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+--- Thread: tire pressure and understeer/oversteer (/showthread.php?tid=475)
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- malammik - 04-14-2007
Today was a blast. My left knee is blistered from rubbing against the door but I will fix myself up before tomorrow with some of the painters tape I ripped off Steve's car
hock:Even on the skid pad, I could not get my car to oversteer. Must be because of those huge rear tires. Understeer no problem. Plenty of that :X.
Should I try to lower the pressure in the front and increase a little in the back? Any specific values that had worked for you in a 997?
C'yall tomorrow.
- Darren - 04-14-2007
malammik wrote:
Quote:Today was a blast. My left knee is blistered from rubbing against the door but I will fix myself up before tomorrow with some of the painters tape I ripped off Steve's car
Even on the skid pad, I could not get my car to oversteer. Must be because of those huge rear tires. Understeer no problem. Plenty of that :X.
Should I try to lower the pressure in the front and increase a little in the back? Any specific values that had worked for you in a 997?
C'yall tomorrow.
You can't get it to oversteer easily. They made it understeer like that because most people don't know how to drive and don't care to learn. Oversteer causes huge liability for auto makers, and Porsche especially has been burned by this. Its not just the huge rear tires, its the lack of front camber more than anything.
Sort of a pet peeve of mine (I'm igniting a debate here) is this whole inflate/deflate understeer/oversteer thing.
The absolute fact is that there is an IDEAL pressure that will cause a tire to have maximum grip. If you inflate, or deflate beyond that point, you will lose grip.
I don't know whether inflating or deflating causes you to lose grip faster, and I really don't care, neither one is an appropriate way to tune a chassis.
If you inflate your rear tires so that they look like donuts, then your contact patch will be, of course, very narrow, and the rear end will come around faster. This will reduce understeer with the expense of reducing overall grip.
I realize this is Autox lore; it is a typical case of fixing the end that ain't broke. The problems with the 996/997 are that the front tires are too narrow (no you cant fix that with narrower rear tires) and the front camber is not adjustible less than about -1 degrees, if you are lucky!
The best solution, IMHO, is lowering the car which will allow you to have more negative front camber. Heck, our cars sit 1" or so higher than how they were designed for Europe, because of U.S. bumper regulations. I also like to run wider front tires, I have 265's on my 996. I don't know the factory 997 front size, but I'll guess 225. 225 is just not enough. The stage 3 step is to install camber plates. I wouldn't recommend this for a 996/997 because the products available are not quite ready for daily driving.
As an aside, any guesses what size tires they run on Grand-Am 997 Cup Cars????
2-20.9 Tire sizes are 280/640R18 front and 305/680R18 rear
This all said (as Kam always reminds me) you learn more by driving what you have, and driving cars that aren't perfect, than by continually making changes. And obviously an understeering car is more safe than any other condition.
- smankow - 04-15-2007
Darren wrote:
Quote:And obviously an understeering car is more safe than any other condition.
I don't think that Eddie Griffin would agree with this
Well stated, Darren
![[Image: thumbup.gif]](http://forums.rennlist.com/rennforums/graemlins/thumbup.gif)
- bobt993 - 04-15-2007
Michael,
Next time on a skidpad. Turn off the PSM for skidpad only. Use the sport mode on. Have an instructor ride with you. Start the session circling until you feel the car start to understeer (plow). Now let the wheel out slightly. Keep letting the wheel out a little until you feel grip on the front again. At this point, your balanced. Lifting throttle will induce oversteer. Last option is for the instructor to add a tug on the hand brake without you knowing. From here you can practice CPR. (control pause recovery). Throttle lifts will induce oversteer in a 996 once you know how to control it. I have taught this for Boxster drivers to induce rotation, but only with a safety measure. One mistake is waiting for the cars response and then it's too late. You must time such an action and best place to learn is the skidpad.
- JimWirt - 04-16-2007
Its a shame, at the novice autocross school the instructors I don't think where faimiliar with the 996s or 997s. These guys spent the whole day understeering all over the place. My 78sc still wants to understeer. I have adjustable koni's and when I dialled them back to full soft I could then get oversteer. When going into a corner if you brake and turn will this load the front and then add some throttle, will this allow some tail out stuff! thinking about it I don't think I saw one 996 or 997 tail out at all durring the school.
- Darren - 04-16-2007
bobt993 wrote:
Quote:You must time such an action and best place to learn is the skidpad.
Even on the skidpad at Summit, I couldn't get the rear end to step out on my 996, on wet pavement, without using the throttle. All you get is understeer. I'd bring it up to, I don't know, 25-30 mph, right at the limit in a nice circle, then lift -- nothing! The car stays on the same circle.
This is because in my constant circle, the rear tires were nowhere near the limit of adhesion and the front tires were right at it. If the rear tires had less adhesion, and I lifted like this, then the weight transfer when I lifted should have unloaded the back a little, reducing the rear grip, and the back end would come out. The problem is that the discrepency of front to rear grip is so great to compensate for the fact that the motor is in the wrong end of the car. Theoretically there is some speed that if I were going fast enough that this would work, but at low speeds on a skidpad, my experience is that it doesn't.
A great/fun exercise with PSM on the skidpad is to get the car to understeer with PSM on and to get an understanding of what PSM is actually doing for you in that case. For the most part, you can mash the throttle and it will compensate to allow you to go in whatever radius circle you want. You can also practice driving to the limit right before PSM kicks on.
- malammik - 04-16-2007
We were on the skidpad and even after the car was balanced lifting throttle would not make it go into oversteer. Should've tried the handbreak. I am pretty sure that would've done it.
I got a really good sense as to where my car would understeer but not oversteer.
PSM was off the whole time. Even during the sunday competition.
- JimWirt - 04-16-2007
What happens if you use left foot braking in your car. i know that is an advanced technique but the instructor was doing it with my car. Wasn't sure what kind of an effect that would have on understeering but I would think it would help some.
- malammik - 04-16-2007
I assume that anything that would make the rear weels spin slower the the car needs them would cause oversteer. I wish my instructor tried something like that.
This why we need Porsche/RTR autox so we can exchange ideas and try them right away.
- TwentySix - 04-16-2007
Left foot braking and PSM don't mix (IIRC even with PSM turned off ) sorry.... I think the handbrake would do it though...