Rick J.

Here's the specs we did for Robs blue beast: front toe: .02 a side rear toe: .09
front camber: -3.5 rear camber: -3.1
this is a full track car with JIC's and ERP arms/ solid everything

jdistefa

Darty means that the car will hunt left and right with changes in road surface contours. Essentially the car can feel very twitchy, especially on high speed straights. You really have to set up for your track or autocross application.

Re. the tire pressure/understeer issue, my understanding is that it's a bit of biphasic curve. You can lower the pressure a bit to get more grip in the front (I do, especially on cold days and/or cold tires), but if you lower the pressure too much, the car can actually understeer more. You have to find the sweet spot through trial and error, feel, and checking tire pressures/temps. IMHO of course .

cabrio993

Yes, tire pressures changes the contour of the tire. This in turns changes the contact patch of the tire against the road. The more pressure you have, the less contact patch you have with the road and therefore less grip.

By decreasing air pressure at the front and increasing at the rear you end up with more grip up front and less in the rear (over-steering).

If you lower the pressure too much, then you begin to have sidewall flex, which counteracts your desired effect.

So, basically, as you well guessed it, it's all about tire contact patch on the road and which one has the most grip, rear or front, to achieve the desired over-steer/under-steer balance.

nile13

That's what most people intuitively think and this is actually incorrect.

Yes, the contact patch changes with air pressure change. But the stiffness of the sidewall also changes and it turns out that for the normal range of pressure for most tires at street/autocross situations (say, from 26 to 46 psi) the sidewall stiffness (think of it effectively as a spring rate adjustment in the suspension) has a much more pronounced effect on handling than any small changes in contact patch.

In other words, increasing the tire pressure definitely increases grip. This is culled from 9 seasons of autocrossing on both street and race tires in many different cars. And talking to fast people who are also smarter than me Counterintuitive, I know, but turns out to be true. At least for 30-40 different tires that I've driven on.

Now, if you go way outside of normal pressure range, different things start to happen. You'd want almost flat tires for driving on sand or in a swamp. But that's outside of this particualr discussion. Just shows that the behaviour is far from liniar.

Interestingly, my 993 has a setup almost identical to Jon's. Same suspension, same sway bars, same width tires, very similar alignment. I rarely autocross it, but when I do, I've played with the pressures a bit (tires ar pretty stiff Azenis RT-615s). I think I've settled on about 32 up front and 36 at the rear. The car feels fairly satble and doesn't seem to understeer unless I overcook it into a turn. Then again, I don't really drive it with the steering wheel. Most of the control comes from the right foot. The steering wheel is kind of there to point it in the general direction, then you can play with the car's butt to put it where you want it to start accelerating. But, again, I would not say that it understeers.

Hopes this helps

Edward

Good comments here re tire pressures and how it affects handling. But one must keep in mind that a given tire has its operating window, so to speak, and any pressure change outside that particular tire's range is going to give you poorer performance, and give you the wrong idea of the affects. Add to this that each tire is different, will be more/less sensitive to pressure changes, and then will respond more/less drastically to said changes. All this to say, don't obsess. Get the pressure within the proper range (ask around for that tire on your car), and concern yourself more with driver input. What you do can more/less mitigate a tire's shortcomings. Conversely, tires set up perfectly can be made nearly completely irrelevant by poor driver's habits (not to say that is you, of course). Just food for thought to balance things out a bit

Edward

flatsixforme

Excellent feedback guys, thanks. I agree, driving experience is most important as my car's ability far exceeds my own. That said, it helps to know as much about the mechanics of the car and how changes in set up inputs affect handling/feel so I can better process in my head what I am feeling and why when on the track. At the very least it helps me formulate questions to ask those with experience

I never thought about the sidewall as a spring rate adjustement, good point Nile. So, if what you are saying is true that increasing tire pressure (and in effect spring rate) then theoretically, if i wanted to set my car up to have as much oversteer as possible, I would find the particular tire's pressure range of optimum performance and make the following adjustments:

1. Set front tire pressure to maximum pressure within optimum range
2. Set rear tire pressure to minimum pressure within optimum range
3. Set front shocks stiffer than rear shocks (or am I confusing spring rate and damping rate)

Now I understand that there are many other factors that play an important role into this equation but in its basic form this seems reverse to what I have pulled from reading many people's thoughts/comments/setups here on RL.

