Rec'd Tire Pressure...why so much conflict?
05-08-2005, 10:19 AM
#16
Addict
Rennlist Member
Rennlist Member
As to your original quetion: Why so much conflict?
I think there are more seat of the pants recommendations for street tires than there are those based on pyrometer readings, IMHO. I believe a pyro would eliminate a lot of the guesswork surrounding this issue. And I'm not certain the shoe polish trick would provide a good guide, either (no offense, TD). Certainly, it's better than nothing but it seems that it might provide false readings regarding tire rollover.
Specifically, much of the polish rubs off during your warm up laps, when there's more flex and less pressure in your tires. As your tire pressures build, there's actually less rollover than when you started but since the polish is already gone, you're only seeing what happens at lower pressures. (Am I missing something?)
Pyrometer aside, there is another recommendation from Redline Man with which I tend to agree: Watch the amount of pressure build up from cold to hot. John recommends that you target an increase between 7-9 psi; 10 or more suggests you're overheating the tires, while anything less would mean you're not generating enough heat.
I think this makes sense since it's something I've confirmed with my pyrometer. Whether it's street tires or R compounds, the treads usually develop appropriate heat when they've picked up around 8 psi from cold to hot.
I recently instructed a GT3 owner and wasn't quite sure where we should be since IT IS a very different car than mine. It had the stock 18" PS2's and we began the first session at 32f/34r. At the end of the session, we were at 37f/43r which is a jump of 5 and 9 psi respectively. The car felt fine and we didn't play with it too much since it was his first track day but it seems a more appropriate starting point for this car should be one that generates a little more heat up front, perhaps 30f/34f cold.
As your numbers suggest, Dell, relatively low pressures can cause a great deal of heat build up, but you don't want too much. Tire pressures and heat are similar in that there is a target range which produces optimum grip. When tires get too hot, they get greasy; too cold, and they remain relatively slick. And when they are overinflated, you're not using the full contact patch; too cold, and you get excessive rollover.
Hope that helps. Good luck!
I think there are more seat of the pants recommendations for street tires than there are those based on pyrometer readings, IMHO. I believe a pyro would eliminate a lot of the guesswork surrounding this issue. And I'm not certain the shoe polish trick would provide a good guide, either (no offense, TD). Certainly, it's better than nothing but it seems that it might provide false readings regarding tire rollover.
Specifically, much of the polish rubs off during your warm up laps, when there's more flex and less pressure in your tires. As your tire pressures build, there's actually less rollover than when you started but since the polish is already gone, you're only seeing what happens at lower pressures. (Am I missing something?)
Pyrometer aside, there is another recommendation from Redline Man with which I tend to agree: Watch the amount of pressure build up from cold to hot. John recommends that you target an increase between 7-9 psi; 10 or more suggests you're overheating the tires, while anything less would mean you're not generating enough heat.
I think this makes sense since it's something I've confirmed with my pyrometer. Whether it's street tires or R compounds, the treads usually develop appropriate heat when they've picked up around 8 psi from cold to hot.
I recently instructed a GT3 owner and wasn't quite sure where we should be since IT IS a very different car than mine. It had the stock 18" PS2's and we began the first session at 32f/34r. At the end of the session, we were at 37f/43r which is a jump of 5 and 9 psi respectively. The car felt fine and we didn't play with it too much since it was his first track day but it seems a more appropriate starting point for this car should be one that generates a little more heat up front, perhaps 30f/34f cold.
As your numbers suggest, Dell, relatively low pressures can cause a great deal of heat build up, but you don't want too much. Tire pressures and heat are similar in that there is a target range which produces optimum grip. When tires get too hot, they get greasy; too cold, and they remain relatively slick. And when they are overinflated, you're not using the full contact patch; too cold, and you get excessive rollover.
Hope that helps. Good luck!
Last edited by joey bagadonuts; 05-08-2005 at 10:43 AM.
05-08-2005, 11:25 PM
#17
Originally Posted by joey bagadonuts
And I'm not certain the shoe polish trick would provide a good guide, either (no offense, TD). Certainly, it's better than nothing but it seems that it might provide false readings regarding tire rollover.
Last edited by TD in DC; 05-09-2005 at 11:04 AM.
05-09-2005, 10:36 AM
#18
Race Director
I just took off a set of 18" Dunlops SSR's of the Cup car that I had corded.. Apparently they need more time to build up heat than I gave them.. 3 out of 4 tires wasted in an afternoon.. 
05-09-2005, 10:59 AM
#19
Still plays with cars.
