex
algorithms which are beyond the scope of this
course (and beyond this instructor’s capability as
well). Therefore, this course will address the
subject through a simple “analysis” that will cover
most automotive cases, especially passenger cars,
and arrive at a limited result with the limitations as
fully identified as possible.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 65
The greatest single complicating factor with regard to
automotive directional stability is that an automobile must be
“dynamically” stable, not just “statically” stable. The defining
difference between those two conditions is that a “statically”
stable system may show the accepted stable state
characteristic of returning to equilibrium position after the
application of a disturbance input, but in returning may
“overshoot” the equilibrium position and then execute a
reverse correction back to the equilibrium point once again,
which results in an even greater overshoot, and so on.
Although the system initially exhibits what would be regarded
as stable behavior, that is not the full story; that initial behavior
degenerates into an ever increasing series of oscillations
ultimately leading to a loss of control. Such a system is
statically stable, but dynamically unstable.
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 66
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 67
Consider the case of a vehicle traveling in a straight
line on a level (slightly crowned) road just before
encountering a “disturbance” such as a sudden change
in the road crown (highly exaggerated) resulting in a
lateral (with respect to the vehicle) component of the
weight vector acting through the c.g. (disturbance force
“ ”):
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 68
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 69
The essence of automotive directional stability may be gleaned from
the illustrative example just given of the three vehicles of varying weight
distribution. However, such an “explanation” falls far short of an
authentic analysis. Such an analysis begins with an assessment of the
geometry of a vehicle undergoing a slight turning action, either as the
result of a lateral disturbance force or an input from the steering wheel.
From the geometry of the situation the differential equations of
motion are written as if the vehicle were a free body moving in space, but
generally in only 2-DOF (1 translational, 1 rotational); these equations
consist of the summation of forces in the lateral direction, and the
summation of moments about the vehicle c.g., in accord with the concept
of “Dynamic Equilibrium”. The analysis leading to the following
discussion was essentially (other modeling results may have been
blended in) based upon the “bicycle model”; the geometry of which is
presented as follows…
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 70
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 71
The resulting differential equations are solved in accord with the
appropriate classical math methodology. Depending on the
complexity of the model, the resulting expressions may include:
1. “ ” ( ) is expressed in radians or degrees
and is a stability indicator.
2. “ ” ( ) provides a means of comparing
designs with respect to the degree of understeer present.
3. “ ” ( ) is a speed at which an oversteering
vehicle can switch from initially controllable (slight oversteer) to
very unstable (high oversteer) behavior.
4. “ ” ( ), as required at the front wheels
to make a steady-state turn of radius “ ” is another measure of
under/over steer.
5. “ ” ( ) is a determinant like “ ” for classifying a
vehicle condition as stable (understeering).
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 72
“ ” ( ), a.k.a. “Understeer
Gradient”, expressed in radians or degrees, presents a
more general way of classifying a vehicle’s stability
condition than comparing specific drift angle values
“ ” and “ ”. If a vehicle is stable (understeering, US)
then “ ”, if neutral (NS) then “ ”, or if
unstable (oversteering, OS): “ ”. This coefficient
can take various forms, of which one of the simpler is:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 73
“ ” ( ) provides a means of
comparing designs with respect to the degree of
understeer present. Mathematically, it is the speed at
which the steering angle “ ” to make a turn of radius
“ ” is equal to “ ” (that is, the steering angle is
twice that determined by the low speed “Ackermann”
steering geometry “ ”):
(Note that the 57.3 is the number of degrees per radian; it is a units
conversion factor…)
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 74
“ ” ( ) is a speed at which an
oversteering vehicle can switch from initially
controllable (slight oversteer) to very unstable (high
oversteer) behavior:
(Note that the system of units conversion factor “57.3” is now
negative; this is in recognition of the fact that an oversteering
vehicle will have a negative “ ”. Since an understeering vehicle
does not have a negative “ ” this particular stability factor is not
applicable (“…there is no critical speed for an understeer car…a
neutral steer car does not have a real critical speed…”).)
