I did a search, but I think I am more confused than before I started reading.

Could someone suggest appropriate DE tire pressures for a 996 with stock suspension with the goal of reducing understeer somewhat.

 

Controversial subject, you'll probably get a bunch of answers.

IMHO there is no single right answer, depends on your tires, driving style, the track, ambient temperature and other mysterious factors (just kidding on the last one). Try reducing front pressures by 2psi and then drive a few laps. Repeat until you achieve the desired result.

Also try posting this on the Racing and DE forum if you want a bunch more replies.

 

What tires do you have?

-td

 

To start off I would use roughly
36front 42rear
measured hot after you pull in from a session. often times tires will become overinflated on track and your contact patch will be reduced, you'll try to drive at the same level as before and over heat the tires and lose even more grip. If it feels like the tires go off towards the end of the session, you can lower the pressure by about 2 psi front and rear.

 

Assuming street tires, I suggest 36F 38 R hot pressure. If you are a newcomer to DE start with cold pressures 4 lbs lower than the hot target. Check thehot pressure as soon as you get into the pits. Bleed down as needed. My experience is that any pressure over 40 will make cause a loss of grip.

Regarding understeer, when does the car undesteer? A 911 will understeer in slow corners, trail braking solves that. A 911 will also understeer if one accelerates before the apex. Easily solved, enter the corner faster but accelerate only after the apex while unwinding he steering wheel.

Regards,

 

+1 What they said

If street tires this is pretty good advice IMHO.

 

PS2 in 225/40-18 front and 285/30-18 rear.

I am doing my first real DE on a track with several very tight corners. I was able to do several laps of this track in the spring when I went to visit a friend who was doing a DE at this track. What I noticed most was that it was really hard to get the car to rotate in the slow corners, hence the desire to reduce understeer. Of course, it doesn't help that my natural tendency is to overdrive anyway, or that my demo ride was in an Elise with a very experienced driver who could do just about anything.

I am not very experienced at performance driving, so things like trail braking are merely vague concepts to me at the moment. I will need to read up on that.

 

It is funny, I went to a Porsche club Performance Driving School this past weekend. Extraordinary people, by the way. Just amazing that so many instructors and volunteers would work so hard only for the chance of infecting neophytes like me with the bug.

Anyway, during the course of the weekend, I heard both that you should increase relative front tire pressure and reduce relative front tire pressure in order to reduce understeer. Per the Tire Rack web page on air pressure for competition tires, I think it is an increase in the fronts.

 

36F/38-40R (hot) is a good starting place. Especially since you are new, I would encourage you to get more hours/experience before trying to change the tire pressure much. You may be able to get what you want through technique.

If you start changing pressures, etc., while learning--you're adding variables to the mix that may cause problems later and make it harder to get a good style. Within a reasonable range, the higher the pressure on a pair of tires, the more they will bite (and wear). So, if you want oversteer, then the balance (frt to rear) will be slightly higher pressure in the front compared to the rear; if you want more understeer, then the balance would be adjusted slightly higher pressure in the rear compared to the front. Similar things can be accomplished with adjustable sway bars. While all this is techincally true, this assumes your technique is solid. I'd encourage you to give yourself more track time before you do change too much.

 

Shoot for under 40 hot. (37-40; probably around 32-33 cold) you will have to bleed the air after every session as you will pick up speed every time and this will put more heat into your tires. Also, check with the locals at the track. The will have good tire pressure info for you.
Practice and technique will fix this. My first guess for the cause of your described understeer is diving the corner and trying to turn with the fronts loaded. Lots of beginners do this. Many times the fix is as simple as, brake a little earlier and get back to gas sooner. Could be more complicated, but I'd give balancing the car a shot.

-td

 

I agree with himself! I usually try to run under 40psi hot but every tire heats up differently, every track is different so you have to check at the end of each session. On my main track for example (Shenandoah at Summit Point), I've to run 2psi lower pressures on the right side wheels as most of the 18 turns are left turns and the right (outside) tires heat up more.

For me the principle "slow in, fast out" works the best on the 911. So, it's better you start the turn a little slower but try to hit the apex right and at full throttle. The 996 has enough power to accelerate out of the turn and the traction is perfect as you'll have so much weight on your rear when you open up the streering wheel between the apex and the end of the turn. Over time you'll automatically get faster at turn-in but you'll have at this point learned the main lesson: hit the apex at the right spot and accelerate out of the corner. As more you have to fight the balance of the car with a (too) fast turn-in (understeer), as more you'll loose by not hitting the apex right (point and speed).

 

Startign with normal street pressures, decreasing your tire pressue will increase the size of the contact patch and increase grip - up to a point. Too little pressure increases flex and decreases grip.

 

I think the key here is that this is your "first real DE". Understeer is one of the safety mechanisms built into the car. I hope this doesn't sound too smart-***, but I would suggest you take all the safety measures you can during your first several DE runs, and this includes the understeeer.

There are so many things to learn at DE, tire pressures should not be one of the first ones. Get the feel of the car, feel what it is like to drive a DE at a racetrack, practice "the line", the braking points, proper braking technique, keeping the head up, proper gear and proper shifting, smoothness. Understeer at turn-in is usually one of the early signs of going too hot into a corner. So, if you notice understeer at turn-in, slow it down and practice the "slow in, fast-out" technique. As you get more seat time, have some basic memory and basis to compare things, then start playing with tire pressures, maybe alignment settings, tires, etc.

 

Not smart *** at all. I very much appreciate what you are saying and your comment. I did a DE on a police academy driving track in my Mustang Cobra (an understeering pig, if there ever was one) a few months back. Very tight track, more like an extended autocross. I know from this and my few autocrosss experiences that my tendancy is to go in to corners too fast, and I need to work on that and all the other points you mention. Still, there is so much understeer in my completely stock car, I doubt that I could eliminate all that much of it with tire pressures, could I?

 

DCP -

i won't go into details much because there are so many competing opinions here, but if the goal is to reduce understeer, you will do best running the front pressures a bit higher than the rear pressures in a stock 996. i know a lot of guys are on record as saying the exact opposite; once you've tried it their way, set your fronts to 34psi cold and your rears to 30psi cold and see how it feels.

a lot of guys say that lowering the tire pressure will increase the contact patch size and, in turn, the grip. this is only true in certain pressure ranges that lie outside of the design range for the tire. when a tire's pressure is within its design range, adding pressure increases the efficiency of the tire; this adds grip.

here's a good rundown on how different changes effect handling:
http://www.rogerkrausracing.com/Tech...verunder.shtml