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Old 09-10-2007, 07:40 AM   #1
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Tire Pressure Question

Sammy and I ran an autocross in Indiana yesterday and we both did really well. I forgot to let the air out of my tires for the way home ( about 90 miles ) and I must say I really liked the way the car felt with the increased tire pressure ( I think I had 36 psi in the front and 38 psi in the rear ). I dropped the pressure down to 29 / 36 once I got home, but I was wondering what would be the disadvantage of running with increased tire pressure ?

Pictures :

Sammy's Front



My Front



Sammy's Friend's Shifter Cart



Sammy's Rear



My Rear



Any input would be appreciated...

Thanks,

Nick
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Old 09-10-2007, 08:07 AM   #2
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I run 36 lbs front and 40 lbs rear all the time with my 18" PS2's and I dont track my car, just for road use. I think I got that redommendation from someone on the fourm a while ago.

Any lower pressure than that and the sidewalls feel too flexible on hard cornering. Having gone through a pair of PS2's running the tires at that pressure, I found no unusual wear patterns, just the usual wear on the inner edge of the tire from the allingment setup.

Ed

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Old 09-10-2007, 09:00 AM   #3
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Thanks for the response, edevlin !

I found some good threads on the subject :

Correct PSI for Pirellis

Tyre Pressure for Boxster ??? (POLL)

I am still confused, though

If Porsche recommends 29 / 36 , and you run 4 to 7 psi above that yet you do not experience any unusual tire wear, are there any other issues to be concerned with ?

Nick
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Old 09-10-2007, 09:59 AM   #4
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Part of the result of running the factory 29/36 is increased UNDERSTEER. By running more air in the front, you reduce the understeer tendancy. While running pressures too high may cause a change in tread life, you may find the benefits outweigh the drawbacks.
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Old 09-10-2007, 10:46 AM   #5
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Quote:
Originally Posted by racer_d
Part of the result of running the factory 29/36 is increased UNDERSTEER. By running more air in the front, you reduce the understeer tendancy. While running pressures too high may cause a change in tread life, you may find the benefits outweigh the drawbacks.
The factory 29/36psi recommendation is that hot or cold?

Your point does makes sense, understeer is always safer for an average driver. How many more psi do you actually put in the front tyres to reduce understeer or whats the ideal front tyre pressure to have a neutrally balanced boxster?
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Old 09-10-2007, 12:28 PM   #6
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Quote:
Originally Posted by panameras
The factory 29/36psi recommendation is that hot or cold?

Your point does makes sense, understeer is always safer for an average driver. How many more psi do you actually put in the front tyres to reduce understeer or whats the ideal front tyre pressure to have a neutrally balanced boxster?
Hi,

Tire pressures are always measured COLD. You cannot eliminate or neutralize the Boxster's steering with Tire pressure alone, you can only make it more neutral. Alignmentspecs and perhaps even a change in some of the suspension hardware would be necessary to make it totally neutral.

I run a higher pressure than OEM spec as well, it gives the car better turn-in and it lowers the rolling resistance which aids acceleration and range (MPG).

But, there is no absolutely BEST Tire pressure. You need to take into account your driving conditions (consider Potholes, Freeway expansion joints, paved vs unpaved, etc.), your driving style, even the type of wheels you use (too low a pressure can cause rim damage more easily). It's best to experiment using an accurate Tire Gauge until you find a pressure that you think gives you the best ride and response...

Happy Motoring!... Jim'99
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Old 09-10-2007, 05:55 PM   #7
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Did he just post "Sammy's rear"??? Funny, I didn't realize my rear was big enough that it needed a license plate.

I really dislike the ride when I'm driving home from an event with the pressures at 38 rear/34 front. I feel like I should be using a mouth piece to protect my teeth from all the jarring that takes place.

Funny thing about this event was I really noticed a lack of front grip. It's always a problem to contend with but usually I notice it when I'm into a turn too deep and push the front or when I'm late on my steering. This was the first event in a long time where I actually took out the spare tire. I wonder if that extra weight actually helped me with grip? I felt that I had a relatively neutral handling car before...
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Old 09-10-2007, 09:06 PM   #8
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Sammy,

I am envious of your rear ( I can't believe I just typed that )

Those clear lights look so awesome following you on the way home !

I actually like the way the car rides with more air in the tires.

Nick
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Old 09-11-2007, 07:06 AM   #9
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Thanks Jim

You haven't answered my question. Obviously you can't improve the handling balance by just increase the tyre pressure but it will help and it might be most cost effective way to do so.