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 75
“ ” ( ), as required at the front
wheels to make a steady-state turn of radius “ ” is
another measure of under/over steer (the smaller the
angle the less the degree of understeer):
An alternative formulation might be given as:
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 76
“ ” ( ) is a determinant like “ ” for
classifying a vehicle’s directional stability nature: “
” is stable (understeering), “ ” is neutral, and
“ ” is unstable (oversteering). Specifically, the
SM is a dimensionless fraction that relates the LCG
placement to the “Neutral Steer Point” (NSP) for that
condition ( ):
AUTOMOTIVE DYNAMICS,LATERAL
AUTOMOTIVE DYNAMICS and DESIGN 77
• FOR
AUTOMOTIVE LATERAL ANALYSIS HAS BEEN
WRITTEN BY THE INSTRUCTOR (TBD) WHICH,
PLUS A COPY OF “
” WILL
BE

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3- AUTOMOTIVE LATERAL DYNAMICS, Rev. A

  • 1. Brian Paul Wiegand, B.M.E., P.E. 1 AUTOMOTIVE DYNAMICS and DESIGN
  • 2. AUTOMOTIVE DYNAMICS,LATERAL There are a number of automotive performance aspects which are associated with motions in the lateral direction: maneuver ( & ), , and . With regard to maneuver, the which can be attained in turning is an important index of performance and safety; the lateral acceleration point at which roll-over can occur is generally at a level significantly greater than this maximum lateral acceleration. However, the level at which rollover could occur is still an important . Before attaining a steady-state condition, a turning maneuver must first go through a phase, which is also a matter of some significance. Lastly, there is the matter of , which has to do with the lateral tire traction force balance front-to-rear, and the of the vehicle tires due to those forces. AUTOMOTIVE DYNAMICS and DESIGN 2
  • 3. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 3 An automobile undergoing directional change is in general plane motion: translation + rotation. Here the radius of the turn is considered large enough that the forces producing acceleration are to be considered purely lateral in orientation:
  • 4. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 4 For this case, the principle of “dynamic equilibrium” requires the following relationships between forces, moments, and the accelerations produced thereby: TRANSLATIONAL: ROTATIONAL: Consider for the moment only the steady-state condition of constant angular velocity (“ ”); in this state the equations become…
  • 5. In the lateral force equation substitute “ ” for “m” and “ ” for “ ”: AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 5 In the moments about the CG equation substitute “ ” for “ ”: Substitute this expression for “ ” in the force equation and do a little manipulation:
  • 6. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 6 Now solve for “ ”, and “ ”, then substitute “ ” for “ ”, “ ” for “ ”, and “ ” for “ ”: So, for steady-state turning the lateral tire traction force(s) at each axle need only equal the weight load at each axle times the lateral acceleration in g’s. However, the generation of those tire forces is a bit more complex than might first be supposed; the matter is not simply a case of applying Coulomb’s friction law…
  • 7. AUTOMOTIVE DYNAMICS,LATERAL Automobiles produce all primary direction controlling forces at the tire/road interface. This force generation is not in accord with Coulomb’s Friction Law: “ ”. Because of the nature of rubber pneumatic tires, the traction force (“ ”) and normal load (“ ”) relation is nonlinear. Empirical studies show that for a tire the lateral traction coefficient “ ” is itself a function of the normal load: AUTOMOTIVE DYNAMICS and DESIGN 7 The coefficients “ ” and “ ” are particular to the type of tire concerned.
