VEHICLE DYNAMICS

Rolling ResistanceCauses:-

•Deformation of Tyres.•Deformation of roads.•Friction between tyre and road surface due to trade slippage.•Road roughness•Friction in wheel bearings.•Air circulation inside tyre.•Fan effect of rotating tyre by the air on outside.

Tyre requirements

• To support the weight of the vehicle and distribute it over the road surface.

• To offer minimum rolling resistance to the motion of the vehicle and thus reduce fuel consumption.

• To contribute to the suspension cushioning of impact forces created by road surface irregularities.

• To permit the generation of traction, braking forces and steering moment on dry or wet road surfaces.

• To confirm safe operation up to the maximum speed of the vehicle.• To provide quiet straight-ahead running and freedom from squeal on

cornering and braking.• To realize an acceptable tread life under varied conditions.• Light weight.• Long and even tread life.

Tyre adhesion• The grip of the tyre on the road is referred to as “tyre

adhesion”, and depends not only on the tyre but also to a greater extent on the road surface condition. It is partly due to the force of molecular attraction existing between the tyre rubber and the road surfaces , and partly due to a mechanical interlocking action occurring as the rubber conforms to the surface roughness of the road . For tyre adhesion ,contact between tyre and road surface must be maintained under all running conditions.

Wheel RequirementsStructure – Wheels should be sufficiently strong and rigid, so that they retain their shape under all operating conditions including abnormal impacts. The dimensional tolerance of the wheel should be sufficiently accurate to provide accurate wheel alignment and balancing. Weight – Wheels should be made as light as possible, to reduce unsprung mass. Tyre attachment – The wheel must be designed so that the tyre can be fitted easily and be firmly and secured. Wheel mounting – The wheel attached must perform the job of locating, securing, supporting the wheel and should allow the wheel to be easily fitted or removed from its axle hub. Cost – Wheels should be low in cost, which can be easily fabricated, cast forged and which then require the minimum amount of machining. The material should not readily deteriorate with age and weathering. If susceptible to corrosion, it must be provided with protection treatments.

Tyre construction

Carcass – The carcass is made from layers of nylon, rayon or polyester plies.

Beads – The edges of the tyre contacting the rim are known as beads and moulded inside each bead is a strengthening endless steel wire cord.

Side Walls – The outside of the tyre carcass, known as the side walls, is covered with rubber compound. Side walls need to be very flexible and capable of protecting the carcass from external damage such as cuts which can occur when the tyre is made to climb up a kerb.

Bracing belt – Between the carcass and tyre tread is a crown reinforcement belt made from rayon cord or steel cores. This circumferential endless cord belt provides the rigidity to the tread rubber.

Tread – The outside circumferential crown portion of the tyre is known as the tread.It is made from a hard wearing rubber compound whose function is to grip the counter of the road.

Radial Lateral Longitudinal flexibility flexibility flexibility

Circumferential flexibility

Twisting flexibility

Effect Of Lateral Flexibility

Cornering power = cornering force (Py)/slip angle (α)

Self Aligning Torque

A characteristic of the pneumatic tyre is that it prefers to roll freely in a straight path, and if compelled to do otherwise it generates its own self-aligning torque. This effect arises because in the presence of a slip angle the trailing portion of the tyre contact patch experiences greater lateral deformation than the leading portion, so that the shift in centre of pressure causes the cornering force to act at a small distance behind the rotational axis of the wheel.

For a typical tyre the vertical load is distributed in an approximately uniform manner. Lateral force increases from front to rear of the contact patch for smaller lateral force and sleep angles.

At larger lateral force, the lateral force is progressively limited by sliding which occurs at the rear of contact patch and spreads forward as the force increases.

Self-aligning torque due to pneumatic trail

The offset between cornering force centre of pressure and the geometric centre of the wheel is known as the pneumatic trail. As the cornering force centre of pressure is to the rear of the geometric centre of the wheel, the pneumatic trail causes a moment about the geometric wheel centre which causes to restore the plane of the wheel with its direction of motion. This moment is known as self-aligning torque.

The self aligning torque increases with increase in contact patch length, as it increases pneumatic trail, and increases with tyre stiffness for a given sleep angle, as it increases cornering force.

The self aligning torque is increased markedly by greater than normal vertical loading and to a lesser extent by the application of a tractive force.

The self – aligning torque is little affected with small slip angles when braking or accelerating, but with larger slip angles braking decreases the aligning torque and acceleration increases it. Over inflation of tyre reduces contact patch length and so reduces self-aligning torque. Driving and braking force also reduce the self-aligning torque.

Self-aligning torque provides the driver with a certain measure of ‘feel’ in handling the vehicle. The driver is able to sense a loss of tyre adhesion by the steering ‘going light’ as a result of the diminishing self-aligning torque. A reasonable but not excessive amount of tyre self-aligning torque is usually sought, so that the steering retains an acceptable degree of feel without becoming heavy in operation.

External Disturbing Forces