Wheelbase - Road

Road

In automobiles, the wheelbase is the horizontal distance between the center of the front wheel and the center of the rear wheel. At equilibrium, the total torque of the forces acting on the car is zero, and thus the wheelbase is related to the force on each pair of tires by the following formula:

where is the force on the front tires, is the force on the rear tires, is the wheelbase, is the distance from the center of gravity (CG) to the rear wheels, is the distance from the center of gravity to the front wheels ( + = ), is the mass of the car, and is the gravity constant. So, for example, when one loads the truck with heavy goods, the center of gravity shifts rearward and the force on the rear tire increases causing it to sink to the extent that depends on the stiffness of the suspension. If the automobile is accelerating or decelerating, extra torque is placed on the rear or front tire respectively, and the equation relating the wheelbase, height above the ground of the CG, and the force on each pair of tires becomes:

where is the force on the front tires, is the force on the rear tires, is the distance from the CG to the rear wheels, is the distance from the CG to the front wheels, is the wheelbase, is the mass of the car, is the acceleration of gravity (approx. 9.8 m/s2), is the height of the CG above the ground, is the acceleration (or deceleration if the value is negative). So, as is common experience, when the automobile accelerates, the rear usually sinks and the front rises depending on the suspension. Likewise, when braking the front noses down and the rear rises.:

Because of the effect the wheelbase has on the weight distribution of the vehicle, wheelbase dimensions are crucial to the balance and steering of the automobile. For example, a car with a much greater weight load on the rear tends to understeer due to the lack of the load (force) on the front tires and therefore the grip (friction) from them. This is why it is crucial, when towing a single-axle caravan, to distribute the caravan's weight so that down-thrust on the tow-hook is about 100 pounds force (400 N). Likewise, a car may oversteer or even "spin out" if there is too much force on the front tires and not enough on the rear tires. Also, when turning there is lateral torque placed upon the tires which imparts a turning force that depends upon the length of the tire distances from the CG. Thus, in a car with a short wheelbase, the short lever arm from the CG to the rear wheel will result in a greater lateral force on the rear tire which means greater acceleration and less time for the driver to adjust and prevent a spin out or worse.

Wheelbases provide the basis for one of the most common vehicle size class systems.