  • 8. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 8 The “ ” coefficient is the basic coefficient of traction; it is dependent upon the type of tire material / road surface, and directly proportional to the magnitude of the contact area. The “ ” coefficient is a measure of the decrease of contact area due to tire distortion under lateral load:
  • 9. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 9 Combining the lateral traction coefficient and Coulomb’s law by substitution for “ ” results in the normal load/potential lateral force relationship Graphically, this function may be depicted for a “typical” set of coefficient values as
  • 10. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 10 A lateral traction force at the tire / road contact area not only causes “curl-up” distortion of the tire in the plane, but also… …causes a distortion in the plane resulting in what is erroneously called the “ ”, but here is called the “ ”…
  • 11. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 11 The “ ” relation, for a particular normal load/ tire/inflation pressure, looks as per the depiction
  • 12. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 12 The lateral force generation potential for an axle is essentially only a matter of adding the lateral force generation potentials of the tires: In the static case the normal loads would be equal, “ ”. However, in a turning situation a “weight transfer” occurs which alters the lateral force generation potential by decreasing the normal load on the tire closest to the turn center and increasing, by an equivalent amount, the normal load on the tire furthest from the turn center.
  • 13. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 13 This situation may be depicted as follows:
  • 14. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 14 From the information given in the figure expressions for the normal loads at the inner and outer tires may be determined: The quantity “ ” is the “weight transferred”.
  • 15. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 15 Substituting “ ” and “ ” into the “ ” equation produces the axle lateral traction force potential taking “weight transfer” into account:
  • 16. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 16 From that equation, further equations for the maximum axle lateral acceleration limits of slide and overturn can be determined:
  • 17. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 17 A plot of traction potential vs. lateral load illustrates the situation:
  • 18. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 18 The “ ” and “ ” points are given directly by the means of the appropriate equations. Observe what happens when these equations are plotted vs. increasing axle weight load (with no change in vertical c.g.):
  • 19. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 19 Plotting those same equations vs. the center of gravity height produces:
  • 20. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 20 A conventional vehicle has two axles in tandem. The static portion of vehicle weight that can be assigned to each axle equation is determined from the total vehicle weight and the longitudinal center of gravity:
  • 21. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 21 Using the same values for axle weight (2000 lb), track, and tire coefficients the effect of varying the longitudinal on the maximum lateral acceleration (slide) may be visualized:
  • 22. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 22 For a conventional automotive configuration (rear drive, same type & size tires/wheels all around) obtaining a maximum lateral acceleration level favors an even longitudinal weight distribution. However, directional stability favors a forward bias, while acceleration and braking favor a rear bias. The longitudinal weight distribution of a particular vehicle depends upon the vehicle’s design intent (family car, race car, sporty road car, etc.)
  • 23. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 23 2o14 Chevrolet Corvette Stingray Weight Distribution Frt/Rr: 49.5%/50.5% Tires Frt/Rr: 245/35R19 / 285/30R20 2013 F1 (typical, per FIA regulations) Wt Distribution Frt/Rr: 45.5-46.7% / 54.5-53.3% Tires Frt/Rr: 245/660R13 / 325/660R13 1980 Ford Fiesta S Weight Distribution Frt/Rr: 63.0%/37.0% Tires Frt/Rr: 155/80R12 / 155/80R12
  • 24. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 24 The two-axle model for static “weight transfer” between axles as used so far was highly simplified; there was no consideration of the effects resulting from the presence of a suspension in a dynamic situation. By design, a suspension allows for deflection under load, and consequently there is some deflection under longitudinal loads (dive, squat) and some deflection under lateral loads (roll). The deflection under lateral loads modifies the “weight transfer” results from that obtained using the previous simple moment balance equations.
  • 25. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 25 The suspension type and dimensions determines the location of the roll center at either front or rear axle line. The line delineated by connecting the two roll centers constitutes the “ ” about which the sprung mass will rotate under lateral load:
  • 26. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 26 To illustrate the matter, consider this depiction of the roll axis and c.g. situation of a 1980 Ford Fiesta S (1.1 liter, European version) in a “4 up” condition:
  • 27. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 27 Some simple weight accounting was used to establish the sprung weight and its c.g. coordinates: Note the unsprung mass c.g. is assumed to be at mid- wheelbase longitudinally, at centerline laterally, and at the rolling radius height of the 155SR12 tires.
