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Old 07-13-2005, 02:15 AM   #1
STIRS
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Default Correct Air Pressure for Aftermarket Wheels+Tires

I just replaced my old tires at work today with brand new 225/45/17 Kuhmo Ecsta MX's on my 17" wheels and I know the factory recommended setting for the air pressure is something like 32 Front and 29 Rear Im not sure if im supposed to inflate the new setup to this spec so I just put 32 all around and I know that incorrect air pressure wears out tires badly so should I keep them at this setting or go with the stock specs?

-Thanks
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Old 07-13-2005, 03:24 AM   #2
Uncle Scotty
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FORGET stock pressures with the MX's ! ! ! !!

Start with 39F/37R....and maybe add a pound or 2 to hit the sweet spot.

When I had 'em I ran 40-41F/38-39R and it was great
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Old 07-13-2005, 03:27 AM   #3
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any particular reason to running more psi in the front? i would think more psi in teh rear would make for better handling
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Old 07-13-2005, 08:05 AM   #4
ripvw
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Quote:
Originally Posted by svek
any particular reason to running more psi in the front? i would think more psi in teh rear would make for better handling
The goal with any impreza would be to enhance grip in the front to improve handling and help to offset the excessive understeer as set by the factory.

the best way to do this would be through the use of proper alignment, i.e. run .5 degree more negative camber up front than in the rear, larger swaybars - ideally matched front/rear such as 22/22 or 24/24. the larger rear bar definitely decreases understeer, while a larger front bar increases understeer a bit but also increases front grip because it improves the front suspension's ability to maintain negative camber settings during hard cornering. That's why a larger front swaybar is so critical - along with lots of front negative camber - to autoXers. Finally, one of the best ways to improve front grip is to add an ALK - which effectively "softens" the front springs when exiting corners to enhance front grip.

you can also adjust grip via tire pressure. it depends upon the tire, of course, but at some pressure setting the tires will have more grip than at others. with an otherwise stock '02 wagon on the RE92's I used to run 35F/30R since this gave me less understeer and more front grip. By comparison, the factory settings for a Porsche Boxster - which has the opposite problem of a WRX - were at 30F/36R for the (then) stock 16" tires. The Boxster needs more grip in the rear than the front. (Porsche is perhaps the only manufacturer who specs tire pressure for performance - not comfort - and who also tests and mandates specific tires for every model. There is no RE92 equivalent when you buy a Porsche - they're all max performance tires.)

Now that I have made modifications to the wagon including proper alignment (-2F/-1.5R), swaybars (20/20), and an ALK - I run 36/36 or 38/38 on my 225/45x17 Falken FK451's. I know that the "right" pressure for these tires on the street is somewhere between 36 and 40psi cold - I just haven't decided exactly what it is. I don't need to adjust tire pressure to compensate for lack of grip up front - I've already accomplished that via other methods. I want both front and rear tires to be at "max grip" air pressure. I'm convinced with this tire and my mods the pressures front and rear work best when the same.

Since Uncle Scotty has thousands of miles of MX experience on his well-modified 02' sedan, I'd listen to the man when it comes to MX tire pressures. He has obviously determined that for max grip with that tire running 225/45x17 the best results come with slightly different pressures front and rear - and at tire pressures well above the stock 33/32 that Subaru recommends for the 215/45x17 tire size.

finally, in general any street tire is likely to "come into it's own" as far as grip is concerned at somewhere around 35psi cold as a minimum. Why? Because street tires do not have anywhere near the sidewall stiffness of a race tire, and need sufficient pressure to maintain sidewall stiffness during hard cornering to avoid rolling onto the outside edge of the tire. Euro Metric tires - like most of the ones spec'd for the Impreza - achieve maximum load capacity @ 36psi for most sizes - and 42psi for "XL" tires. I'm convinced that you need to run close to or slightly above this pressure setting with most tires to get the sidewall stiffness to the point where the tire can achieve max grip during hard cornering.

In support of this "theory", a recent article in Road & Track had a quote from one of Michelin's tech guys stating that running 35/35 cold pressure is a really good "starting point" when to comes to high performance tire pressure settings. In addition, John Falstead - our resident 'Ring runner in the Motorsports forum - recommends 40/40 "hot" pressure for street tires - which is going to be in that same ballpark when converted to cold pressure. He was running Bridgestone S02's when he made that comment.

Due to the nature of the Impreza's strut suspension design - which tends to gain camber during cornering - running higher pressures than the factory "comfort specs" is critical in my opinion - but it does depend upon the tire. The new Falken RT-615 is a design that may work best at pressures below most other tires, for example - based upon comments some of the racers have made about this tire in the Motorsports forum as well as here.
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Old 07-13-2005, 03:58 PM   #5
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maybe im completely wrong, but you say to increase grip you increase tire pressure. in my mind that worsens grip. i would think that a tire would be more likely to slide at higher pressures. i would think that less psi in the front would be better. the way i see that is you are going around a corner fast you want the front to grip and the rear to slide slightly. granted i have 0 racing experience and im not trying to disprove your point. thats just the way i invisioned it.


note: i run 40/40 on mine because im lazy
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Old 07-13-2005, 07:58 PM   #6
Airboy
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I think he is refering to lateral grip, where higher pressure would keep the sidewalls from 'rolling over'. At least that is what I got from his post.
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Old 07-13-2005, 08:47 PM   #7
madkayaker
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It would depend a lot on the road surface. If you where off road, lower pressure would give you much more traction. On the road is different. Too low or too high psi would be bad. You've seen how a tire with too low psi wears the outside edges of the tire faster than the middle. And when you have one with too high of a psi it wears the middle faster. I think that it would be ideal to find the perfect medium in which there would be even contact to the pavment accross the entire tread.
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Old 07-14-2005, 12:50 AM   #8
ripvw
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Quote:
Originally Posted by madkayaker
...Too low or too high psi would be bad. You've seen how a tire with too low psi wears the outside edges of the tire faster than the middle. And when you have one with too high of a psi it wears the middle faster. I think that it would be ideal to find the perfect medium in which there would be even contact to the pavment accross the entire tread.
yeah - that's it. if you start @ 35psi cold and increase the pressure, at some point the tire will begin to lose grip - and the same is true if you lower the pressure too much. another problem is that as you increase pressure and eventually go beyond the "ideal" pressure for grip the tire becomes more responsive to driver input - steering feel is enhanced at the expense of maximum grip.

best to go slow and be aware that when the tire "feels right" in terms of steering response you may actually get a little more grip if you drop the pressure a couple of psi.

obviously, I'm talking about street pressures here - autox in particular seees to have it's own special rules when it comes to this issue and I would not presume to tell racers what pressures to run on the track...
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