Rec'd Tire Pressure...why so much conflict?
05-09-2005, 04:15 PM
#31
Mr. Excitement
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Just goes to show that there are very few absolutes in world.
There are on the other hand many variables involved. Tire volume, relative humidity of air fill, tire type, driving style, and suspension setup, track temps the list can go on and on.
I recommend that you ask for and find someone at the track with you that has some tire experience to help you understand your car and tire needs. Someone with good basic knowledge of tire reading and pressure adjustments will get you close in no time.
There are on the other hand many variables involved. Tire volume, relative humidity of air fill, tire type, driving style, and suspension setup, track temps the list can go on and on.
I recommend that you ask for and find someone at the track with you that has some tire experience to help you understand your car and tire needs. Someone with good basic knowledge of tire reading and pressure adjustments will get you close in no time.
05-09-2005, 04:40 PM
#33
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Hey, John H - for once I do not agree with you. If you take a 911 with 36/44 cold out on street tires and lap a road course on a 70 degree day, within ten minutes or so the pressure will have built up to somewhere north of 50 lbs and dangerously close to the maximum inflation pressure of the tire (usually around 55 lbs). As described above the handling will feel "greasy". Bleeding down to 40 hot cures the problem instantly. While I agree on what causes the heating (twist in the tire itself induced by cornering forces and slip angles) I would NEVER suggest someone add air to a tire running 17 or 18 inch street rubber which was measured at over 40 lbs hot. Never.
Rgds,
Rgds,
05-09-2005, 04:54 PM
#34
Originally Posted by Bob Rouleau
Hey, John H - for once I do not agree with you. If you take a 911 with 36/44 cold out on street tires and lap a road course on a 70 degree day, within ten minutes or so the pressure will have built up to somewhere north of 50 lbs and dangerously close to the maximum inflation pressure of the tire (usually around 55 lbs). As described above the handling will feel "greasy". Bleeding down to 40 hot cures the problem instantly.
TD in DC
05-09-2005, 05:09 PM
#35
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TD - I haven't seen the OG ones. F1 cars use automatic bleed off valves (or at least they did when I had somoething to do with one of the teams). The tricky bit is calibrating the things apparently. The ones I saw replace the usual valve and don't require an additional hole. The F1 versions cost $1,200 each. Ouch. For that price I'll check and modify pressure after every run!
Rgds,
Rgds,
05-09-2005, 06:04 PM
#36
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Originally Posted by BobbyC
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...
What cold pressure should I start with? Thanks...
You need to know the cold and hot pressures for a single session. Often, your cars will pick up pressures as temps rise during the day, especially in the morning. My guess is that your starting pressure for the second session was probably higher than your original starting point.
05-09-2005, 06:19 PM
#37
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Bobby, the trick is to check your pressure after the run and (on street tires) if the pressure is over 40 lbs, bleed them down to 40. That;s why you see so many guys jumping out in the pits with tire gauges in hand.
Rgds,
Rgds,
05-09-2005, 08:42 PM
#39
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Hey Guys;
First, I did not intend to start a flame war. I just strongly dissagree with some of the info offered, and I only read a couple notes.
TD -
This is a very common phenomenon. What feels good to the driver must be seperated from what is best for the tire. The tire comes first, for if it fails, so too does the driver!
Then, if we are talking maximization of lap times, one must go further and divorce oneself from what "feels good" because it is probably SLOW. To be more precise, the driver is.
As a general rule, and as a for-instance, I will offer to you that an R-compound tire should be started somewhere around 30psi front on a 944, with a couple pounds down for the shoes on the rear (I honestly have not fiddled with a 911 to know). This is a pressure that will most likely work to the tire's advantage in terms of performance and heatlh. Whether that makes the driver happy depends on their ability to wheel the car.
A driver that is less agressive will likely find that the car feels skatey at these pressures. They will also not be able to get the tires up to temp. This is simply because they do not drive hard enough. The lower pressures feel more comforting because the tire is more resilient and less edgy.
If this driver seeks to up the ante and increase their speeds, they will likely report that what used to feel good now does not yield the grip to do what they want. They are now overheating their tires because the cold pressures are too low. Again, the increase from cold-to-hot will tell the tale. They are also lacking mechanical grip because the tire is not strongly supported, and therefore will not resist cornering forces.
If you are feeling that the car does not stick well at a higher pressure, but feels more comfortable at a lower pressure, it is likely that if you were to find the wherewithall to drive faster (and in control, of course), those settings would magically prove effective.
----------------------------------------------------------------------------------------
Now while I'm typing I see many people chiming in. Many of these comments are aimed at things not even spoken about, or attribute things to me I did not offer or support. As I mentioned, what I started with was just scratching the surface. Bob and Kurt bring up more detail, and these things are key to really splitting all the hairs.
