Forum: HoseHeads Sprint Car General Forum (go)
Moderators: dirtonly  /  dmantx  /  hosehead

I've always been under the assumption that when pavement cars swerve back and forth under caution, it's to get heat in their tires.  So why do dirt cars do it?  Same reason?  Or to 'warm up' suspension or some other reason?

   I'm not a crew chief and I didn't stay at a Holiday Inn last night......

     I've always understood it was to warm up the tires, the same reason they sometimes will stomp the gas to spin the tires in the corners during a yellow. Watch how some guys that are really running well struggle the first couple of laps after a restart. I think they're faster once the tires warm back up again.

If you have low pressure or a possible bad bleader or a leak the heat helps keep some pressure in the tire.

I tell it like I see it.

For the pavement Racers there's a 2nd reason to swerve back and forth under caution.  Once the pavement tires get hot under race conditions, slowing to caution speed can allow the tires to pick up rubber from the track and other various debris (pebbles, etc.).  Swerving helps knock the junk off so there's forward and lateral grip on the restart.

For the dirt racers what Murphy and ILSPRINTS have said is true.  The tire warmth helps prevent the rear tires from "sealing over" as they cool off from racing conditions.  Essentially this is a thin layer of rubber that glazes over as it cools and becomes really hard.  The tire grip is greatly reduced and can take several laps to work back to proper tire conditions on the restart.  If you get a pit pass you'll notice crews grinding tire surfaces on used tires to prevent this from happening at the start of a race.

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As tires heat up the air pressure goes up too. Under cautions the air in the tire can cool making the air pressure go down. Bleeders let air out as the tire pressure goes up to keep the pre set pressure in the tire. But no way to bring pressure up after tire cools. So scrubbing & spinning keeps air in tire hot so air pressure doesn't fall to levels that make the cars handling terrible on restart.

Cool thread, last two posts explain a lot. I only ran a micro, never had bleeders and I can tell you starting a race with 3.5 to 4 lbs of air in your tires make things interesting for the first couple of laps. Good times.

What would the tire pressure be at the beginning of a race and how much would be in the tire an hour after the race once it cooled off and the bleeders released Air during the race?

Even though I may not know you, I 
care what most of you think!

All I can tell you is tire manufacturers suggest pressures no lower than 6.5 on rt rear. I think maybe 4 or 4.5 on left. Not sure, its been a while since I read the sign on the Hoosier truck at Knoxville.

Good questions.  I've seen those pressures be as low at 4.5 - 6.0 pounds at the start but never seen what the pressures are when they come off the track after a race.

Tires pressures are a big adjustment imo. It can be the difference in getting your tire to a proper operating temperature or not and it can also influence stagger. Wheel spin and track conditions can change your tire pressures at the end of the race. I don't have my notes anymore but I think I probably gained 1-3 lbs of air pressure by the end of a race without bleeders. When I started messing around with lower pressures is when I found out how a race tire is supposed to wear to get optimum traction, as with anything there is an edge that goes to far. How many racers use nitrogen in their tires?