Tire psi ?

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TracAction

Tire psi ?

Postby TracAction » Mon Jul 23, 2007 1:29 am

Ok so I am running with 31.5psi in the fronts and 35.5 in the rears for this one. I have only had the car a week so didn't want to muck with too much untill I had run it once. I do think I need to up the pressure a little more like 38 front and 42 rear or so? any ideas? I included a pic from 7/21 that may give a visual... It looks worse in the original image or maybe it's just me


Image
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Postby Loren » Mon Jul 23, 2007 9:36 am

The photo doesn't really answer the question. For street tires, first priority is keeping the tire from rolling over onto the sidewall. From there (assuming you don't have to run abnormally high pressure to keep them off the sidewall) you can go to standard "race tire" tuning. That involves taking tire temperatures.

The goal is temperature that is even across the tread within about 10 degrees. Too much heat on the inside or outside of the tread can indicate an alignment problem. Too hot in the center indicates too much pressure, too cool in the center not enough pressure.

If you don't have a tire pyrometer and a good place to test (autocross runs are usually not long enough to even get the tires to operating temperature, much less give any indication of proper heat distribution), once you've got the pressure set to keep the tires off the sidewalls, you can adjust pressures to suit the handling characteristics of the car.

To do that, start with EQUAL tire pressures all around (or, it's more common for RWD cars to have slightly LESS pressure in the rear than the front, and FWD cars to have MORE pressure in the front... this is due to weight distribution). Find that "baseline pressure" based on what the sidewalls are doing. (or if you have tires like Azenis or an actual race tire that don't roll over no matter what... start with something in mid 30's or ask others what they've experimented with) Then drive the car and see if it has a tendency to oversteer or understeer. In general, just like in suspension tuning, you want to "soften" the end of the car that is not gripping enough. Do that up to the point that your sidewalls will allow, but if that's not enough, "stiffen" the other end.

Good luck!

With a big heavy car like that, I'd start with 38-40 psi all around.
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TracAction

Postby TracAction » Mon Jul 23, 2007 11:02 am

well I have a tire pyrometer on order already (will prob show up tomarrow) and as for a big car, I wouldn't call the TT one, but it is heavier than it looks (about 2,600lbs)

It is the AWD Quattro which does have a slight understeer.

SO when the pyrometer does get here is just normal driving a way to test temps or what, all you said was if i didn't have a pyrometer and a good place to test. but didn't say what is a good way to test.
rippin mazda

Postby rippin mazda » Mon Jul 23, 2007 11:22 am

Your best bet for tire temp testing is to go to a test and tune event and have a helper or two, take the temps and pressures right after you finish a run. That will be the most accurate. Taking the temps after driving on the street won't really tell you what you want to know, if anything useful at all.

Ryan
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Postby Loren » Mon Jul 23, 2007 11:29 am

Don't take offense, but I didn't really look at the CAR in your photo... just focused on the outside tire. I glanced quickly at your avatar at one point and the profile kinda looked like a 350z, so I went with that... d'oh!

2600 pounds, unless it's front heavy, probably start around 36-38. If it's front heavy, definitely add 2 psi to the front.

As for "inherent" understeer, you're not going to cure it with tire pressure, but you might help it a little.

How to test tire temps...

Straight line tire temps mean nothing. (unless you're drag racing, I guess) You need CORNERING tire temps. There are two schools of thought. One is to do it on a skidpad, which I think it certainly best for initial tuning. The other is to do it on the track.

Start with the tires "warm" or "cold", but not "hot" (either start cold or after a brief rest from normal driving). Drive at max G-load in a tight circle. I've done it effectively in a circle of about 50 feet and 5 laps. Run in one direction, then IMMEDIATELY check the temps of the outside tires. The inside tires don't matter so much, you're going to tune the outside pressures. Do the same in the opposite direction.

If your temps aren't rising enough (I don't remember what you're shooting for, I want to say 150-180 degrees), do more laps.

If you've not had an agressive alignment done, it is most likely that you'll see high temps on the outside of each tire. If that's the case, all you can do is tune the tire pressure to try to get the center of the tire close to what the outside is (I doubt that you'll get them the same without overinflating and causing weird tire wear issues)... and schedule an alignment.

The other school of thought for pyrometer testing is geared more to track driving and it's more accurate... but also track dependent. Just set a baseline pressure and go drive a few laps. Immediately pit in and check the tire temps on all four corners.

The pitfall there is that some tracks don't lend themselves to accurate readings using that method. Sebring for instance, gives you about a half-mile straight followed by a high-speed right-hander that takes you to the pits. Unless you have serious problems, your tire temps will equalize on that 1/2 mile straight, and then you'll get somewhat elevated temps on the left side tires from turn 17. So, if using "real world" conditions to check tire temperatures, you really have to consider your testing environment.
Loren Williams - Loren @ Invisiblesun.org
The "Push Harder, Suck Less" philosophy explained:
Push Harder - Drive as close to the limit of your tires as possible.
Suck Less - Drive something resembling a proper racing line.
Anonymous

Postby Anonymous » Mon Jul 23, 2007 12:28 pm

TracAction wrote:and as for a big car, I wouldn't call the TT one, but it is heavier than it looks (about 2,600lbs)

It is the AWD Quattro which does have a slight understeer.
2000 Audi TT Coupe (180 quattro)
Curb Weight: 3131 lbs.
Weight Distribution (front/rear %): 60/40
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Postby Alizarin » Mon Jul 23, 2007 1:01 pm

Not sure if it'll help, but your car's weight is pretty close to mine and is AWD too.

On my street tires I've had pretty good success lately (the last 2 events) running 38psi in the front, and 30 to 32 in the rear. Seems to be the best handling of all that I've tried thus far.
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Postby Dave-ROR » Mon Jul 23, 2007 1:44 pm

Jeremy wrote:
TracAction wrote:and as for a big car, I wouldn't call the TT one, but it is heavier than it looks (about 2,600lbs)

It is the AWD Quattro which does have a slight understeer.
2000 Audi TT Coupe (180 quattro)
Curb Weight: 3131 lbs.
Weight Distribution (front/rear %): 60/40
:thumbup: I was going to say there's no way a quattro TT is weighing in at 2600lbs in street trim. My S2000 is lighter, but still well over 2600lbs.

Personally, I ran whatever the car was delivered with in March of 06 until this last event. As it turned out, it was at about 29lbs ft and 29.5lbs rear, so I bumped them to 36f/38r which worked out well.
-Dave
I drive really slow cars... really slowly.
TracAction

Postby TracAction » Mon Jul 23, 2007 1:57 pm

YES SORRY typo the GROSS vehicle weight is 3760 (off the door) I typed it wrong when I put it in there = ) but at any rate, AND on top of that I worked out a test/skid pad area to do some testing. So this will work out good.
Last edited by TracAction on Sat Aug 18, 2007 11:24 am, edited 1 time in total.
Anonymous

Postby Anonymous » Mon Jul 23, 2007 2:19 pm

Gross vehicle weight rating (GVWR)

The weight specified by a manufacturer as the recommended maximum weight of a vehicle when fully loaded.
So, that means you can only safely load 500 pounds of stuff in your car, including passengers. The car only actually weighs 3131 lbs.
TracAction

Postby TracAction » Tue Jul 24, 2007 12:22 am

lol yea looking at the trunk/hatch that is about right.

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