jop

(Not sure whether to post this in Performance or Racing)

I'm starting to run a TT in auto-x and I'm finding some gaps in my knowledge of tire pressures and handling. Unfortunately, my research so far has produced a lot of conflicting information. I don't know exactly what questions to ask, so I've compiled my current knowledge into a few paragraphs; if you read this and see any flaws in my understanding, please point them out. I'm just posting what I believe to be correct and hoping to have it confirmed or corrected.

Anyway...

I see four factors that influence how much tire pressure to use:

1) minimum tire pressure (heat issues, rim rollover)
2) maximum tire pressure (rated limit, 51 psi or whatever)
3) desired slip angles
4) desired contact patch area

The last two are the ones I'm concerned with.

From what I've read...

Slip angles are defined as the difference between the wheel heading (not the tire) and the ground beneath it. Slip angles are a meaningless concept if the tire is sliding.

If the slip angle is 0, the tire is not "walking" the tread; there shouldn't be a squeal from the tire. As the slip angle increases, the tire has less traction due to the walking effect (part of the tire is sliding on the ground and thus has less traction).

Raising a tire's pressure will reduce tread deflection, in turn reducing the slip angle. This provides a rotational force on the car, attempting to reduce the slip angle towards 0; I imagine this would cause the slip angles on the other end of the car to increase. So increasing the tire pressures at one end of the car reduces the slip angles at that end of the car, and also increases the slip angles at the other end of the car.

Given that the end with smaller slip angles will have a more traction -- increasing tire pressures increases traction at that end of the car, while actually reducing traction (in absoluted terms) at the other end.

Raising tire pressure also reduces the area of the contact patch, though ... which means that after a certain point traction will actually be reduced.

So higher *relative* pressures increase traction because of the reduced slip angles, but higher *absolute* pressures reduce traction because of the smaller contact patch.

Which suggests that you want the lowest possible pressures (limited by rollover/heat), and then you use tire pressures to "balance" the car ... it's the relative pressures that affect slip angles, not the absolute pressures.

So to tune tire pressure on a front-heavy car (w/ understeering stock suspension), you want to:
(1) Find the lowest front tire pressure that prevents excessive sidewall rollover.
This will give you the biggest possible contact patch.
(2) Start with the rear tires at a similar pressure and then reduce it to dial out understeer.

Side note: I've seen quotes to the effect that understeer is defined as having larger slip angles up front; this must assume equal tires/tire pressures front and rear? Or are those variables already present in the slip angle itself?

Again, everything in this post is based on my limited understanding; I'm just looking for confirmation or correction.

-Jon

smallTTs

But I'm afraid you don't really understand slip angle too well. It's a little more complex than what you've said, and a bunch of what you've said isn't exactly correct. No flame intended. Do you understand airplane wings, angle of attack, and stall? Tires perform VERY similarly to airplane wings. Also, different configurations of wings and tires have very different performance curves.

Now the good news is that you don't really need to understand slip angles; the tires do. You just have to use them as they like to be used.

Tire pressures for auto-x depend on a number of factors. Primarily the particular car and particular tire. Lets assume an A4 quattro with a decent sized high-perf tire (S03's 730's P-zero, etc) which will actually generate some lateral force (side load, or that which turns the car). If you drive aggressively, (and most people I've watched lose at auto-crossing don't drive agressively enough), rolling the tire under enough to lose most of the contact patch is the BIG problem on the front, or most heavily loaded tire. This is even worse with FWD. Inflate until you are not getting more than a half-inch or so onto the sidewall past the tread. Chalk or white shoe polish or paint on the outside of the tire shoulder works well as an indicator. You are most correct here that you need to prevent the rolling under.

For an example: 2000GTI VR6 with 225-45/17's on 7.5 wheels, 45 psi was minimum to keep the 730's fronts from rolling under. This car had 4 of 4 STS class wins and 2 of 4 PAX firsts and 2 of 4 PAX seconds. It was driven agressively.

On the rear it's a different story. Remember a front heavy car ('most every Audi & VW) works the front tires MUCH harder than the rears. On the FWD GTI, he'd carry the inside rear in many turns. We found that 45 psi in the rears also was good! This is counter-intuitive, but we felt that the three-wheeling actually put a significant load on the outside rear...or perhaps the high rear pressure crowned the rear tires enough to lessen their grip and let the rear come around a litte (less understeer: more oversteer). Whatever the explanation, it worked and fitted the very aggressive, but controlled, driving style well.

