yes yes yes anouther question by that no-nothing antinull ( ^_^.)
i have aftermarket wheels and rims and dont know what psi i should have in my tires
i cant even put the stock amount in because i have NO idea what that is (door tag is gone)
its a 85 se v6 rims are 16" if that helps?
http://69.14.49.129/id/cars/fiero/02.jpg

[This message has been edited by antinull.com (edited 04-21-2007).]

Essentially you don't fill them past the amount of psi listed on the tire.

Personally, I don't use the standard 30 psi because if I do, I scrub the shoulders off (I like to corner). I recommend 35 psi. 40 and up will give you a harsh ride and wear the center down to fast. 35 seems optimum for me.

Arn

ok i will try 35 psi
when i go autocrossing i will be filling them up a bit more i suppose
like 40-45

i curb rashed my tire/rim backing out of my garage today i feel so dumb

I run 32psi all around in my V6 and 32psi up front and 20 psi in the rear of my V8
Dave

------------------
[
1999 Mercedes ML430 1987 Fiero GT 450hp 1986 Fiero SE
certified master technician

funny thing i checked the psi in each tire there all at 32 even lol
even after winter
guess theese tires arnt leaking

ha, i had leaky tires before winter. they were so bad, i parked funny on a curb (half asleep when parked it), when i went for a drive the next morning, plop, right off the rim. Had to store it on the spare because it started snowing.

ewwww
well i have to air up my truck atleast every month

The correct air pressure will depend on the tire. Stiffer tires will require more air pressure. For example, with 215/60-TR14 tires, I found the optimal air pressure to be 32PSI front / 36PSI rear. With 215/45-ZR17 tires, it's 40PSI front / 45PSI rear. I try to make the air pressure follow the front/rear weight balance. It makes the tires wear more evenly.


BTW, here's my methodology: I fill the rear tires to 5PSI less than the maximum cold pressure stamped on the tire. Then I fill the front tires to about 10% less than the rear (weight balance). After that, I check tread wear every week or so, and adjust air pressure accordingly.

[This message has been edited by Blacktree (edited 04-22-2007).]

quote Originally posted by antinull.com: what psi should i fill my tires to?

That's a very good question... One that I've got a vested interest in as well.

I'm no expert, but I assume that the correct answer is "Whatever pressure results in even treadwear across the surface of the tire". The problem with this answer, however, is that you don't know you're running the wrong pressure until you've messed up your tread enough to determine that it's uneven. By the time the treadwear is uneven enough for me to detect the problem, it's been thousands of miles.

Personally, I'd like a way to figure this out BEFORE I drove thousands of miles and messed up my tires.

So does anyone know a real scientific method of determining this?

Would it be correct to assume that even temperature distribution across the tread would be a good predictor of even treadwear? Could you go for a drive and then quick jump out and check all across the tread surface temps with an infrared thermometer? Hot in the middle, pressure is too high. Cooler in the middle, pressure is too low. Would that work?

You could try it. But I suspect the alignment settings might make things a little complicated.

quote Originally posted by FTF Engineering: does anyone know a real scientific method of determining this?

Assuming that a tire is mounted on correctly sized rim (i.e. the rim width is the tire's "reference rim width" a.k.a. "measured rim width"), here's a good starting point for most tires:

Pressure = (rated max tire pressure) X (square root of (actual tire load / rated max tire load))

i.e. the load factor is the square root of the relative tire load

Note that this method requires that you know the actual load on each tire ... or at least a close approximation.

To answer your second question ... Yes, temperature across the tread is a very good indicator of proper tire inflation and wheel alignment for the specific test conditions. Results may vary depending upon speeds, cornering loads, road surface, etc.

[This message has been edited by Marvin McInnis (edited 04-23-2007).]

quote Originally posted by antinull.com: funny thing i checked the psi in each tire there all at 32 even lol even after winter guess theese tires arnt leaking

Unlike my left rear.... I fill it up to the 32PSI like always, the next morning I come out and the thing is 100% flat. I was thinking "It did not leak on my dads car why does it leak on mine?"
I still cannot find the hole but no biggie. I am getting new tires, or should I say three new tires. One rim already has a new Yokohama 225 60 15 on it.

I run most of my cars at 32PSI.

[This message has been edited by skstibi (edited 04-23-2007).]

quote Originally posted by skstibi: Unlike my left rear.... I fill it up to the 32PSI like always, the next morning I come out and the thing is 100% flat. I was thinking "It did not leak on my dads car why does it leak on mine?" I still cannot find the hole but no biggie. I am getting new tires, or should I say three new tires. One rim already has a new Yokohama 225 60 15 on it. I run most of my cars at 32PSI.

