Autocross Handling Help Needed

So I did a test and tune yesterday, and got some good (in quality, but bad in result) information, and I'm turning to your folks for some help.

Car

1996 BMW 318ti

Stock style limited slip - could probably use a rebuild

Tires: 245/40 Hoosier A6s - well worn on 17x8.5 K1s with 12mm spacer in front

Conditions

Low to mid 60s, sunny, all day... lots of wind

Airport asphalt, bump in a few places

Starting Setup

Front

Vorshlag camber plates, max caster, max camber (-3.2ish) with 1/8" toe out

6" 550lb Eibach springs on M3 struts with new Koni yellow inserts (off the shelf for 1996 M3)

Rebound at one full turn down from full stiff

Eibach 28mm sway bar on full stiff using M3 endlinks

Ride height - 13" (from wheel center to fender lip)

36psi

Rear

650lb TCK springs

3-year old Koni singles (off the shelf for 1996 318ti)

EDIT - Rebound at 25% from full soft

Stock Sport rear sway bar (16mm)

Ride Height - 12.625" (from wheel center to fender lip)

Alignment is wherever those settings put me - can't do much about that

32psi

End-of-day Setup

Front

Set the sway bar to full soft

Rebound set to half turn up from full soft

34psi

Rear

Ride Height - 12.875"

31psi

So now that you have information overload, I'll get to the problem.

I had several people drive the car, and we all had the same initial impression - too much push on the front end. The car wouldn't rotate, and if driven hard, would just understeer. Car was really picking up the inside front, and in several spots felt like it was trying to get on two wheels. The back of the car was very bouncy going over bumps in the "grippiest" corner. The car felt so bad that I raised the rear ride height about a 1/4" to in the event there was contact making the car want to get up on two.

Data for the morning session showed a peak steady sate G-load at a laughable .83g's.

After lunch, I put the front sway bar to full soft, then softened the front springs, then lowered pressure. All of these things helped. The car wasn't picking up the inside front as high, and grip was better. Car was still bouncy in the back, and still "felt" like it wanted to get on two wheels in the harder corners.

Data for the afternoon showed an improved, but still not-so-great peak G-load of .95g's.

On this same surface, in slightly warmer conditions, I've pulled 1.1g's on 225mm street tires.

Despite lessening the push, the car still would not rotate, even with power (or what little the 318ti has).

I'm planning on upping the rear rebound to try to keep the back end from bouncing so much and in an attempt to get more rotation (couldn't adjust at the track), but does anyone else have any ideas as to why the car seems to be handling so poorly? The worst part is that the car feels like it wants to go over - it really shakes your confidence.

I know I'm pushing spring rates on off the shelf Konis, but I didn't think it should make things this bad.

Thanks!

Any reason you lowered the rear tire pressures? Normally you increase them for more rotation.

The pairing of a huge front bar with a factory rear bar might be causing it, too.

take this for what it's worth since I drive a FWD... but generally accepted thought is to bar the opposite end from the drive wheels... i.e. on a FWD you increase the size of the rear bar , make the front smaller even remove it... conversely with a RWD conventional thought says increase the size and stiffness of the front bar and decrease/remove the rear, also (as Tommy says) you can lessen the amount of grip generated by the rear by raising the pressures in the rear

you can also "hunt" for more grip in the front by "playing" with the pressures in the front

Tommy Suddard wrote: Any reason you lowered the rear tire pressures? Normally you increase them for more rotation. The pairing of a huge front bar with a factory rear bar might be causing it, too.

It was a long day and I was tired? You are right about increasing pressure for rotation, but you can also lower for rotation (but I don't like how that feels).

wbjones wrote: take this for what it's worth since I drive a FWD... but generally accepted thought is to bar the opposite end from the drive wheels... i.e. on a FWD you increase the size of the rear bar , make the front smaller even remove it... conversely with a RWD conventional thought says increase the size and stiffness of the front bar and decrease/remove the rear, also (as Tommy says) you can lessen the amount of grip generated by the rear by raising the pressures in the rear you can also "hunt" for more grip in the front by "playing" with the pressures in the front

I do need to play with front pressures some more, but my initial wear/surface infrared pyrometer shows that the pressure is pretty close.

BMW's <3 a big front bar, as it helps keep camber in check. The probably was I was creating a solid front axle, and one tire grips less than two.

The rear had a TON of grip, and I'd love to increase front grip to match, but I'm not sure I can do much to that effect.

I did up the rear rebound this evening, and rotation feels better on the street tires, and the rear feels more under control. I'll be running the same surface and will likely have a very similar corner to compare that change.

The big bars on the fronts of RWD Mac-p strut cars are for limiting camber loss due to roll to gain traction. Sounds backwards, but upping my mustang from a 22mm to a 35mm front bar did wonders for it (rear wasnt changed)

Gimp, did you up the front spring rate from last season? I forgot to ask you earlier..

could you list setup changes from last season? (when it was handling reasonably, granted on street tires)

Apexcarver wrote: Gimp, did you up the front spring rate from last season? I forgot to ask you earlier.. could you list setup changes from last season? (when it was handling reasonably, granted on street tires)

Sure thing. Last year's setup:

225/50/15 Dunlop StarSpecs on 16x7" wheels

Front

448lb springs on non-M style coilovers stuck on full soft

Same sway bar on full stiff with non-M endlinks (attached to the control arm instead of the strut)

Same alignment and camber settings on the plates, but the front ride height may have been a little lower

Rear

560lb springs

Same rear Konis on 25% stiff

Same rear bar

Ride height was a little higher

How tough will it be to go back to the 450 front springs?

