Help with possible alignment issue...

background
I have had my car for almost 3 years and 40k miles. It's a 1999 M Coupe. This is the first time I have been less than thrilled with the handling, in fact I'm kinda bummed.

I put a Ground Control CO susp on it 20k miles ago. Koni SA and Eibach f#375 and r#450. I lowered the car around 1". Could have lowered it more, I remember that. I took it to my shop and they used the string method to align the car. The shocks are near full firm. Definately not full firm, but near.

I just ran through a set of S0-3 tires and could not have been happier with the performance. Prolly too much understeer, but I was ok with that for street use. The car had mad amounts of grip and was very solid feeling at speed. The car had very predictable habits. Letting off the gas felt like a mild rise in the rear, and gettin on the gas just simply felt planted.

The tires were 235/40/17 and 265/40/17. Just loved them.

the situation
2000 miles ago I put PS2 tires on the car. 235/40/17 and 255/40/17. People are just raving about them and they are pounds lighter per tire. I took it to my shop to get an alignment. This time they used a hunter. Same guy, different technique.

Here are my alignment specs from off the Hunter:

FRONT l/r
camber -1.5 -1.1 degrees
caster 7.7 7.5 degrees
toe .05 .03 inch
total toe .07 inch

REAR l/r
camber -2.4 -2.6 degrees
toe -.02 .07 inch
total toe .05 inch

Basically, in higher speed sweeping curves, the car does not feel very good at all. I have some bounciness. Also, the car seems way too sensitive to throttle input. Instead of the nice lift and plant characteristics, it seems darty under throttle variations. Also, I do not seem to have as much grip as I should have. The car just feels much much less planted and for the first time I feel the end could just fly off on me.

I was running the tires at f35psi r35psi and have since changed that to f35psi and r32psi. That seemed to help take out some of the bounciness, but basically, especially in comparison to what I was used to with my S0-3 tires and that set-up, the confident feeling and the grip are just not there.

I was also advised to maybe reduce some firmness in the rear shocks. This could help, as reducing the rear air pressure did help calm things down a little, but, I am really leaning to my alignment being off. I don't see how merely softening my rear shock is all of the sudden going to produce wonderful amounts of confidence and much higher grip levels.

The car has just never felt this insecure before. When I got the car it had 275/40/17 rear tires, then I moved to the 265 rears and now to the 255 rears. I'm weening myself off the width, so I do expect some different characteristics, but this just feels wrong.

Any tips on the alignment based on the above numbers?

Thanks!

Reply » Help with possible alignment issue...

With the rear you cannot do too much unless you get some extra hardware. Now you mentioned about stability in a corner so my advice will be based on that. High speed stability in a straight line will be affected negatively.
1. Set the toe to 0 (I am actually suprised you have different toe in the front left and right - since it is adjustable a good shop should be able to set it to 0 both sides).
2. As you said reduce the rebound a bit in the rear. Depending on how much you do that you will have to reduce the rebound in the front too a bit.
3. How does your ride height front rear look like? You said the car feels darty and unstable but in what way? Understeer, oversteer etc? Ideally you you like a slight raked look with the front slightly lower than the rear.
4. Shim the front struts for about -2 camber. That will help quite a bit. Or get camber bolts when you align the car next time.

You know that I am running that TCK setup with 500/500 springs. Car is lower than stock by about 3/4 inch. The front is slighty lower than the rear. I shimmed my front struts for -2 negative camber and set the front toe to 0. My shocks are at about a quarter from full stiff. I have stock tire sizes. The result is very nice and confidence inspiring. I can finally trail brake the car without a problem. It is really well balanced.

Other things to consider would be stiffer subframe bushings. And the optional butt strut. Those should help with the rear staying more planted and car responding/settling faster.

Did you consider that you were to used to the understeer from the previous tires combo you had and now you are not used any longer to how the car feels with closer to stock sizes tires?

Reply » Help with possible alignment issue...

What is up with my rear toe?

====

Florian-

Darty and unstable like it's just not progressively planted. Before I had TONs of predictable and linear grip. Now I don't feel like that.

Where do you shim the front struts? Got a pic? I've seen a few hot set-ups that use shims to get -2 in the front.

I guess what is really bugging me is that the S0-3 wet-up was heaven, and now it's so yucky. Tires and alignment are the only changes, and now I'm messing with shocks and shoving shims in there and all this kind of stuff.

Yes, I expected some change in going to a skinnier rear, but this just feels too dramatic. It just feels skittish in higher speed curves. It never gets that planted feeling. It's more like I'm curving and arcing and instead of feeling planted and building what feels like linear grip, I start to get some bounce and some wander. It's not a little bit different than with the 265 rears, it's a different car.

At lower speeds it's ok. I can tell the difference coming out of tight corners and stuff like that. At higher speeds it's borderline scary compared to what I'm used to.

40k miles on this car and now I'm shimming the front for -2 degrees? hmm.

That's why I kinda want to get the alignment happy before I start messing with air pressure and shock rebound to much.

PS How you liking your set-up? I assume your happy?

I'm glad someone is happy!

No seriously.

Reply » Help with possible alignment issue...

Quote:

What is up with my rear toe?

