Tire Pressures
12-26-2004, 09:37 PM
#1
Instructor
Thread Starter
Tire Pressures
Happy Holidays...wanted to pick your brains on pressures and the relation to understeer and oversteer (or as we Nascar fans say... tight and loose). I'm planning on returning to do some DE's this year, and wanted to see if you had any suggestions for sites that can relate the pressures in front/back/left/right and the handling relationship it makes on your car. I've seen the links to pressures from our friend, Doug, in Ozzieland, but wanted to get some more, so that I fully understand 'tightning up the car or loosening up the car' with tire pressures.
I started the session last Fall with the recommended cold pressures of 36F/44R and started adjusting from there... I think I got too low, but wasn't sure what I was doing, and my instructor didn't know what to suggest, since he was a 944 driver.
Best wishes and thanks for any help,
Roland
I started the session last Fall with the recommended cold pressures of 36F/44R and started adjusting from there... I think I got too low, but wasn't sure what I was doing, and my instructor didn't know what to suggest, since he was a 944 driver.
Best wishes and thanks for any help,
Roland
12-26-2004, 09:56 PM
#2
Nordschleife Master
This is a huge issue both in terms of importance and complexity.
The "right" pressure depends so much on the tires, ambient conditions, car set-up and driver style that there's no one right pressure. Competition or street tires? Cold cloudy day or hot and sunny? Got stock springs and Boges or Eibachs and Konis?
The best advice is to get a starting pressure and then adjust the pressure after, and maybe during, each run. Chalking the tires is a good, low tech approach. Then you can see if the tire is rolling over too much, or is too hard. Checking air pressure at the end of the run is a good idea as well.
I think the biggest danger is to have the tires too hard to start and have them balloon up during the session as they get hot. This will cause a significant loss of traction.
36/44 psi strikes me as too hard for the track. I'm assuming those are street tires as well. (Waaay to hard for competition tires.) There's no allowance for heating is those numbers.
The "right" pressure depends so much on the tires, ambient conditions, car set-up and driver style that there's no one right pressure. Competition or street tires? Cold cloudy day or hot and sunny? Got stock springs and Boges or Eibachs and Konis?
The best advice is to get a starting pressure and then adjust the pressure after, and maybe during, each run. Chalking the tires is a good, low tech approach. Then you can see if the tire is rolling over too much, or is too hard. Checking air pressure at the end of the run is a good idea as well.
I think the biggest danger is to have the tires too hard to start and have them balloon up during the session as they get hot. This will cause a significant loss of traction.
36/44 psi strikes me as too hard for the track. I'm assuming those are street tires as well. (Waaay to hard for competition tires.) There's no allowance for heating is those numbers.
12-26-2004, 10:01 PM
#3
Instructor
Thread Starter
Tire Pressures
Yes, running street tires (Michelin Pilot Sports). When I lowered the pressures to 32F/40R (cold) it seem to get away from easier, but wasn't sure what I was feeling was loose or tight. I understand the concepts of over/understeer, but need help. Does lowering the pressures in the front/back create over or understeer, and if so..which tires on which sides?
Too many questions I'm sure, but I'd love to be able to understand and adjust my car. When I hear the announcers say...'they'll take a pound out of the back to tighten up the car'...or something like that; I'd like to be able to undertand it.
Best wishes once again,
Roland
Too many questions I'm sure, but I'd love to be able to understand and adjust my car. When I hear the announcers say...'they'll take a pound out of the back to tighten up the car'...or something like that; I'd like to be able to undertand it.
Best wishes once again,
Roland
12-26-2004, 10:23 PM
#4
Nordschleife Master
The dynamics of a car cornering are pretty complex with the shocks, springs, tires, sway bars and alignment all coming into play. (Not that I really understand it well even with my ME degree)
The simplified approach is that tightening an end of the car, be that with stiffer springs, shocks, tires or sway bars, reduces traction. So having a tighter front end makes the car understeer and a tigher back end makes it understeer. Making an adjustment in tire pressure can bring these into balance, or give the desired effect. Some P-cars have easily adjusted sway bars and can be balanced that way. And then there are adjustable shocks.
Lot's of ways to improve and tune the car. And spend $$$.
The simplified approach is that tightening an end of the car, be that with stiffer springs, shocks, tires or sway bars, reduces traction. So having a tighter front end makes the car understeer and a tigher back end makes it understeer. Making an adjustment in tire pressure can bring these into balance, or give the desired effect. Some P-cars have easily adjusted sway bars and can be balanced that way. And then there are adjustable shocks.
Lot's of ways to improve and tune the car. And spend $$$.
12-27-2004, 11:45 AM
#5
Racer
Join Date: Sep 2004
Location: Moore, SC
Posts: 438
Likes: 0
Received 0 Likes
on
0 Posts
. . . . So having a tighter front end makes the car understeer and a tigher back end makes it understeer.
I believe that having a tighter back end (reducing traction so that the rearend breaks free first) would make the car oversteer not understeer.
