Correct Pressure or Correct Temperature?
01-14-2004, 03:35 PM
#1
Burning Brakes
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Correct Pressure or Correct Temperature?
Which is better, if one had to choose.
Obviously, one would like to have both. But what happens if the only way to achieve the recommended operating temperature is to run a lower pressure.
Thanks in advance.
John
Obviously, one would like to have both. But what happens if the only way to achieve the recommended operating temperature is to run a lower pressure.
Thanks in advance.
John
01-14-2004, 09:31 PM
#4
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The pressure / temperature senerario should happen in sequence as there is a tire design criteria that has a target optimum pressure to give the tire carcass it's intended "shape" or integrity as well as the tread compound it's intended properties. As example, the tire engineer knows the design dimensions, the materials used and the components necessary to give the tire it's desired result. And we're speaking of race tires here (DOT or slicks) as the materials list can have some fairly exotic materials and construction techniques - as example the required belt material to give sufficient strength, resistance to growth (at highest speeds) yet be as light as possible is a very particular balance and a tire load consideration then relates to the air pressure target. So if the tire engineer gives a target HOT pressure (range) that is the first step. Tune the vehicle to achieve that range. The second step is to then achieve the tread slab temperature as the working range is normally quite large (commonly a minimum of 160 - 220).
Now anticipating the reason the question was posted in the first place ... what do I do if I can't get to the minimum range? The variety of answers could be; 1) drive harder, 2) consider that you may not have maximized the tire & wheel package to the car (don't try to stuff as much tire under the car & pay attention to maximizing the the to the wheel - you only have to look as far as the ALMS competitors to see that the physical width of their tires is fairly small (240 or 250MM front and 270MM rear on the GT3's yet the maximize the wheels for a trapezoidal appearance giving a very quick and predictable turn-in and stable mid corner feel for the driver + the fact that it's easier to 'tune') or 3) spending the proper time on vehicle set-up and testing to get the maximum from the variety of suspension components (the obvious camber, caster & toe coupled with spring rate, corner balance, sway bars matched to the springs etc) which is what the professional teams do at the track in their testing and pre qualifying stints. It's a balance but don't skip the basic steps.
Now anticipating the reason the question was posted in the first place ... what do I do if I can't get to the minimum range? The variety of answers could be; 1) drive harder, 2) consider that you may not have maximized the tire & wheel package to the car (don't try to stuff as much tire under the car & pay attention to maximizing the the to the wheel - you only have to look as far as the ALMS competitors to see that the physical width of their tires is fairly small (240 or 250MM front and 270MM rear on the GT3's yet the maximize the wheels for a trapezoidal appearance giving a very quick and predictable turn-in and stable mid corner feel for the driver + the fact that it's easier to 'tune') or 3) spending the proper time on vehicle set-up and testing to get the maximum from the variety of suspension components (the obvious camber, caster & toe coupled with spring rate, corner balance, sway bars matched to the springs etc) which is what the professional teams do at the track in their testing and pre qualifying stints. It's a balance but don't skip the basic steps.
01-15-2004, 09:58 AM
#5
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Hey;
Great stuff from Brown. If I may expand...
To tire test, you need a crew that knows what to do, and you need to drive hard but consistently between sessions. It helps to have a track with little time between hard corners and pit lane so the tires do not have a chance to cool.
If you do not have the ability to do tire testing, by all means you want to make sure you get the right hot pressure. Priority 1 is making sure the tire is well supported mechanically so that it remains healthy. All these guys running around with 22-24 PSI in their R tires are courting disaster. They "feel" good at those soft pressures, but they are not optimized for performance by an stretch, and could be dangerous.
Pay particular attention to the pressure rise from cold to hot. This tells you how hard the tire is working. 10psi or more change may indicate the need to add some air so that the tire does not work too hard. 5psi change and you likely need to drive harder, or let some air out to make the tire work a little harder and therefor heat up a bit more. If you have the means to measure tire temps, so much the better.
Most R tires like to be right around 40psi hot. Michelin recommends 34-36.
If you are doing pyrometer testing, this is generally to tell you whether your suspension setting are correct for your driving style.
Great stuff from Brown. If I may expand...
To tire test, you need a crew that knows what to do, and you need to drive hard but consistently between sessions. It helps to have a track with little time between hard corners and pit lane so the tires do not have a chance to cool.
If you do not have the ability to do tire testing, by all means you want to make sure you get the right hot pressure. Priority 1 is making sure the tire is well supported mechanically so that it remains healthy. All these guys running around with 22-24 PSI in their R tires are courting disaster. They "feel" good at those soft pressures, but they are not optimized for performance by an stretch, and could be dangerous.
