Z-man

Mickey:
Now that's a loaded question! I don't know if this is best, but this setup works for me. Bear in mind that I run in stock PCA classes:
1. Shocks: I have the 474 option - Sport Shock absorbers (Koni Yellows)
2. Sway bars: Stock front stablizer bar (27mm) and 968 M030 19mm rear stablizier bar set to full firm.
3. wheels Tires: I typically run either 225-50/16's front, 245-45/16's rear, or 245-45/16's at all four corners, if the rules allow me to run 8-inch wide wheels up front. Tires are Pirelli P-Zero C R-compounds. My tire pressures vary, but I run on the higher side: 35 to 37 psi cold: usually the same pressures front and rear.

Does it work? In my PCA region (NNJR), I ran undefeated in my stock class in 2003. (Truth be told, it my class isn't that competitive! ) In the PCA Zone 1 Autox last October, I took second in class behind a very well driven 968, beating out another 968 by 0.097 seconds. I'd say my setup suits my driving style well.
( See my autocross stats here ) <-- Shameless plug

IMHO, a super firm setup won't be as effective in autox. (Another setup misconception!!) At an autocross, since the car is travelling at a slower speed, the car cannot take full advantage of a firmer suspension, as it can on the track. Thus, a super firm car will dart about and not be as easy to handle through an autocross course.

As always, YMMV,
-Z-man.

M758

Well,
A superfirm suspension will be more difficult to drive in an autocross. I have noticed that my spec car does not give me the feel back at autocross as it does on the track. The stiffer suspension and lowered ride height is the issue. I know that after I lowered the car further I had more difficult feeling if the car was at the limit. I as a driver felt less weight transfer (body roll) and therefore had more challenging time "reading" the car. I since have been autocrossing a stock 83 944. This car pitches and rolls like boat, but is still quite fast. I can't say faster than my spec car mostly because it run on street tires vs R's for the spec car. But I can feel what the car is doing so much more. It is actually quite fun especially the looks on people's faces after the run. The often looks like it is pitching rolling everywhere (and it is), but since its nicely balanced I can hustle it around rather qucikly. Nice thing is that I can also easily feel when there is understeer or oversteer and give input to react accordingly. The only problem is that I to learn that I can do it too qucik as car response more slowly than my 944-spec. So must learn to anticipate a little more.

Z-man the biggest issue may be that most autocross sites probably do not have surface as smooth as tracks. Thus a stiff suspension can lose contact with the ground causeing rapid handling changes.

jonnybgood

I don't know the details of how a jake brake works but I know it increases compression specifically for the purpose of braking. Truckers need all the help they can to get 85klbs safely down the road and their requirements are not applicable to racing a 944!!!

I have read these posts and the previous thread and I have learned from the EXPERIENCE of these guys who actually race. The benefit of your maintaining your position about engine braking has a positive in that it is making these guys explain their technique. I must say though that it gets frustrating when you do not have a car, have not driven autocross or race and you continue to state things as if you are an authority.

just my 0.02

poindex30

Z-man,

Thanks for the information. I currently have the stock suspension with 113k miles on it. I plan on using Koni yellows all around, 200 lb. spings up front, m030 968 sway bars front and rear and stickier 225 tires all around. I would like to do this by March when the autocross season starts.

M758 makes an interesting point about having a softer suspension for autocross. I also autocross my 540 and I usually do very well with that car and I'm only so-so in my 944. The 540 has a sports package and is a 6 speed but it feels like a big boat compared to the 944. The extra weight of the car and the pitching and rolling cause me to corner more slowly and the V8 torque enables me to take off like a rocket out of every corner.

There is no question that I am faster on the same track in my 540 than I am in my S2. I feel like there is also MUCH better grip in the 540. I can barely push down the corners of the 540 and I can easily push down the corners of the 944 especially the rear. That's why I'm going to go stiffer in the 944.

I want to stay in the stock class. Would R compound tires move me up in class?

Thanks,

Mickey

M758

You need to check your local PCA region Autocross rules. In SCCA no it would not. The springs would bump you a class in SCCA. Again need to check rules for PCA.

Z-man

Mickey:
IIRC, option 474 (Sports package) had upgraded springs: around 180lbs, so 200lb springs should be good. I am not sure what difference in spring rates I have in the rear of the car with option 474, but make sure you balance the front and rear spring rates!

Regarding R-compounds: In most cases, the use of R-compound tires will not bump you out of stock classes. Check with your local autocross org. How much autox / DE experience do you have?

-Z-man.

poindex30

I participated in about 11 races last year. Seven races were with the PCA and 4 races with the BMWCCA.

I don't plan on using springs in the rear. I plan on using yellow Konis set to full stiff. Does that sound reasonable? I think tires will make the biggest difference since I sometimes feel like I'm riding on ice. I inflate my tires to 38 lbs at every corner. I've tried as high as 41 and as low as 36 and 38 gives me the best grip plus I can tell when the car is about to lose grip.

