Rear Tire Diameter


What you do:

Choose a rear tire diameter that gives you the drive wheel torque, center of gravity and top speed you want.

Factors to consider:

1. Tire diameters range from 20 - 40 inches.

2. The larger the tire, the more it weighs.

The HiPerMath equation is:

tire weight = tire diameter + tire width - 10

3. The larger the diameter of the tire, the higher the top speed of the car will be.

The HiPerMath equation is:

miles per hour =                           (rpm)(tire diameter)                          
                             (transmission gear ratio)(rear end gear ratio)(336)

This assumes your engine has enough power to push the car to that speed.

4. Tire diameter affects center of gravity and weight transfer.

The HiPerMath equations are:

stance = rear tire diameter - front tire diameter


front wheel weight added (to rear wheels off the line) = (level wheelbase)(car weight)(0.55) - (car weight)(0.55)

center of gravity = (level wheelbase)(wheelbase)(front wheel weight added) + front tire diameter
                                                        (stance)(car weight)                                                2

drive wheel torque = (torque)(transmission gear)(rear end gear)(0.85)

weight transferred = (center of gravity)(drive wheel torque)
                                    (wheelbase)(rear tire diameter / 2)

5. Rear tire diameter affects wheel thrust to the pavement.

The HiPerMath equations are:

drive wheel torque = (torque)(transmission gear)(rear end gear)(0.85)

wheel thrust to the pavement =     drive wheel torque    
                                                    (rear tire diameter / 2)

mph = miles per hour
rpm = revolutions per minute