I don't think you want to do this with quattro, so your ideas are spot on: Get the fronts up there enough to prevent roll-under, then tune with rear pressures. Go to the max number (51 psi for example) on the fronts if you have to to prevent roll under. IMO, if that's less than about 45 on the A4, GTI or FWD TT, you're not driving hard enough.

As far as heat goes, auto-x doesn't last long enough to even get most tires up to a good operating temp. If you can hold your hand on the tire tread for more than one-half second right after a run, your tires are still below optimum operating temps.

Autocrossing a street car demands more of the tires than almost any kind of normal driving. You are loading them at the outer envelope of their capabilities; if you are not, you aren't going as fast as the car is capable of. Sorta like flying a fighter "at the edge of the envelope".

Understeer IS correctly defined as having higher front slip angles, but it's not about equal or non equal tire pressures, nor even equal or non-equal tire sizes. It's about how the front tires are working relative to the rears. I'll look for a good "slip angle" discussion and post it when I find it.

My $.02

BTW: What's your car?

ryoung

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and adding to smallTTs excellent comments.

You don't want the rear tire rolling over either, the rear of the car will feel horribly squirrelly, so I'd suggest starting with at least 42 in the rear. Since your car "naturally" understeers, raising the rear pressure above the optimum for the maximum contact patch without a tire rolling over does two things,

1. The tire crowns slightly, reducing the size of the contact patch, reducing grip, and therefore reducing understeer.

2. The higher pressure raises the spring rate of the tire (it's as much of a spring as that steel coil). Raising spring rate in the rear also reduces understeer, and I've seen data on race tires showing that a 1 psi increase raises the spring rate of the tire about 20 lb/inch. If you've noticed tire pressure adjustments in the pits during a Nascar race, they're using tire pressure to fine tune handling by changing the spring rate.

jop

Trust me, I have no problem admitting I don't understand slip angles very well -- 3 days ago the only thing I knew about them was the existance of the term "slip angle". :-) I've done several hours of research (and thinking), but I'm still trying to understand what's actually happening with the tires. I know so little about this particular topic that I can't distinguish the good information from the worthless stuff. I'd love to see any info you have on slip angles; I've read a lot of your posts and you seem to really understand this stuff.

I actually do understand understeer/oversteer (empirically, at least); I think I may have simply chosen my words rather poorly.

The car is a 2001 Mk2 225TTQC. I've been running 40 psi front (a little higher than necessary, but better safe than sorry). I gradually boosted the rears up to 48 psi; this helped a lot but still wasn't enough ... I put in some new shocks today, and the car is much more balanced in transients (and now has the ability to steer even during hard braking)... the next autocross I'll do will be 4/7, so I'll see how everything works out.

smallTTs

I guess I'd have to throw in the Vette and TT and some cash also, huh?

I agree with your comments. Suspension systems are complex little devils to fine tune. Remember that the suspension spring and the tire 'spring' are in series, not parallel. On street cars the tire spring rate is something like 5-10 times the effective wheel rate (the spring rate actually felt by the car at the wheels).

An interesting experiment is to take a small weight, attach it to a "screen door" (extension) spring and let the weight bob up and down at it's natural frequency. If you choose carefully, you'll get something like 1 cycle per second, which is just about the ride rate of a street car. Now, add a stiffer (5 times minimum) spring between the weight and the first (softer) spring and set the thing in motion. Usually the motion of the weight (mass) changes from a nice sinusoidal one to one not so nice. Now imagine tuning by just changing the stiff spring (tire). Of course, the Cup car wheel rate is a lot closer to the tire rate than a street car.

Most of the rest of the racing world allows cockpit adjustable antiroll bars, but not NASCAR. It's cool that Cup cars' handling can be adjusted by such a small change in tire pressure. Of course those minor spring changes change more than handling; the change how and where the power gets down (forward bite).

Larrytt

Running like this with both RE-730's and now my Nokian "snows". lol

I can win just about every time out, can't get PAX FTD because of the class I run in and the other people at the events.