I believe that having a tighter back end (reducing traction so that the rearend breaks free first) would make the car oversteer not understeer.
12-27-2004, 05:35 PM
#6
Nordschleife Master
Umm....No posting after heavy holiday meals...
That should be:
"So having a tighter front end makes the car understeer and a tigher back end makes it oversteer. "
That should be:
"So having a tighter front end makes the car understeer and a tigher back end makes it oversteer. "
12-27-2004, 05:36 PM
#7
Rennlist Member
Guys, 44psi????????????????? what are you nuts?
try starting out with 34psi front and 35 psi rear and start from there. depending on the type of street tires you have, you will probably end up with 32 psi cold.
remember, the pressures you are looking at are the maufact. recomendations for max pressures. 44 hot will be more like 50psi , and that is not only a formula for a loose car (if on the rear tires) but a dangerous pressure for blowout risk.
tires can effect push and oversteer, but pressures are really for fine tunining. however, a tire with the noted pressures will be more than undrivable (if really driven to the limits, as those limits will be quite a bit lower than if the tires were properly inflated)
In speed GT we ran Toyo T1S (275 tread wear )and then proxy RA1 (40 tread wear) most teams ran the pure street tires in the 32psi cold range in the end, and the RA1 race DOT tires in the 27cold range ( 38hot street / 35ish psi hot race tire)
and yes, if you are pushing, you can lower front pressures and get some improvement. if you are loose, you can lower rear pressures. sometimes an opposite amount of little of both can help too.
mk
try starting out with 34psi front and 35 psi rear and start from there. depending on the type of street tires you have, you will probably end up with 32 psi cold.
remember, the pressures you are looking at are the maufact. recomendations for max pressures. 44 hot will be more like 50psi , and that is not only a formula for a loose car (if on the rear tires) but a dangerous pressure for blowout risk.
tires can effect push and oversteer, but pressures are really for fine tunining. however, a tire with the noted pressures will be more than undrivable (if really driven to the limits, as those limits will be quite a bit lower than if the tires were properly inflated)
In speed GT we ran Toyo T1S (275 tread wear )and then proxy RA1 (40 tread wear) most teams ran the pure street tires in the 32psi cold range in the end, and the RA1 race DOT tires in the 27cold range ( 38hot street / 35ish psi hot race tire)
and yes, if you are pushing, you can lower front pressures and get some improvement. if you are loose, you can lower rear pressures. sometimes an opposite amount of little of both can help too.
mk
Trending Topics
12-28-2004, 07:36 PM
#8
Addict
Rennlist Member
Rennlist Member
Join Date: Oct 2003
Location: Gone. On the Open Road
Posts: 16,496
Received 1,638 Likes
on
1,069 Posts
Originally Posted by wino5150
...wanted to pick your brains on pressures and the relation to understeer and oversteer (or as we Nascar fans say... tight and loose).
http://www.tirerack.com/tires/tirete...n/racepres.jsp
12-28-2004, 09:19 PM
#9
Addict
Rennlist Member
Rennlist Member
Re: tire pressures, I agree with Mark that 38/44 psi cold is way WAY to high for track use. I ran 18" Michelin Pilot Sports at 32f/30r psi hot which meant I was starting off the day at 26f/24r psi cold! This number was determined by the previous owner who worked with Michelin engineers when running the One Lap of America. Now your numbers may vary due to running 16" and different suspension but this will at least give you an idea of the range. For most street tires, I'd shoot for 38-40 psi hot, meaning 32-34 cold. You're better off trying to change the car's balance via adjustable antiroll bars. You can really eat up tires if you try to change the balance of the car with radical pressure differences.
Re: NASCAR terminology, as far as I can tell:
Loose = oversteer
Free = neutral or mild oversteer
Tight = mild understeer
Push = understeer
Pushin' like a dump truck = severe understeer
The NASCAR guys use tire pressures to adjust their total springrates at each corner. This is only a fine tune adjustment as they've already done lots of testing and have years of experience to set the shock damping and spring rates. Unlike most racing series, NASCAR doesn't allow driver adjustable sway (antiroll) bars, which is a more common way of changing a car's balance. All the car's adjustments are made in the pits so that's why you see them using wrenches at the rear of the car. They can adjust cross weight (wedge) and roll center height (raise/lower the track bar) to make bigger handling changes.
Hope that helps.
Re: NASCAR terminology, as far as I can tell:
Loose = oversteer
Free = neutral or mild oversteer
Tight = mild understeer
Push = understeer
Pushin' like a dump truck = severe understeer
The NASCAR guys use tire pressures to adjust their total springrates at each corner. This is only a fine tune adjustment as they've already done lots of testing and have years of experience to set the shock damping and spring rates. Unlike most racing series, NASCAR doesn't allow driver adjustable sway (antiroll) bars, which is a more common way of changing a car's balance. All the car's adjustments are made in the pits so that's why you see them using wrenches at the rear of the car. They can adjust cross weight (wedge) and roll center height (raise/lower the track bar) to make bigger handling changes.
Hope that helps.