Slow speed understeer

[450knotOffice]

It seems that whenever I really push my '84 very hard in slow second gear turns, the front understeers pretty dramatically. In mid to high speed sweepers the car is very neutral.

The cars has 22/28 Torsion bars, Bilstein Sports on all four corners, new Polyurethane bushes in front and stock rubber in back, a Rennline strut brace, a bump steer spacer for the rack, and stock Carrera anti-sway bars. The tires are Bridgestone S03's (205/16 and 225/16 on 7's and 8's). The car sits at just slightly lower than Euro height and has had a slightly more aggressive alignment than stock.

How the heck do I get rid of that low speed push? I hate it.

Thanks guys.

[slodave]

Does this have something to do with friction (or tire grip with wider front tires) and no power steering?

Dave

[Jeff Alton]

Scott, when you upgrated your torion bars you essentially maintained the balance the car originally had. Yes, it is stiffer, but the front/rear spring rate ratio is essentially the same. You can monkey with tire pressures and alignment to a degree (very small degree). But, you probably will need an adjustable sway bar on one or both ends of the car or a larger rear torsion bar.

You will need to stiffen the rear roll resistance or lessen the front roll resistance or a combination of both.

Cheers

[Steve@Rennsport]

Scott:

Questions:

1) What are your current alignment settings?

2) What are your current tire pressures?

3) Do you have an LSD?


Remember, these cars WILL have some low-speed push to prevent the car from getting loose at high speed. This is why Porsche racecars employ front splitters and rear wings,....

Adding adjustable swaybars as Jeff mentioned provides the ability to change the understeer/oversteer balance to your skills & preferences.

[stlrj]

The car sits at just slightly lower than Euro height and has had a slightly more aggressive alignment than stock.

I think you just answered your own question. I think you found the trade off.



Cheers,

Joe

[Quicksilver]

You can adjust the push:loose balance with adjustable sway bars or very easily by messing with the tire pressures but I would very much keep in mind what Steve said about highspeed oversteer though...

My car used to be really nimble at low speeds. I guess I would call it "just loose enough to make it really easy to get it to point". At higher speeds it was just waiting to bite your head off. It was loose enough that I couldn't really let anyone else drive it. (a few that did had eye opening experiences!)

Last year I did just about what you did except I put polybronzes on the rear too. I can let other people drive the car now without fear. On slower corners I can drive in a little more aggressively and carry a touch of trail braking into the corner. (Your feet get pretty busy down there...)

I do find that the car is much more sensitive to tire pressures now. I used to adjust 2lbs at a time and now I usually am adjusting in one pound increments.

BTW - Did you get the wipers figured out?

[javadog]

Well,

The biggest problem you face is that, for some reason, Porsche forgot to weld a proper roof on the car, when it was built. Hard to fix that now....

When you upgraded your torsion bars, you actually increased the rear stiffness a little more than the front, which is good if you have too much understeer. At the risk of repeating what the others have said, if you don't want to play with roll bars, you might consider increasing your front tire pressure, running a little more front camber and making sure you don't have too much toe-in in the front. From pictures I have seen of your car, I'd prefer to run the rear ride height a little higher, compared to the height you have set the front. I like my ride heights a little higher than most people on this board.

If you have limited slip, which is unlikely on a Cab, I'd think, you may be stuck with a bit more understeer than you'd like. You can also play with your corner entry speeds and when you get on the gas to help reduce the problem. These cars will tend to want to push in slow speed corners with a lot of throttle. That's just the way Porsche set them up. It's much worse in my opinion, on the Targas and Cabs. I have similar tires on my wife's '86 Targa and after getting out of one of my coupes and into her car, I am amazed at the difference. More than once, I have been unpleasantly surprised at the tight left turn at the bottom of the small hill we live on.

