Weight, Mass Load
and Load transfer in
a Vehicle~ Rohan Sahdev
~Dept of Mechanical and Manufacturing Engg.
~Manipal Instt, of Technology
• The concept of “WEIGHT, MASS LOAD AND LOAD TRANSFER
IN A VEHICLE” is the basis of the field of VEHICLE DYNAMICS.
• Vehicle Dynamics is simply the study of the forces which affect
wheeled vehicles in motion and of the vehicle's responses, either
natural or driver induced, to those forces.
• In many cases it is sufficient to understand the cause and effect of
the forces and responses without establishing finite values or
• So, on considering a vehicle, we need to analyse the various aspects
such as how the vehicle response is while in a corner, how the load
transfers when accelerated, or when applying a sudden brake. Also,
what are the parameters which govern such behaviour in a vehicle.
• On understanding the very basics of load transfer, one can get an
insight of the various design parameters while designing the
vehicle suspension and steering.
LOAD ON FRONT
LOAD ON REAR
THE CENTER OF GRAVITY of any body
is defined as that point about which, if the
body were suspended from it, all parts of
the body would be in equilibrium-i.e.
without tendency to rotate. It is the three
dimensional balance point of the race car.
All accelerative forces acting on a body
can be considered to act through the
center of gravity of that body.
We want our race car's cg to be just as low
as we can get it.
THE MASS CENTROID AXIS is related
to the cg sort of. If we were to slice the car
into a series of transverse sections-like a
loaf of bread-each section would have its
own center of gravity, or centroid. If, in
side view, we were then to draw a line
joining each of these centroids, we would
have the mass centroid axis. Figure (1)
applies. This axis will not be anything that
resembles a straight line, even if we were
to go to the considerable trouble of
calculating it. However, a reasonable
straight line approximation can be
intuitively arrived at that will give an
indication of the distribution of the
vehicle's mass in the vertical plane.
THE ROLL CENTER of a suspension
system is that point, in the transverse
plane of the axles, about which the
sprung mass of that end of the vehicle
will roll under the influence of
It is sort of a geometric balance point. It is
also the point through which the lateral
forces transmitted from the tire's contact
patches act upon the chassis.
THE ROLL AXIS is the straight line
joining the front roll center with the rear
THE ROLL MOMENT is the linear
distance between the roll center at one end
of the vehicle and the concentration of
mass at that end of the vehicle. For the
vehicle as a whole the roll moment is the
linear distance between the roll axis and
the vehicle center of gravity measured in
the transverse plane at the center of
gravity. Figure (13) applies.
DYNAMIC LOAD TRANSFER DUE TO THE
FORCES GENERATED AS THE VEHICLE BRAKES,
ACCLERATES AND CHANGES DIRECTION.
Such a load transfer occurs in the longitudinal plane under linear
acceleration or declaration.
Since the c.g.is necessarily located at some distance above the track
surface, any acceleration is accompanied by a longitudinal shift of
load, rearward in the case of acceleration and forward in the case of
braking. The total weight of the vehicle does not change; load is
merely transferred from the wheels at one end of the car to the wheels
at the other end.
The amount of longitudinal load transfer that will take place due to a
given acceleration is directly proportional to the weight of the vehicle,
the height of its center of gravity and the rate of acceleration. It is
inversely proportional to the length of the wheelbase.
LONGITUDINAL LOAD TRANSFER
In the case of braking the effects of load transfer are several-and all bad.
1. First off, by unloading the rear wheels, the amount of braking energy that
they are capable of transmitting to vehicle is limited by the traction
potential of the smaller front tires.
2. At the same time, since the load transfer increases the vertical loading on
the front tires, it also compresses the front springs which cambers the tires
in the negative direction (in at the top) which may help in a cornering
situation but does nothing good for braking (or acceleration).
3. These same front tires, if the suspension geometry should be less than
optimum, may also be caused to scrub transversely across the track as they
move into the bump position due to the compression of the springs.
4. Compression of the front springs from the load transfer uses up some
portion of the available suspension bump travel and brings the nose and/or
chassis into perilous proximity to the race track.
So, braking has the following effects
The chassis may bottom on the track-
which makes a nasty noise.