You're stuck with it unless you add toe adjustment (KMAC eccentric trailing arm bushings, Ireland toe kit, etc.). Stuck with it in the sense that there is no adjustment in the rear, you get what you get. It will change slightly with ride height.

Quote:

====

Florian-

Darty and unstable like it's just not progressively planted. Before I had TONs of predictable and linear grip. Now I don't feel like that.

Where do you shim the front struts? Got a pic? I've seen a few hot set-ups that use shims to get -2 in the front.

Washers between the kingpin and the strut at the bottom (2 bolts per strut). More washers -> more negative camber. You may or may not have much room to do this with 235 tires before the tire hits the spring perch.

Quote:

I guess what is really bugging me is that the S0-3 wet-up was heaven, and now it's so yucky. Tires and alignment are the only changes, and now I'm messing with shocks and shoving shims in there and all this kind of stuff.

Yes, I expected some change in going to a skinnier rear, but this just feels too dramatic. It just feels skittish at higher speeds. Like in a nice left sweeper, I'll move to the outside and attack the corner. It never gets that planted feeling. It's more like I'm curving and arcing and instead of feeling planted and building what feels like linear grip, I start to get some bounce and some wander. It's not a little bit different than with the 265 rears, it's a different car.

At lower speeds it's ok. I can tell the difference coming out of tight corners and stuff like that. At higher speeds it's borderline scary compared to what I'm used to.

40k miles on this car and now I'm shimming the front for -2 degrees? hmm.

Mileage has nothing to do with it, for what that's worth.

Quote:

That's why I kinda want to get the alignment happy before I start messing with air pressure and shock rebound to much.

PS How you liking your set-up? I assume your happy?

I'm glad someone is happy!

No seriously.

From your previous setup, and for that matter your current camber settings, I'm gonna guess you prefer understeer. Most of us don't, and we dial out what's there from the factory to make it easier to rotate the car. Not everyone wants the same thing here. Those that track their cars want to be able to steer with throttle. That's harder to do with 275 rear / 235 front than it is with 255 rear / 235 front.

I don't think your rear camber is where you want it, and I think you're riding on the inside of your rear tires most of the time. A smaller contact patch than you want for your driving (no hard cornering forces). Raise your ride height, or get yourself some adjustability in the rear and bring the negative camber back to -2 or stock (-1.75). I also don't think your front toe is where you want it. Factory is about .125" toe-in. The car will feel more darty at high speeds with less toe-in, in terms of steering response. That toe-out on one side in the rear isn't helping your high-speed stability, but the only way to correct it w/o adding more parts is to change your ride height. IT shouldn't be a significant factor here since your total toe is in in the rear.

Unless you're sliding, you can't really answer the oversteer or understeer question. Since I suspect you're not (as you shouldn't be at high speeds on the street unless you have a desire to wad up another beloved M coupe), I suspect this is mostly about your rear camber, the rear subframe's tendency to move around with stock bushings, and springs/damping.

The bounciness is damping and springs, and to some extent tire pressure and corner weights.

The off/on throttle thing is weight transfer (what's your caster?) and changes in suspension geometry. If you're not running Ireland subframe bushings, you really should be. It's also possible you need new trailing arm bushings.

It may seem like "All I changed was tires!", but... they're the most significant part of the grip equation, and you made a significant change. You changed your unsprung weight, you changed the width in the rear, and you changed your front-to-back weighting (PS2 255/40-17 is .3" smaller in diameter than an S-03 265/40-17 if I'm to believe the TireRack numbers). You also changed tire compounds and sidewall stiffness. And you changed your alignment, yes?

When you change tire sizes, you get different behavior. Which often necessitates alignment changes and suspension changes. This is why it's important to write down your old numbers (everything), as well as how the car drove, and get some advice on what to do if you want to change the way it drives. And be able to communicate the desires to someone else. "so yucky" doesn't help us help you.

Hindsight being 20/20, if you liked the way the car drove before and didn't want to have to make other changes, you should've just bought new S-03's in the same size. Not what you want to hear, I know.

I'll say it again like I did on bimmerforums... find someone near you with a tire pyrometer that knows how to use it, and take them out with you. Take a bunch of measurements after these sweepers you've been mentioning. If the owner of the tire pyrometer knows how to use it, you'll make progress with just tire pressure and they'll point you in the right direction for other changes.

Reply » Help with possible alignment issue...

I think Daniel's response was really good and helpful.

From there let me say it again. Get those stiffer subframe bushings from IE and get them installed. David can do that in 4 hours and his hourly rate is not too bad.
Also, play with the rebound as mentioned. Look at the ride height as well.

If you want me to drive your car and give some ideas we can do that. Peachtree BMWCCA will have their April meet in Dahlonega at a winery there. If you want you can come and I can drive/look at it.

Florian

Reply » Help with possible alignment issue...

Quote:

I also don't think your front toe is where you want it. Factory is about .125" toe-in. The car will feel more darty at high speeds with less toe-in, in terms of steering response. That toe-out on one side in the rear isn't helping your high-speed stability, but the only way to correct it w/o adding more parts is to change your ride height.
IT shouldn't be a significant factor here since your total toe is in in the rear.

How can I tell the Count's Rear TOTAL Toe equates to IN @ .05? Does a positive number mean toe out or toe in?

Thanks for the great answer.

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