Dubai944

Van, I never have wear on the inside, but I run Dunlop race slicks that generate some decent grip. It depends on your setup, tire temps will guide you to best camber setup for each wheel.

Here's a front left that's been running at 3.6 degrees with 1000 pound front springs. This tire has done one full race weekend, which is a thirty minute practise session, 20 minute qualify and a 45 minute race, as well as another 20 minute practise and 20 minute qualify at a second round.



Wear is even across the tread, no wear on the inside, with some heating and wear on the outside shoulder normal for a front left on predominantly right hand turn corner tracks.

And here's a picture of the same tire getting a workout during the race

333pg333

I think I should deal with the separate issues in isolation. So the dartiness can be bump steer and too much toe out, or just toe out over neutral.

The rear getting squirmy and nervous under brakes could be lack of toe in, lack of rear neg...perhaps ride height/levels. What about brake bias? I have switched over to the 928 5/33 rear proportioning valve but have had this for some time so I don't think this has anything to do with it. Shocks have both bump and rebound adj. Currently they're all up about 3/4.

At this stage there hasn't been any mid corner 'numbness' Van. The rear end stuff could also be me coming in too hot to a corner and the brakes are really strong now, especially the fronts. I might just be upsetting the balance with these symptoms, but when the pro drove my car last weekend he was concerned with the rear squirm so perhaps it's not just me...and he drove it like he stole it.

944CS

if you drew a line from the center of the torsion bar to the center of the hub, at your current ride height, what is the angle it makes with the ground, assuming the ground is the x-axis and positive Y axis is toward the sky?

Van

You could just measure from the ground to the center of the torsion bar cap; and from the ground to the center of your wheel hub, and we could figure out the angle.

Dubai, I run about -2.5 degrees negative front camber with 550 lb springs - and my last set of BFG R1s lasted approx 21 track days (about 80 heat cycles) before I corded the inside. But, I'm not the fastest guy out there, so I'm more gentle on tires than most.

Patrick, if a pro gave you the opinion that the car was squirmy under braking, what was his opinion on how to fix it?

Dubai944

Patrick,

Ride height and bar changes will affect the overall steer characteristics, but shouldn't really be making the rear end 'squirmy or nervous'. Straight line instability is most likely toe settings and or bump steer.

If it's actually oversteering on corner entry, but not in steady state cornering I'd be suspicious that something is loose or you have a broken shock or something.

95ONE

Toe out in the front eh. Maybe for autocross. I'm curious if you guys hit 125mph or higher much at your tracks? I'm usually at 120-160 on 4 straights at TWS. I prefer the high speed stability. At lower speed tracks, I could understand a LITTLE bit of toe out. -Front only. That said, I still run zero toe in the front. 1/8-1/4 {toe in} the back. Lots of Neg camber all around. same size tires all around. Light springs though. 4-500lb. Car is light. 2300lbs. This has produced well over 1.2g's consistent. Not professing to be magical at these cars.. Just what I've got, and what I've noticed. And it's all just a memory at this point.

944CS

I agree that would be the way to figure it out, if you knew your reference points were the same.


What Patrick needs to do here is set up his car with zero toe front and rear. He also needs to have the tie rod parallel to the control arm. Then find the camber settings that produce the tire temps he is looking for. I would also start with the highest positive caster settings that I could maintain left to right. Then fine tune the balance with the sway bar settings or the spring rates. If you are very close with your spring rates (as you should be) all it takes is a little adjustment with the sway bars.

944CS

yes sorry + caster

Larry Herman

You are assuming that the tie rod and control arm operate in the same radius. Typically they do not (I don't know this for certain in a 944/951) and so the only proper way is to bump-steer it.

333pg333

I've never dealt with bump steer and don't really understand the concept. Have to read up on that...but what am I trying to achieve when adjusting it?
Same with caster. From what I understand the more you increase it the more stable the car becomes and you can also run less camber, but then it's at the expense of turn in which I don't want to sacrifice too much of...now that I'm have it. I don't want to sound greedy but also feel there is a way of improving things too.

Van, these guys just hop from one car into the next. They're not familiar with each one and not the front engined versions either. Essentially while expressing concern, he didn't come up with a plan. Maybe there is a legal thing going on? He's just a hired gun for the day. He asked again the next day if I'd had it looked at. I know that he was close to losing the car once or twice but skill stepped in. It was probably a couple of things, one of which might have been the broken rear sway bar mount.

Larry Herman

Bump-steer is when the direction of the front (or rear) wheel changes as it moves up & down through it's range of motion. Think of when you go over a bump and as the suspension compresses that wheel toes out. Kind of makes things unstable, right? You can reduce the bump-steer by raising or lowering the tie-rod end or by stiffening the suspension to get less movement.

Caster (or king pin inclination) is the rearward tilt of the strut. This puts the actual axis of the steering ahead of the contact patch. It helps with self-centering and also with adding camber as you turn the wheel. It also causes weight jacking and heavier steering because caster raises the outside wheel. This can be a detriment in manual steering cars. Also too much caster can cause instability, but I honestly don't know what too much is as the 996/997 cars can run with up to 9 degrees of caster and have remarkable stability.

BTW I am available to help with your car for just 2 plane tickets from Philly to where ever you are.

333pg333

Haha, thanks Larry. I'd love to have that sort of money. I do hope to be back in the US sometime next year to deal with another project so who knows what may eventuate. All this stuff we're talking about will have some effect on the project though.

So what I will do is get a measurement of all my settings now and work off that. I even am wondering if some of this may be down to my front brakes being so strong now and overwhelming the rear and general balance of the car? I can literally just 'big toe' them in general street driving now so perhaps...

Larry Herman

Just let me know. As long as I bring my wife I can go anywhere.

As to your front brakes, if they are overwhelming the rear brakes, then you should have more stability as locking the fronts will keep the car on a straight line whereas locking the backs will create instability.

disasterman

I don't think strong front brakes are going to cause an instability problem. Consider additional downforce via an improved spiltter. I recently dealt with the same problem (loose rear end under braking). I added a custom/large and low splitter which definitely helped when threshold braking from high speed. If you want to adjust brake bias, consider a tilton bias adjuser which will make sense under rain conditions and when the fronts start loosing grip.

Dubai944

Patrick,

Another thing to check is that your caster is even on both sides. i don't think brakes are your issue.

You can do a quick and rough bump steer check on a lift by measuring front toe on the floor and then just keep jacking it an inch at a time and remeasureing toe until you reach the droop limit. This won't show you your toe out on compression, but if there is much change even in the droop direction, you can be pretty sure you have a geometry problem. To do it properly you really need to remove the springs and allow the suspension to lower to full bump also.

If you can find a shop with the knowledge and tools to check this accuurately do so. Personally I just use laser string against each front rim giving two vertical lines on a wall in front of the car. I stick tape on the wall and mark it with the string line at ride height. Then use the lift to bump it up and down through the travel range marking any movement left or right of the original mark. Obviously if the lines are moving outward the front end is toeing out and inward toeing in. Ideally you don't want any toe change or at least avoid it until the vey extremes of travel. To achieve that on a car with dropped arms you will have to drop the tie rods. Getting this right really makes a difference to how the car feels.