Banking on experience, I started with 30/30/28/28 on a new set of Toyos over the weekend. I got 42/40/39/36 hot. I bled off 2 from the LF to meet the target, and 2 from the RR to bring it up a tad in temp. I then got 40/40/39/37 and called it a victory. Right Rears never get hot on my car, even at Mid-O. The tires looked happy and performed very well.
If you bleed off to 40 hot after a run, you should then keep track of your pressure increase from cold to hot for the next run. Keeping track of that increase is a key to keeping tires healthy. The tire should come first.
When you get right down to it, you must experiment to find what works. It has been only in the last 3-4 years that I've hammered tires hard enough to matter, so I've really started paying attention.
You NEVER want to underinflate, you NEVER want to see large cold-to-hot increases, and you NEVER want to be anywhere near 50psi hot on ANY tire.
Consider the can or worms duly stirred!
First, I did not intend to start a flame war. I just strongly dissagree with some of the info offered, and I only read a couple notes.
TD -
This is a very common phenomenon. What feels good to the driver must be seperated from what is best for the tire. The tire comes first, for if it fails, so too does the driver!
Then, if we are talking maximization of lap times, one must go further and divorce oneself from what "feels good" because it is probably SLOW. To be more precise, the driver is.As a general rule, and as a for-instance, I will offer to you that an R-compound tire should be started somewhere around 30psi front on a 944, with a couple pounds down for the shoes on the rear (I honestly have not fiddled with a 911 to know). This is a pressure that will most likely work to the tire's advantage in terms of performance and heatlh. Whether that makes the driver happy depends on their ability to wheel the car.
A driver that is less agressive will likely find that the car feels skatey at these pressures. They will also not be able to get the tires up to temp. This is simply because they do not drive hard enough. The lower pressures feel more comforting because the tire is more resilient and less edgy.
If this driver seeks to up the ante and increase their speeds, they will likely report that what used to feel good now does not yield the grip to do what they want. They are now overheating their tires because the cold pressures are too low. Again, the increase from cold-to-hot will tell the tale. They are also lacking mechanical grip because the tire is not strongly supported, and therefore will not resist cornering forces.
If you are feeling that the car does not stick well at a higher pressure, but feels more comfortable at a lower pressure, it is likely that if you were to find the wherewithall to drive faster (and in control, of course), those settings would magically prove effective.
----------------------------------------------------------------------------------------
Now while I'm typing I see many people chiming in. Many of these comments are aimed at things not even spoken about, or attribute things to me I did not offer or support. As I mentioned, what I started with was just scratching the surface. Bob and Kurt bring up more detail, and these things are key to really splitting all the hairs.
Banking on experience, I started with 30/30/28/28 on a new set of Toyos over the weekend. I got 42/40/39/36 hot. I bled off 2 from the LF to meet the target, and 2 from the RR to bring it up a tad in temp. I then got 40/40/39/37 and called it a victory. Right Rears never get hot on my car, even at Mid-O. The tires looked happy and performed very well.
If you bleed off to 40 hot after a run, you should then keep track of your pressure increase from cold to hot for the next run. Keeping track of that increase is a key to keeping tires healthy. The tire should come first.
When you get right down to it, you must experiment to find what works. It has been only in the last 3-4 years that I've hammered tires hard enough to matter, so I've really started paying attention.
You NEVER want to underinflate, you NEVER want to see large cold-to-hot increases, and you NEVER want to be anywhere near 50psi hot on ANY tire.
Consider the can or worms duly stirred!
05-09-2005, 08:47 PM
#40
Nordschleife Master
Thread Starter
Originally Posted by RedlineMan
Consider the can or worms duly stirred!
Great info and it is much appreciated! Thank you again.
Cheers,
Dell
05-09-2005, 09:02 PM
#41
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Originally Posted by Bob Rouleau
Hey, John H - for once I do not agree with you. I would NEVER suggest someone add air to a tire running 17 or 18 inch street rubber which was measured at over 40 lbs hot. Never. Rgds,
If I saw a tire that had more than a 10psi increase, say 15psi... this would suggest to me that the tire was undersupported, and that I should then fear for its health and longevity. Raising the cold pressure in the tire would therefore support it better, reduce flex-induced heat, and drop the hot pressure and pressure increase.
My Hoosier example from this weekend proves - even without the numbers to back it up (due to my laziness) - that the strategy can be effective. My highly tuned ****-o-meter tells me so!