  • 28. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 28 Looking at the vehicle cross-section through the sprung c.g., the situation is as depicted:
  • 29. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 29 The roll angle “ ” of the sprung mass under a 0.5 g lateral acceleration “ ” can be determined from the fact that the roll resistance (roll angle “ ” times the roll stiffness “ ”) has to equal the roll moment (lateral force “ ” times the roll moment arm “ ”) at equilibrium: Into this simple equation plug all the necessary parameters and solve for “ ”:
  • 30. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 30 However, the roll angle determination not quite so simple. The roll height “ ” decreases by an amount “ ” during the rolling action, which would tend to make the roll angle “ ” less than 3.9 degrees, but the rolling also moves the sprung mass laterally by an amount “ ”, which would tend to make the roll angle “ ” more than the 3.9 degrees:
  • 31. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 31 There is further complication in that the unsprung masses at the front and rear axles also make their contributions to the sprung roll moment as transmitted through their linkages (if the suspension is of an independent type, otherwise the unsprung mass moments are absorbed internally). From this fact, a more complex reality may be expressed by the equation: The symbolism in this equation is as follows…
  • 32. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 32 = The sprung mass roll angle, degrees. = The total vehicle roll resistance, lb-ft/deg. = The weight of the sprung mass, lb. = The lateral acceleration, g’s. = The sprung mass roll moment arm, ft. = The front axle unsprung mass weight, lb. = The front axle unsprung mass vertical c.g. (approx. the rolling radius), ft. = The rear axle unsprung mass weight, lb. = The rear axle unsprung mass vertical c.g. (approx. the rolling radius), ft.
  • 33. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 33 Even with all this complication, the resulting value for the roll angle will still just be approximate as matters such as free surface effect of liquids, lateral shift of the unsprung mass c.g.(s), and various secondary deflections (including shift of the RC’s in roll) are still unaccounted for. However, the roll angle value as determined may have all the accuracy that is needed for early design studies. A simple iterative spreadsheet solution yields , which is reasonable, but probably a bit low considering certain effects still were not taken into account.
  • 34. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 34 Now that the roll angle “ ” has been estimated, the normal loads “ ” and “ ”, can be determined at the front and rear axles. First, consider the IFS which is a MacPherson strut type: Rolled sprung mass normal load equations:
  • 35. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 35 Plugging the appropriate values into the new sprung equations produces the following results: If the previous simple equations were used, i.e. if roll stiffness was not considered, the results would have been: Nif = (1241.7/2)–(12(4.3)218.3)/52.52)–2138.4(0.5)7.28(48.8)/(90×52.2)= 326.0 lb Nof = (1241.7/2)+(12(4.3)218.3/52.52)+2138.4(0.5)7.28(48.8)/(90×52.2)= 915.7 lb
  • 36. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 36 Now that the front axle has been considered, let’s turn our attention to the rear axle, which is non-independent of the dead beam type: Rolled sprung mass normal load equations:
  • 37. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 37 Plugging the appropriate values into the new sprung equations produces the following results: If the previous simple equations were used, i.e. if roll stiffness was not considered, the results would have been: Nir = (1057.7/2)-(12(4.3)154.9/52.01)-2138.4(0.5)7.52(41.2)/(90×52.01) = 304.4 lb Nor = (1057.7/2)+(12(4.3)154.9/52.01)+2138.4(0.5)7.52(41.2)/(90×52.01) = 753.3 lb
  • 38. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 38 Note that without roll, the little Fiesta’s normal loads in a 0.5g maneuver would have been much more even front-to-rear on the heavily loaded outside wheels. The effect of the Fiesta’s relative roll stiffnesses, which is the result of the spring rates and moment arms, is to have much more of the roll moment resisted by the front suspension than by the rear. Consequently, when roll is taken into account the Fiesta’s normal loads are much more skewed toward the front outer wheel. This is by design; the suspension roll centers (roll axis only slightly down towards front), spring rates (greater roll stiffness at front), and tracks were chosen for this effect.