As for taking driving conditions into consideration, Porsche obviously has done the homework for you already. They only have one recommendation for most continents and mostl driving conditions for standard size summer tyres which is 29psi at the front. In other words this is the most ideal tyre pressure for most driving condition regardless Potholes, Freeway expansion joints, paved vs unpaved, etc. So if Porsche didn't worry about using different pressures in different conditions why would you worry? Also, don't worry about "too low a pressure can cause rim damage more easily" because in this scenario we are trying to increase tyre pressure not decrease.

Having said that, however, it won't hurt to increase the front tyre pressure a little over the factory recommendation to improve understeer (not eliminate) Hence my question again to those who have actually tried various front tyre pressures what do you think is the most ideal front tyre pressure over the factory recommended 29psi?


Quote:
Originally Posted by MNBoxster
Hi,

Tire pressures are always measured COLD. You cannot eliminate or neutralize the Boxster's steering with Tire pressure alone, you can only make it more neutral. Alignmentspecs and perhaps even a change in some of the suspension hardware would be necessary to make it totally neutral.

I run a higher pressure than OEM spec as well, it gives the car better turn-in and it lowers the rolling resistance which aids acceleration and range (MPG).

But, there is no absolutely BEST Tire pressure. You need to take into account your driving conditions (consider Potholes, Freeway expansion joints, paved vs unpaved, etc.), your driving style, even the type of wheels you use (too low a pressure can cause rim damage more easily). It's best to experiment using an accurate Tire Gauge until you find a pressure that you think gives you the best ride and response...

Happy Motoring!... Jim'99

Last edited by panameras; 09-11-2007 at 07:12 AM.
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Old 09-11-2007, 07:22 AM   #10
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I have been running 35/35 cold for a while now and tire wear is very even across the tread. I do notice a little more (detail) up front on rough roads. These pressures have helped to preserve the outer shoulders on track days also.

Track hounds will tell you that maximum dry traction is achieved at the lowest pressure you can run without the sidewall beginning to roll over. This pressure varies greatly between drivers. Super smooth, very experienced drivers can run lower pressures than a guy who really tosses and slides his car around. A novice who does a lot of sliding can destroy a set of tires in one day with too low pressure.
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Old 09-11-2007, 07:27 AM   #11
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yeah, porsche recommends 29 psi front. but i use 31 psi front and 36 psi rear and I am happy with it. i think it is all based on personal preference and see how you like the handling.
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Old 09-11-2007, 07:56 AM   #12
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Quote:
Originally Posted by panameras
...As for taking driving conditions into consideration, Porsche obviously has done the homework for you already. They only have one recommendation for most continents and mostl driving conditions for standard size summer tyres which is 29psi at the front. In other words this is the most ideal tyre pressure for most driving condition regardless Potholes, Freeway expansion joints, paved vs unpaved, etc. So if Porsche didn't worry about using different pressures in different conditions why would you worry?
Hi,

I think you have to take a second look at Porsche's objective here. Their recommended Tire Pressures (as are the Tire Pressure recommendations from virtually all manufacturers) are derived as a compomise to best achieve several competing objectives such as Safety, Ride Quality, Performance, Reliability, and Range (MPG). As such, it is not the best pressure for actually fully achieving any one of them.

This is why your particular road conditions, Driving Style and Driving abilities can often be better served by adjusting the pressures to closer achieve what your individual objectives may be.

Not everyone who owns this car is Tracking it, carving out the Twistys, or bouncing off the Rev Limiter. In fact, given the Boxsters Original Buyer's demographic - established Business Person (probably a little older), second car, Leasee, etc., I suspect that at least 50% of their Original owners have never Tracked the car and probably drive it mostly for commutes and personal (rather than Performance) driving. For these people, the OEM spec Tire Pressures work just fine. But for someone who's skill is greater, or whose objectives are different, deviating from OEM spec gives them what they are looking for.

So far as Front pressures are concerned, I personally find that with my Pirelli P-Zero Rossos, on 18" Turbo wheels, 32 PSI gives me the best response while still preserving some modicum of Ride Quality. Those times I have Auto-X'd it, I've gone as low as 27 PSI, tried 26 PSI and the response seemed too sluggish. On the Track, 26 PSI might actually be OK, but the radii on Auto-X curves/turns tends to be much tighter than the Track...

Happy Motoring!... Jim'99

Last edited by MNBoxster; 09-11-2007 at 08:05 AM.
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Old 09-15-2007, 12:10 AM   #13
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I was going to start a thread about tire pressure but thanks for the search forum tools
I can't believe that they put 45 all around on the tires that they just installed today on my car after reading this thread

So it's safe to say to go
225/40 18 Front 31psi
265/35 18 Rear 36psi

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