Friction in Vehicles: Importance and Impact on Performance

FRICTION
Friction is a force which opposes motion between two parts which are in contact and
moving. As a result, it slows down the moving objects and noise. Friction force is
categorized into two broad categories. Static and dynamic friction. Static friction is
felt when there is no motion between the two bodies. Dynamic friction occurs when
there is relative motion between the objects. The dynamic friction is further divided
into three sub categories. Sliding friction, rolling friction and the pivot friction. Pivot
friction affects rotation of motion such as the one happening on bearings. Rolling
friction happens on rolling elements such as the wheels of a car. (Shedwood, 2012).
Sliding friction occurs when an object slides over a plane. Other ways in which
friction can be classified in according to the nature of the surfaces. Are they
lubricated or not? Boundary friction is the kind of friction which occurs in rubbing
surfaces. It is also called non viscous friction or the greasy friction. Fluid friction is
commonly known as viscosity. (Serway et al, 2002)
There are a number of laws about friction.
Static friction laws.
a) Friction force is always opposite the force causing motion.
b) The force which is enough to just cause the objects to move is equal to the
friction force.
c) The static friction is dependent on the surface roughness.
d) There are of contact does not affect static friction
e) The ratio of the limiting force to the normal force is always a constant.
Laws of kinetic friction.
a) Friction force is always opposite to the force causing motion.
b) The kinetic friction remains constant with moderate speeds.
c) The ratio of kinetic friction to the normal reaction is a constant.
FRICTION IN VEHICLES.
Friction affects all parts which are in motion. A vehicle having so many parts which
are moving, friction also occurs in vehicles. Friction in cars is inevitable. Some car
elements require friction for them to function properly while others do not require
friction at all. The figure below shows the forces acting on the vehicle. (Popova et al,
2015)

Fig: forces acting on a car. (Fishbane et al, 2013)
The forces acting on a vehicle are the weight, reaction force due to weight, the
driving force air resistance (drag force) and friction. Friction is almost in every part
of the vehicle. (Hibberler, 2007).
Car elements where friction is undesired are usually lubricated. For instance, in gear
boxes. While elements which require friction are not lubricated at all. (Goedecke,
2014)
Fasteners
A vehicle has many threaded fasteners of different types. All threaded fasteners rely
on friction to hold them into position. Friction is the most critical property of a
fastener. Friction is very important to ensure there is adequate clamping of
members at a joint. (Feynman et al, 2014). When one is turning a bolt, just 10% is
used in tensioning while the rest 90 % results to friction.
Specific places where threaded fasteners which require friction are wheels and
engine blocks.
Fig: car wheel (Popova et al,
2015)

Fig: engine block (Popova et al, 2015)
The image above shows automotive (car) wheel. Four studs are used and 3 nuts in
this case to hold the wheel. This is a good example where friction is needed very
much. Other examples where the threaded fasteners are used include in the engine
blocks, chassis. Etc. Almost every assembly of a car have threaded fasteners. Thus,
friction is needed in all of those. (Dehg et al, 2012).
Jackscrews
Friction plays an important role in screw jacks. Jackscrews are capable of lifting tons
of load by just rotating the handle either clockwise or counter-clockwise. The
mechanical advantage of most jackscrews is usually between 30% and 50 %. How
does a car jack lock itself? The answer to this question is friction. You increase the
friction between the nut and the screw, then the screw will remain stationary and
will not unwind itself when lifting the car (non-hauling). Friction is increased by
using screws whose α > φ (helix angle is greater than friction coefficient). The effort
required to lift the car is a function of the friction coefficient. (Simo et al, 2012).
fig: a car screw jack. (Martins et
al, 2015).
Fig: scissor screw jack (Martins
et al, 2015).
All this are examples of jack screws used to lift cars where friction plays a major
role.
Road traction
The friction created between the road surface and the drive wheel is called traction.
It is created by the car tires. Traction makes it possible for the car to grip the road

without slipping and facilitating acceleration, navigation and braking. You increase
traction, you improve on grip. Traction is all about friction. When a car is
accelerating on snow, the drive force exceeds the static friction and as a result, the
wheel loses grip and starts to spin. Centrifugal forces which are encountered when
turning or taking a corner, exceeds the static friction between the road and the tire
and the inertia of the car causes the car to proceed straight forward despite turning
the steering wheel. Another form of friction comes in when static friction is
exceeded and it is until dynamic friction comes in will the car stop (braking).
There are factors that influence traction.
a) Material of the tire and road material. Rubber and tarmac have the best
coefficient of friction. Various rubber grades exist for application in different
applications.
b) Texture of the material. Rough road and a rough tire have the best traction.
Traction in wet and icy roads is the worst.
c) The weight of the car. Friction increases with the weight of the car. If your car
has a rear drive, you can add some luggage at the rear carriage to increase
traction.
There are instances when traction is not needed, such as when drifting. During
drifting, rolling friction (dynamic friction) is kept higher than the static friction
making the wheels to ‘lock’ and they slide along the road.
Friction clutches
A friction clutch is an important assembly in power transmission of machines which
are frequently stopped and started including vehicles. Friction force is required to
start from rest the driven shaft and to bring in to, without slipping or sliding, the
required speed. (Allmaier, 2011).
For friction clutches, it should be observed that friction force enough to hold and
keep the load should be developed on contact surfaces. Heat friction is undesired
and should be rapidly transmitted out of the assembly or dissipated out. (Allmaier,
2012)
Friction clutches are of three types. Disc clutches (multiple or single disk), cone
clutches and the centrifugal clutches.Single disk clutches and dual disk plates