(WOMR file photo)
Just like many things associated with sprint car racing, the tires are very unique. If you take a quick look at a sprint car, it may look like the right rear tire is bigger than the left rear tire. Low and behold your are actually correct! This is called stagger.
Sprint Cars use a very large right rear tire. The average size right rear tire is about 21 inches wide, has a circumference of 105 inches, and a 33.2 inch diameter on a 15 inch diameter wheel.
Another interesting thing that you may notice about the tires is that they look relatively flat. This is intentional. Sprint cars use a very low tire pressure in the rear tires, somewhere between 4 psi and 6 psi in the right rear! The low tire pressure increases the amount of tire that is in contact with the racing surface, which in turn, increases the grip level of the tire. If the track is very tacky, a higher tire pressure is needed. If the track is dry or slick, a lower pressure is required. Moving to the front of the car, the front tires of a sprint car are usually inflated to around 10-12 psi.
The design of the sprint car tire is with a very soft side wall. This allows the tire to distort during cornering, creating a very large contact area with the racing surface. Again, this give the front tires “more bite”, or traction.
As I mentioned earlier, sprint cars use stagger, meaning the left rear tire is smaller than the right rear tire. Why this stagger, you might ask?
Stagger has everything to do with making the sprint car turn left, cornering. Your car has a differential that allows your rear end to turn at different rates, that allows the car to make right turns and left turns more easily. Because the tire on the outside of the turn must travel farther and faster to make the turn, without that differential the car would not turn or corner very well because both tires would be traveling at the same speed.
A sprint car, however, has a one-piece rear axel connecting both rear wheels, without the differential. Therefore, the rear wheels turn at the same rate, and, if the tires were the same circumference the race car would not turn very well, possibly putting the car and the driver head on into the fence! Think of stagger like a coke bottle, bigger on one end than the other end. When you roll a coke bottle on the ground, the bottle turns towards the smaller end. That is stagger!
The amount of stagger used varies upon the track and its conditions. On a tacky track, more stagger is used, on a dry track less stagger. Other considerations for the amount of stagger include the size of the track. A small track would require more stagger than a larger track. The banking of the track dictates that amount of stagger, higher banked track will require more stagger that a relatively flat track.
If you walk through the pits, you will see race crews measuring the rear stagger by running a tape measure the center of the rear tires, measuring the rear tire circumferences. The difference between the two tires (left and right rear) circumferences is the amount of stagger that they have in the race car.
Another thing that you may notice while you are in the pits is the sound of air guns or wrenches whirring and buzzing away. This is the sound of the crews removing the wheel’s bead locks so that they can fit different circumference tires on the rim to achieve the required amount of stagger. This is often done just before the race car moves out onto the track for the race.
Due to the extremely low tire pressures used, sprint cars use bead locks to ensure that the tires stay on the rim or wheels. A bead lock is simply a ring that is bolted onto the outside of the wheel locking the tires into place. Without this handy and simple little device, the tires would easily peel off of the wheels, with this unbridled 800 plus horsepower engine driving those rear wheels!
The next segment will deal with those different looking wings!
TIL NEXT TIME, I AM STLL WORKING ON MY REDNECK!