Scrubs is what
racers call tires that have been briefly brought up to full racing temperature then taken off the track. The reason tires are 'scrubbed' is that a
racing tire is very different from the sort of tires sold for street cars. Most racing tires are delivered without curing the
rubber. Curing hardens rubber and makes it more durable, but soft rubber provides more 'grip' as softer rubber sticks better to rubber left on the track and oozes into the small imperfections in the track surface. Soft rubber offers more control over the car than mere
friction might provide. Since the object is to go as fast as possible it is desirable to have soft tires because the more grip the harder the car can turn, brake or accelerate. (see
traction circle)
The fresh, raw, uncured rubber is soft as the tire can be, and useful for getting the fastest possible lap. Once the tire passes one hundred degrees its grip increases as the tire softens. But it will only stay that way for a short period of time. If the tire gets too hot (and race tire temperatures often reach between 180 and 220 degrees) it will begin to become 'greasy' and lose grip. Usually a fresh tire has only a few hot laps before its grip begins to diminish. So while a fresh tire might be the fastest tire, its peak grip doesn't last very long. So the raw tire might be ideal for qualifying where the fastest possible time is rewarded, but not so very good in a race that may last for many miles. You want a race tire to last longer.
So drivers will go out and 'scrub' their tires. Scrubbing a tire consists of doing one to three 'hot' laps then immediately bringing the car in and removing the tires. Well funded (and professional) race teams may do this more than once during a session. The tire is given one and only one heat cycle. The scrubbed tire is then allowed to cool. Bringing the tire up to temperature activates the carbon black within the rubber and begins the curing process. Curing makes the tire last longer. By giving only one heat cycle and immediately cooling the tire you end up with a tire that has almost all the grip of a raw tire, but is more stable so it can withstand the heat for a long period of time. When a tire begins to lose grip it is 'going away' and if a tire goes away too soon a racer may lose positions to competitors whose tires retain more grip. Tire life is very real factor road race or longer stock car races where a tire may be asked to last for 100 miles or more. Multiple heat cycles simply make the tire harden faster, so the fastest type of race tire is a 'scrubbed' tire whose next use is for a race.
Racing tires can be quite expensive ($2K for set of four is not atypical) so sanctioning bodies like the FIA, NASCAR and Indy Car often regulate tire use for reasons of cost control and safety. Racing teams may be restricted in both the number and types of tires used. Often the sanctioning body will specify a particular tire . The utility and possibilities for scrubbing may be affected by racing rules, but when permitted tires are usually scrubbed.
When a driver swings his wheel back and forth during a pace lap they are not 'scrubbing' their tires in this sense. The driver is trying to clear the surface of small rocks and debris and increase the tire temperature as much as possible, though such violent motions do not bring the tire up to racing temperatures. Autocross tires generally test between 100 and 180 degrees, while road racing tires operate at between 140 to 220 degrees. Tire temperatures are measured with a tire pyrometer which ranges up to 350 degrees fahrenheit or 175 celsius.