  • 39. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 39 Speaking of lateral performance in the current context of roll is a natural lead-in to the subject of “roll gain”. Roll gain is the steady state equilibrium amount of roll, usually in degrees, per lateral acceleration, in g’s, as illustrated:
  • 40. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 40 The subject of roll gain is closely inter-related with the topic of transient response in maneuver; there are a number of aspects to the subject of transient response of a vehicle in maneuver, and roll gain is just one such aspect. There is also an angular acceleration “ ” associated with the transient condition at the initiation or termination of a turn (or with the application of the accelerator or brakes in a turn, or with a turn of varying radius). The yaw inertia of the vehicle “ ” times this angular acceleration represents an inertial moment which must be equaled by moments generated by forces at the tires in order for the transition from straight ahead to turning to occur…
  • 41. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 41 In the above equation, “ ” is the case when the vehicle is initiating a turning maneuver. When the vehicle comes out of the turning maneuver there will be a less intense shorter lived reversal of this situation, i.e., “ ”. Then, as is often the case in slaloms, chicanes, or lane changing maneuvers, a turning action in the opposite direction may commence causing a situation of “ ” once again, only now the forces will be pointing in the direction opposite from before!
  • 42. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 42 With all this fluxing of forces and moments inflicted upon a damped spring-mass system it should not be surprising that there would be some oscillatory behavior observable. A plot of vehicle yaw velocity “ ” versus time “ ” for the transient phase at the commencement of a turn leading up to a steady state condition illustrates such yaw oscillation behavior and its two phases, “rise” and “decay”, which sum to the total vehicle “response time”:
  • 44. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 44 For the same specific vehicles as noted in the previous figure, the over-plot of transient responses is illustrated by:
  • 45. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 45 Just as the or “ ” is an important factor in the pitch motion of the sprung mass, the or “ ” (a.k.a. “ ”) is an important factor in the yaw motion of the entire vehicle as it determines an “oscillation center” ( ) about which the vehicle will initially tend to pivot. The value of this important yaw motion factor has three possibilities:
  • 46. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 46 Each of these possibilities corresponds to a certain physical reality with a particular location for the oscillation center:
  • 47. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 47 (1) As soon as the front wheels begin to steer the tendency to pivot about the will generate lateral reaction forces at the rear tires in the same direction as when the steady state condition is reached; (2) The tendency to pivot about the rear axle means that steering angle input at the front wheels will not immediately generate lateral reaction forces at the rear tires; . (3) As the front wheels begin to steer the pivot about the will generate reaction forces at the rear tires in opposite direction from steady state condition; there will be a reversal of forces from transient to steady state. .
  • 48. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 48 The transient condition has terminated and the steady state has begun when the sprung mass has attained some constant roll angle, the vehicle has reached some constant yaw velocity (no yaw acceleration), and the associate fluctuation of traction forces at the tires has ceased. This is the condition generally sought while circling the circumference of that test course known as the “skidpad”.
  • 49. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 49 A common skidpad in plan view
  • 51. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 51 As noted earlier, rollover seldom occurs for modern passenger cars as the slide point is generally reached first. However, this does not mean that rollover is not a concern. Although rollover was involved in less than 3% of passenger vehicle accidents in the US for 2009, rollover was involved in about 35% of all fatalities (23,437 total fatalities, 8,296 roll-over fatalities). Of the 8,296 fatalities about 66% failed to wear seatbelts, with many resultant ejections from the vehicle. However, that leaves 34% who were properly belted in, yet who fail to survive anyway, which still represents a disconcertingly large proportion of all fatalities at about 12%. (Highway Loss Data Institute, Insurance Institute for Highway Safety, “Rollover and Roof Strength”, ‘www.iihs.org’, March 2011, pg. 1).
