Why would racing slicks be burned with a torch then scraped right before a race?

Answer 1

You usually don't see that BEFORE a race, unless it's during a practice session. What they are doing is scraping off rubber that was picked up on the hot tires in order to find the wear indicators. These are little "holes" in the tire surface. By measureing how deep the hole is, it tells them how much rubber is left on the tire, which then tells them how much tire wear there was on that run. There are about 5 holes across the tire, and at about 6 places around the tire. So this can tell them a lot about how the car set up is working. By heating and scraping off the build-up, it gives them a more accurate measurement, by making sure they are measureing the surface of the tire, and not the build-up. The build-up is usually picked up when the car drops down to the apron to come into the pits. I have seen teams ruff up tires with a wire brush or in some cases steel for traction