Cold Tire Pressure
09-01-2009, 11:19 AM
#18
Rennlist Member
I'm a bit of a maverick when it comes to tire pressures on my '99 C2 coupe.
I use the factory lable setting at all times: 36 front, 44 rear. I check them monthly and top them off if they need it. I check them in the morning before I drive the car. I check them monthly and top them off if they need it (note: intentional repeat). I use N-spec tires exclusively, and make certain the build dates match up. I use the same tires for street and track (DE) events. I use the same tire pressures for both.
I have the alignment done every year.
No uneven wear. Ever.
OBC reads over 30MPG on flat sections of highway (Florida, near sea level, 93 octane). At DE's, I run in the Red Run Group (Instructor). When on the track, I don't have any problem keeping up with traffic on street tires, or getting out of the way of Turbo's and race cars, if necessary.
I have the alignment done every year (intentional repeat).
I replace the rear tires about every 16 months (I do about 6 DE's a year) and the fronts every 32 months.
If you drive and maintenance your 996 like a Camry (nee refridgerator-like appliance), you will be disappointed with the results.
I use the factory lable setting at all times: 36 front, 44 rear. I check them monthly and top them off if they need it. I check them in the morning before I drive the car. I check them monthly and top them off if they need it (note: intentional repeat). I use N-spec tires exclusively, and make certain the build dates match up. I use the same tires for street and track (DE) events. I use the same tire pressures for both.
I have the alignment done every year.
No uneven wear. Ever.
OBC reads over 30MPG on flat sections of highway (Florida, near sea level, 93 octane). At DE's, I run in the Red Run Group (Instructor). When on the track, I don't have any problem keeping up with traffic on street tires, or getting out of the way of Turbo's and race cars, if necessary.
I have the alignment done every year (intentional repeat).
I replace the rear tires about every 16 months (I do about 6 DE's a year) and the fronts every 32 months.
If you drive and maintenance your 996 like a Camry (nee refridgerator-like appliance), you will be disappointed with the results.
09-01-2009, 07:26 PM
#19
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those are just a starting point for a generic set of tires.
move up/down based on your butt-o-meter.
for trac use i would try to keep hot temps in the 40psi range.
move up/down based on your butt-o-meter.
for trac use i would try to keep hot temps in the 40psi range.
09-01-2009, 08:52 PM
#20
Instructor
09-01-2009, 09:01 PM
#21
Instructor
im using kumho exsta xs's at 40 rear 32 front.
if you dont know what pressure to run go by the sticker. regardless of where you drive the car you can always check the tire roll with chalk. this will tell you if your pressure is too low if you rub the chalk off the sidewall and too much pressure will not only not rub off chalk but you will lose traction and get center tread wear. (the latter only noticable over time). there is always a few psi leeway up and down from factory psi.
if you dont know what pressure to run go by the sticker. regardless of where you drive the car you can always check the tire roll with chalk. this will tell you if your pressure is too low if you rub the chalk off the sidewall and too much pressure will not only not rub off chalk but you will lose traction and get center tread wear. (the latter only noticable over time). there is always a few psi leeway up and down from factory psi.
09-01-2009, 11:01 PM
#22
my car was made in 1999 with 1999 era tires.
Having mused all of that now, methinks a bit lower tire pressures may be warranted, with a close look at wear patterns.
09-01-2009, 11:21 PM
#23
Instructor
Join Date: Jul 2008
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And lastly, Altitude
http://www.tirerack.com/tires/tirete...jsp?techid=167
http://www.tirerack.com/tires/tirete...jsp?techid=167
In essense, I guess I'm actually running 2-4 below recommended, which makes sense that mine equates to. 32-33F and 40-42R
Cool - so, I'd chime in w/ my PS2s that 33/41 is optimal... my $0.02 on the topic.
Regarding using "sidewall pressures" - now that's just crazy.... isn't that max pressure recommended by the mfgr?
09-02-2009, 03:14 PM
#24
Three Wheelin'
+1 ...furthermore, tyre manufacturers can't recomend pressures...the weight of the car, the tune of the suspension, and the location of the engine are key variables.