  • 52. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 52 Therefore, even though rollover is unlikely, it still merits serious concern. The NHTSA used to (NCAP 2001-2003) rate vehicles for rollover resistance based solely on a mathematically derived figure of merit called the “Static Stability Factor” ( ). The is exactly the same as “ ” as calculated by the previous rigid model equation: Where: = A figure numerically equal to the lateral acceleration for overturn (in “g’s”). = The average vehicle track width, front plus rear divided by two. = The vehicle center of gravity height above the ground plane.
  • 53. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 53 The can’t be a totally accurate means of comparison between vehicles for reasons touched on earlier: under lateral inertial load a rolling movement of the sprung mass will occur through some angle “ which will reduce the “ ” by some amount “ ” and cause the sprung weight to shift laterally by some amount “ ”, all of which is among a number of suspension dependent changes which will render the real overturn point somewhat less than “ ”. Still, while the difference between the actual “ ” and the is significant, the is simple to determine and exhibits a strong statistical correlation with the incidence of roll-over accidents, as determined by NHTSA statistical analysis. Since 2004 the NHTSA has been using the combined results of the calculation plus the “Fishhook” test for new vehicle rollover resistance ratings.
  • 54. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 54 The “Fishhook” name comes from the shape of the path taken by the vehicle during the test. The Fishhook invokes the rollover tendency of a vehicle by approaching as close as possible to actual rollover through a rather harsh maneuver. The Fishhook uses steering inputs that approximate the steering a panicked driver might use to regain lane position after dropping two wheels off the roadway onto the shoulder, but is performed on a level pavement with a rapid initial steering input followed by an over correction. The original version of this test was developed by Toyota, and variations of it were adopted by Nissan and Honda.
  • 56. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 56 NHTSA’s test version includes roll rate measurement in order to time the counter-steer to coincide with the maximum roll angle each vehicle takes in response to the initial steering input. The test utilizes an automated steering system programmed with inputs intended to compensate for differences in vehicle steering gear ratio, wheelbase, and stability properties. To begin, the vehicle is driver controlled in a straight line. The driver releases the throttle, coasts to the target speed, which starts around 35 mph (56 kph) and is increased in 5 mph (8 kph) increments for each run (until “termination” is achieved), and then activates the auto-pilot which commences the maneuver. The test runs conclude when a “termination” condition is achieved involving two inch or greater lift of the vehicle’s “inside” tires (fail), or if the vehicle completes the final run at maximum speed of 50 mph/80 kph without lift (pass). If needed, further testing is undertaken to confirm the exact speed at which lift occurs, and that the lift point is repeatable.
  • 57. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 57 Vehicle directional stability is a subject that was initially developed for, and by, the aircraft industry starting from that industry’s earliest days. Although the advent of the automobile preceded the airplane, it would be more than 35 years before the science of stability would even start to be applied to ground vehicles. Stability as it pertains to a vehicle can have many aspects, mainly yaw (directional) stability, roll stability, and pitch stability. The emphasis of this course segment on lateral acceleration narrows the focus to just directional stability and roll stability, and to a limited extent the latter has already been dealt with.
  • 58. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 58 What held back the development of directional stability theory as applied to automobiles was the lack of information regarding tire behavior. The behavior of an airplane in flight is determined by aerodynamic forces, and the effort to understand such forces was present at the very dawn of the airplane; Orville and Wilber Wright built a wind tunnel to study aero effects long before their first airplane ever left the ground. The behavior of automobiles is predominantly determined by tire behavior, and the first tire testing machine was possibly the rotating steel drum tire tester of Becker, Fromm, and Maruhn circa 1930.
  • 59. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 59 Those German researchers generated tire data as a prerequisite for their investigation of the great automotive problem of the time: steering “shimmy”. In this they were following up on the French researcher George Broulheit who had identified the tire characteristic of “slip angle” in his investigation relating to shimmy. A notable follower in the footsteps of these Europeans was R.D. Evans of the Goodyear Tire and Rubber Company who continued the investigation of the physical properties of tires via another drum tire tester.
