Like most things in Sprint Car Racing, Sprint Car Tires are unique. Take a quick look at a Sprint Car and it may look as though one rear tire is bigger than the other, that’s probably because it is! This is known as Stagger, we’ll talk more about this later.
Sprint Cars use very large right rear tires, in fact an average right rear tire is around 21 inches (53.3 cm) wide, has a circumference of 105 inches (266.7 cm) and a 33.2 inch (84.3 cm) diameter on a 15 inch (38.1 cm) diameter wheel.
Another thing you may notice about Sprint Car Tires is that the rear tires sometimes look flat; this is not a mistake by the pit crew! Sprint Cars use very low tire pressures in the rear tires, sometimes as low as 4 PSI in the left rear and 6 PSI in the right rear. These low pressures increase the amount of tire that is in contact with the track surface, in turn increasing the grip. If the track is very tacky or hooky higher pressures are needed, if the track is dry or slick lower pressures are required. The front tires of a Sprint Car are usually inflated to around 10-12 PSI.
Sprint Car Tires are designed with very soft sidewalls, this allows the tire to “distort” during cornering, creating a larger contact area with the track surface. This once again increases the contact area creating more bite.
As mentioned earlier Sprint Cars use stagger, meaning that the left rear tire is smaller than the right rear tire. Why do they use stagger? Well the best way to explain this is to think about your road car, it has a differential that allows the rear wheels to rotate at different speeds when cornering. If you turn left, the right wheel will turn faster than the left wheel and vice versa, without this your car would not corner very well. Sprint Cars have a once-piece rear axle connecting the left and right wheels and do not have differentials, therefore if the rear tires were the same size, the driver would probably end up in the fence! The amount of stagger used varies from around 11-20 inches (27.94-50.8 cm), depending on track conditions. On a tacky or hooky track more stagger is required, on a dry or slick track less stagger is needed. Other considerations for the amount of stagger is the size of the track, a small track will require more stagger than a large track. The banking of a track is also another factor that should be considered, high banked tracks will require more stagger than flat tracks.
If you take a walk through the pits at a Sprint Car show you’ll see the crews busily measuring a car’s stagger by running a tape measure around the center of the rear tires to measure their circumference, the difference between the two tires is the amount of stagger. Another thing you will notice while you are in the pits is the sound of air wrenches whirring away, this is the crews removing the wheel’s bead locks so that they can fit tires of different circumference to achieve the correct amount of stagger. This is often done at the last minute after assessing the condition of the race track.
Due to the very low air pressures used, Sprint Cars use Bead Locks to ensure the tires stay on the wheels. Bead Locks are simply a ring that is bolted to the outside of the wheel locking the tires into place. Without these the tires would easily peel off of the wheels.
Sprint Cars use devices called Bleeder Valves to regulate the air pressure in the tires. As a Sprint Car Tire heats up, so does the air in the tire, this increases the air pressure in the tire. This can be a major problem in a feature race, where the heat built up over many laps can more than triple the air pressure in the tire, dramatically affecting the setup of the car. Bleeder Valves are inserted into the wheel and are set to the required pressure, once the tire reaches that pressure the Bleeder Valve releases the excess air, ensuring the tire maintains the optimum pressure.
Several compounds of Sprint Car Tires are available, from very soft to hard, although compared to pavement racing, even the hard compound tires would be considered soft. One of the major Sprint Car Tire manufacturers is Hoosier Racing Tires, their softest tire is the D10 and their hardest tire is the F55. Compound choice is critical, if you go too soft you may literally tear the tire apart or worst still wear it down until it goes bang! On the other hand, if a tire is too hard for the track conditions, the car may not have enough grip and the driver will find himself heading backwards in the pack.
The tread pattern of Sprint Car Tires varies between manufacturers, most comprising large block patterns. The most important part of the tread pattern is the edges of the block, good sharp edges provide forward and side bite. As the tread blocks make contact with the track surface they move, generating heat. If you ever get the chance to feel a Sprint Car’s tire after a feature race you will find that it will be very warm to extremely hot. Some manufacturers have placed holes in the middle of the tire’s tread blocks called “heat dispersion vents” which help to dissipate this heat build up which can cause blistering. Front tires tend to have an “S” pattern as they do not require forward bite.
Hoosier right rear tread pattern
Hoosier front tire tread pattern
Another way to dissipate heat and improve a Sprint Car Tire's grip is by grooving the tires. Tires are grooved with special tools which heat up and slice their way through the tire’s tread. Large tread blocks can be grooved into smaller blocks, the more blocks the better the cooling efficiency of the tire as well as creating more edges for the tire to grip on. More tread blocks also assist with “tread cleaning”, meaning that any dirt that may build up on the tire is channeled out. Tread blocks should not be grooved too small or they will tend to tear from the tire. Grooves can be made horizontally and vertically across the block as well as diagonally. Diagonal grooves will give both forward and sideways bite.
Siping the tire
Siping means to cut the Sprint Car Tire's blocks with a razor blade. Siping will assist the tire build up heat quicker which will help the tire reach its optimum operating temperature. Once this temperature is reached siping will also assists in heat dissipation. Siping is usually done on hard compound tires, the sipes will help create graining of the tire, which will stop the tire sealing over. Very soft compounds should not be siped. Special siping tools are available which allow the depth of the cut to be controlled. Sipes should not exceed a depth of more than half of the tread block.