So, if we project ourselves down the track
to the corner whose rapid approach caused
the braking in the first place, we find
ourselves entering the corner with the
nose scraping the ground, the rear jacked
up and the tire cambers all over the place.
• At some time in the corner, the driver will see his way
clear to push on the throttle and start accelerating. More
longitudinal transfer will now occur-but in the opposite
direction. Load will now be transferred from the front
wheels to the rear.
• The rearward load transfer will supply this extra tire
capability in the form of increased vertical load.
Naturally, we don't get something for nothing. The cost,
in this case is that the rearward load transfer now
compresses the rear springs, uses up suspension travel,
and cambers the rear tires in the negative sense.
• While a bit of negative camber is, as we have seen, a
good thing, the probability is that we will get too much
especially if the driver jumps on the throttle instead of
getting the car up on the tire.
Anti-dive and anti-lift are tricks that can be
applied to a car's front suspension geometry to
control brake dive and acceleration lift.
ANTI DIVE AND ANTI SQUAT
• As the wheel moves, the upright tends to move
with the wheel. But, since the upright is rigidly
mounted, it tends to negate the Torque. As a
result, when accelerating, there is a forward
thrust on the lower A-arm and the upper A arm
will be pulled in the reverse direction.
• As the IC is in front of COG and slightly above
it, the rear end squat will be reduced and the
front end will rise higher due to more weight
shifted to the rear.
• Changing the upper control arm offset negates
the lifting force created.
• This will result in less rise of the front end, and
less weight would be transferred to the rear.
• Lateral load transfer is caused by forces very similar to those which cause
longitudinal transfer-with the operating axis turned ninety degrees.
• In any cornering situation, centrifugal force, acting through the vehicle's
c.g. tends to throw the car out at tangent to its intended path.
• This centrifugal force is resisted by the lateral forces developed by the tires.
• Since the vehicle's c.g. is necessarily located above the track surface, the
tendency of the c.g. to fly sideways while the tires roll on their curved path
gives rise to a moment of force which transfers some of the load from the
inside tires to the outside tires.
• Lateral load transfer is a bad thing. The transfer of load from one tire of a
pair to the other reduces the total tractive capacity of the pair.
Lateral load transfer
• When the vehicle is cornering, the tire
cornering forces comes into picture.
• These forces can be obtained by referring to
the data provided by the tire manufacturer.
• The basic relationship here is very simple: the greater the lateral
acceleration the greater the centrifugal force and the greater the
tire side forces we must will take place.
• The tendency of a given vehicle to roll due a given lateral
acceleration will vary directly with the vehicle's roll moment
and the amount of mass involved.
• The part of the car that is going to roll is the sprung mass.
• Centrifugal force, being an acceleration, will act through the c.g.
of the sprung mass.
• The greater the vertical distance also between the roll center
and the c.g., the greater will roll couple produced by a given
lateral or centrifugal acceleration.
• Jack forces is an effect of lateral load transfer.
• For a given rate of lateral automobile tends to "jack
itself up" as it goes around a corner. However, any
independent system with the roll center above ground
level will jack to some extent.
• The effect is caused by the fact that the reaction force of
the turn must act through the roll center.
• If the roll center is above the ground, then the line of
action between the tire contact patch and the roll center
will be inclined upward toward the vehicle centerline.
• This being so the side force developed by the tire will have a
vertical component which will tend to lift or "jack" the
• This lifting action, in addition to raising the e.g., will also
move the suspension into droop with unfortunate results in
the camber department. The higher the roll center (and the
narrower the track), the steeper the inclination of the line of
action and the greater the jacking force.
• Jacking is to be avoided on any car and is the single major
reason why today's projectiles feature very low roll centers.
As it is very clear from the presentation, that,
Vehicle Load Transfer has got an extensive
application in the design of a vehicle.
Hence, the study of Vehicle Load Transfer can
help us to achieve the optimum configuration
of the suspension system, steering system, and
most importantly, the static load distribution in
Tune to win ---Carrol Smith
Chassis engineering---Herb Adams
Race Car Vehicle Dynamics--- Milliken & Milliken