Bleeding tire at HOT ?

[highboost]

Hi guys,

I've always read you must bleed tire pressures when the tire is cold. However...

Lets say I'm at the track and after my session the tires are 3psi above my target, so I bleed 3psi when I come in the pits before letting the tires cool.

I am probably missing something very basic, but I'm having a difficult time understanding why this is a bad idea?

I really appreciate any insight here, Thanks

 

[patprimmer]

Because you have to set the pressure at a set temperature.
Cold is consistent, hot is variable.

You set the correct cold pressure to give the correct performance when it heats up and the pressure builds. If you then let it down you are below the pressure that proved best.



Regards
Pat
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[CapriRacer]

You initially set the tires cold, then there is heat buildup due the heat being generated within the tire. The technical term is hysteresis. That results in a pressure buildup: PV = nRT

The amount of heat buildup - and therefore pressure build up - is going to depend on the track and a few other things. What you want is a target HOT pressure that results in a target tread temperature - which needs to measured with a pyrometer. Once you determine what the hot pressure is supposed to be, then you set the cold pressures so that you reach the target hot pressure at the end of your session.

You NEVER want to reset the pressure back to the cold pressure when the tire is hot. You MIGHT want to bleed off (or add) some pressure to reach the target HOT pressure as dictated by the results of the pyrometer.

 

[NormPeterson]

Lets say I'm at the track and after my session the tires are 3psi above my target, so I bleed 3psi when I come in the pits before letting the tires cool.

A 20 minute session should be more than long enough for the tires to reach equilibrium (three 60 second autocrosses spaced 15 minutes apart gets close). So if you have already established that you want xx psi (hot) in a given tire and you come in with it at xx + 3, there isn't much reason for not bleeding the 3 off while the tire is still hot.


Always checking/setting pressures when cold assumes that the temperature and pressure rises from cold to operating will be consistent over an unknown range of ambient temperatures, so that you'll end up with the intended hot pressure under operation once it stabilizes.

For the average car owner (for whose benefit most of what's written on this topic is intended), specifying "cold" removes his having to guess whether the tire is "cold", "hot", or somewhere in between, and by doing so reduces the amount of information that he needs to be given. Even that's more than what some people are willing to deal with.


Norm

 

[evelrod]

Setting a "hard" cold pressure is quite difficult if you are racing at several different tracks. Some tracks are harder on tires than others, thus making the 'hot' settings different if you simply stick to a given 'cold' temp setting. Also, a rainy day will necessitate a much higher cold temp since the rain will not allow the tire to reach normal operating temp.

From a practical point, I set my hot temps during test/practice sessions using my tire pyrometer (not those cheap infrared deals, either). From experience, I can determine what cold temp is needed for each track that I race at. At a new track, I'll need to do a bit of testing and that may very well mean I'll need to bleed off or add a little 'hot' pressure to maintain the optimum setting.

Some racers are still using the automatic bleeders that were so popular in the 80's...You set the tire press at the 'optimum' and as the heat/pressure increases the bleeders keep the press at optimum....Obviously, that means between sessions it is necessary to add pressure as needed.

Rod

 

[patprimmer]

But Rod

I expect you don't start with a cold pressure then try to maintain that hot.

I expect you start with a cold pressure that you estimate will give the best hot pressure, then tune the pressure to suit the track on the day. You tune by performance, not by maintain a set number re pressure.

Regards
Pat
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[evelrod]

I understand what you are saying, Pat. However, the idea of maintaining the 'start' pressure throughout the event, even as tire heat increases, is exactly what the 'bleeders' were intended to do. You set your required max press cold and as the tire heat increased the pressure it simply kept bleeding off to maintain the max op press. I never tried them, but several nationally competitive teams did. Guess they did not work out all that well since I never see them in race wheels any longer.

Rod

 

[patprimmer]

Rod

If they started cold with their desired hot pressure and set the bleeders at that, I expect the tyres would take longer to warm up which is a disadvantage, but the car might be faster when they are cold. To much trouble for to little if any gain?

Regards
Pat
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[MikeHalloran]

NASCAR tried bleeders a few seasons ago.
Everybody had them.
Everybody crashed.
They don't use them now.


Mike Halloran
Pembroke Pines, FL, USA

 

[LionelHutz]

I think one of the main issues with the bleeders was once they started to bleed pressure they had a tendency to just keep bleeding off the pressure until there was none left.