IcemanG17

I was thinking about the handling "feel" of all the 928's have owned & driven, especially on track at the "limit" (yes I drive flat out)

My general impression is the early body cars with supposedly worse aero and "square" tire setup handle the "best" (meaning the most neutral)....starting with that assumption here is my impression on a 928 by 928 basis that I drove at Thunderhill:

1st: 1988 S4 auto, 235 front 265 rear street tires, full street car "Sharky"..drove it for my 1st 6 events.....while I had a blast the car was heavily biased towards understeer....very front end grip limited, had great power (286whp) but open diff held it back......it just seemed like it took forever to turn in, probably due to weight and stock supension-alignment plus staggered tires

2nd: 1989 5 speed "Black Widow" full race car..295 front 335 rear.....the weight reduction with proper alignment increases front end grip quite a bit, but still understeered at the limit and it was hard to use throttle to rotate with those monster tires out back.....still very fun to drive....but could use more front end grip....

3rd: Seans 87 S4 5 speed street-track car....600-400 bilsteins with carls sway bar, 265 front 295 rear tires.....the best "street" 928 I have driven on track....but still heavy with slow turn in and biased towards understeer....you could use power to help rotate, but I never really tried that much (not my car)

4th: 1984 "Estate" lemons racer with its station wagon rear end + S spoiler on top of that...."square" tire setup with proper alignment, cut eibachs on bilsteins...110mm front 140mm rear ride height.... air dam, vented hood....this car had the most front end grip of any 928 I drove to date....kept the rear sway bar on max stiff to help turn in.....still would understeer at the limit.....my guess is the downforce off the estate rear increased rear grip similar to the S4 setup......could use power to rotate when the LSD held.....

5th 1988 S4 auto (speedtoys)...very powerful full street car with tired suspension when I drove it.....but its overall balance felt very similar to Sharky....which makes sense, the tires were nearly the same size stagger...

6th: #51 "Casper" 1979 5 speed with LSD..... no aero (yet), custom bilsteins on hypercoils (very stiff) devek front sway bar, 2720lbs, square tires 275's all around with 25mm spacer in rear, adjustable rear sway bar, 95mm front 140mm rear, 3' camber front with slight toe in, 2.5' rear with zero toe.....while trying to limit the worn LSD from slipping I eventually disconnected the rear sway bar....while this made the LSD work better, it really didn't do anything to the balance of the car!! Excellent front end grip and turn in...but controllable....all of the other 928's seemed to increase grip at speed at the rear of the car......The Estates front end grip got much BETTER once I installed the simple air dam.... This 928 will need a wing once I install the splitter!!

it appears the aero and/or tire stagger of the S4+ increases understeer vs the earlier cars.....

I have a couple of thoughts to improve the aero of my racer.....a splitter is easy with a vented hood (like the estate) and a wing will help in the rear.....I also have been thinking about a rear diffuser under the rear bumper....or possibly modifying the bottom of the rear bumper to incorporate a diffuser.... I do like the side skirts that Carls racer has.....that with a proper undertray should provide a bunch more downforce.....

Flame suit on.....ready for others thoughts-ideas-experience?

Cheburator

I am fitting a front splitter and the hood is vented, all the while the engine is sealed from the front. The air passes over the splitter, through the gearbox cooler, engine oil cooler and radiator and then is deflected upwards away from the engine and through the hood by a near hermetic shield. The underbody has a flat floor, while the back has a diffuser and a banana shaped carbon fibre adjustable GT2 wing, on a level roughly in line with the roof line. Hoping to get pics of some progress in the next two weeks

Contrary to what Kibbort will tell you, even the 928 with its double wishbone setup needs a lot of camber upfront to actually turn into corners. Thus I agree pretty much with all of your observations. One word of caution - on a higher powered car - mine was around 420chp, you will need the width of the rear tires. Now that I am projecting 430whp, I am certain I will need all of the 305s I can muster.

When you race prep a 928, you inevitably end up with more weight taken out of the back. Moreover, I am yet to see a good shock setup at the back with a reasonable price tag. All the setups you have mentioned are frankly average. I say that because even my LEDAs struggled to control the rebound at the back, and they are on a different planet compared to a Koni or a Bilstein shocks in your list of examples. The LEDAs themselves cannot hold a candle to the Ohlins that BIG928 has or the Penske that Adrian Clark has in his car here in England. You want to control the car on the throttle with less meaty rear tires. All good, until you have power, a high speed corner and average shocks. Food for thought?

That's where my racer struggled, and why I have gone down the route of proper big aero...

RKD in OKC

When you say basically all the cars understeer, where is it understeering? Turn-in, mid-corner throttle steer, corner exit power on?

destructo_low

My car is an 84 model. I have never driven it on the track, but there is one turn in town that is a sharp 90 degree at probably a 90 foot radius in the outside lane. When I have taken the turn quick enough to skid, it always seems like the front and back break free at the same time. It is very controllable. You just let off the gas, and it grabs. I have 255 rear and 235 front, so they are staggered, but not heavily.

