Calculation of Maximum Acceleration, Maximum Tractive Effort and Reactions for Different Drives (Automobile)

31.10.

Calculation of Maximum Acceleration, Maximum Tractive Effort and Reactions for Different Drives

31.10.1.

Front Wheel Drive

The forces acting on the vehicle and giving rise to dynamic equilibrium are shown in Fig. 31.9.
Forces acting on a vehicle in motion.
Fig. 31.9. Forces acting on a vehicle in motion.
If b = wheel base
h = height of C.G. from the road surface / = distance of C.G. from rear axle
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31.10.2.

Rear Wheel Drive

Here the tractive effort acts only on rear wheels. Hence eliminating Ff and applying Fr to the rear wheels in Fig. 31.9, maximum tractive effort becomes,
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31.10.3.


Four Wheel Drive

This may be with or without third differential.
(i) Without Third Differential. Here both Ff and Fr come into play. Assuming that limiting friction occurs at all the four wheels simultaneously, the maximum tractive effort,
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(ii) With Third Differential. The torque at the front the rear wheels becomes equal with the application of third differential. Slip occurs at the wheels where the normal reaction is smaller and thus limits the tractive effort. In case, the load distribution to the front and rear wheels is equal, the slip has to occur first at the front wheels because the static normal reaction at front wheels is reduced due to inertia effect.


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After finding out the values of Rr and Rf from above equations, it can be ascertained whether the assumptions made was correct or not. If it is not, the solution of the problem should be based on the other assumption.
The above article deals with the cases of vehicles moving on level road. Similar analysis holds for the vehicles moving on grades. This involves another variable, 9, the inclination of the grade. Example 31.5 can be referred for this case.
Example 31.7. A motor car with wheel base 2.75 m with a centre of gravity 0.85 m above the ground and 1.15 m behind the front axle has a coefficient of adhesion 0.6 between the tyre and ground. Calculate the maximum possible acceleration when the vehicle, is (a) driven on four wheels
(6) driven on the front wheels only
(c) driven on the rear wheels only.
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