Lifetime Rennlist
Member
LVDell-
Street tires run best at around 40 lbs HOT. Depending on how new the tires are (new full treads will heat up faster and increase pressure to the point where the car will feel unstable), you need to start at a lower pressure. I'd start at 32 cold all around and monitor pressure carefully. How mcu heatyou generate is a function of temperature, track surface and how hard you drive.
The factory pressures are compromised because low profile tires are prone to pinch flats - there are no potholes on a race track (I hope) so pressures can be optimized for grip. If you run factory cold pressures, you tires will heat and pressure build to well over 50 lbs in back and handling will be twitchy to say the least.
If you're running stock camber settings you will wear the outside edge of the tires quickly
Rgds,
Street tires run best at around 40 lbs HOT. Depending on how new the tires are (new full treads will heat up faster and increase pressure to the point where the car will feel unstable), you need to start at a lower pressure. I'd start at 32 cold all around and monitor pressure carefully. How mcu heatyou generate is a function of temperature, track surface and how hard you drive.
The factory pressures are compromised because low profile tires are prone to pinch flats - there are no potholes on a race track (I hope) so pressures can be optimized for grip. If you run factory cold pressures, you tires will heat and pressure build to well over 50 lbs in back and handling will be twitchy to say the least.
If you're running stock camber settings you will wear the outside edge of the tires quickly
Rgds,
05-09-2005, 12:32 PM
#20
Nordschleife Master
Thread Starter
Thanks Bob.....I started them out at around 29-30 and ended up at full temp when I came off the track to take another reading at about 38-39. Next time I will start about 32 and go from there.
As for the pyrometer, I have no idea what it does, how to use it, and what it tells me. I will start looking into picking one up but still kinda in the dark.
Thanks for everybody's patience! Now that I feel more confident on the track I would really like to learn the technical side of the track experiecne to understand fully everything that goes into setting up my car perfectly.
Cheers,
Dell
As for the pyrometer, I have no idea what it does, how to use it, and what it tells me. I will start looking into picking one up but still kinda in the dark.
Thanks for everybody's patience! Now that I feel more confident on the track I would really like to learn the technical side of the track experiecne to understand fully everything that goes into setting up my car perfectly.
Cheers,
Dell
05-09-2005, 12:42 PM
#21
Originally Posted by LVDell
Thanks Bob.....I started them out at around 29-30 and ended up at full temp when I came off the track to take another reading at about 38-39. Next time I will start about 32 and go from there.
As for the pyrometer, I have no idea what it does, how to use it, and what it tells me. I will start looking into picking one up but still kinda in the dark.
Thanks for everybody's patience! Now that I feel more confident on the track I would really like to learn the technical side of the track experiecne to understand fully everything that goes into setting up my car perfectly.
Cheers,
Dell
As for the pyrometer, I have no idea what it does, how to use it, and what it tells me. I will start looking into picking one up but still kinda in the dark.
Thanks for everybody's patience! Now that I feel more confident on the track I would really like to learn the technical side of the track experiecne to understand fully everything that goes into setting up my car perfectly.
Cheers,
Dell
A one lb change is significant. If you started out at 29-30 and got to 38-39, try going to 30-31, or just leaving it alone, rather than straight to 32. Sounds like you may be running just about right, I mean, if your target is 40 hot, then 38 and 39 isn't all that bad.
TD in DC
05-09-2005, 12:46 PM
#22
Nordschleife Master
Thread Starter
Thanks for the tip Todd
In all honestly, I am not sure what my target is. Sounds like it should be about 40? I feel so lost in the technical stuff. I think I am going to go pick up a book at Borders that talks about all the components that go into a car on the track (pressure, tires, suspension, etc.) so I can start to make better sense of it all.
In all honestly, I am not sure what my target is. Sounds like it should be about 40? I feel so lost in the technical stuff. I think I am going to go pick up a book at Borders that talks about all the components that go into a car on the track (pressure, tires, suspension, etc.) so I can start to make better sense of it all.
05-09-2005, 12:51 PM
#23
I have been told that 40 hot for street tires is a good target. However, as with most things, this is a matter of car feel so you should listen to what your car, through the tires, is trying to tell you.
05-09-2005, 01:40 PM
#24
Addict
Rennlist Member
Rennlist Member
This came from a presentation another Rennlister got from Michelin. How much is specific to their tires I'm not sure, but probably a good starting point for any:
Rear axle slip angle:
Decreasing the rear pressure will increase the potential of the rear axle
Little effect on rear axle slip angle from front pressure changes
Overall vehicle handling behavior and cornering attitude:
Big effect from front pressure changes
Steering gain decreases 20-25% between 26-38psi
Small effect from rear pressure changes: increasing the pressure will tend to reduce the understeer slightly
Overall recommended hot pressure targets:
Front: 32-34psi
Rear: 37-40psi
Forgive the funny formatting, I just cut and pasted. The key answer to your question is at the bottom. Hope this helps.