09-02-2009, 07:31 PM
#25
I tried many combinations and settled at 36/38. My current set of rear PS N1's I ran at 36/38 and have over 16k on them. Previous set of rears ran at 36/40 and also got over 16k. At 40cold the rears wore evenly across the tire with the most wear dead center. At 38cold the rears wore most on the inner edge, less on the outer, and the 38 seems a bit more neutra. On ROW 030. Can steer the car with the throttle at AX. -.4/-1.4 camber (mild). Two sets of rears for each set of fronts.
09-02-2009, 08:23 PM
#26
Burning Brakes
Wow! Guess it's obvious, but I've never thought much about altitude. I tend to drive at 2k - 4k altitude - and I tend to run 1-2psi below recommended, generally... I get even wear except for an aggressive alignment - I tend to get some wear on the outside with some added neg camber.
In essense, I guess I'm actually running 2-4 below recommended, which makes sense that mine equates to. 32-33F and 40-42R
Cool - so, I'd chime in w/ my PS2s that 33/41 is optimal... my $0.02 on the topic.
Regarding using "sidewall pressures" - now that's just crazy.... isn't that max pressure recommended by the mfgr?
In essense, I guess I'm actually running 2-4 below recommended, which makes sense that mine equates to. 32-33F and 40-42R
Cool - so, I'd chime in w/ my PS2s that 33/41 is optimal... my $0.02 on the topic.
Regarding using "sidewall pressures" - now that's just crazy.... isn't that max pressure recommended by the mfgr?
09-03-2009, 08:49 AM
#28
Drifting
My 2002 Targa was wearing out rear tires in the middle so that they were down to the wear bars at 6-8000 kilometers. Not happy -- it was expensive.
So, I went down to 38 on the rears and now the tires wear evenly over the width of the tread -- am now at 16000 kilometers and not half worn -- same brand and type of tire.
Porsche dealer checked the alignement -- on all 4 wheels and said -- perfect -- not even a fraction of -- all 4 to within 1% of middle of spec.
Where did Porsche get 44 for the rear?
Go figure!!!
So, I went down to 38 on the rears and now the tires wear evenly over the width of the tread -- am now at 16000 kilometers and not half worn -- same brand and type of tire.
Porsche dealer checked the alignement -- on all 4 wheels and said -- perfect -- not even a fraction of -- all 4 to within 1% of middle of spec.
Where did Porsche get 44 for the rear?
Go figure!!!
The short answer to your question, though, is safety and liability. Every tire has a specific pressure for a given load where it offers the best grip. Below this, and it deforms under load, can't effectively press all that tread to the pavement, and grip quickly reduces- in the worst case, you get roll-under. Above this pressure, and it balloons out and grip is lost. Generally speaking, grip is lost quicker at low pressures than high pressures. Wider tires grip better in corners than narrow (it's related to contact patch and slip angle).
Case in point is the Boxster- perfect 50/50 weight distribution, so you'd expect the tires to be identical sizes and pressures, right? Instead, Porsche specced out smaller tires in the front. You might think the front tires would therefore need to be at higher pressure than the rear since the contact patch is smaller, and each square inch is carrying more weight. But not Porsche- they again specced out lower pressure in the front than the back that cause the car to understeer like a FWD! Swapping larger tires in the front and pumping the front pressure up reduces the tendency, and gives the car neutral handling.
With so much weight in the rear of a 996, Porsche had to go to a lot of effort to keep it facing the right way at the edge of traction. So, they specced out tire sizes and tire pressures that maximize rear grip. If you enter a corner with 44psi in your rear tires, the weight transfer towards the outside will compress them flat and give excellent traction. At lower pressures, the tire will deform and give less traction. In fact, this is what Porsche did in the front- while the rear tires are getting excellent traction, the front tires are deforming under load at stock pressures, and break lose before the rear tires under nearly all conditions, giving you nice, safe, boring understeer. If you raise the front tire pressure, you will find your car performs completely differently at the edge of traction and your back end breaks lose more, simply because you're now able to push the car harder with the better front grip.
In short, 44psi = good for car control for the general public, bad for tire wear.