This may or may not work, but it is a counterintuitive and viable option that most would not think of. I never suggested that you simply keep adding air in perpetuity, approaching 50psi. Certainly not. No one is looking for the grip of a Flinstone-mobile.
If you can't cool down your tires by properly supporting them with the right air pressure, you either better pull some plug wires or get a better setup!
05-09-2005, 09:04 PM
#42
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Originally Posted by kurt M
Just goes to show that there are very few absolutes in world.
05-09-2005, 11:48 PM
#43
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BobbyC - Yes I suggest aiming for 40 hot all around. Then you can fine tune to find what works best for you. Lower pressure in front will mean a bit more understeer - some people prefer that. I prefer to get all 4 tires working evenly and then adjust for balance. After tuning most prefer a 2 lb differential on a 911. For a 944, ask John H, I am ignorant of 944 tuning whereas he's a guru - a fact I acknowledge even though we don't agree on tire pressures. 
05-10-2005, 12:33 AM
#44
John,
I believe you when you say that increasing pressures can help under certain circumstancres, and I know that I can, and someday will, drive significantly faster than I drive today. However, I do not think that I am having a problem heating my tires up, if for no other reason than not driving as smoothly as I should despite my best efforts. Between monitoring (1) chalk marks; (2) hot and cold pressures; and (3) temperatures using a probe pyrometer (not to mention making adjustments 1 psi at a time), I lseemed to learn that I needed to start with pressures that were below the recommended stock pressures of 36/44. I mean, it is not really that surprising that my tires would have hot pressures that far exceeded 40 psi hot when starting with pressures of 36/44. At the stock pressures, my tires felt extremely greasy, and the chalk mark would remain well onto the tread. Only after I started decreasing the pressure did the chalk mark start moving down the side of the tire (at no time did I even reach wtihin 1/2" of the sidewall marker), the hot pressures approach 40 psi, and did my tires start feeling good. I know that I am using my tires somewhat because my data logger shows that I am exceeding 1 G lateral forces on a regular basis (I am fairly consistent, even with my errors), which I assume will heat up the tires sufficiently. Based on my conversations with the local 996 drivers, the readings after runs, and the feeling of the car, I found that performance was best (at least at Mid Ohio) when my cold pressure was about 29 psi. Since the chalk marks did not exceed the sideway markings, I assume that I am not experiencing any sidewall flex. I can very well imagine that if the sidewalls were flexing too much that I would get a significant increase in temperature/pressure that would be better corrected by adding, rather than subtracting, air.
My novice instincts tell me that overpressure can give you the greasy feeling because your tire patch is too small the same way that underpressure and the accompanying overheating can give you the greasy feeling. I imagine that, if you weren't paying attention, you could easily make the greasy feeling worse if you misinterpret which end of the spectrum is causing the problem.
Thanks for the information and fostering discussion, which helps expand my understanding of the possible causes for the greasy feeling that wrong pressures/temperatures can cause.
TD in DC
P.S. Will you be at Watkins Glen for ARPCA's event in two weekends?
I believe you when you say that increasing pressures can help under certain circumstancres, and I know that I can, and someday will, drive significantly faster than I drive today. However, I do not think that I am having a problem heating my tires up, if for no other reason than not driving as smoothly as I should despite my best efforts.
Between monitoring (1) chalk marks; (2) hot and cold pressures; and (3) temperatures using a probe pyrometer (not to mention making adjustments 1 psi at a time), I lseemed to learn that I needed to start with pressures that were below the recommended stock pressures of 36/44. I mean, it is not really that surprising that my tires would have hot pressures that far exceeded 40 psi hot when starting with pressures of 36/44. At the stock pressures, my tires felt extremely greasy, and the chalk mark would remain well onto the tread. Only after I started decreasing the pressure did the chalk mark start moving down the side of the tire (at no time did I even reach wtihin 1/2" of the sidewall marker), the hot pressures approach 40 psi, and did my tires start feeling good. I know that I am using my tires somewhat because my data logger shows that I am exceeding 1 G lateral forces on a regular basis (I am fairly consistent, even with my errors), which I assume will heat up the tires sufficiently. Based on my conversations with the local 996 drivers, the readings after runs, and the feeling of the car, I found that performance was best (at least at Mid Ohio) when my cold pressure was about 29 psi. Since the chalk marks did not exceed the sideway markings, I assume that I am not experiencing any sidewall flex. I can very well imagine that if the sidewalls were flexing too much that I would get a significant increase in temperature/pressure that would be better corrected by adding, rather than subtracting, air.My novice instincts tell me that overpressure can give you the greasy feeling because your tire patch is too small the same way that underpressure and the accompanying overheating can give you the greasy feeling. I imagine that, if you weren't paying attention, you could easily make the greasy feeling worse if you misinterpret which end of the spectrum is causing the problem.