Rear axle slip angle:
Decreasing the rear pressure will increase the potential of the rear axle
Little effect on rear axle slip angle from front pressure changes
Overall vehicle handling behavior and cornering attitude:
Big effect from front pressure changes
Steering gain decreases 20-25% between 26-38psi
Small effect from rear pressure changes: increasing the pressure will tend to reduce the understeer slightly
Overall recommended hot pressure targets:
Front: 32-34psi
Rear: 37-40psi
Forgive the funny formatting, I just cut and pasted. The key answer to your question is at the bottom. Hope this helps.
05-09-2005, 01:59 PM
#26
Still plays with cars.
Lifetime Rennlist
Member
Dell - Some people like a differential front to rear. Lower pressure in front tends to make the car more neutral in slow corners (sway bars also affect this). For autocross a 4-ish pound differential is a good starting point. On a track with a lot of high speed corners I prefer the car to understeer a bit. I trail brake to compensate in slower corners but I dislike a car that oversteers (loose tail) in fast ones. That's a subjective preference mind you.
A pyrometer is a useful tool to suspension setup and tire pressure. The idea is equal temperature across the tread measured at three locations, inside, outside and middle. Correct camber will provide results within about 10 deg. C, although toe in will cause the inside to heat up more. Once the camber and toe is set correctly, temps can be used to set the pressure. The idea is to get the tire temperature into the range which traction is best. This varies from tire to tire and input from the manufacturer is needed. If the temps are too low (below optimum range) decrease pressure to make the tire twist more and get hotter, if too high, add pressure to do the reverse.
This sounds pretty easy but getting consistent temperature reading is not that easy. An infrared pyromter is easy to use (point it at the tire and read the digital display) but many observe that you have to be consistent about the distance between the sensor and the surface. Pros use probe type equipment which sticks a needle into the tire. They also have several people doing this as soon as the car comes in because the temps can reduce quickly and give false readings if a couple mins goes by between reading the left front and the right rear ... experience counts.
Hope this helps!
A pyrometer is a useful tool to suspension setup and tire pressure. The idea is equal temperature across the tread measured at three locations, inside, outside and middle. Correct camber will provide results within about 10 deg. C, although toe in will cause the inside to heat up more. Once the camber and toe is set correctly, temps can be used to set the pressure. The idea is to get the tire temperature into the range which traction is best. This varies from tire to tire and input from the manufacturer is needed. If the temps are too low (below optimum range) decrease pressure to make the tire twist more and get hotter, if too high, add pressure to do the reverse.
This sounds pretty easy but getting consistent temperature reading is not that easy. An infrared pyromter is easy to use (point it at the tire and read the digital display) but many observe that you have to be consistent about the distance between the sensor and the surface. Pros use probe type equipment which sticks a needle into the tire. They also have several people doing this as soon as the car comes in because the temps can reduce quickly and give false readings if a couple mins goes by between reading the left front and the right rear ... experience counts.
Hope this helps!
05-09-2005, 02:14 PM
#27
Addict
Rennlist Member
Rennlist Member
Originally Posted by LVDell
a 5-6 psi delta between F&R hot?
I've found, at least in my car, that the rears gain heat/pressure faster than the fronts too, so I set cold pressure closer to what I want at hot in the front than the rear. Hope that makes sense.
05-09-2005, 02:24 PM
#28
Addict
Rennlist Member
Rennlist Member
Hey Dell;
Reading just the first few posts is enough to show why you can never get consistent info on tire pressures. I'm not going to wade through two pages of "differing" opinions. Like Paul Harvey, I'll give you the Rest of the Story. If anyone agrees, then you know tires. If you don't... you DON'T. Sorry. No offense intended. Just me being ... well, me.
NEVER IN A MILLION YEARS... should you run lower than recommended pressure in a tire. This is assuming the recomendation is correct. Some of the factory numbers are absolute gibberish. If your car is over 5 years old, the info does not match the current tire technology. Think about it. Get your info from the TIRE company.
RULES -
1) Air is what supports a tire. DUH, right? Well, why would you short change the support of your tires? There is not a tire on this planet - intended for a "sedan" - that should be run under 25psi. If you drive like a grandma, then you can "get away with it." If you drive hard, you are an accident waiting to happen, and are not as fast as you could be.
