During many televised auto races, commentators occasionally say a particular driver has gone into the marbles. This means a driver has steered into a slick patch of track around the outer edges, particularly in the corners. The marbles themselves are actually pill-shaped pieces of rubber shaved off the cars' tires as they race. Track maintenance crews use street sweepers to remove these pieces of rubber between races, but little can be done to prevent their formation during a race.
The tires used for both NASCAR and IROC-style racing are completely different from the standard road tires on passenger cars. In order to provide maximum grip between the tires and the track, manufacturers create wide, grooveless racing tires called slicks. Besides providing more contact area, these slick tires also use a softer form of rubber, which literally melts from the heat of the track and the friction of driving. The surface of a race track can be very abrasive, but this provides additional grip for the drivers.
One problem drivers face with these soft rubber tires and the abrasive track surface is the formation of marbles. If there are 30 cars in a race, for example, this translates to 120 tires losing bits of rubber. Over the course of a 500-mile race, the build-up of rubber marbles and other debris from the tires can become significant. Understeering into a marbled corner can mean a sudden loss of traction or even a crash into the retaining wall.
The formation of marbles is considered a necessary evil in the sport of motor racing. Drivers prefer the control and grip provided by the softer rubber tires, but the abrasions caused by an asphalt track inevitably create a pilling action. Most experienced drivers recognize the dangers of driving in the marbles during a race, so they instinctively steer away from the outer edges whenever possible. A significant number of accidents are caused by inexperienced drivers losing control in these areas and sliding across the track.