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Old 09-23-2012, 05:20 AM   #1
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Tire Pressure

I've been running 30/36 F/R in my 225/255 17" tires. Lately I've read various threads in which others are running higher psi (i.e. 35/40). What is the reason for the higher psi and should I be doing the same? Thanks.
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Old 09-23-2012, 06:11 AM   #2
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That seems a bit low to me. My tire?wheel man recommends 36/36 minimum. He also says that with the low profile tires when you hit the inevitable potholes there is more chance that the tire will fully compress and damage the expensive porsche rims if you have lower pressure. And lower pressure is suppose to affect tire wear, fuel milage and handling. so they say.
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Old 09-23-2012, 06:12 AM   #3
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Everything is a tradeoff.

Depends on the make/model of your tire and what you're trying to acheive. There is no single "right answer" for tire pressures and many people have different preferences in terms of ride quality, balance (understeer vs oversteer), tire wear, and several other key handling characteristics that can be somewhat altered by varying the tire pressure.

A gross example is the difference between the Nitto NT-01 and Toyo R888, both R-compound racing tires. The Nitto's typically run best at 36-38 psi hot (all four corners) but the Toyo's, with a stiffer sidewall, run best around 30psi (all around). Running either tire at 5 psi more than these values results in a greasy, loose feeling, and loss of ultimate grip. Running at 5psi less than these values usually results in rolling the tire too far over in hard corners. Within this range, there is a lot of room for variation and preference.

Read the posts and then try making some minor changes to tire pressures to see if you feel any difference or improvement in the direction that you want to go. If so, then move a little more. Eventually, you'll reach a point of diminishing returns or where a negative tradeoff starts to become apparrent.
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Last edited by thstone; 09-23-2012 at 06:16 AM.
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Old 09-23-2012, 11:47 AM   #4
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Unless you are trying to make some change to the handling, there's little reason to not use the factory recommended pressures. 30 is not too low at all, and 6 more pounds won't help that much if you nail a big pothole. Also, your 17's are not particularly low profile tires.

PS I have been playing with pressures, and am underwhelmed with the results. The factory pressures gave me good tire wear, so I went back to that.

Last edited by stephen wilson; 09-23-2012 at 11:50 AM.
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Old 09-23-2012, 02:25 PM   #5
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Tire Pressure

Thanks everyone.
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Old 09-23-2012, 02:50 PM   #6
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The factory recommended pressures are based on getting a lot of understeer, which is what most people who change their suspensions are precisely trying to get rid of.

What I have found is that equalising pressure front to rear, or even going with a lower pressure in the rear versus front is the easiest and cheapest way of dialing out some understeer, Look at the Guide to Performance Handling in this link. Tire Tech Information - Air Pressure for Competition Tires

I generally run 36 square on the street, on the track I run about 4 psi lower in the rear than the front.
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Old 09-23-2012, 03:16 PM   #7
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That's the odd thing, everyone says it will reduce understeer, but I just didn't find that to be the case. It did help make the intitial turn-in more crisp, but still ended in terminal understeer, which was actually worse than before.
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Old 09-23-2012, 03:34 PM   #8
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The porsche shop that did my wheel balancing had set it to 40 all over. Front felt like it was sliding all over. Factory recommended has the right feel and control IMO. But, I'm still trying a few pounds change at a time to find the optimum for my tires.
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Old 09-23-2012, 05:34 PM   #9
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Quote:
Originally Posted by stephen wilson View Post
That's the odd thing, everyone says it will reduce understeer, but I just didn't find that to be the case. It did help make the intitial turn-in more crisp, but still ended in terminal understeer, which was actually worse than before.
So it did turn faster with higher tire pressure in the front? Yes , with a stock suspension, it will understeer when really pushed to the limits, but a good crisp turn in is what you want.
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Old 09-23-2012, 06:02 PM   #10
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Stephen,

What he said ^

Try playing a couple of pounds up or down front or rear - preferably in a repeatable situation. Don't expect miracles however. These cars are designed with terminal understeer in mind.

I do not know anything about you or your driving skill level so please read this as nothing more than my opinion on how to deal with the "terminal understeer" associated with the Boxster.

No amount of tire pressure adjusting will overcome the understeer on its own. Learning advanced driving skills may however. These cars handle extraordinarily in the right hands and a little throttle off induced oversteer followed by a controlled application of judicious throttle will completely overcome the understeer if you are brave. If you are currently finding that the car is simply plowing when you try to go through a corner then I would be almost certain that you are running in a little quick, turning in and getting on the throttle a little too early and expecting it to do something different to what it did last time. Try running up to the apex under a gentle trail brake waiting for the steering to bite while the weight is still on the nose and then once the tail starts to naturally come around (as it will) get on the throttle hard enough to keep the tail slipping. The result will be a quite natural feeling oversteer. In this respect the Boxster is not terribly different from any Porsche. Slow in, careful transition from brake to throttle, fast out.

If all of the above means little or nothing go and spend some time at the PCAs DE program.
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Old 09-24-2012, 06:03 AM   #11
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Well, if I didn't have other areas demanding my $$$ right now, the best solution would be a pair of adjustable anti-roll bars. I do have racing experience in a Formula Ford, and was using a very long exit ramp as a skid pad for "testing". It's long enough to pretty much eliminate any transient affects. overall the balance is almost where I want it since I've gone to a 245/265 setup. It would likely oversteer if I ran it on a track, since I'm "tuning" for the slower corners I like to play on ( 2nd gear tops ).
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Old 09-24-2012, 03:02 PM   #12
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Tire Pressure

I have the 17 inch rims with 205/50's and 255/40's and I run 29 psi in the front and 36 psi in the rear for regular street riding. This works for me.
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Old 09-24-2012, 11:31 PM   #13
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i always ran with mine at 32/36. after one mechanic visit i thought the car was going to shake apart. i found the pressures at 38/40. the tech had reset it. now he knows to leave it. no harm, no foul.

picked it up from the alignment shop a couple weeks ago, and talking with the shop owner, he says "oh by the way, i set your tire pressures at 44/46" what?!?

dammit, leave it alone. this is my DD and i like it lower for a softer ride...
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Old 09-25-2012, 04:42 AM   #14
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Why in the world would he set the pressures that high!?
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Old 09-25-2012, 04:48 AM   #15
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I'm running 19's at 30F 31R that is what the selling dealer recommended and set them to. I'm with Stephen, IMO 36-38 is going to cause cupping.

PS I think checking the tires with a known good gauge is useful even with TPS which I think reads artificially high.
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Old 09-25-2012, 05:26 AM   #16
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i run 29/36 as per porsche on the street. i take that pressure to the track and, when the tires get hot, they swell to 34/40. at 34/40 the fronts are starting to roll onto the sidewall and the rears are only running on their middles, so after a few laps to heat the tires i modify to 36/38 and the car drives like it is on rails and the tires wear evenly.
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Old 09-27-2012, 05:20 PM   #17
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I run factory pressures. My rear tires seem fine. My front tires just basically have 'grab' wear on them. Tread wear seems fine. I'm thinking About how you all drive to experience the wear on your tires you all are talking about....?
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Old 09-27-2012, 05:20 PM
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