I just bought a new set of Michelin Sport Pilot A/S 205/55 16" for my
2002 WRX. The decal on the door says to inflate tires to 32/29 front/back. The guy at the tire store said to leave them at 35/35 because he was worried this specific tire would wear unevenly and that seeing it was getting colder here the inflation would drop in any case (as if I couldn't add air and remove it as necessary during my regular bi-weekly tire checks).
So, what should I set the pressure in my tires to? Should I listen to the guy at the tire store, or follow the guidlines on the sticker?
You can adjust the pressures up and down to suit your preferences for ride vs. handling, but it's important to maintain that 3-psi differential from front to rear. Too much in the rears relative to the fronts can promote excessive oversteer in the WRX. Perhaps try 35/32 F/R and make adjustments from there.
If it were me, I'd leave them at 32/29 and check the tire wear at the next rotation interval. Underpressure tires will show more wear towards the sidewalls, Overpressure shows more center wear. You won't see too much difference over the rotation interval and you can adjust the pressure then.
I have always understood that increasing tire pressure decreases the slip angle (improves the grip.) If that is true, then increasing pressure in the rears would make the car UNDERSTEER. Only thing is, I don't know whether All Wheel Drive makes a difference.
On Sat, 15 Oct 2005 02:08:15 GMT, snipped-for-privacy@worldnet.att.net wrote in news: snipped-for-privacy@g47g2000cwa.googlegroups.com:
Increasing tire pressure makes the contact patch smaller which reduces available grip but makes the tire firmer and more responsive. Although going in either direction is not linear and depends on rubber compounds and tire construction.
Generally increasing rear pressure will *increase* oversteer. However, if the tire is sloppy at lower pressures and prone to snap oversteer as the tire loses its shape. Increasing pressure will make the loss of grip more progressive and more predictable leading you to believe you are getting more grip.
Recommended pressures on the door sticker are for comfort rather than handling. As someone has already said, they should be used as a low starting point. Work up in pressure (cold) until the ride becomes stiffer. That point will vary with tire construction and age. I have found that point to vary with different brands of tires in the same size on the same car, based on my own preferences. Keep the same ratio of pressures as on the sticker unless you are on the track experimenting with the handling and looking for optimum balance rather than a safe dose of understeer as is typical for recommended pressures.
The number on the side of a tire is a max (cold) for that tire, and has nothing to do with any particular application of the tire or the car and rim on which it is mounted. Don't use that number for anything other than not inflating it to a higher pressure than that number.
My regular daily driving pressure is usually 5 to 10 psi above the sticker/manual recommended pressure. Autocross pressures up to 15 psi above recommended.
My experience (limited, of course!) with Michelins is they seem to respond better to slightly higher pressures than some other brands I've used (mirroring advice from a tire dealer friend to always run Michelins at 2-5 psi higher than the door sticker.) OTOH, they've often seemed "slippier" across the board of pressures I used, where some brands were "sticky" at lower pressures and lost a little as they were inflated higher, others became more "sticky" at higher pressures. As a general rule, I'll run about 2 psi over the door sticker's "everyday" recommendation, and usually maintain whatever frt/rr differential is recommended. Only time and driving will tell, but if you're checking pressure bi-weekly, you should be able to give yourself enough time to get used to one "feel" before trying the next.
I've been running them at 3psi higher keeping the ratios between front and rear. I kept the ratios because I had read somewhere that handling could be affected, but I couldn't remember if it was oversteer, or understeer. Thanks to 'Hear and There' for clearing that up. It seems to work fine at 3psi over the sticker. I'll try varying it higher. I did notice a lighter colour band of dirt in the center of the rear tires as if that part of the tire wasn't making complete contact, or as much as the the outer portions of the tire. There wasn't a similar band on the fronts which are at a higher pressure, though. I just noticed it as I parked the car for the day, so I'll see if it is something that varies with tire pressure.
Great tire, though. I never had any complaints with the OEM RE92's, but the difference these tires make is tremendous. So much quieter and steering response is instantaneous, but not twitchy. The RE92s seemed wobbly and vague in comparison and didn't inspire the confidence the Michelins do. The car feels like a different vehicle with the Michelins on.
The main reason I asked the question is the tire guy initially ordered the wrong tires. I asked for the pilot's and more than once asked him to confirmation yet he managed to order and install on the car Michelin Energy MXV4's. Don't ask me why he thought I'd want grandpa touring tires on my WRX, nor why he was surprised I wasn't happy with 'his' selection of tires for my car. At that point I wasn't going to take his word for what the tire pressure should be.
The WRX is an all-wheel-drive vehicle. It's *very* important to maintain the same tire circumference on all four wheels. For example, if you have half the tread left on all your tires and one is damaged beyond repair, you should replace *all* the tires at once.
There is a slight front-to-rear weight difference on your car with more weight on the front. The "rolling radius" of a tire is affected by the weight on that axle. That's why the slight (32/29 is slight) bias front to rear is recommended. It's to make the tire circumference the same front to rear.
The decal is there to tell you important stuff. Not just to make the door jamb interesting reading.
On Sat, 15 Oct 2005 16:06:02 GMT, James Meyer wrote in news: snipped-for-privacy@4ax.com:
The tire companies recommend ensuring less than 1.5 inches of circumference for AWD/4WD vehicles, while in the U.S. people report here that Subaru insists on 1/4 inch. Do the math.
Tire companies quote Revs per Mile because with radial tires the revs per mile is virtually constant. The distance travelled by the tire under load is dependent on the belt length. The variable is load which causes tread squirm, which has a very small variance. (less than 1%)
The sticker recommendations are to create a similar size contact patch from front to read based on weight ratio. While the revs per mile is fairly constant, the size of the contact patch is directly related to the weight on the wheel and the pressure in the tire.
Information is more valuable when you know how the numbers are actually derived and why they are important.
i am NOT an expert, so i'll stay away from the technical stuff.
my preference is to overinflate 10% over the door sticker, purely fora little extra fuel economy. that works out to about 3 psi, which should be well within safe limits.
but a dramatically overinflated tire (like the way they ship new cars) can be VERY dangerous - on my first new car ('86 accord 2-door) i didn't notice how overinflated the tires were, until the first stretch of wet highway, when it started hydroplaning at about 70 km/h! it seems the dealer had missed a thing or two in the pdi.
This is good question! Does AWD make a difference? Anyone know?
When I took my BMW 328i to a weekend Safety Handling Course at a local track, the lecture portion covered tires. The response to one question was almost uniformly wrong. The question was: In snow, repeat snow, are you better off to have your tires a bit over-inflated or under-inflated?
Almost everyone answered under-inflated. Wrong! The correct answer is a bit over-inflated.
Wonder if that applies to AWD. BMWs are or course rear-wheel drive, although they are available, in some models, as AWD.
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