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PSA - Do NOT go by the PSI rating on your tire.

 
Old 04-18-2012, 06:49 PM
  #16  
Bertrand Daoust
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I would say that 32 to 36 PSI is the way to go on modern tires AND at speed we all drive on our road in North America.

There is no place here that we can drive over 200 Km/h for a long period of time.

That's the main reason why the RDK system is useless on our car here.

Last edited by Bertrand Daoust; 04-19-2012 at 09:15 AM.
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Old 04-19-2012, 03:51 AM
  #17  
RKD in OKC
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If you go to the trouble to actually call the manufacturer of your tires...they will tell you to run the pressures recommended by your automobile manufacturer for your car.
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Old 04-19-2012, 07:53 AM
  #18  
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928 or not, I alway run 36psi, on all four corners, on ALL my cars.
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Old 04-19-2012, 09:29 AM
  #19  
Gary Knox
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Rule of thumb for high performance "street" tires when used for driving on the track is to check the pressures when the tire is HOT. Pressure of 40 PSI hot is usually the best traction performance, and allows max rubber contact in cornering.

Tires like Hoosiers and Michelin Pilot Sport Cups use less pressure to get max performance.

Gary--
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Old 04-19-2012, 11:00 AM
  #20  
RKD in OKC
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The old rule of thumb for racer used to be 10% gain cold to hot. If a tire gained more than 10% pressure it was too low, if less it was too high. That and the chalk line thing go out the window with most all modern radial tires. Street tires take plus or minus 3 psi to make any difference in the tire's handling as opposed to F1 tire's 1/4 psi. At least that is what the tire tech guy at Bridgestone told me that was their F1 tire guy, and races in the the open road racing when I called Bridgestone. He is the one that recommended the factory tire pressures.

If you want to maxumize your traction get a temp probe, drive the car long enough to heat up the tires (street or track) and measure the pressures across the tread perpendicular to tire rolling. If the temp in the middle of the tire is higher than the edges, too much pressure. If lower too little. Also if the outside is hotter than the inside you can use more negative camber and vice versa less. If the temp probe is one of the infra red guns, the tire temps must be taken as quickly as possible after the car is being driven the tread will cool quickly with the car stopped, (like at a stop light).

If I let the Dunlop Direzzas I run get over 33psi HOT they will blister the center of the tread in just one or two autocross laps. They are specifically designed to heat up quickly and have been the most sensitive street tire I have ever autocrossed on. I run them at 32 cold for street, and 28 cold for racing. Racing, I watch the pressures and do not let them get above 32. The next morning after racing they are typically at 27 to 28.
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Old 04-19-2012, 02:44 PM
  #21  
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Clark Howard - if you familiar with the consumer advocate who is on CNN and elsewhere - used to tell people to fill their tires up to the max PSI because it saves gas. I remember this from when he was a local Atlanta talk show host. It was stupid advice (still is stupid advice) as it can really screw up traction, especially on very small/light cars.

Kind of like the local oil change guy trying to sell me nitrogen for my tires. I tell him I have an 80%/20% mix of nitrogen to other gases. He has twice now asked me where I bought it.
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Old 04-19-2012, 02:50 PM
  #22  
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Originally Posted by IcemanG17 View Post

A friend had a 1997 Viper with 335 rear tires.....he was complaining at how little traction the rear tires had.....I asked him how much air pressure he put in it....he said "the tire says 51psi".............I told him drop it too 32psi.....he came back the next day and was stunned at the increase in traction and ride comfort!!!
That is downright scary, that a guy driving such a monster around is so clueless about performance. Another reason to give Vipers a wide berth.
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Old 04-19-2012, 03:07 PM
  #23  
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Originally Posted by RKD in OKC View Post
The old rule of thumb for racer used to be 10% gain cold to hot. If a tire gained more than 10% pressure it was too low, if less it was too high. .
LOL I was going to post this yesterday but I didnt want to get flamed . When I was racing motorcycles this was the general rule.

ALso using a temp probe is a good idea if you are serious about even temps. The little 'holes' on the tire surface is for this (probe).

Motorcycle tires are very sensitive to air pressure and when you are racing and rounding turn 10 at Summit Point at > 110 MPH (and WFO) that is not the time to figure out your high speed weave is being caused by low rear tire pressure.

Street tires are so forgiving of the wide range of neglect they suffer you can get away with 28-36psi however for my car I find a slight bias of 36front 38 rear to be very reasonable. For long trips on the highway I run 40-42 (max pressure on the tire is 44PSI) .

The 10 % rule is a good one, but requires people to read cold vs hot temps.
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Old 04-19-2012, 03:10 PM
  #24  
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Originally Posted by Gary Knox View Post
Rule of thumb for high performance "street" tires when used for driving on the track is to check the pressures when the tire is HOT. Pressure of 40 PSI hot is usually the best traction performance, and allows max rubber contact in cornering.

What I was saying is you need to SEE what that cornering contact point is.

Its not _a_ pressure.

Its YOUR pressure.