Thanks for the information and fostering discussion, which helps expand my understanding of the possible causes for the greasy feeling that wrong pressures/temperatures can cause.
TD in DC
P.S. Will you be at Watkins Glen for ARPCA's event in two weekends?
05-10-2005, 08:33 AM
#45
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Some other points...
- "40 Hot" is a number that derives from where R-compound tires generally like to be, and may indeed have no correlation to street tires in anyone's particular situation. Further, this is again only when a tire has achieved a proper increase from cold to hot. Most R tires like to be somewhere between 175-200 degrees carcass temp to optimize the rubber compound for grip, which is about what you will see if you have the above. Street tires don't necessarily work that way, as the compound is not really designed to get sticky. They are relying more on "mechanical" sidewall grip.
- Tire pressure graphs are like a mountain with a rounded top. You can be under-inflated, which would put you on the uphill side of the graph, looking left-to-right. Overinflated would see you on the downhill side of the graph. Nearing proper temps would see you up around the peak somewhere. What adjustment you make to achieve a desired effect depends on what side of optimum you are.
- Indeed, overinflation will make the car skate around quite a bit. That graph of a mountain would show that the uphill side (underinflated) had quite a bit less slope than the downhill (overinflated) side. Only a few pounds underinflated will not have as dramatic an effect on grip as a few overinflated, for most intermediate drivers. However, hot shoes will quickly overheat an underinflated tire where intermediates won't.
As a for-instance; My new Toyos this weekend. I set them at 35 cold for proper heat cycling and went out for the first session on Friday, also re-learning the track again after a year away. I was shocked to find the car had absolutely no grip at all. I was not expecting "on rails" mind you, but it did not even feel like the car was connected to the road at all. In the back of my mind I'm thinking "I did not spend all this money on suspension to have the street car feel again!" Even though I knew the tires were green, I couldn't help feeling a bit bummed.
Putting my old set of Hoosiers on for the second session was a revelation. I felt every seam, every ripple... this was my car again. I was hammering!! Putting the Toyos back on for Sunday morning had me a bit unsure, but not to worry. Set to proper pressures (30/30/28/28 to start), they performed quite well for it being the first hot session on them. I was a happy camper!
TD - you have a perfect regimen there. You have all the bases covered, and while all that may be a bit of overkill for street tires (simply watching the "roll-over" marks will generally suffice), you will be all set for a possible future switch to R tires. Your process has all the bases covered!
- "40 Hot" is a number that derives from where R-compound tires generally like to be, and may indeed have no correlation to street tires in anyone's particular situation. Further, this is again only when a tire has achieved a proper increase from cold to hot. Most R tires like to be somewhere between 175-200 degrees carcass temp to optimize the rubber compound for grip, which is about what you will see if you have the above. Street tires don't necessarily work that way, as the compound is not really designed to get sticky. They are relying more on "mechanical" sidewall grip.
- Tire pressure graphs are like a mountain with a rounded top. You can be under-inflated, which would put you on the uphill side of the graph, looking left-to-right. Overinflated would see you on the downhill side of the graph. Nearing proper temps would see you up around the peak somewhere. What adjustment you make to achieve a desired effect depends on what side of optimum you are.
- Indeed, overinflation will make the car skate around quite a bit. That graph of a mountain would show that the uphill side (underinflated) had quite a bit less slope than the downhill (overinflated) side. Only a few pounds underinflated will not have as dramatic an effect on grip as a few overinflated, for most intermediate drivers. However, hot shoes will quickly overheat an underinflated tire where intermediates won't.
As a for-instance; My new Toyos this weekend. I set them at 35 cold for proper heat cycling and went out for the first session on Friday, also re-learning the track again after a year away. I was shocked to find the car had absolutely no grip at all. I was not expecting "on rails" mind you, but it did not even feel like the car was connected to the road at all. In the back of my mind I'm thinking "I did not spend all this money on suspension to have the street car feel again!" Even though I knew the tires were green, I couldn't help feeling a bit bummed.
Putting my old set of Hoosiers on for the second session was a revelation. I felt every seam, every ripple... this was my car again. I was hammering!! Putting the Toyos back on for Sunday morning had me a bit unsure, but not to worry. Set to proper pressures (30/30/28/28 to start), they performed quite well for it being the first hot session on them. I was a happy camper!
TD - you have a perfect regimen there. You have all the bases covered, and while all that may be a bit of overkill for street tires (simply watching the "roll-over" marks will generally suffice), you will be all set for a possible future switch to R tires. Your process has all the bases covered!