Pay particular attention to the pressure rise from cold to hot. This tells you how hard the tire is working. 10psi or more change may indicate the need to add some air so that the tire does not work too hard. 5psi change and you likely need to drive harder, or let some air out to make the tire work a little harder and therefor heat up a bit more. If you have the means to measure tire temps, so much the better.
Most R tires like to be right around 40psi hot. Michelin recommends 34-36.
If you are doing pyrometer testing, this is generally to tell you whether your suspension setting are correct for your driving style.
01-15-2004, 10:56 AM
#7
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Ok, I'm with you guys so far, but this discussion has been for track tires.
Could I sidetrack the thread for a moment over to street tires?
My stock Pirelli P-zero's that are on my 2002 Boxster, I'm running 32/34 right now. What psi should they go to hot? Anyone know?
Could I sidetrack the thread for a moment over to street tires?
My stock Pirelli P-zero's that are on my 2002 Boxster, I'm running 32/34 right now. What psi should they go to hot? Anyone know?
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01-15-2004, 02:06 PM
#8
Three Wheelin'
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Boy you guys are way technical on the racing forum. I usually get hammered for being too technical in the other forums. I like it and won’t give an overly simplistic answer again.
I agree with everything Redlineman and brown said. Pressure first supports the mechanical stability/function of the tire. Fine tuning to reach overall desired optimum temperatures, and desired temperature consistency across the carcass is where slight changes of inflation, suspension adjustments, or even driving style changes comes in.
I agree with everything Redlineman and brown said. Pressure first supports the mechanical stability/function of the tire. Fine tuning to reach overall desired optimum temperatures, and desired temperature consistency across the carcass is where slight changes of inflation, suspension adjustments, or even driving style changes comes in.
01-15-2004, 02:20 PM
#9
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Hey mike;
Oh fine... steel the thread!!
The focus changes with street tires from being optimization of temp for what I call "chemical grip" (sticky rubber) to simply keeping the tires healthy. You obviously will not effect the chemical grip of a street tire to any great degree since this is not an essential part of their makeup. You simply want to optimize them mechanically in terms of health and safety.
In the most general sense, You want a pressure that will keep them off their sidewalls. The shoe polish trick will help you there, and you don't need blobs of it everywhere. One or two small dabs strategically placed will do.
To get more specific, look to the pressure increase from cold to hot. As is the case with an R tire, you do not want to see much if any over a 10psi increase.
It is not commonly understood, but the vast majority of total heat in a tire stems from the mechanical flexure of the component belts, not from surface friction. If you see a tire that is increasing 10+psi, that means the tire is working very hard.
The counterintuitive part of this is that this would call for an INCREASE in air pressure because it is the air that supports the tire's structure. This will allow the tire to work more efficiently in a mechanical sense, with more support, and therefor at a lower temp. If you saw a low cold-to-hot pressure increase, letting a little air out would allow the tire to flex more, build more heat, and therefor more pressure.
If anyone would like a deeper discussion of this stuff (and a lot more!), I have a whole bunch more info in my
Pilot Emeritus series, located at:
http://redlinerennsport.homestead.com/DriversEdEd.html
Oh fine... steel the thread!!
The focus changes with street tires from being optimization of temp for what I call "chemical grip" (sticky rubber) to simply keeping the tires healthy. You obviously will not effect the chemical grip of a street tire to any great degree since this is not an essential part of their makeup. You simply want to optimize them mechanically in terms of health and safety.
In the most general sense, You want a pressure that will keep them off their sidewalls. The shoe polish trick will help you there, and you don't need blobs of it everywhere. One or two small dabs strategically placed will do.
To get more specific, look to the pressure increase from cold to hot. As is the case with an R tire, you do not want to see much if any over a 10psi increase.
It is not commonly understood, but the vast majority of total heat in a tire stems from the mechanical flexure of the component belts, not from surface friction. If you see a tire that is increasing 10+psi, that means the tire is working very hard.
The counterintuitive part of this is that this would call for an INCREASE in air pressure because it is the air that supports the tire's structure. This will allow the tire to work more efficiently in a mechanical sense, with more support, and therefor at a lower temp. If you saw a low cold-to-hot pressure increase, letting a little air out would allow the tire to flex more, build more heat, and therefor more pressure.