I run dramatically less pressure in winter, than summer..because of how the tire works with how/where I drive it.
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Old 04-19-2012, 03:56 PM
  #25  
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Originally Posted by Speedtoys View Post
I run dramatically less pressure in winter, than summer..because of how the tire works with how/where I drive it.
Could you elucidate that a bit? Doesn't it still come down to the pressure where your tire has even temps across it, wears evenly and doesn't roll under? How does that change with the ambient temp?
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Old 04-19-2012, 04:37 PM
  #26  
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Originally Posted by Erik N View Post
Could you elucidate that a bit? Doesn't it still come down to the pressure where your tire has even temps across it, wears evenly and doesn't roll under? How does that change with the ambient temp?
You want more compliance in your tires/suspension on wet cold roads..more "give" in between the skips and hops where you're not sticking as well as dry+warm would. A stiffly reacting tire & suspension on a wet road..is gonna lose grip easier, and regain it later..than a compliant soft setup.


So in my STREET CAR..I measure how the tire is working on the corners and switchbacks and adjusting pressures to adjust how the tire is working. IMHO, its about 1/8" of edge wear up or down the tread for every 2psi of change.

I have never, ever, had uneven wear across the face of the tire..just not a going factor for me on the street.

I would gather..that this talk about cold this, hot that..if you just drove the car in a session on the track, and adjusted pressure to meet the edge of the tread face nicely (I like to go NO deeper than the tip of the arrows molded into the edge of the tread face)..you would find you're at a pretty good place without needing a pyrometer. Beyond that..you have to adjust toe and other crap to obtain this temperature nirvana...but thats a race setup.

In the race car, we undo both sway bars, and drop down tire pressures a ton to stay soft and rubbery on the track...you want the sidewall to roll over more. You cant go as fast..so you have to run softer pressures to use the treadface properly..and you want the suspension softer to keep the movement of weight from one side of the car to the other slower and with less kick.

Jackie Chan fans..I relate this to Drunken Master fighting style.

I could care less about tire temps across the tire on my street car..its folly..im not changing alignment to meet tire temp goals every time the weather changes and the tire gets used differently. I just want it to wear nicely, with a compromise towards performance.

Racing will care a lot about that, but many race classes have limits to what you can do with alignment (either by actual measurement, or capability of the stock hardware)..so in the end you're much more worried about pressures and measuring sidewall roll-over.


Autox taught me how to read a tire..what it wants. Thats all im sayin'..winter is a lower pressure, summer is a higher one...the pressure you end up with depends on how the tire's being used under those conditions...how you drive it, etc.

When you're good, you can feel which tire is down 2psi too..that roll-out of the throttle rotated diffrently...etc. Very subtle..but once you learn to use the mass of the car to rotate it -separate- from using steering input, pressures and how the tire works for you become important..and you wont care what the number is..just what its doing by visual inspection and feel.

Like Ive said in other posts...learn to race in a crapcan spec class car, and you can drive _anything_ to it's potential so so SO much easier.
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Old 04-19-2012, 05:28 PM
  #27  
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Originally Posted by Speedtoys View Post
So in my STREET CAR..I measure how the tire is working on the corners and switchbacks and adjusting pressures to adjust how the tire is working. IMHO, its about 1/8" of edge wear up or down the tread for every 2psi of change.

I have never, ever, had uneven wear across the face of the tire..just not a going factor for me on the street.

That's how I do it too. Using the "white shoe polish" marker on the corner.

I leave that pressure in the car cold as well (36/34). Tires wear fine, always ready to take a corner. Bleed out some as they warm to keep 36/34. The only tread wear issues I've had are chunking, cording, and flat spotting
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Old 04-20-2012, 05:46 PM
  #28  
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Originally Posted by IcemanG17 View Post
I think stock S4's were 36 front 44 rear? 36 front is fine for a street car....44 rear is a bit high.....
3.0 bar was ok at the time when S4 were new and thats why Porsche used 3 bar RDK sensors in rear Flat Disk and Design 90 wheels. By time GTS came along rears were changed to 2.5 bar sensors as tires had developed in those few years.
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Old 04-20-2012, 06:20 PM
  #29  
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Erkka,
Mine are 3 bar all round - for certain. I cannot believe they were changed (stock wheels) so I assume it was delivered this way - I had assumed all GTSs were like this since mine was one of the last RDK cars built.

However the specs suggest it should have had 2.5 bar sensors all round - it definitely does not.

(So maybe they had no 2.5 bar sensors left...?)

That is why I run an aftermarket wireless TPMS (initially Smarttire - now Orange).

Alan
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Old 04-20-2012, 06:27 PM
  #30  
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Very interesting. 4 sets of ROW GTS wheels I have seen have all had 2.5 in them, 3 sets of summer and one winter wheels. ROW owners manual also says 2.5 but it seems US version still says 3 for rear winter wheels. So its ROW vs US thing. I think same tires were used in both types. So factory saw need to change only ROW setup. In any case US front wheels should always have 2.5's.

By the way. Owners manual PDF named as '93 US MY is really '92 MY printed in late 1991 meaning its meant for early '93 US cars. Maybe there is different manual content for real '93 MY?
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