JR

[jluetjen]

You can also look at your driving style. If you pick up the throttle too hard in mid corner you'll unweigh the front end which will cause it to push. Having a limited-slip diff will make this even worse. The trick is to roll more speed through the entry and mid-corner on part throttle, and then pick-up the throttle hard once you've passed the apex and can see the corner exit.

That's one reason why 911's are a little bit harder for novices to drive, but then reward the experienced driver. Most novices don't feel comfortable rolling more speed through the entry, and as a result over-brake, and then use too much throttle on exit -- both of which upset the car's handling.

[}{arlequin]

either loosen the front or tighten the rear.

w/in what you have, lower front tire press, or increase rear to get it to step out more or just enough to 'defeat' the push.

bigger rear sway may do the trick too. don't need to go big bucks for an adjustable bling... just get the next size up that factory offered. maybe a late g50 sway is bigger... probably a 930 sway would be bigger as well... or, if you can still live w/ it, go up to 30's in the back...

[jluetjen]

Scott; as the others have aluded to, keep in mind that you have a cab which is by definition a flexible chassis structure. As a result you will have diminishing returns if you stiffen the rear or soften the front for the simple reason that no matter what you do, the chassis between the two ends of the car will flex when you apply hard cornering forces (not to mention that this flex is not dampened either). If you're going to drive a 911 hard around corners, it's best to get a coupe, or to a lesser degree a Targa.

[Wil Ferch]

I've quickly scanned all the replies and as accurate as they are...we're missing a really big aspect here....

Almost ANY car's understeering / oversteering characteristics will be different in low speed ( high steering angle) situatii\ons ... vs... high speed ( low steering wheel angle) situations. For a low speed turn, the steering wheel will be cranked over pretty hard and you're running really big ( for lack of better term) slip angles on the front tires. It's not really "slip" angles but a combo of lots of steering lock and tread squirm/classic "slip" angle. But.... you get the idea.

If you *truly* are able to "tune-out" low speed understeer, you will be almost guaranteed to have pucker-inducing high speed oversteer ( ! ) ...or at least high speed neutrality...when the same levels of g-force is applied with minimal steering angle.

Back "in the day"....Bruce Mclaren had a terrible time trying to get his M8 Can-Am cars to have front end stick around low speed corners. In the end...they never did get it figured out.....and I fear neither will we here.

- Wil

EDIT--- oh, BTW...a lot can be accomplished ( as already stated...it seems) by the "method" you feed-in power during low speed/high steering angle turns. The more gentle the power application, the less "push" you're likely to induce...and the better balanced your car will be.

[ivath]

It could be interesting to hear what speed people consider as "low speed corenering" . In other words, at what speed should you expect neutral handling. I never do hard cornering above 60 mph and I wish I had less understeer on my 87 Carrera.
Is it then dangerous to increase the rear roll stiffness a bit to get a more neutral handling?

[dfink]

My cab does the same thing. I had to learn to slow down in the corners and you will actually be going much faster as once you are pushing it really sucks the speed. Back off on the throttle and then also back off on the steering input and the car will start turning. They teach you this in the evolution schools. It really is counter intuitive to reduce steering when you are pushing but you have to reduce the angle for the tires to grab. Also my cab has limited slip.
Take it the track and drive in the rain. I bet it pushes at speed then also.

[john walker's workshop]

got posi?

[87 clubsport]

Scott,one easy way to reduce u steer , on your standard carrera rear sway bar drill 1 inch c to c hole and get a adjustable drop link fr Pelican, than you can adjust the sway bar and since you lower the car also can reduced per load, it will help you to improve understeer, hope this may help.
Steve

[pwd72s]

Originally posted by 450knotOffice
[B

How the heck do I get rid of that low speed push? I hate it.

Thanks guys. [/B]

Unhook your front sway bar. But if you have ZF limited slip, it won't help much. My '70 T, as set up, was a much better autoross than track car. My '72S was a better track/road car than an autocross car. It's next to impossible to accomplish both. That said, good luck. Be sure to tell us if you pull it off...

