Knowing Pit Road Speed in the Rain

This weekend, we learned that the real weather challenge for the NASCAR Nationwide Series isn’t rain. It’s not enough rain. It wasn’t raining hard enough to put on rain tires, but it wasn’t quite dry enough to safely race on slicks. (I’ve written before about why racing in the rain is hard.) But they managed to pull it off, put on a great show and @Brendan62 finally got that long-sought-after win.

When the teams switched to rain tires, the crew chiefs had to remind the drivers that their tach readings for maintaining pit road speed would be different .  I’ve written quite a bit about tach readings and speed limits:

• Pit Road Speeding FAQ (Pocono 2012)
• Pit Road Speeding Explained (Indy 2009)
• Explanation How to Calculate Pit Road Speed

If you know the  gear ratios in the transmission and the rear end, you can convert the engine speed (in revolutions per minute) to the rotation rate at the wheels.  How far the car moves each revolution of the axle depends on the distance around: the bigger the tire, the further the car moves. This is why your odometer and speedometer get screwed up if you don’t use the right size tire on your car.  You can check this out yourself using two tumblers with different sizes.  If you lie them on their sides and start at the same place, they roll them each one revolution, the larger tumbler will have gone further.

Rain tires have thicker tread and a larger circumference. The car moves further along with each rotation of the tires. If you don’t compensate for this in your tach readings for pit road speed, you’ll end up with a speeding penalty, even though you would have been fine at the same tach readings with the slicks on.

I couldn’t find the difference in the circumferences, but remember that teams are always pushing to get as close to the pit road speed limit as possible, so even a relatively small change in the relationship between speed and circumference can put you at the tail end of the longest line.