Thanks,

Mickey

gnosis

Many newbies sit too far back from the wheel, with their seats angled back too far, stretching their arms out straight to reach the wheel. Not only does the reduce their strength because they haven't got bent elbows with which to generate an effective turning force on the steering wheel, they also run the risk of serious injury in the event of a head-on collision. Reflex action tends to cause people in this situation to stiffen their arms to brace for the impact. However this is the exact opposite of what you should do. The result can be that the impact through your straightened elbows will cause so much force to be transferred to your shoulders that it can quite easily smash your shoulder blades, and in the worst cases push the bones of your upper arms straight out of your back. Nasty, I'm sure you'll agree.

So please sit a little closer, keep your elbows bent, and you will have much better control and safety.

Clayton

gnosis

Most cars that use high levels of downforce require spring rates somewhere in that region. The worst were the original ground effects F1 cars of the early 80s. They were so hard that the relatively smooth surfaces of race tracks (and particularly street circuits) would cause vibrations so bad that the drivers' visions were blurred. This is due to the natural freqency of the spring.

Any of the top drivers from that era (Nelson Piquet, Alan Jones, etc.) would bitch constantly about how annoying the cars were to drive.

Active suspension helped, but that was banned. The problem still exists that in order to maintain effective downforce, ride heights must be extremely tightly controlled - we're talking a few millimetres here. Enormously stiff springs are the only way to achieve this.

I don't know the exact rate the F1s and Indy Cars are using these days, but I'd say they're not too different from that.

Clayton

turbo944

You mean just using torsion bars (still springs of a nature) in the back or no torsion springs and no regular springs? You cannot run with just shock absorbers/dampers, you do need some sort of spring.

gnosis

Damping rates have more of an effect on how long it takes a car to "take a set". They control the rate of weight transfer.

Spring rate will determine the amount of body roll. Low speed compression damping will determine the rate at which the weight transfers and results in body roll. If your car seems lethargic when you turn into a corner, try higher low speed compression damping rates. If you only have single compression adjustment (as opposed to two way), then you will also have to suffer higher compression damping for high speed bumps too. This can make the car unsettled on rough surfaces, but it'll be crisp to turn in. Again, it's a compromise, and one you have to worry less about the smoother the road surface is.

This is why two way compression damping is a good idea (although few people seem to care all that much about it - personally, I think it's the hot set up). Low speed damper movements (from 0 - 2 inches/second) occur when your car rolls in corners. High speed damper movements (above that, all the way up to 50 inches/second for rally cars) occur when you hit bumps on the road surface. The ability to tune both separately is extremely valuable, especially for road cars where bumps cause damper velocities of 2-15 inches/second).

Spring and damping rates tend to go hand in hand, but there is a huge range of tuning that can be done in this area.

Clayton

FSAEracer03

1) I'm not refering to a jake brake, or engine braking in semi's. Sorry if that wasn't clear.

2) I'm glad you've learned from their experiences. Kind of reminds me of how I learned from the experiences of of people affiliated with the shop I've worked at.

3) Are you trying to tell me that noy owning a Porsche somehow degrades my statements or that I then "do not have a car"?? There are other cars out there johnny!

4) Yes, I did admit that I have never been bumper-to-bumper racing before, but if you read carefully, I have autocrossed.

Ok, now what I was originally posting for...

This clip from Carroll Smith's "Drive to Win" shows the "stability" I was trying to explain when refering to Andretti:

mpw

Races or DEs? Hopefully I'll see you at some of the PCA events this year.

- Matt

FSAEracer03

Another quote on the engine braking force from "Drive To Win," pg. 2-23:
"...one used the friction of the engine (not, as commonly believed, the compression) to slow the car." -Carroll Smith


Here's the sketch of the jacking force change I was refering to... this is what happen's when you get bored in classes like Science/Technical Communication... well obviously I don't pay enough attention because I cant explain what I mean for ****

The equations on the righthand side are simple statics force balances... let me know if I made a mistake (not surprising ). Similar diagrams and equations can be seen in "Tune To Win" IIRC, though if you want any scans or page numbers, I'll have to reread some stuff and post photos for ya...

gnosis

I cannot see how there is any time when compression braking <edit: I mean engine braking, not that truck thing that happens down steep hills> could be used even if it were to have some positive effect in terms of stability (which I doubt completely since any increase in retardation at the rear of the car versus the front will encourage a pendular moment that will try to spin you).

You should be either accelerating HARD or braking HARD. There is no inbetween, save for the minimum time wasted trying to make the transition as smooth as you can.

In order to compression brake, you have to avoid using the wheel brakes, which amounts to coasting. You can't coast and drive fast. If you want proof, ask the bloke who outbrakes you (with his brakes) while you compression brake.

Clayton