User Advice & Scrubbing In
For most purposes, racing tires will benefit from an appropriate 'scrubbing in' procedure. This provides the best combination of performance and longevity under race conditions.
There are several difficulties that arise when using racing tires if they are not scrubbed in prior to use. The most common is "Cold Graining", where the layer of the tread compound in contact with the track, fails in shear with the layer below. The result is a very visible low frequency, high amplitude rippling effect. This is more common with new tires particularly when used in wet, damp or greasy conditions.
Cooper Tire Motorsport recommends that a standard scrubbing in procedure be used whenever possible (conditions and regulations allowing). This consists of subjecting the tires through one gentle heat cycle, gradually loading them up whilst avoiding drifting the car. This should take about three to four laps of a circuit where the lap time is in the region of 60 to 100 seconds. The last lap should only be about 80% race speed. If possible, scrub in at least one new set of tires during free practice, and put these aside for the race. In this way you will know that they have been balanced correctly, and have no slow punctures etc. Getting this done early is important as the qualifying session may become wet or be red flagged, which could force the use of new tires in a race.
When regulations or circumstances do not allow the above procedure to be carried out, then the following should be borne in mind: -
- Graining of the loaded front tires can be avoided if it is ensured that they are fully up to temperature before pushing hard. It is relatively easy to generate temperature in the driven tires as they are transmitting power most of the time. The front tires, however, will need to be given more time and be loaded up progressively before they will be 'In' fully. It should be noted that it is the loading of tires that introduces the significant heat, not sliding or wheelspinning.
- For qualifying, the best results have been shown to have been achieved when the front and rear tire temperatures are the same when measured at the base of the tread. If pushed too early, the rear will come in before the front causing understeer, and taken to the extreme, cold graining as detailed above.
- It is quite possible to get the rear tires to go off before the front tires come in, which will lead to a car that is never balanced, making set-up and qualifying very difficult. Thus to get the best from a set of scrubbed tires, look after the rear tires by not using full power out of corners, and instead work the front tires progressively without inducing large amounts of understeer. When the fronts are fully up to temperature, go for a time (really hooked up lap). Push hard for 2 to 3 laps, and then back off the pace for a lap to allow the tires to recover, and to get some clear track. Repeat this procedure for the best results.
It should be noted that there is always going to be a slight performance peak from new tires, but it will only be possible to take advantage of this if the car is balanced on its tires. A correctly scrubbed in set of tires will always give more consistent performance over its lifetime than a set that has been used hard from new, even if it hasn't grained.