Engineering Tip #18: Rain Dance

Suspension and tire management for racing in the rain can be a very different exercise than when dealing with a dry track.

“Having a good, solid platform to start with, a car that’s reasonably well balanced and easy to drive, works even better in the wet,” Ken Payne, Motorsports Technical Director at BFGoodrich Tires says. “If your car is already going to be set up for that last fraction of a second and loose in the dry, you’re potentially setting yourself up to be really troublesome in the wet. If you have the luxury of softening the springs, softening the bars up a little bit, that’s always helpful. My preference in the rain was always to have a bit of mild understeer in the car.”

Pressure can be tricky when the track is wet. For one, Payne notes, the ideal range may be narrower to maintain the shape of the contact patch. For another, pressure will have a much smaller increase than on a dry track.

“You can hurt the performance of the tire by going too low or too high [on pressure], because it alters the contact patch to a shape that’s not optimum. Typically the hot temperature for rain tires isn’t going to be that much different than the temperatures for dry tires. For instance, if target hot pressure is the 38, you’ll start with a little higher cold pressures because you’re not going to get the level of pressure buildup in the wet that you might in the dry. If you’re targeting 38 hot and it’s relatively flat temperature conditions in the rain, you might only go down to 36 for your cold pressure as a starting point,” he says.

Of course, races that start wet don’t always stay that way. Anyone that has watched an F1 race that goes from wet to dry conditions has seen drivers veering toward the puddles on a straight in an effort to keep their tires under them until it’s time for the next pit stop. The reason racing tires either come with little tread or are shaved to a smaller tread depth – or both – is about longevity as much as performance. When a tire starts to lose chunks of tread, its performance is dropping rapidly, and rain tires are much more prone to that than dry tires.

“[Conserving rain tires] is very much a function of how much of the race is left and the tire itself,” says Payne. “Certainly if you can hit some puddles and damp spots without risking an unfortunate moment, that can be very helpful. Especially in sprint races, you’re  going to see some falloff, but everybody’s going to experience the same thing, so the best thing is to experiment: Follow the pace that you think is right; try to adjust your lines accordingly as it dries out. If the classic racing line is really going to be the line you need, follow it. You may not be able to use those rain tires very well the next event, but they should hold up with you for the duration of a typical sprint race.”