The rolling radius of a tire is the distance between the center of the wheel and the bottom of the tire. Typically, the larger the diameter of the tire, the greater the rolling radius. Understanding this is important for designers of vehicles since this will impact the vehicle's functions, such as the speedometer, the transmission shift points and the Revolutions Per Minute (RPM) of the engine's crank shaft. In racing applications, the radius is used to set the stagger on oval track vehicles. On drag racing vehicles, the rolling radius can affect the speed at which the front tires break the timing lights.
Different types of radius actually develop in a rolling tire. There is the radius of the wheelslip, radius of kinetics and thirdly, the radius we refer to here, the distance between the center of the wheel and the bottom of the tire. Rolling radius is not constant. It varies as the tire rolls and it's calculated with dynamic measurements.
On a two-wheel drive vehicle, the rolling radius of the front and rear tires can be greatly different. A smaller tire on the front of a two-wheel drive vehicle will roll much faster than the larger rear tire and will make the vehicle react very quickly to steering commands. It will not, however, hinder the vehicle's performance. This is not the case on a four-wheel drive vehicle. Many systems are adversely affected by operating a four-wheel drive vehicle with different rolling radius tires on the front axle and on the rear axle.
In this situation, the front axle is going to be operating at a different speed than the rear axle. This will tear the gears out of the transfer case as it struggles to drive both axles at the same speed. It can also damage the ring and pinion gears in both axles as the axles fight against each other to maintain a common speed. In mud bog-type vehicles, the operator will often place a slightly larger rolling radius tire on the front axle in an attempt to make the front tires spin faster and stay up on top of the mud.
In circle track racing applications, the outside tire will need to turn slightly faster than the inside tire in order to cover the further distance around the outside of the turn. Operators achieve this by placing a tire with slightly more stagger, or a larger rolling radius, on the outside of the vehicle. In a dirt racing vehicle, this does not affect the ability of the vehicle to drive straight on the track's front and back stretch due to a slight sideways slide which the race car remains in all around the race track. On a paved track, the race car is set up with an open differential, which allows the two tires on the drive axle to spin independently of each other.
