Q: Inner liners are required at Dover, which is one-mile track. What is an inner liner and why is Dover the only one-mile track at which it is required?
A: If you look at a picture of a tire (and this one is from a race at Lowes Motor Speedway), you’ll find that there are two valves on it. This is how you know the tire has an inner liner.
An inner liner is a tire inside a tire. Instead of there being a single chamber for air, there are two chambers. The outer valve fills the space between the tire and the inner liner. The valve closer to the center of the wheel fills the inner liner, which is usually about 12-25 pounds per square inch (psi) higher pressure than the outer liner. The inner liner gives the driver a chance to be able to keep control of the car if the outer liner goes flat. At Dover, the left-front tire has a minimum pressure of 23 psi, so the inner liner on that tire would be inflated to somewhere between 35 psi and 48 psi. The right front tire has a minimum recommended pressure of 49 psi, which means that its inner liner would be somewhere between 61 psi and 74 psi. (Thanks Jim for the heads up about my not being able to add correctly. Should be OK now.)
Inner liners are required at all tracks greater than one mile in length, so they are not run, for example at Phoenix. They are, however, run at Dover because Dover has fairly tight turns and 24 degree banking. Phoenix has 11 degree and 9 degree banking in turns 1/2 and 3/4 respectively. The turn length for Dover is about 782 feet. Remembering that there are 5280 feet in a mile and multiplying 782×4 to account for all four turns, you get 3182 feet of turns. That means drivers are turning about 60 percent of each lap. Phoenix has longer straightaways, so the drivers are only turning about 30% of the time there.
The amount of time the car is turning is important because the car experiences a lateral (or centripetal)acceleration when it turns. At Dover, the turn radius is approximately 500 feet. The 24-degree banking means that drivers can be going 130 mph into the turns, which corresponds to a centripetal acceleration of 2.2g (See calculation). (Saturday’s practice showed cars heading into the turns at 150 mph (which corresponds to 3.0g). The driver experiences that acceleration, but so do the tires. This acceleration is comparable to what the car experiences at a track like Texas Motor Speedway, which means that the tires have to stand up to the same forces at Dover as they would at an intermediate track and that’s why inner liners are required.
NOTE: I couldn’t find the turn radius for Dover on the web, so I estimated it as follows: On a map of Dover taken from Google Earth and imported into PowerPoint, I drew a line along the straightaway. The Dover track tells us that the straightaways are 1076 feet long. That gives me a scale: how many inches in my drawing equals 1076 feet. I then drew a semicircle along turns 1 and 2 and measured its radius, scaling by the factor I got from the straightaway. It’s not pretty, but it should be at least in the ballpark.
By request, here is some additional information on centripetal vs. centrifugal acceleration and force.