  • 60. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 60 The war years of 1939-1945 brought most research into tire and automotive behavior to a halt. However, in Germany at Junkers Aircraft the researchers Von Schlippe and Dietrich developed a simple structural model of the pneumatic tire which after the war would tie in with the research of Dr. A.W. Bull and (later) S. A. Lippmann at US Rubber. Essentially this had to do with the lag time of drift angle formation after application of a side force being a function of the distance rolled, and hence of velocity and time. They found that, for highway speeds and small drift angles, the transient phase of the tires adjusting to the lateral forces could be neglected and the modeling of vehicle behavior for just the steady state condition would still have broad validity. This made possible all the subsequent steady state modeling investigations into vehicle directional stability.
  • 61. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 61 One of the first to attempt a mathematical analysis of automotive stability was Prof. Yves Rocard of the Sorbonne in 1954. Prof. Rocard’s model was 4-wheeled but rigid; no weight transfer or roll effects were included. In 1955 M.A. Julien and G.J. Arnet presented a paper based on an analysis of a less simplified model in that it separated the vehicle into a sprung and an unsprung mass. Maurice Olley at General Motors had begun research into the “shimmy” problem in the early 1930’s, and by the 1950’s he and his GM compatriots had progressed to major revelations regarding automotive stability, benefiting greatly from the tire work of Evans, Bull, Gough, and others.
  • 62. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 62 Also around this 50’s period William F. Milliken, David W. Whitcomb, and Leonard Segel of the Cornell Aeronautical Laboratory would do a great deal to advance the understanding of tire behavior and automotive directional stability, including the construction of the AF-CAL on-road tire tester. The stability model utilized by the CAL investigators was a four wheel model of sprung and unsprung masses, so the effects of lateral “weight transfer” and roll were accounted for, and steady-state conditions prevailed. Tire drift angle and other functions were linearized such that the stability analysis results were valid only up to about 0.3 to 0.4 g’s.
  • 63. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 63 Since the heady period of the 1950’s many new approaches to understanding automotive directional stability have been taken. Mathematical vehicle modeling and empirical tire research, utilizing newer and ever more realistic test machines, would continue, but computer simulations would come to play an ever more prominent role. At first the computer simulations were largely through the use of analog devices intended to serve in validation of theory, and vice versa. However, analog computers were limited in capability, and were to be replaced by more versatile digital machines. Since early digital machines were typically slower than analogs, for a while in the early 1970’s hybrid computers found favor. As the computational speed and other characteristics of digital computers rapidly improved they came to dominate the simulation world, initially as just mainframes but ultimately also as “personal computers”. The change in platforms also coincided with corresponding changes in simulation software; specific purpose built software was to a large extent supplanted by general multipurpose dynamic simulation and finite element analysis (FEA) codes.
  • 64. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 64 The result has been a modern plethora of highly detailed and complex modeling attempts. A full understanding would involve a long exposition involving a good deal of higher math and complex algorithms which are beyond the scope of this course (and beyond this instructor’s capability as well). Therefore, this course will address the subject through a simple “analysis” that will cover most automotive cases, especially passenger cars, and arrive at a limited result with the limitations as fully identified as possible.
  • 65. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 65 The greatest single complicating factor with regard to automotive directional stability is that an automobile must be “dynamically” stable, not just “statically” stable. The defining difference between those two conditions is that a “statically” stable system may show the accepted stable state characteristic of returning to equilibrium position after the application of a disturbance input, but in returning may “overshoot” the equilibrium position and then execute a reverse correction back to the equilibrium point once again, which results in an even greater overshoot, and so on. Although the system initially exhibits what would be regarded as stable behavior, that is not the full story; that initial behavior degenerates into an ever increasing series of oscillations ultimately leading to a loss of control. Such a system is statically stable, but dynamically unstable.
