It is the unending task of a racer to attempt to maximize the consistency which the car is able to produce. Race cars that have greater consistency in the E.T.'s will win more rounds of competition.
This requires monitoring and then managing a multitude of details on the vehicle, some of which may seem trivial at first glance. One such detail is the condition of the slicks that are providing the traction to propel the car down the track, specifically their operating circumference. It is common, even for new slicks at identical air pressures, to have different operating circumferences.
Most race cars operate with a locked rear differential, which means the tires on both sides of the car will turn an equal amount as the car goes down the track. However, if the tires are not the same circumference, the actual distance the chassis is pushed down the track will be different on the left as compared to the right. This effect will result in the chassis not being propelled straight down the lane, essentially slowly turning either left or right. It can be such a minute effect that the driver may not be aware that he is countering this action by slight movements of the steering wheel. While it can be minute in a short distance, it is cumulative. A 1/4" difference in circumference at 60' will be nearly 2" that one tire gains on the other. This will build up to 20" at midtrack.
There is an abundance of negatives that result from allowing this condition to exist. A condition where the driver is continually turning the steering wheel is not optimum since he will never do this the same way twice. Also, the cross lane drift will move the driving tires out of the track's traction groove resulting in a reduction of traction. Tire slippage will then make the chassis drift even more. All of this will result in less consistency.
The fix is easy. A sunny day and a piece of low stretch string that is wrapped around the center of each inflated slick. Verify that both slicks are at their normal operating air pressure. Measure both around the same line at the center of the tread width to compare the circumferences. Identify the larger slick and mark that length onto the string. The smaller slick needs to be adjusted to match this length. Increase the air pressure in the smaller slick by 30% and place it out in the sunlight for 10 minutes. Wait patiently. Reduce the air pressure back to operating and compare the circumference to the marked string. Repeat this technique as required to sneak up on the desired circumference. If required, larger changes can be achieved by using more air pressure or larger time increments. Once the slicks are equal they can be mounted on the car to be tested.
Craig Dienes checking the tires on the TIP Dragster
Tested....for what? Well, it is likely that if this condition has existed on the chassis for some time that the driver has been driving around the chassis and/or there have potentially been suspension adjustments to try to eliminate chassis drift. Any chassis adjustments that had been made to account for the chassis drift will need to be undone. A series of test passes at the track will allow confirmation that the car tracks straight.
This discussion would not be complete if managing the operating circumference at the track was not also covered. While the car is in the pits, one side will have sun exposure while the other is shaded. This condition continues while the car sits in the staging lanes. Cover the sunny side slick to limit heat gain.
Whenever one side is very hot, the air pressure rises. Since a hot tire is also more flexible, the extra pressure will create a condition where the tire could gain circumference. The changes are small, but over a period of months they build up.
Consistency on each of the car's settings will result in consistency in the final E.T., which will result in a race car that will be deeper in the rounds on every outing.
~ Matt Dalton (aka Triggerman)
Written by Matt Dalton, also known amongst Pontiac enthusiasts as Triggerman. After years of racing his daily driver at the local dragstrips, he graduated to full blown race cars. He purchased and refurbished an original 1968 Firebird F/C and spent 10 years bracket racing throughout central Florida, where an overly analytical attitude was parlayed into extreme consistency