hn

I was having a hard time keeping my tail from sliding during the fun drive. I recently had pss9 and 18" wheels and like how the car drive on the freeway and the interchange. But when it comes to the real twisties, it can be a little scary. Here is the setup :

Pss9: 8f/6r
Tire pressure: 35f/37r
Tire size: 225f/265r on 18"

Also, my tires have only 300 miles and I am not sure if they are properly broken in.

I guess I can adjust one of these 3 to reduce oversteer. I think the easiest would be pumping up the front pressure to 37 (also to save the front wheels from pot holes). Second choice is to set pss9 to the same level on both front and rear. Changing tires is not practical.

I'd appreciate your advice as which one is more efficient.

Thx

Mark in Hermosa

I assume you meant oversteer? Loosen the rear end a bit to reduce oversteer, tighten to create oversteer. I know there are more options, but this is what I do. I don't mess with pressure.

TheOtherEric

Aren't those tire pressures a bit low? Doesn't the manual say like 40-something psi for 18" wheels? I'm running like 38/42 but granted I don't know what the heck I'm doing.

914und993

The manual says 44 psi for rear 18" tires. That is pretty high, probably to help protect that wide rear rim from potholes, maybe also to make oversteer less likely.

Try running higher pressures in your rear tires. You can also soften up the rear damper setting even more.

Chip

Anir

With all due respect to Chip, adding more air to rear tires (especially anything near the recommended 44 psi) will only make the tail more prone to sliding, i.e. accentuate the oversteer.

I run 32-34 front and 34-36 psi rear on the street, and less at the track (when I've actually got a functioning 993, that is ). I think your pressures are fine. You might get a little more improvement by tweeking the PSS-9's. Adjustable rear RS bar?

Good luck!

viperbob

hn,

What other items, like sway bars, are you running along with the PSS9s? Also, what are your alignement settings? Did the place where you had the alignment done CHANGE THE KINEMATIC TOE?? With the reduction of ride height with the PSS9s, you could be experiencing some toe change if this was not done. Just one of MANY things to look at before any hypothesis can be made.

In general, if that is all you have done to your car, there should still be some understeer left in it. It is tough to get the 993s to be tail happy. Sure you weren't lifting a little on the turns, especially with the new suspension?

Anyway, here is something that you might find useful.

http://users.telenet.be/miata/englis...suspension.htm

User 4621

Since it's quick and free, I would first try a 1 click difference in the PSS-9's, instead of 2. Mine are 8F/6R street and 3F/1R track, but that's on a C4S with 285 rear tires.

cfdarch

Tire Rack has a nice list of conceptual ways to adjust oversteer / understeer

Guide to high Performance Handling


I was experiencing understeer / push when autocrossing and found by running @ 2 psi lower in the back than the front I could achieve a modicrum of oversteer. BTW a 4 psi differential was downright scarry. I run Kuhmo ECSTA V700's.

914und993

The question of how a change in tire pressure will affect oversteer or understeer is not really so simple, and if my first statement implied it, my bad.

There will be an optimum air pressure for a given tire under a given set of circumstances that will give the smallest slip angle while cornering. Raising or lowering the tire pressure from that point will increase the slip angle, and if we are talking about the rear tires it will increase the bias toward oversteer.

What we really don't know is where on that curve hn is at 37 psi. If he is at that optimum pressure for his road runs, raising or lowering tire pressure will increase his oversteer. If at 37 psi he is still below that "minimum slip angle" pressure, then raising it some will reduce oversteer.

I suppose my suggestion should actually have been to try raising air pressure or lowering it in increments to see whether a change in either direction will have a favorable result. Anir may be right, going higher might be the wrong direction.

Fred Puhn's "How to Make Your Car Handle" explains this, and lots of other really good stuff, in an interesting fashion.

Chip

fbfisher

Hn - any time you want to test your new settings let me know and you can chase me through the canyons some more.

Don't lift

hn, if you are SERIOUS about learning high speed car control, setting up your car, fine tuning your suspension and tire pressure.....etc. The ONLY safest way to go is joint POC and do couple of Streets of Willow events to start! Good luck!

kary993

In my experience with these cars I would *not* change the tire pressures to reduce over or under steer. You only run the risk of wearing the tires out prematurely. Also, you can ruin sidewalls with too little pressure, especially with 18" wheels.

PSS9's do offer some relief for over and under steer by adjusting the setting with some offset. Small results though. Having adjustable sway bars will provide the best result if you have them.

Having over steer in a 993 is a very desirable thing if you want to go fast. Ability to pivot a car and use the horsepower/torque to direct the car will yield very fast times (lot's of fun). Most people feel they have over steer because they lift at the wrong times, usually in the corner causing them to feel scared of what they think is an over steering car. In reality it is not over steering, they just pitched the car, now they have to catch it.

I agree with the above post. Go out to a track or school where you can learn more about your cars dynamics in a safe and controlled environment. You will be glad you did and will enjoy your 993 even more than you already do!

hn

I forgot to mention the M030 bars. My alignment setting is conservative for street, camber rear=-1.0 and total toe in=3/32" and front=-0.4 and 1/16" respectively. May be it has something to do with my driving technic : during the first 1/2 hr of the run where all the twists were so close to each other, I was in 2nd gear the whole time and used the gas pedal to control the speed (maybe that slowed me down but I felt safe though). I rarely used the brake and definitely didn't lift during the twist if I remember correctly. I just can't imagine changing gear/ heel and toe every few seconds. Now Fred (or anyone that think what I did was all wrong), you are familiar with the run you know what I am talking about, how did u do it? what gear were you on and at what speed?

What track you guys in OC/LA go to?

Thanks guy for your input and ViperBob and Cary, I'll print the articles to read tonight, thanks for the links.

fbfisher

Hn, you did just fine. Take your time, go at your own pace and come out with us more often to get familiar with the roads.

That said, most of us were in 2nd most of the time. Uphill runs are much easier because gravity and compression help slow you down. Compression and brakes on the down hill side. Don't feel rushed, our runs are not a race or a time trial - and no one drives anywhere near the limits of the car or their ability. We all keep it safe, and sane so we all arrive with a big smile on our face - should have seen Addison's brother - ear to ear grin at every stop!

cfdarch

hn

You have more variables than I do....FWI...I have Gert's ROW M030 w/ Bilstein HD set-up...i.e. no suspension adjustment so when I go to auto-X the only variable I've got is tire pressure. As I am not a "street" racer, I run around town on 17" cup wheels w/ Bridgestone S-03's at Porsche's recommended 36 PSI f/r. I drive my car to and from the auto-X on Kumho Ecsta V700's (225f/265r) on 18" turbo twists also @ 36psi f/r. There was a thread sometime ago regarding Porsche's recommended 44psi rear pressure on 18" wheels.The board concensus as I recall was this was to high and probably was the lawyers, not the engineers @ work trying to dial in as much understeer as possible. Anyhow...it's only while auto-Xing that I lower the rears relative to the fronts and always at a tire pressure that is within reason. In your case it seems if you have too much oversteer you might try adding air to the rears and see how that goes prior to adjusting the PSS9's. BTW how did you determine/ select the your current settings??