You could have a hole in the tread as you suspect. But you could also have the same problem I had. Mine leaked around the wheel weights, mostly in cold weather. My last two sets of tires all leaked around the wheel weights. I had them re-balanced with stick on weights. Problem gone. If you have hammer-on style weights, put some soapy water around them & see if that's your leak.

[This message has been edited by Formula Owner (edited 04-23-2007).]

For what it's worth.
This is the reply from Dave Martin of the Hankook Tire Company after I purchased my tires and wheels from Wheel Max online.

quote The inflation pressure for larger aftermarket tire and wheel combinations is usually the same as what's listed in the owners manual of your vehicle for the original equipment tires. Based on the weight of the vehicle, automobile manufacturers determine the recommended tire pressure, and those values don't necessarily change when switching to larger tires and wheels. The maximum pressure listed on the sidewall of the tire corresponds to the maximum load listed on the sidewall. The tires on your vehicle aren't carrying the maximum rated load, so you don't need to run the maximum rated inflation pressure. It's best to use the tire pressure that was recommended by the vehicle manufacturer. This wouldn't be true if you put smaller, narrower tires on a big, heavy car, but that's not the case because you're replacing the O.E. size with a larger tire/wheel package.

Thanks for the info guys.

So let me see if I got this formula right...

"Proper" Pressure = (rated max tire pressure) X (square root of (actual tire load / rated max tire load))

My rear tires say 1433 lb max load at 51 psi max pressure, and and according to this thread:

quote https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20050818-2-062324.html: Left front 619 Right front 588 left rear 799 right rear 836 no driver, 2842 total left front 694 rt. front 599 left rear 878 rt. rear 840 with driver 3011 total

the corner weights are in the ballpark of 600 lbs for each front and 825 lbs for each rear.

Plugging that info into your formula: "Proper" Pressure = (51 psi) X (sqrt (825 lb / 1433lb))

Works out to 38.6 psi......

Sounds a little high... Am I doing that right?

quote Originally posted by FTF Engineering: Sounds a little high... Am I doing that right?

Yes, you did the calculation correctly ... and I agree that 38.6 psi sounds a little high, but the max inflation pressure of 51 psi is higher than usual, too. (Are you running VR rated tires, perhaps?) Most street tires have rated max inflation pressures in the low to mid 40s. Also consider that the calculated value is only intended to be the starting point for your wheel/tire combination, on your car. Assuming the same sized tires all around, 33 psi front and 38 psi rear is just a good starting point for you.

One small point: These calculations assume that the tire is mounted on a properly sized rim, one with the "reference rim width" (a.k.a. "measured rim width") specified for your particular tires. Rim width has a significant effect on the exact tire inflation pressure needed for uniform contact patch pressure. In general, a rim width less than "reference" requires reduced inflation pressure, while a rim wider than "reference" requires higher inflation pressure.

Finally, here's an example using my '88 Fiero. The car is placarded thus: Front - max gross axle weight 1295 pounds, 205/65R15 tires, 30 psi (+4 psi hot) ... Rear - max gross axle weight 1932 pounds, 215/60R15 tires, 30 psi (+4 psi hot). Using tire rating information posted on TireRack's web site, here are the calculated values for the same axle weights and tire sizes for a couple of HR rated tires: Front ~31 psi ... Rear ~36 psi. For comparison, by trial and error I have found that 32 to 34 psi (cold) gives me the most satisfactory overall results in street use.

[This message has been edited by Marvin McInnis (edited 04-23-2007).]

Haha!! Forget the placard. I'm running 265/35WR-18's on appropriately sized nine inch rims.

The placard us useless at this point and that's why I'm interested in a more versatile theory based approach. That rubber ain't cheap and I don't want to wait until I can SEE the problem.

A starting point isn't the problem... I've already done that. What I want now is fine tuning without ruining my tires in the process.

When the tires are brand new, you have the perfect opportunity to observe the wear pattern. The fresh rubber is going to be nice and smooth. Once that rubber hits the road, the road surface will rough it up. Observe the pattern and adjust accordingly.

If your tires are already broken in, then obviously that method won't work. You might try painting/drawing a line across the tread and see which part of the line disappears first. It's just an idea. :shrug:

[This message has been edited by Blacktree (edited 04-24-2007).]