  • 67. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 67 Consider the case of a vehicle traveling in a straight line on a level (slightly crowned) road just before encountering a “disturbance” such as a sudden change in the road crown (highly exaggerated) resulting in a lateral (with respect to the vehicle) component of the weight vector acting through the c.g. (disturbance force “ ”):
  • 69. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 69 The essence of automotive directional stability may be gleaned from the illustrative example just given of the three vehicles of varying weight distribution. However, such an “explanation” falls far short of an authentic analysis. Such an analysis begins with an assessment of the geometry of a vehicle undergoing a slight turning action, either as the result of a lateral disturbance force or an input from the steering wheel. From the geometry of the situation the differential equations of motion are written as if the vehicle were a free body moving in space, but generally in only 2-DOF (1 translational, 1 rotational); these equations consist of the summation of forces in the lateral direction, and the summation of moments about the vehicle c.g., in accord with the concept of “Dynamic Equilibrium”. The analysis leading to the following discussion was essentially (other modeling results may have been blended in) based upon the “bicycle model”; the geometry of which is presented as follows…
  • 71. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 71 The resulting differential equations are solved in accord with the appropriate classical math methodology. Depending on the complexity of the model, the resulting expressions may include: 1. “ ” ( ) is expressed in radians or degrees and is a stability indicator. 2. “ ” ( ) provides a means of comparing designs with respect to the degree of understeer present. 3. “ ” ( ) is a speed at which an oversteering vehicle can switch from initially controllable (slight oversteer) to very unstable (high oversteer) behavior. 4. “ ” ( ), as required at the front wheels to make a steady-state turn of radius “ ” is another measure of under/over steer. 5. “ ” ( ) is a determinant like “ ” for classifying a vehicle condition as stable (understeering).
  • 72. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 72 “ ” ( ), a.k.a. “Understeer Gradient”, expressed in radians or degrees, presents a more general way of classifying a vehicle’s stability condition than comparing specific drift angle values “ ” and “ ”. If a vehicle is stable (understeering, US) then “ ”, if neutral (NS) then “ ”, or if unstable (oversteering, OS): “ ”. This coefficient can take various forms, of which one of the simpler is:
  • 73. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 73 “ ” ( ) provides a means of comparing designs with respect to the degree of understeer present. Mathematically, it is the speed at which the steering angle “ ” to make a turn of radius “ ” is equal to “ ” (that is, the steering angle is twice that determined by the low speed “Ackermann” steering geometry “ ”): (Note that the 57.3 is the number of degrees per radian; it is a units conversion factor…)
  • 74. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 74 “ ” ( ) is a speed at which an oversteering vehicle can switch from initially controllable (slight oversteer) to very unstable (high oversteer) behavior: (Note that the system of units conversion factor “57.3” is now negative; this is in recognition of the fact that an oversteering vehicle will have a negative “ ”. Since an understeering vehicle does not have a negative “ ” this particular stability factor is not applicable (“…there is no critical speed for an understeer car…a neutral steer car does not have a real critical speed…”).)
  • 75. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 75 “ ” ( ), as required at the front wheels to make a steady-state turn of radius “ ” is another measure of under/over steer (the smaller the angle the less the degree of understeer): An alternative formulation might be given as:
  • 76. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 76 “ ” ( ) is a determinant like “ ” for classifying a vehicle’s directional stability nature: “ ” is stable (understeering), “ ” is neutral, and “ ” is unstable (oversteering). Specifically, the SM is a dimensionless fraction that relates the LCG placement to the “Neutral Steer Point” (NSP) for that condition ( ):
  • 77. AUTOMOTIVE DYNAMICS,LATERAL AUTOMOTIVE DYNAMICS and DESIGN 77 • FOR AUTOMOTIVE LATERAL ANALYSIS HAS BEEN WRITTEN BY THE INSTRUCTOR (TBD) WHICH, PLUS A COPY OF “ ” WILL BE