Resistances to vehicle motion include aerodynamic drag, gradient resistance from inclines, rolling resistance from flexing tires and road surfaces, and inertia forces during acceleration and braking. A gearbox is needed to reduce the high rotational speed of the engine to slower wheel speeds required for starting, stopping, and slower travel while increasing torque. Gears provide increased torque through speed reduction to help overcome resistances when starting from a stop, and shift to faster gears as speed increases to handle higher loads without overstressing components.

1. Resistances to vehicle motion
&
Need for a gearbox
K.G.MaheswaranK.G.Maheswaran

2. Physics rule of motion
• Object is in static status
It needs a larger force to overcome friction
• Object starts moving• Object starts moving
the needed driving force becomes smaller and
smaller, and the speed becomes faster and faster

3. Single stage gear reducer.

4. Engines need to operate at a relatively high
rotational speed, which is inappropriate for
starting, stopping, and slower travel.
The transmission reduces the higher engine
speed to the slower wheel speed, increasing
torque in the process

5. A diagram comparing the power and torque bands of
a "torquey" engine versus a "peaky" one

6. When we are to roll from standstill, we need torque
(force) to move the vehicle.
In this case we want our 1st gear (the largest in size,
also the ‘torque-giving’ gear) to provide us increased
torque by converting the extra engine speed into force.
As we go faster, the load increases on gear as it’s made
to rotate faster but it’s not designed for it. It gets
stressed, engine growls and we push 2nd gear which is
smaller and faster. This way we go on as speed
increases because beyond a certain limit, the gearbox
cannot bear the increased rotation and we keep on
transferring the role to next gear

7. Resistance to motion
Resistance a vehicle faces while attempting to move
from a stall condition or while accelerating
To sustain motion-This resistance must be overcome by the
powerplant of the engine
Power produced < Resistance to motionPower produced < Resistance to motion
Vehicle will gradually slow down.
We must have experienced the slowing down of bicycles if we stop
pedaling.
The bicycle also slows down if we go uphill or if wind blows from
front. A poorly inflated tire also causes the vehicle to groan more and
slow down. These are the resistances that force the vehicle to slow
down under their effect.

8. Broadly the resistances can be categorized into the following categories:
Aerodynamic drag
Gradient resistance
Rolling resistance
Inertia
All the above produce a restraining force working against the tractive
force. The tractive force must be greater than or equal to the resistive
forces in order to maintain a sustainable motion. We can balance them as
F = F = F + F + F + FF = F req = FA + FG + FR + FI
where
FA= Force due to air resistance
FG = Force due to gradient of a slope
FR = Force due to rolling resistance
FI = Force due to moving or static inertia
The last one FI comes into the picture only when the vehicle accelerates or decelerates,
while the first three always offer a resistance even when the vehicle is moving at a constant
speed

9. Air resistance/ Aerodynamic drag:
When a body travels within a dense medium, the molecules of the
medium collide with the moving object and thereby absorb some of
the energy. This is felt as a resistance to the moving object. If the
medium is denser, then the resistance is more. Also when the
object moves at a faster speed, the resistance increases
proportionately. Mathematically it can be expressed as:
FA = Cd × A × V²
where
Cd = Co−efficient of drag
= 0.6
A = Frontal Area Of Vehicle
V = Velocity of the vehicle

14. AERODYNAMIC LIFT

15. Flow seperation and reattachement

16. AFTER FLOW WAKE

18. CD for various shapes
of truck leading edges

20. • When the vehicle travels uphill, a component of its weight
works in a direction opposite to its motion.
• If some energy is not supplied to overcome this backward
force, then the vehicle would slow down, stall and roll
Gradient resistance:
force, then the vehicle would slow down, stall and roll
backwards.

21. • If the vehicle is trading uphill at a slope of θ, then the
weight of the vehicle, W has two components: one
perpendicular to the road surface (with a value W·Cos
θ) and the other along the road surface (with a value
W·Sin θ).
• The component along the road surface is the one that
tries to restrict the motion
The gradient resistance is given by: FG = W·Sin θ
θ

22. Rolling resistance
• When a vehicle rolls, it rolls with its tires in contact with the road
surface
• Relative motion of two hard surfaces produces a friction.
• Further, neither the road, nor the tire are perfectly rigid. Hence, both
flex under the load slightly.flex under the load slightly.
• As there is a gradual deformation at the contact between the road
and the tire, greatest at the bottom most point and least at the entry
and exit points, the slip of the tire w.r.t. the road produces another
type of loss of energy which results in a resistance.

23. • Rolling resistance is composed of the following
components:
• Tire Rolling resistance: FR,T
• Road rolling resistance: FR,Tr
• Resistance due to tire slip angle: FR,αR,α
• Resistance due to bearing friction and residual
braking: FR,fr

24. • Hence the rolling resistance offered may be written as:
FR = FR,T + FR,Tr + FR,α + FR,fr
• The tire rolling resistance FR,T is a result of the resistance
due to flexure of the tire, air resistance on the tire and
friction of tire with the road. These three can be summed
up and written as:up and written as:
FR,T = FR.T.flex + FR.T.A + FR.T.fr.
• In a simplified manner the total rolling resistance can be
related to the vertical load on the wheels and can be
written as:
Co−efficient of rolling friction, kR or Crr= F
R/FZ.w

25. Contact Type KR OR Crr
Steel wheel on rail 0.0002...0.0010
Car tire on road 0.010...0.035
Car tire energy safe 0.006...0.009
Tube 22mm, 8 bar 0.002
Race tyre 23 mm, 7 bar 0.003
Touring 32 mm, 5 bar 0.005
Tyre with leak protection 37
mm, 5 bar / 3 bar
0.007 / 0.01

26. Rolling Resistance And Drag Forces Versus Velocity

27. Inertial or Transient Forces
• Transient forces are primarily comprised of acceleration
related forces where a change in velocity is required.
These include:
• The rotational inertia requirements (FI ) and the• The rotational inertia requirements (FI ) and the
translational mass (Fma).
• If rotational mass is added it adds not only rotational
inertia but also translational inertia.

28. resistance
Power Demand CurvePresistance
Vehicle Speed