Fred, I was trying to keep up but in no way I'd would take any risk. I was fine in the back and enjoyed the drive as it was my first time. It was amazing that everyone else was able to stay together and Max managed to take pictures too . I am glad I had the idea right about using 2nd gear.

Cary, I set the tire pressure based mainly on the recommendations from others here. Most people run with lower pressure in the front than the rear to reduce understeer and I also understand that it also has a lot to do with how hot the tires get to give maximum grip. I don't track my car (yet) or drive it that fast to know what best pressure is. So for now, I just set up my car strictly for fun (street), not competition.

Thx

 

HN

My experience w/ both the Bridgestones and the Kumho's is the rears heat-up more than the fronts. Anyway the concept as my not so nimble mind understands it is as follows:
+ to increase oversteer; increase air in front or decrease air in the rear
++ to increase understeer; decrease air in front or increase air in the rear.
or to say it conversely:
+ to decrease understeer; increase air in front or decrease air in the rear.
++ to decrease oversteer; decrease air in front or increase air in the rear.

Am I missing something???

 

Maybe I am reading some of these posts incorrectly, but here is how tire pressures affect over and under steer.

If a car is experiencing under steer you can reduce the tire pressure in the front. What this does is takes the contact patch of tire on the road surface and begins to force the tire to ride on the edges of the tire. This is of course referenced from an optimally pressured tire that has a flat contact patch at operating pressure. When you reduce the pressure you are wearing the outside edges because the tire is no longer flat to the ground. Conversely if you were to increase pressure in the front, the contact patch would decrease, thus providing less grip than before. This condition normally creates more under steer.

The same result happens in the rear. It is all based on contact to the ground.

Over steer: decrease rear tire pressure and/or increase front pressure
Under steer: decrease front tire pressure and/or increase rear pressure

I would do neither of these since the tire is no longer optimally contacting the ground and you will prematurely wear the tire unevenly.

 

Some very good and sometimes conflicting information and opinions on over/understeer in this thread. A subject close to my heart.

In my opinion hn's set-up should not have that much understeer though. 2 clicks on the PSS-9 and 2 psi difference? Something could be "off". How about comments made early in this thread about the kinematic toe setting, Hn have you checked it? Is your alignment shop experienced in setting up 993's with all the correct tools etc.?

And what about those new tires? Any chance of the rears being greasy at 300 miles?

Enjoy

Nol

 

Nol, What I think I have is too much oversteer. I forgot the question about the kinematic toe, no my shop didn't adjust it. I was assured not to worry, it's not needed for my application.

I am also wondering about the new tires too.

 

Kary

In the real world what is static and thereby possibly optimum???...the camber and toe change w/ dynamic loading, tire pressure is constantly changing due to thermal forces...so in fact how does one determine optimum anything???particularly one component of a synergistic system.....I humbly suggest that one be congniscent of their operating circumstances, what they are trying to achieve and make adjustments accordingly all within material/operational tolerances. It seems that it's a balancing act and at the end of the day whether it is "optimum" or not is determined by the driver. Anyway hn is already running below and outside of Porsches recommendations....so to add 2psi in the rear he will reduce the difference from 7psi to 5psi...is this a big deal??? If it feels better to hn I'd say yes!!! Anyway...for those looking for optimum mileage for their tires a 993 maybe a poor choice of a vehicle to put them on...cheers, Cary

 

N Wrote"I forgot the question about the kinematic toe, no my shop didn't adjust it. I was assured not to worry, it's not needed for my application. "

I would go to another shop if they told me that. Oftentimes an independent won't have the equipment to do the kinematic toe. I would be very uncomfortable not knowing if it was right or not.

 

hn, I think it does matter. I only have the dealer/importer here to deal with, who have the tools at least. But if you search the archives you will see testimony by the "guru's" on the importance of "993" experience in setting up your car, including kinematic toe.