2) Heat in a tire comes primarily from the mechanical flexure of the carcass, not from surface friction. When you see racers weaving, they may be scrubbing junk off their tires to some extent, but they are primarily FLEXING the tire to build heat. Take air away, and the tire heats up more. Add air and it heats up less. Write it on the back of your hand if you must... right next to Righty Tighty, Lefty Loosey.
3) Pressure increase from cold-to-hot is the KEY parameter you are trying to control. Using rules 1 & 2, you can surmize that the more a tire works mechanically, the more heat it will make, and the more the pressure will increase. You can then further extrapolate that there must be a limit to how much of this expansion you want.
4) Pressure increase of over 10 pounds require you to look closely at your situation. This means the tire is working very hard, and instead of decreasing the hot pressure to what the optimum might be, you should ADD air to better support the tire, therefore reducing heat buildup, and the pressure increase.
I'll tell you right here and now that a street tire likely needs between 34-36psi cold to start, period. Street tires are not designed to be optimized for what I call chemical grip by getting to the right heat range. Heat generally reduces the grip of a street tire in very short order. They usually offer simply what I call mechanical grip; a stiff sidewall to keep them from rolling over.
As such, you want to properly support the tire so that it remains healthy. If it is rolling no farther than the line that most tires have on the corners of the outer tread blocks, you have enough pressure for your driving ability. If it is rolling farther, you need more air to better support the tire. If it is not reaching that point, and you are at 34-36 cold, then this is a signal to remove the board from under the gas pedal and start denting the carpet under there a little more frequently!
Let me offer a perfect air-pressure-tuning case-in-point. Mid Ohio this past weekend. Old Hoosiers. Started at 30/30/28/28 (always refer to pressures LF/RF/LR/RR, by the by). When they warmed up after a couple laps, the were doing well. Frankly, I was too lazy to take the hot pressures. The tires are old and smoked, so I really didn't care either.
Problem; I noted that after 8-10 laps, the car was going a bit loose in the rear. Let's say a bit looser than optimal. It was slowing me down. What to do? The fact that it happened later in the session suggested to me that the rear tires were overheating, and losing grip, and it was consistent in both directions. I still didn't get motivated to take pressures. I simply made a pressure adjustment, but which way?
If you've been paying attention, then you know I ADDED air, in this case 1lb per side. Problem completely solved in the next session. Just the right amount of oversteer to help me get around those blasted off camber turns that litter that joint! If I had bothered to use the air gauge, it would have confirmed that by adding air and better supporting the tire, I REDUCED the pressure increase from cold to hot, and therefore reduced the heat in the tire, increasing the grip.
This is just a scratch on the surface, but its a CORRECT start.
Reading just the first few posts is enough to show why you can never get consistent info on tire pressures. I'm not going to wade through two pages of "differing" opinions. Like Paul Harvey, I'll give you the Rest of the Story. If anyone agrees, then you know tires. If you don't... you DON'T. Sorry. No offense intended. Just me being ... well, me.
NEVER IN A MILLION YEARS... should you run lower than recommended pressure in a tire. This is assuming the recomendation is correct. Some of the factory numbers are absolute gibberish. If your car is over 5 years old, the info does not match the current tire technology. Think about it. Get your info from the TIRE company.
RULES -
1) Air is what supports a tire. DUH, right? Well, why would you short change the support of your tires? There is not a tire on this planet - intended for a "sedan" - that should be run under 25psi. If you drive like a grandma, then you can "get away with it." If you drive hard, you are an accident waiting to happen, and are not as fast as you could be.
2) Heat in a tire comes primarily from the mechanical flexure of the carcass, not from surface friction. When you see racers weaving, they may be scrubbing junk off their tires to some extent, but they are primarily FLEXING the tire to build heat. Take air away, and the tire heats up more. Add air and it heats up less. Write it on the back of your hand if you must... right next to Righty Tighty, Lefty Loosey.
3) Pressure increase from cold-to-hot is the KEY parameter you are trying to control. Using rules 1 & 2, you can surmize that the more a tire works mechanically, the more heat it will make, and the more the pressure will increase. You can then further extrapolate that there must be a limit to how much of this expansion you want.
4) Pressure increase of over 10 pounds require you to look closely at your situation. This means the tire is working very hard, and instead of decreasing the hot pressure to what the optimum might be, you should ADD air to better support the tire, therefore reducing heat buildup, and the pressure increase.