[Dueller]

here's an oldie but a goody...lottsa great info from Mr. Ferch, et al

more t/bar stuff - anyone got experience/comments on 21/31 combo?

[BK911]

Sorry if this is a dumb question, but could you adjust the ride height in the front so it's heavier?

[450knotOffice]

Guys, thanks for all of the responses. I'm sorry I wasn't able to answer some of the questions as I had a long day of flying today that started at 4 AM. I literally just got back home an hour ago at 7:00 PM. Long day.

One more day tomorrow like today.

Anyhoo...

I am by no means an expert driver (not at all), however I have been driving sports cars fairly aggressively on mountain roads for most of my driving life and have been having a ball learning to drive the '84 cab and now my 996 coupe in the twisties. In fact, I have tracked the 996 a few times at Willow Springs on the big track and I do fairly well there for a track newbie ( ). I am familiar with the concepts of throttle steering, trail braking, etc. In fact, in the cab I normally use some trail braking as I enter 2nd and 3d gear corners which really allows me to set the front end and turn in nicely with very little understeer. I'll then roll in some light throttle to balance the car mid corner and it normally feels great. As I begin to exit I roll in some more throttle as I unwind the wheel and the back plants and off she goes. It really is a hoot.

What I have noticed however, is a strong propensity for big understeer if I cook it into a slow corner (say 40 mph) and then turn in very tightly and apply what I consider balancing throttle right at the corner entry (which I know should induce some understeer because I'm unweighting the front end a little), the car plows like it's on ice. Basically the only way to get it back in line is to drop the throttle or to just scrub off the speed with more steering angle.

I suppose my low speed technique sucks but I would think a car as capable as this with these very grippy S03's I have on it would be able to handle a tight fast corner a little more gracefully.

As I said however, if I get into a nice rhythm with the car and trail brake into those same tight corners the front end sticks very nicely. However, I sometimes feel as if I might be playing with fire, trail braking as much as I do with this car in order to get the front end to point.

I don't know if I have a limited slip diff but I doubt it (I'll check one of these days). As for your suggestions re: sway bars, I may go with the upgrade if you guys really think it'll help balance my car.

On a side note, the 996, with lousy Yoko AVS ES 100's sticks better in those same tight corners with a stock alignment than the cab does. That car is beautifully pointy and also stable and neutral in high speed sweepers (120 mph in turn 8 at Willow feels pretty easy with that car)

Thanks again for the help guys!

[joseph222]

Scott,

Lots of good info in this thread. I don't really have anything to add other than it comes down to a few choices. Change your car (again), or change your driving attitude (or do a bit of both).

If you're running in sanctioned events, you might want to check the rules before doing some of the suggested changes.

Seems to me you have a pretty typically "correct" suspension setup (barring any bent a-arms or other anomalies). And we'll assume you're running 6-8 lbs more air in the back than in the front.

If you have just recently made the suspension changes to your car (say over the last 12 months), you might consider leaving your car as it is currently setup before continuing on with even more changes. It takes me longer than I'd like to truly come to grips with suspension tuning changes. And the more changes made at once, the longer the adjustment takes.

However having said that, if you're autocrossing, I would suggest that you add 2-4 lbs of air to your front tires to increase front end grip, which in effect will induce a bit of oversteer for you.

Now think about how you can adjust your driving style to suit your current setup and eliminate that push.

For a 2nd gear slow speed turn, try a faster entry than usual and as soon as you introduce steering input, lift off the accelerator (or apply trail brake), to induce oversteer. Once the car has rotated into the turn and is almost angled towards the exit, roll back on the throttle. This can be done in a more subtle smooth manner, or it can be done very aggressively with lots of power throttle oversteer (the latter being more fun but not as fast as subtle & smooth). It will take a bit of practice.

If you're not autocrossing, I'd suggest finding a big empty parking lot on a few sunday early mornings and giving it a try. Less peril for yourself and others.

Let us know how it goes!