If anyone would like a deeper discussion of this stuff (and a lot more!), I have a whole bunch more info in my
Pilot Emeritus series, located at:
http://redlinerennsport.homestead.com/DriversEdEd.html
01-22-2004, 10:23 PM
#11
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Michael,
While this has been a very informative and education thread, I'm not sure your question was answered. I ran P-Zeros at the track last Fall after my MPSC's corded on day 1 of a two-day DE. For my car, suspension and driving style, I determined that 38 psi cold all around was the right starting point. My C4 heats up fronts and rears at similar rates at the track and I ended the sessions with hot pressures of 44-45 psi.
Pyro readings across the treads indicated I was right on the money. Also, I had never heard the "greater than 5/less than 10" target for pressure changes at the track but that's exactly what I got. Man, these guys are good!
FWIW, my corner exit and max straight-line speeds were virtually the same between the street and R compounds. It's given me cause to reconsider the whole "track tire" concept altogether.
While this has been a very informative and education thread, I'm not sure your question was answered. I ran P-Zeros at the track last Fall after my MPSC's corded on day 1 of a two-day DE. For my car, suspension and driving style, I determined that 38 psi cold all around was the right starting point. My C4 heats up fronts and rears at similar rates at the track and I ended the sessions with hot pressures of 44-45 psi.
Pyro readings across the treads indicated I was right on the money. Also, I had never heard the "greater than 5/less than 10" target for pressure changes at the track but that's exactly what I got. Man, these guys are good!
FWIW, my corner exit and max straight-line speeds were virtually the same between the street and R compounds. It's given me cause to reconsider the whole "track tire" concept altogether.
01-23-2004, 09:47 AM
#12
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Hey Joey;
Street tires have indeed reached an amazing performance level. I have been truly impressed with some of my student's performances on street tires.
In many cases I myself could drive their car faster, as one would expect. However, I also know full well that I could take their car, throw on a set of Rs, and REALLY fly.
I do not mean this to sound condescending at all here, please. Perhaps at the level you or your car are comfortable and/or used to driving at, there may not be a worthwhile difference, and that is perfectly fine. In fact, if you are happy and satisfied, it is a totally enviable position to be in!!!
However, I'm pretty confident that if you were to ratchet up your level of committment and tweak the cars setup accordingly, you would once again realize the "need" for the grip that only Rs can offer.
Street tires have indeed reached an amazing performance level. I have been truly impressed with some of my student's performances on street tires.
In many cases I myself could drive their car faster, as one would expect. However, I also know full well that I could take their car, throw on a set of Rs, and REALLY fly.
I do not mean this to sound condescending at all here, please. Perhaps at the level you or your car are comfortable and/or used to driving at, there may not be a worthwhile difference, and that is perfectly fine. In fact, if you are happy and satisfied, it is a totally enviable position to be in!!!
However, I'm pretty confident that if you were to ratchet up your level of committment and tweak the cars setup accordingly, you would once again realize the "need" for the grip that only Rs can offer.
01-23-2004, 10:28 AM
#14
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Speaking from the aircraft point of view, the Air Force uses nothing but Nitrogen to fill their aircraft tires. Main reason I was told was because it makes the presssure more stable over different temp ranges, plus you don't want the moisture freezing way up high.
I don't exactly know how this would equate to racing tires, but I would think that the moisture would make the pressures wander depending on the outside ambient air temperature.
Just wanted to lend a little avaiation theory...
I don't exactly know how this would equate to racing tires, but I would think that the moisture would make the pressures wander depending on the outside ambient air temperature.
Just wanted to lend a little avaiation theory...
01-23-2004, 12:50 PM
#15
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RedlineMan,
No offense taken. I hear you on the R compounds but I'm wondering if it was the particular track which tainted my opinion more than I realized.
The back-to-back comparison I referenced above took place at Road America, Elkhart Lake, WI. For those not familiar, it's a 4-mile circuit which features fast sweepers and several long (half mile) straights. It's a wide course, too, with passing opportunities at virtually every point on the track.
It's not what you'd call a momentum track, though, and so much of it is driven with the pedal on the floor. Am I correct in thinking that grip makes less of a difference on such a track? I know without R's, I'll have my #ss handed to me at autocross but are there certain tracks where the lack of stick makes a bigger difference than at others?
No offense taken. I hear you on the R compounds but I'm wondering if it was the particular track which tainted my opinion more than I realized.
The back-to-back comparison I referenced above took place at Road America, Elkhart Lake, WI. For those not familiar, it's a 4-mile circuit which features fast sweepers and several long (half mile) straights. It's a wide course, too, with passing opportunities at virtually every point on the track.
It's not what you'd call a momentum track, though, and so much of it is driven with the pedal on the floor. Am I correct in thinking that grip makes less of a difference on such a track? I know without R's, I'll have my #ss handed to me at autocross but are there certain tracks where the lack of stick makes a bigger difference than at others?