I'll tell you right here and now that a street tire likely needs between 34-36psi cold to start, period. Street tires are not designed to be optimized for what I call chemical grip by getting to the right heat range. Heat generally reduces the grip of a street tire in very short order. They usually offer simply what I call mechanical grip; a stiff sidewall to keep them from rolling over.
As such, you want to properly support the tire so that it remains healthy. If it is rolling no farther than the line that most tires have on the corners of the outer tread blocks, you have enough pressure for your driving ability. If it is rolling farther, you need more air to better support the tire. If it is not reaching that point, and you are at 34-36 cold, then this is a signal to remove the board from under the gas pedal and start denting the carpet under there a little more frequently!
Let me offer a perfect air-pressure-tuning case-in-point. Mid Ohio this past weekend. Old Hoosiers. Started at 30/30/28/28 (always refer to pressures LF/RF/LR/RR, by the by). When they warmed up after a couple laps, the were doing well. Frankly, I was too lazy to take the hot pressures. The tires are old and smoked, so I really didn't care either.
Problem; I noted that after 8-10 laps, the car was going a bit loose in the rear. Let's say a bit looser than optimal. It was slowing me down. What to do? The fact that it happened later in the session suggested to me that the rear tires were overheating, and losing grip, and it was consistent in both directions. I still didn't get motivated to take pressures. I simply made a pressure adjustment, but which way?
If you've been paying attention, then you know I ADDED air, in this case 1lb per side. Problem completely solved in the next session. Just the right amount of oversteer to help me get around those blasted off camber turns that litter that joint! If I had bothered to use the air gauge, it would have confirmed that by adding air and better supporting the tire, I REDUCED the pressure increase from cold to hot, and therefore reduced the heat in the tire, increasing the grip.
This is just a scratch on the surface, but its a CORRECT start.
05-09-2005, 02:41 PM
#29
John,
I have a few questions for you, not because I know enough to challenge what you are saying, but rather to understand what you are saying.
I used to start with the stock recommended pressures on my 996, which are 36/44 (cold). At that recommended pressure, my tires felt greasy and slippery after the first couple of laps. The guy who tuned my suspension recommended that I instead try 34/36 (cold). The grip was much, much better, and the tires did not start getting greasy until 3/4 of the way through the run group.
I started talking to the local 996 gurus at potomac and they suggested that I remove even more air. I played around with this until I got down to 29 psi (cold) all around. The hot pressure was roughly 40 psi all around, and the car felt much, much better. The probe pyrometer showed that I had roughly a 10 degree spread in the front, about a 12-15 degree spread in the rear (insides hotter), and the chalk showed that I still had about 1/4 to 1/2" to go before I hit the sidewall markings. I know that I am not driving as fast as you guys do, but the seat of the pants feeling of the car told me that it was right to remove air rather than add. This was also at Mid-Ohio.
Why would it feel so good to drive with so much less air in my tires?
I have a few questions for you, not because I know enough to challenge what you are saying, but rather to understand what you are saying.
I used to start with the stock recommended pressures on my 996, which are 36/44 (cold). At that recommended pressure, my tires felt greasy and slippery after the first couple of laps. The guy who tuned my suspension recommended that I instead try 34/36 (cold). The grip was much, much better, and the tires did not start getting greasy until 3/4 of the way through the run group.
I started talking to the local 996 gurus at potomac and they suggested that I remove even more air. I played around with this until I got down to 29 psi (cold) all around. The hot pressure was roughly 40 psi all around, and the car felt much, much better. The probe pyrometer showed that I had roughly a 10 degree spread in the front, about a 12-15 degree spread in the rear (insides hotter), and the chalk showed that I still had about 1/4 to 1/2" to go before I hit the sidewall markings. I know that I am not driving as fast as you guys do, but the seat of the pants feeling of the car told me that it was right to remove air rather than add. This was also at Mid-Ohio.
Why would it feel so good to drive with so much less air in my tires?
Last edited by TD in DC; 05-09-2005 at 03:21 PM.
05-09-2005, 03:42 PM
#30
Rennlist Member
Now I'm confused too! Reading the foregoing posts has left me wondering what I should do at my next DE? Help, please!
I have the street Bridgestone S02A's on my car. At my last DE at Lime Rock I started with factory recommended cold pressures, 36F/44R...and after my second run noted that pressures both front and back had risen to 44F/52R!
What cold pressure should I start with? Thanks...
I have the street Bridgestone S02A's on my car. At my last DE at Lime Rock I started with factory recommended cold pressures, 36F/44R...and after my second run noted that pressures both front and back had risen to 44F/52R!
What cold pressure should I start with? Thanks...