What are marbles and why do NASCAR drivers try to avoid them?

­To understand how marbles are formed, let's look at the science behind racing tires. Goodyear Racing Eagles, like all racing tires, are made from a rubber compound containing elastomers (elastic polymers) that can stretch and return to their original shape [source: Britannica]. Other components that go into a tire include fillers and rubber attaching agents. These filler agents determine the tire's softness and its grip.

Tires lose traction throughout a race. During a tire stint, tires go through a change in composition. Heat is the main reason for this change or tire wear. When heat is added to a tire, several chemical reactions occur. For one, the tire vulcanizes, or cures. Tires delivered to the track from the Goodyear plant in Akron, Ohio are designed for that specific track. Some tires are cured longer than others, depending on the amount of heat the track will put into a tire. Whatever the case, a tire undergoes a chemical change when it cures: Since heat changes the chemical properties of the elastomers, the molecules change their orientation. When this occurs, the tire has no choice but to wear away. The excess or worn rubber forms the marbles.

Since the tracks at Darlington and Bristol have been repaved, tire wear has decreased over the past few years. But as the weather seasons a track, its abrasive characteristics return. It's just a matter of time before a track like Darlington, traditionally considered the most abrasive surface in NASCAR, recaptures its reputation as a cheese grater on tires. For the most part, NASCAR drivers run the bottom edge of the track. In other words, the racing groove, or fast lane, is around the bottom. When this occurs, the top groove can be very treacherous. Drivers refer to this area as the dirty part of the track. That's where the marbles are.

Needless to say, drivers do not like to race in the marbles. More often than not, a driver that drives in the high groove or in the marbles is forced there against his or her will. Two cars cannot occupy the same spot on the track. Something has to give, right? Drivers have two choices at this point: Crash or drive in the marbles. When a driver is forced onto the dirty part of the track, they tend to back off the accelerator and drive cautiously for a lap or two until their tires are clean. It's only after a driver feels like the tires are free of debris that they will step back onto the gas and start racing hard again.

Now, about that weaving technique: This behavior is just one of the techniques NASCAR drivers employ to combat marbles. We've already seen how heat changes the tire compound -- tires become very soft at high speeds. When drivers are traveling at pace car speed under caution, they're only driving between 60 and 70 mph. Racing tires are designed to travel at high speeds and depend on heat for grip. When they cool they undergo more changes. Hot, sticky tires will pick up all the dirt and debris they run over -- sort of like duct tape picks up lint. Drivers will drive side to side in attempt to scrub off as much debris as possible before the race resumes. Only then will a driver feel confident the car is ready for running at 200 mph, and with any luck, into victory lane.

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