Enjoy

Nol

 

Hn

I'm w/ Scott and NOL...having spent the $$$ you did on your suspension upgrade get the kinematic toe checked out..... I am under the impression from the local dealer that once the kinematic toe is set and you are not out banging into curbs (don't ask me how I know), changing out the suspension (again), or raising/lowering the car height you will not need to do this w/ every alignment.

 

Cary, fwiw, I have only 2 psi difference between the front and the rear, 35f/37r.

Nol, I know how this board feels about the kinematic toe and that's why I posted to see how they feel about what I was told. Oddly, I went all the way to Johnson's Alignment also because of the recommendations from here . I haven't noticed any strange movement yet and what I experienced over the weekend I think is just lack of experience .

thx

 

hn

the factory recommends 36f/44r...an 8 psi difference.....so by adding a couple of psi to the rear (35f/37r to 35f/39r) you are still well within their margin of front to rear pressure differential. At one time Robin Sun was running 32/36r...something magic about a 4 psi differential...perhaps?????

 

My $0.02 on this subject:

When I used to autocross my Rabbit GTI, I would add more pressure in the front tires to reduce the natural understeer of that car.

This makes sense since, in the 993, the rear tires are set higher to tame some of the car's tail happiness. For better or worse, I don't fiddle with the tire pressures to adjust handling since my PSS-9's and adjustable swaybars allow me a measure of control.

Steve Weiner has an excellent handling "cheat sheet" that I've saved on my computer: Rennsport Systems Handling Table. This resource states that front tire pressures should be increased in small increments to decrease the push. As Kary said, I also think adding waaaay too much pressure in the front tires can cause the contact patch to be reduced and increase the understeer. I hear lots of different recommendations for tire pressures (some say follow Porsche's pressures, others say go lower), but I would think 30-35 cold psi is a good place to start for the street. I always find this to be a bit confusing and vague, so I end up guessing, hoping I'm not ruining a good set of street tires.

On my new MPSC's, which I just tried this weekend, I was told to go for 32-34 hot psi pressures. This means I started out at about 28 psi cold (varies with ambient temperature.)

 

Hn,

2 psi between front to back is certainly reasonable. I have run hoosiers(38/40), and pirelli slicks (29/31) that way with great success. On the street I tended to have a greater spread up to 4 psi given that the front tires are only 225 and rears at least 265 (sometimes 275). I drive my car on the street sometimes, but not that much any more. When I do I have P-Zero's set at 37 and 41 hot. That works ok given my set up which is much more aggressive than the average street car. Since my camber is greater and 3.5 degrees, lowering tire pressure is the least of my worries in terms of tire wear. For you, tire wear will become obvious more quickly with a near stock setup.

Many of these posts are correct in that there are many variables here. Tire size, camber, toe in and out, suspensions, sway bars, rake of the car, tire brand, and driving style. I think you might just find yourself a local shop that knows how to set up the car to factory specs or RS specs and then you have a baseline from which to discuss the issues you are having.

Good Luck and keep us posted!

 

Thanks guys for your informative posts.

 

HN and all,

There is some really good info in this thread, though it is all focused on the car. I suggest you also consider the driver.

Based on what you have driven, your style, experience, etc, you will have a certian way that you prefer the car to feel. Within that, you can control what the car is doing. It is all about managing weight (or if you want to impress everyone, call it platform management, LOL). You can make a car understeer or oversteer within limits. Various corners induce one or the other.

I think it would serve you well to get out to a track event where it is much safer to explore what your car is doing. Also, you can have an instructor you trust drive your car - without even pushing it very hard, they should be able to tell how the setup is.

It is really hard to talk about under/oversteer as everyone has a different idea of what a loose car is or one that pushes. For example, I like a car that is a little loose, based on my definition of loose. Maybe DJ will chime in here and give his description of what I call a 'little loose'.

Again, get in an environment where you can explore what you like and what you don't. It will really help you get the car the way you want it.