ryoung

... sorry, here I go again.

I'm trying to improve the handling of my S8 for both street and track; and since it is both, I definitely don't want a track only setup. Corner definitions first,

Phase 1: turn in

Phase 2a: from turn in to apex at constant throttle

Phase 2b: from turn in to apex with trail braking

Phase 3: apex, or constant radius carousel at constant throttle

Phase 4: from apex to corner exit under heavy to full throttle

The following is my assessment of S8 handling in various phases of a corner on the track. Kjella, you've certainly got much more track time in an S8 than I do, and I would really appreciate your critical comments here. Correct me if you believe some of the problems are a result of my driving style ... and tell me how to do it right! As you probably know, I'm the crew chief for a formula car and responsible for adjusting the handling of the car based on the driver's comments. I have MUCH less experience in the driver's shoes.

Phase 1: OK to slight understeer under throttle, very good under braking

Phase 2a: Moderate understeer, but radius can be adjusted nicely with the gas pedal.

Phase 2b: Severe understeer if braking is reduced too quickly as weight transfers out and to the rear (a shock problem?).

Phase 3: Moderate understeer; but again, the car can be easily steered with the gas pedal.

Phase 4: Extreme understeer, and adding additional steering only makes the problem worse. You're forced to either unwind, reduce throttle, or endure.

My first response would normally be to just add a stiffer rear roll bar; the S8 is far far from having a loose rear end. The bar would be the simplest, lowest cost, have a minimal affect on ride, and I wouldn't have to worry about ride height. Track photos show a problem; one example (plus also the sig photo),

<img src="http://pictureposter.audiworld.com/17157/turn8_lift.jpg">


The inside rear tire is already almost off the ground. A stiffer rear roll bar will make this worse, and once the inside tire is in the air, traction control will take over and reduce throttle. Even with Quattro, the Torsen center differential cannot transfer more than 75-80% of the torque to the front axle. Anyway, the only way to increase rear roll resistance is with stiffer rear springs (or a softer front, but the car already rolls almost too much).

The following is my understanding of front/rear spring rates.

stock A8: 280/250 lb/in

stock S8: 390/350 lb/in

Intrax: 400/360-500 lb/in

I believe these Intrax springs are currently used by Hohenester. The rears are progressive, increasing from 360 to 500 lb/in after 2" of compression; but except for that, they're almost identical to S8 spring rates. These also lower an A8 by one inch, just a bit more than the stock S8 springs. [I don't yet know the spring rates used in the H&R, KW, or MTM coilovers.] It seems to me that the Intrax springs are a reasonable solution, though perhaps 50-100 lb/in stiffer both front and rear would also be satisfactory (I need those H&R, KW, and MTM springs rates!) And I'm still sorting out shock options.

Opinions/comments/suggestions (especially from Kjella)?

KjellA

Thank you for sharing your experiences and thoughts Randy, it is very valuable!

I'm no racer myself, well, not cars anyway. My racing experiences are with motorcycles and go-karts. So I'm sure you would get a lot better comments on the S8's handling letting one of your team drivers take it for a spin.

Anyway, my experiences with my car on track are similar to yours, exept that I don't think the understeer in phase 4 is quite that bad. I really think it is worse in phase 2b. And I agree with, you that this probably is a rear shock (or maybe too soft spring?) problem, it is too easy to upset the balance in this phase. The only solution I found is to reduce speed a bit more (and earlier) before turn-in, but this shouldn't have to be neccessary.

I think the phase 4 understeer can be corrected to some degree by trying to loose the rear a bit. In other words, upset the balance on purpose. It is very likely this won't make you go any faster, but it kind of feels better anyway! :-)
Simply try to provoke oversteer (which you won't get, but it will reduce understeer).

The picture of your car lifting the inner rear wheel is very interessting, I wasn't aware of this happening. But with the ESP turned off, the traction control wouldn't reduce throttle even if the wheel was in the air (I'm not sure if it would do it even with the ESP on). The quattros don't have the throttle reducing function (ASR), they have EDL (Electronic Differential Lock). EDL is actually not a differential locking as the name indicates, but simply a system braking down a spinning wheel using the brakes. ABS sensors are used to detect wheel spin. But this function is disabled at speed of more than 80 km/h (~50 mph), so it would probably not help a lot on the track.

In this situation it would be little resistance at the inner rear wheel, so it would get most of the rear axle torque, spinning faster than the other wheels. As long as the ESP is turned off, and the EDL disabled by speed of more than 50 mph that is. Then the the torsen diff would move as much torque as possible to the front. This will add understeer, since 75-80% of the torque would be on the front, most of the rest spinning the inner wheel. Very little power would get to the outer rear wheel. So trying to keep the inner rear wheel to the ground seems like a very good idea!

I had the same thought as you, a stiffer sway bar. But after seeing your picture, I totally agree that this will make it worse. Progressively stiffer rear springs sound like a good solution, or of course fully adjustable rear shocks with added compression damping (and less rebound?). But that is much more expensive, if at all available.

The Intrax springs sounds very interessting. I will try to get some information on the other spring rates, and also on Eibach.

I know that H&R uses Bilstein shocks in their coil-over kits, but I don't think they are adjustable. So a set of adjustable Konis coupled with Intrax progressive springs might help this situation.

KW has a coil-over kit with adjustable shocks (adjustable compression damping). The shocks are manufactured by Al-Ko (don't know them). More information here:
http://www.kw-gmbh.de/e_version/gewinde/gewinde.htm

They don't give any information on spring rates, neither do H&R or Eibach. I will check this a bit further and let you know if I find anything.

ryoung

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Twice I did forget to turn it off. On the 3rd lap in both cases, I could smell brake pads burning and turned it off. Now how could this be?

The S8 never oversteered anywhere, so ESP could only have been correcting for the frequent understeer. OK ... then it brakes an inside rear wheel to help rotate the car. Fine ... but it doesn't seem likely that I could smell a burning REAR pad on the track, does it?

ryoung

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BTW, in addition to the photos in the original post, I have two others showing similar wheel lift. Two different turns, but all four were taken either near the apex or in a carousel; i.e., phase 3. Also, the spring rates listed were provided by a tuner who simply measured them himself.

You're right, understeer is actually worse releasing the brakes on entry than on corner exit under heavy acceleration (phase 2b vs 4). The only reason I said it the other way around was because of the affect on lap times. Since being able to exit a corner faster lets you carry the additional speed all the way down any straight that follows, the exit is normally the most important part of the corner.

In one corner, a long 180 degree carousel, I also did try to provoke oversteer. The track crests slightly just past half-way thru the corner; and as I crossed the crest I'd do a quick complete throttle release. Enough weight momentarily transferred off the rear and onto the nose that the car would rotate toward the apex. As you said, it was nothing approaching oversteer, but it did help.

I also forgot that EDL is disabled above ~50 mph. Some of the corners are below 50 mph, and some are well above. Like you, I was unaware of the wheel lift until I saw the photos, and I never did feel anything approaching wheel spin or EDL activation in either the low or high speed corners. And that was a very good point you raised about the workings of the Torsen -- as the inside rear loses traction, torque is transferred to the front, making understeer worse. Perhaps this is self limiting; the increased understeer limits cornering g's enough that the inside wheel never lifts quite enough to activate EDL?? ESP was always off (well, except for twice when I forgot).

Hmmm, do I know a race driver that I would trust with my S8 on the track? I'll have to think about that one.