Lizard928

I run 245/275 front to rear on the street.
As well, I've tracked my cars and I never really have under steer.

I run a stiffer suspension and do heavy trail braking to keep the weight to the front.
So much so that I used to brakes to help rotate the back end.

I keep hearing about under steer, but I can't say I've overly seen it.

The 275 on the rear are R888 right now and provide more than enPugh grip for well over 400whp. But the way the turbo runs/spools that's to be expected.

mickster

I guess they engineered it towards understeer instead of oversteer later in life. About a week or so ago I lost PSD due to the battery shorting (fixed and talked about in another post). Pulled out and made a left at a corner and floored it-wanted to see how it worked without PSD. Major league oversteer!

Car was 90 degrees in a second and wanted to keep on going *** first before I corrected-that was FUN!

I did see PSD come on for the first time at highway speeds today. Must have lifted into the apex...that's the problem with driving front and all wheel drive cars all the time...

I keep on saying to myself-don't lift, don't lift, don't lift...

I am still learning what this car can do--quite remarkable handling--far better than expected.

S4ordie

Having a "square" tire setup (all wheels/tires the same) probably has as much to do with front end grip as anything. Widen the front fenders and run 305's all around, I suspect your results will be better. Your older cars all run square, S4's on, staggered.

RKD in OKC

When I first got my GTS and started driving it in anger it understeered. Has Hypercoil springs and externally adjustable Koni reds and DEVEK front sway bar. Did several things to get it balanced. First went to 235/275 stagger instead of 225/275.

Then checked mid corner balance for sway bar settings, ie. drove around skid pad of average corner size and slowly accelerated trying to maintain turn. To get that balanced i had to increase the rear sway bar with Louie Ott Drop links and soften the DEVEK bar in front for where it was set before.

Then to control the understeer on turn-in I softened the rear rebound until braking on turn-in would let the rear rise enough to lighten the rear but not so much that getting back on the throttle wouldn't set it in a 4 wheel drift. This also lets the rear rise so the fronts grab better when lifting on throttle to turn in more in mid corner 4 wheel drifting the apex.

To control understeer on corner exit I adjusted the front firm enough the front didn't lift into understeer on throttle application to exit the corner but not so firm that the rear tires would spin out into a power oversteer to easily.

Even with a well balanced setup there are limits. Smaller radius corners have a tendency to understeer, and larger oversteer.

IcemanG17

Great comments.....but keep in mind you are still running a full weight GTS...so think HEAVY....however defining exactly where the handling issue is, is an excellent way to describe the handling..... What are your alignment settings? I wonder if autoX requires more stock-ish alignment since you aren't out there long enough to cook tires? So here is my impressions:

Street S4, with staggered tires: On anything more than a short quick corner, where the front tires can hold on for only a second at best, its not bad after thats its pretty much terminal understeer......for example T2 at Thunderhill is around 7 seconds in a street 928 once you turn in and the car takes a set, adding more steering input will result in understeer....adding throttle can help IF there is an LSD...but typically just increases understeer (my open diff 928's did this).............the same street 928 did BETTER in higher speed corners, where you could turn in early and anticipate the understeer which carries you to exit.... In comparison "Casper" can pull over 1.4G through this entire corner and will under-oversteer based off throttle

Race 928: In theory I want a slightly loose car on low speed corners and slightly tight on high speed corners....another aspect I like is how the handling changes depending on how you turn in.....IF you turn in "Hard" then its loose the whole time.....if you turn in "slow" its slight understeer....Casper is this way now & its really nice...since if you need to adjust you line due to traffic while racing you can.....

dr bob

Different street tires seem to "want" different camber settings. Sometimes more than normal street driving will tolerate without getting twitchy on road surface changes. What camber settings do you use, and have you made other changes in front to counter the effects of the added camber? I've added a bit of camber in front on PS2's, still 225/255 stagger. and it's helped a tiny bit on turn-in. Biggest benefit is keeping more tire on the ground in faster turns. But the penalty in slow corners is scrubbing. Everything is a compromise.

RKD in OKC

Currently my alignment is -1.5 front and -1 rear with toe as close to zero on the negative side with out actually being zero, about -.2. Trammels a little (follows road bumps and grooves on street, not too bad) but does turn in nicely. Also my suspension is not stock either. I've got hyper coils, externally adjustable Koni reds, Devek front sway bar, and Louie Ott Steroid rear drop links. And Yes, I have blistered both front and rear tires at an autocross.

Have setup both a 90GT and my current GTS after learning setup on a 944 Turbo with a very good teacher of both experience and a very very good mechanic. The experience was that I could only afford to upgrade one thing at a time, rear springs, rear sway bar, front springs, front sway bar. AND did several competitions with each step learning what each setup change did in a wide variety of circumstances. After all the updates and adjustments were made corner balancing took 2 seconds off my already fast lap times.

