Stock Car Science

Tire problems again, but the issue here was different than the tire problems we saw in Atlanta earlier in the year. At Atlanta, the tire was too hard. The drivers complained that they didn’t have enough grip to race. That wasn’t the problem today.

Let’s look at the anatomy of a tire. The outermost layer is the tread. When you hear people talk about the ‘tire compound’, what they mean is the particular rubber recipe used for the tread. Immediately underneath the tread are belts, which are usually made from Kevlar or other strong fibers. The belts are laid on the tire at an angle. Underneath the belts are the cords, which are made from copper that may have metal reinforcements.

The harder the tire tread, the less the tire will wear, but the less grip the tire will have. Goodyear has to find that perfect compromise between safety and wear – the same challenge I discussed on my Atlanta blogs. At Atlanta, Goodyear erred on the side of hard tires for the sake of safety.

This weekend, if you saw the pictures of tires on the television after they were on the car later in the race, you saw a grayish area worn in the center of the tire and some copper color on one edge of the tires. (NOTE: Ryan McGee’s blog has a really good picture of this.) The grey material you saw was where the tread was worn away, exposing the belts. The copper was where both tread and belts had been worn away, exposing the cords. Early in the race, we were seeing cords across the tire. The edge of the tire is likely to wear faster when the tire has more camber.

Like Atlanta, Indianapolis is a very rough track. When practice starts, wearing down tires to the cords isn’t unusual. In the past, as practice goes on, rubber transfers from the tires to the track. Rubber builds up on the track and the rubber on the track decreases its roughness, so tire wear decreases.

This is the first time we’ve run the new car at Indy. The center of gravity of the new car is higher, which means that more weight is transferred from the inside to the outside of the car when turning. Since we turn left, this is why all the problems involved right-side tires.

Some commentators suggested that Goodyear misjudged the load that the new car would place on the tires; however, the problem appeared to be more that the track never rubbered up. The Nationwide and Truck series ran at O’Reilly park, across town, so they weren’t helping getting the race track prepared.

The track gets rubbered because friction between the tires and the track heat the tread. Small amounts of the tread melt. Some of the tread gets deposited on the track and some rolls up with other grit and dirt on the track and forms ‘marbles’.

What was unusual this year is that the tire debris at Indianapolis was extremely fine. The tire tread was coming off as very small pieces. During practice, some of this very fine rubber got under Robby Gordon’s car and caught on fire. Normally, rubber doesn’t catch on fire easily; however, if you make any material small enough, you make the surface area much larger, and surfaces are where chemical reactions like oxidation start. Another problem was that the small pieces of rubber were getting everywhere: radiators, into the car, into the driver’s mouths, etc.

As far as I’ve been able to discern, there haven’t been any major changes in the track in terms of levigation (i.e. diamond grinding). The track was last ground in 2002. The different manner in which the tread wore off the tires suggests that there was a problem with either the nature of the tread compound used, or with the tire processing. The tread compound is the same one that was used last year. If the wear rate was faster due to the additional loading caused by the new car, wouldn’t that cause the track to rubber up faster? Even after 370 miles, the tires were still showing significant wear and NASCAR felt that 10-11 laps of green-lap racing was appropriate before they mandated a tire change. There were six competition cautions, and 52 out of 160 were laps run under caution. That makes 108 green-flag laps. Assuming 10 sets of tires, that’s an average of 10.8 green-flap laps per tire.

To be fair to Goodyear, Indy is an anomaly. It isn’t like any other track: It is 2.5 miles and flat (9 degrees banking in the corners). They did a tire test with three teams, but there was not a mandatory test at Indy as there has been in the past. Indy is a one-off tire, one that Goodyear has to develop with a minimum of data.

One interesting thing I don’t have an answer for is that the radio commentators mentioned that, during the first competition caution, the right front tires looked really bad. After that, the majority of the problems were on the right rear tires. I don’t know if the drivers were able to change their braking style, since the right front is loaded going into a corner and the right rear is loaded accelerating out of the corner, or if perhaps that part of the track rubbered up a little more for some reason.

This is part of the frustration of trying to figure out answers from in front of the television. I had the choice of going to Indy or Daytona and I picked Daytona. Hindsight is 20/20: I wish I had picked Indy because I would have like to have seen the actual tire wear patterns myself. I think NASCAR did an admirable job under very difficult circumstances, but my heart aches for teams like the 29 that probably were differentially impacted by the tire issues. This race was decided more on the basis of chance than engineering skill and driver talent and that’s never the way I want to see a race go.

UPDATE for Red: Got an answer to your question about engines and small particulate matter from Tommy Wheeler, who is the Technical Director at Evernham Engine Technology. Tommy says that, because they tear down the entire engine after each race, they don’t really worry about particulate matter in the engine. The biggest problem regarding small particles and the engine on Sunday was clogging the fuel filters and causing problems with the fuel flow to the engine. Also, because the rubber wasn’t sticking to the track, it was getting all over the cars and Tommy says that teams were a little worried about under-car fires. Thanks for the question, Red and to Tommy for the answer.