09-04-2009, 07:42 PM
#29
Instructor
i actually learned something here about deflection and the need for higher psi at high speed=
http://www.tirerack.com/tires/tirete....jsp?techid=72
http://www.tirerack.com/tires/tirete....jsp?techid=72
09-05-2009, 02:05 PM
#30
Three Wheelin'
How accurate was the gauge you're using? You could have been inadvertently overinflating above 44psi and making the problem even worse.
The short answer to your question, though, is safety and liability. Every tire has a specific pressure for a given load where it offers the best grip. Below this, and it deforms under load, can't effectively press all that tread to the pavement, and grip quickly reduces- in the worst case, you get roll-under. Above this pressure, and it balloons out and grip is lost. Generally speaking, grip is lost quicker at low pressures than high pressures. Wider tires grip better in corners than narrow (it's related to contact patch and slip angle).
Case in point is the Boxster- perfect 50/50 weight distribution, so you'd expect the tires to be identical sizes and pressures, right? Instead, Porsche specced out smaller tires in the front. You might think the front tires would therefore need to be at higher pressure than the rear since the contact patch is smaller, and each square inch is carrying more weight. But not Porsche- they again specced out lower pressure in the front than the back that cause the car to understeer like a FWD! Swapping larger tires in the front and pumping the front pressure up reduces the tendency, and gives the car neutral handling.
With so much weight in the rear of a 996, Porsche had to go to a lot of effort to keep it facing the right way at the edge of traction. So, they specced out tire sizes and tire pressures that maximize rear grip. If you enter a corner with 44psi in your rear tires, the weight transfer towards the outside will compress them flat and give excellent traction. At lower pressures, the tire will deform and give less traction. In fact, this is what Porsche did in the front- while the rear tires are getting excellent traction, the front tires are deforming under load at stock pressures, and break lose before the rear tires under nearly all conditions, giving you nice, safe, boring understeer. If you raise the front tire pressure, you will find your car performs completely differently at the edge of traction and your back end breaks lose more, simply because you're now able to push the car harder with the better front grip.
In short, 44psi = good for car control for the general public, bad for tire wear.
The short answer to your question, though, is safety and liability. Every tire has a specific pressure for a given load where it offers the best grip. Below this, and it deforms under load, can't effectively press all that tread to the pavement, and grip quickly reduces- in the worst case, you get roll-under. Above this pressure, and it balloons out and grip is lost. Generally speaking, grip is lost quicker at low pressures than high pressures. Wider tires grip better in corners than narrow (it's related to contact patch and slip angle).
Case in point is the Boxster- perfect 50/50 weight distribution, so you'd expect the tires to be identical sizes and pressures, right? Instead, Porsche specced out smaller tires in the front. You might think the front tires would therefore need to be at higher pressure than the rear since the contact patch is smaller, and each square inch is carrying more weight. But not Porsche- they again specced out lower pressure in the front than the back that cause the car to understeer like a FWD! Swapping larger tires in the front and pumping the front pressure up reduces the tendency, and gives the car neutral handling.
With so much weight in the rear of a 996, Porsche had to go to a lot of effort to keep it facing the right way at the edge of traction. So, they specced out tire sizes and tire pressures that maximize rear grip. If you enter a corner with 44psi in your rear tires, the weight transfer towards the outside will compress them flat and give excellent traction. At lower pressures, the tire will deform and give less traction. In fact, this is what Porsche did in the front- while the rear tires are getting excellent traction, the front tires are deforming under load at stock pressures, and break lose before the rear tires under nearly all conditions, giving you nice, safe, boring understeer. If you raise the front tire pressure, you will find your car performs completely differently at the edge of traction and your back end breaks lose more, simply because you're now able to push the car harder with the better front grip.
In short, 44psi = good for car control for the general public, bad for tire wear.
One point though..it sounds as though you are saying for AX I should leave the rear pressure at 44 (optimum pressure) but increase the pressure at the front...to 38 or maybe 40. Is that a correct interpretation?
Also - what about adjusting pressures when the tyres have some heat in them? If the outer tyre pressure increases because of the extra loading then should I air down to bring it back to optimum?