?

Paul902

RJ, do you have a strut tower brace?

cabrio993

It is not by intuition that I know what I know . At least we agree that we disagree...

RallyJon

Yes.

FWIW, I've pretty much given up on this as I need to keep the car compromised for winter driving.

cgfen

Exactly, IMO........ we need to find a basic reference point, ( front tire pressure), to vary from to determine if lowering pressure = > understeer or < understeer.

I agree, it IS about contact patch, but not ALL about contact patch since as other have noted that a fatty patch with wimpy sidewalls, (due to too low of pressure), is bad. Not trying to bust your chops, just continuing the conversation.

hmmmmmm at first i was going to say, "you are crazy", but then you listed the tire pressures that you run.
These are nearly identical to what i shoot for as HOT pressures when i AX.

If you, or anyone with similar equipment, started @ 26psi front and bled air pressure, i agree, they would probably lose performance. If you added air, to say 30 - 34 psi, then i agree with you when you say "In other words, increasing the tire pressure definitely increases grip."


first thing to do is determine what that optimum range is.
may i suggest with your equipment to try;
28 - 33 psi hot for front.
30 - 36 psi hot for rear


I don't think you will get maximum oversteer with the conditions you list above, once you are in the optimum range to achieve the desired "spring rate", then contact patch size again comes into play, less air = more rubber on the ground.
I'll have to ponder a bit more to formulate an opinion on the rear tire scenario.

for a tight, semi-low speed AX course, i don't think you want stiff front shocks.
IIRC increasing damping has the same effect as increasing anti-roll bar thickness, but i might be 180 degrees out of phase on this.
I"ll do a bit more reading when i go home.

One last thought? I think we need these tools to determine optimum range for our situation;
1. pressure gauge
2. pyrometer
3. shoe polish to gauge sidewall rollover.

Next local AX is the 25th.

cheers

Craig

TheOtherEric

You can't really say that. Tire grip is like a bell curve when plotted against pressure. When underinflated, you lose grip due to all the sidewall flex. When overinflated, you lose grip due to the narrower contact patch.

Last year I asked Randy Pobst when we were at the Nurburgring how I could get my rented Mini Cooper to oversteer more. I figured he'd know since he was driving a similar F/F in World Challenge. His response: "throw in another 15 psi in the rears... that's what we have to do in World Challenge".

cabrio993

That's correct, no chops busted.. That's why if you read my post, i said:
..."If you lower the pressure too much, then you begin to have sidewall flex, which counteracts your desired effect. "

ZombiePorsche44

The pressure gauge and shoe polish will do for the tire pressures and sidewall rollover. A pyrometer will not do you much good at an AX since you really won't be making a lot of suspension adjustments, some people may disagree with me but I have never used one at an AX.

As for me, I keep my tires at normal cold pressures, run 17's w/205's up front, 255's in back and never really mess with anything. My car has a slight bit of oversteer even with just 205's because that's what I wanted when I had the suspension modified. I guess having the suspension up correctly by an expert to start with helps more than trying to dial it in yourself.

ZP44

Mark in Baltimore

Oh, yeah? I asked Michael Schumacher when we were at Spa how I could get my Bugatti Vayron to understeer more. His response: Have your damn mechanic put harder compound tires on the front!"


j/k, Eric. That's cool that you got to talk with Pobst at the Ring.

nile13

Cabrio993, let's compare notes. I've always been fascinated by this subject, but my experience is with autocross only (and mostly with Miata and other cars, not 993).

What pressure would you run in 993? What increase or decrease in pressure would you recomend to make it more neutral?