Running the pressures as high as you are talking about in a TTQ is a bit much. Even when we were tuning Mike's car with the PSS9's and ran out of rear adjustment before he added the rear bar, we never got the rear pressures over 40 psi if I remember right. Car came around nicely like that.

quattro5

also trying to hone my understanding of SA's...

won't increased rear pressure also decrease rear S.A. therefore increasing understeer? or is this a 'to a point' kind of thing, so that the curve is essentially peaked - like "increases understeer by reduced slip angle until reduced contact patch and spring rate begin to override the difference in slip angle" ?

smallTTs

The slip angle (alpha) is the angle between where a tire is pointed and where it is going. Just like angle of attack (AOA) of a wing (also called alpha, by the way) is the angle between where the chord of the wing is pointed and where the aircraft is going.

You have some direct control of front SA by turning the steering wheel. Of course you don't know the actual SA because it varies with traction, velocity, turn radius and normal load (weight on the tire). In other words, the SA is a RESULT of conditions the vehicle is experiencing, just like AOA is a result of aircraft flilght conditions. Different shaped wings, like different tires have different shaped alpha curves. That's why putting a race tire on your street car changes not only the max cornering g but also the response to steering inputs.

The rear, non-steered, wheels will generate lateral force depending on how the car is turning. If the rear is loaded less than the front, and the fronts are also propelling the vehicle (we're getting into "Traction Circle" theory now), the rears won't need as much SA to generate lateral force, so you'll feel the rear stick better, and the car understeers, because the fronts need to do more work, so they need more SA.

A typical SA plot for a tire shows more lateral force is generated as slip angle increases up to maybe 6-8 degrees. At low SA's the increase is quite linear (2 times as much lateral force at 2 degrees as there is at 1 degree). As you approach the maximum lateral force capability of the tire, say around 10 degrees, an extra degree may only give a tiny bit more lateral force, then the next degree might give no more increase, then the next degree might give a DECREASE. If you add even more SA, the decrease increases(nice terminology, huh?)rapidly. This is you sliding toward the outside of the turn...front end first in an Audi.

Notice that the lateral force doesn't all of a sudden go to zero, it just decreases. If it went to 0, it would be like hitting a patch of black ice: the car would cease turning completely and just travel in a straight line the direction the center of gravity was heading when you hit the ice. This doesn't happen on roads without ice; the lateral force generated by the tire decreases so the radius of the turn must increase.

Don't confuse spring, damper antiroll bar rates, etc with tire SA. Those things help determine how much load is on the tire, which is a big factor in determining how much lateral force it can generate.

Try this. At 30 mph put your hand out the window like a small airplane wing (like we all did as kids). If it's parallel to the road, you shouldn't feel any lift. Now by angling the leading edge up a bit you'll get a lifting force. More angle equals more lift, until it stalls, say about 45 degrees. Increasing it more actually reduces the lift, right? When you get to 90 degrees, you have no more lift, just lots of drag. This is similar to how a tire works at some load.

You can simulate what more load does to the tire by increasing the car speed in our little hand-out-the-window experiment. The parallel is quite close. I hope this helps a bit.

ryoung

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Quoting from smallTTs below, "the rears won't need as much SA to generate lateral force, so you'll feel the rear stick better, and the car understeers, because the fronts need to do more work, so they need more SA." Also, remember that the slip angle increases as the tire approaches the maximum lateral force that the tire is capable of generating.

Raising rear pressure is not the ideal solution; it's made necessary because of the heavy front weight bias of the Audi. Since raising pressure reduces the size of the contact patch, we're reducing the maximum available lateral force that the rear tire is capable of generating. As a result, as we approach the limit, the rear tire runs with a higher slip angle, understeer is reduced, and the cars feels more balanced. [Also of course, the higher rear spring rate also helps.]

If the car had a 50/50 weight balance, and pressures are starting from below that needed to maximize the size of the contact patch ... you're right. Raising rear pressure would reduce slip angles, and the car would oversteer.

In the end, though, all of this really does take care of itself. Set front pressure to maximize the size of the contact patch (and maximize lateral grip), and raise rear pressure to reduce understeer as much as possible.

Hope this helps.

quattro5

cool - thnkz!