My local autocrosses are a mix of autocross and track. We have both high speed and low speed corners and slaloms with straights of up to 100mph. The typical difference between track and autocross is not just speed and corner radius, but there are also a LOT more corners in an autocross course. Because there are more corners corner speeds make a more difference in lap times. And due to the lower speeds aero does not really help much to increase corner grip, unless maybe if you do something like this...



I found with the high number of corners overall weight increases corner grip / corner speeds by about the same amount lost in acceleration and braking. Kinda like aero downforce increases drag for lower top speed versus increasing speed and grip in the corners. I used to take out my spare, tool kit, manage fuel, etc. to make the car a light as possible. Then one day I set fastest lap of the day, then took a 300 lb passenger for a lap. My lap time with the 300lb passenger matched my fastest lap even though I noticed my top speed at the end of the fastest straight was 10mph slower. I've also set a fast lap of the day, then gutted 140lb out of the car (even removed passenger seat) and was 1 second a lap slower than my fastest lap. Since then I have stopped gutting my cars and focused on maximizing handling for corner speed.

The major determining factor of corner speed is the balance mid-turn or apex. Balance allows you to throttle steer through the apex while maintaining a 4 wheel drift. The 4 wheel drift maintains the maximum slip angle of the tires. Throttle steer is lifting throttle to let the weight move forward enough that the front tires get a little more grip, letting the drift turn in or rotate more, or adding a little throttle to increase speed and carry the car out wider as opposed to actually turning the steering wheel. This balance is determined by front/rear tire grip, weight distribution, front to rear spring rates, and sway bar settings. The challenge of setting up for this balance is that it is a moving target. Low speed corners tend towards understeer and high speed corners trend towards oversteer. This means setting up a car for high speed corners will produce lots of understeer in low speed corners and visa versa. The best setup is to set your car for size (speed/radius) corners most seen at the track or course to get the best times.

How I like to setup a car for mid-corner balance is to slowly increase speed around a skid pad the size of my average corner until it starts drifting and try to maintain a 4 wheel drift. Whichever end is looser than the other I stiffen the sway bar until it just goes out wider, and the nose tucks in and rotates when I lift. Once balanced I increase the sway bar on both ends noting the speed it starts drifting until the speed that it starts drifting is slower, then back them off a little.



If you have it balanced and adding throttle in a 4 wheel drift makes it understeer instead of just drifting out more increase rebound firmness on the front dampers. It is too firm if it goes into throttle oversteer (rear tire spin) too soon and doesn't give you any throttle steer room.

On turn-n trail braking should be very short lived. I typically lift off the brakes just as I turn-in with the steering wheel. The idea is to keep the weight on the front just long enough to START the car rotating, then get back on some throttle to stop the rotation. If you have to counter steer to catch the rotation you either trail braked too long or the rear rebound is too soft. If it is too firm you will have to trail brake past turn in to get it to rotate. Remember setting up mid-corner balance the car should be balanced under slight throttle so the car should continue to rotate until you add some throttle to stop it. Not rotate only as long as you are trail braking. You want the car balanced under some throttle because it takes throttle to keep the slip angle from scrubbing speed as you drift through the apex.



This is what works best for me, however, there are lots of different driving styles out there. You may have a different style that requires a different setup.

GlenL

Where's Mark when we need him?

I've driven a few but not on the track. For my tastes, it's early cars and manual trannies all the way. Lighter, more responsive and more controllable.

On a related note, I'm loving the Hankook DOT track tires. Work better for me than Hoosiers and at 2/3 the price.

dr bob

I increased front camber by 50% to 45' negative (this is 3/4º) vs factory recommended -30' +/- 10'. It's more sensitive to road irregularities than original, but not enough to be annoying. I can go with a bit more negative, just figured that I'd stick with small changes and work up.

For front toe, I'm at 10' total toe-in. Less toe-in and the car turns in better for sure, but there's more noticeable scrubbing on slow-speed corners for some reason. Ride height in front is 175mm static. Eibach springs on Boge/Sachs dampers. Factory anti-roll bars with no leverage adders yet. It needs a tad more rear bar stiffness for my little backyard mountain tour route, maybe a set of Ott-equivalent links would do that for me with existing LSD.

I probably need to borrow Mark A's scales for a corner-balancing clinic soon.


Thanks for all the tips!

Lizard928

I should mention I run as much caster as possible 1.5 or so camber, and 1/16" toe in for front alignment.

IcemanG17

My caster also is quite high.....4.5' I recall......

Interesting "autox" videos......different (better) than the endless sea of cones in a huge parking lots we get out here......still a bit slow for my tastes, as it surely requires a different setup do to the high number of slow-er corners that require far more steering input than the 60-100mph corners I typically see.....

RKD have you tried "Harrys Lap Timer"......its a $20 Iphone app that will data log your laps.....I am interested to see how much lateral G forces you are pulling?