In order to overcome the road resistance which we discussed so far, we require high drive power at the wheels. To drive upon upward inclines or to accelerate the corresponding reserve power is necessary. The vehicle with high reserve power can only reach higher average speed on hilly stretches and where lot of overtaking is required.


For efficient transfer of drive power and for safe deceleration an adequate power transmission between the tyre and the road surface is required. The static friction must be correspondingly high. In case the forces from the tyre contact area of the vehicle cannot be completely transferred to the road surface there is loss of road grip and the vehicle swerves.


The greater the torque provided by the engine M the greater the propelling force FANT  that takes effect on the contact surfaces between the tires and the road.

FANT = (M . i) / r

The propelling force is directly proportional to the overall ratio “i” and indirectly proportional to the Tyre radius “r” or the Tyre diameter.

The product of rear axle ratio and gear box ratio is called overall ratio. Both are geared to the indented application of the vehicle. The gear box ensures gradual increase in propelling force with increase in vehicle speed and with raising road resistance on upward inclines or during acceleration.


On wet road surfaces problem will arise in transmitting the power from tires to road surfaces. The co-efficient static friction depends on the driving speed, condition of the tyres and the road surface. The co-efficient of static friction falls   drastically on the wet road surfaces as part of the overall friction consists of fluid friction. If the tyres begins to slide on the film of water, this is referred to as aquaplaning.

During aquaplaning neither the braking nor the steering forces can be transferred to the road surface as pure fluid friction is involved.



LEGEND : 1-Tyre, 2-Wedge of water, 3-Road Surface

The tyre being lifted from the ground by the wedge of water which slides under the complete tyre. The effect is more on the vehicle with poor tyre tread, on the more worn out tyre. The higher the vehicle speed the greater the depth of water on the road surface.

The tendency of the aquaplaning reduces with the high pressure forces between tyre and road surface due to more vehicle weight.

Next session: Driving characteristics and cornering ability.

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