Do you have a pyrometer? You may have too much front neg camber.

Even though today was frustrating, it sounds like you have the right approach.

Rog

Ran some math with him.. the short version is, looked up ratio to calculate wheel rates, found that the old front and rears were 81% and 86% of news respectivly. (would have needed either a 490 front or a 690 rear to maitain same spring balance)

that is when I learned the difference that the swaybar mount change made on its effective rate.. BIG difference...

I've been reading more about the effect of changing the front sway bar mounting location, and I'm seeing that it make make the bar up to 50% stiffer when you go from the non-M to the M locations. This was way more than I expected, so I'm thinking about doing what I always do - changing too many things at once.

I've already upped the rear rebound, but that was more to keep the rear end under control and keep the inside rear from hopping over bumps in the sweepers.

I have a set of 448lb springs sitting on the bench. I'm thinking about swapping these in place of the 550lb springs I have in the front, and setting the front back back to full stiff. My hope is that this gets me closer to my old balance.

The rear had a really nice amount of grip, so I'd rather add grip to the front than remove grip from the rear to achieve better balance. If the car is too tail happy after that change, I can always unhook the rear bar to calm things down.

Thoughts?

Tires: 245/40 Hoosier A6s - well worn

Are you sure this wasn't causing most of your difficulties? Testing on toasted tires can be a frustrating experience.

Since it's still understeering in a steady-state corner, it sounds like you need more rear roll stiffness. It's your call if you want to do it with stiffer springs or more bar. I'd probably go the bar route if you have the option to go bigger.

Emod might be on to something too with the too much negative camber. Since you said the car was responding to a softer front end, it might be because the car is rolling more, which adds positive camber, and might mean the tire is squared up to the ground more. Try taking a degree or so out of the front camber and see if that helps. Plus that should help give you a bigger contact patch for braking.

Once you get the steady state sorted out, it should liven up the car more in during transition. If you still want more rotation on turn-in, dial up the firmness of the rear shocks or soften up the fronts. What that does is change how quickly each axle transfers load from one tire to the other. By stiffiening up the rear relative to the front, you be temporarily taking away some understeer to help it rotate. Once you're steady state, the bars and springs are doing most of the work. If you're happy with the turn in, but it's bouncing around too much in the middle of the turn, soften up both the front and rear dampers. It's their relative stiffness that will have the biggest effect on turn-in.

All good points, and I'll take it all into consideration.

As far as front camber, smarter, better, faster people than me have proven that the E36 LOVES front camber - the more the better.

http://corner-carvers.com/forums/showthread.php?t=31227

My 3+ degrees of camber is about where it should be in the front. Terry Fair has gotten away with over 4 degrees in the front of his LS1 powered alpha car (if I remember correctly).

Even on the most stiff setting, front tire temps and wear looked good.

Braking, even with all that camber, is a non-issue on this car. It's probably the only thing the car does really well.

Rear bars are out there, but cost money. Considering this car is my commuter, and I have a project car I should be putting that money towards, I'm more inclined to work with what I already have (softer front spring). New rear springs are about 100 bucks, and the same rule applies.

I promise I'm not ignoring any advice, but swapping to softer front springs only costs me time.

jstein77 wrote: Tires: 245/40 Hoosier A6s - well worn Are you sure this wasn't causing most of your difficulties? Testing on toasted tires can be a frustrating experience.

This was a thought of mine, but Mustang Boy (apex carver) and I talked about it, and wondered if it would really have an impact on balance. We can see an impact on overall grip, but not so much balance.

Apexcarver wrote: The big bars on the fronts of RWD Mac-p strut cars are for limiting camber loss due to roll to gain traction. Sounds backwards, but upping my mustang from a 22mm to a 35mm front bar did wonders for it (rear wasnt changed)

This is true for a stock-class E36/E46, too, but he's got camber plates that are allowing him to get massive negative camber up front (based on what's shown in the first post). I suspect that in this case reducing the size of the front bar may help, especially if it's really lifting the inside front hoof in hard cornering.

Duke wrote: ...especially if it's really lifting the inside front hoof in hard cornering.

All witness and photo accounts say 4-6 inches in the front, with the inside rear body coming up a good bit (but there is lots of droop travel, so the tire stays down).

Once I softened the front bar, the inside front was probably reduced to 2 inches max, and the rear felt a little better.

Duke wrote: This is true for a stock-class E36/E46, too, but he's got camber plates that are allowing him to get massive negative camber up front (based on what's shown in the first post). I suspect that in this case reducing the size of the front bar may help, especially if it's really lifting the inside front hoof in hard cornering.

http://forums.bimmerforums.com/forum/showthread.php?t=1342943

Terry Fair's E46 DSP build - camber plates and big front bar - and he usually knows what he's doing.

Final front camber was roughly -4° front, -2.5° rear

...I have started off with the stock swaybars and 600# front, 750# rear spring rates...

...The stock 24mm front bar was replaced with a 27mm Eibach adjustable bar + Eibach poly mounts...

From the looks of the photos, it's set on full stiff.

...The car on the 600F/750R springs had more roll than I like...

...The 60mm x 6" x 600 #/in fronts and 60x5.5x750 rear Hyperco springs were removed. We put the rear springs up front, which allowed us to lower the ride height another 1/4" on that end (it finally "looks" right) and then we went with 2.25" x 5" x 900#/in Hypercos in the rear...

A bit apples and oranges because of the E36 vs E46, but the theory is there.

Might it be possible that the front tires were heat cycled more drastically than the rears (higher temps) and therefore are now of a harder durometer than the rears?

I think he rotates front to rear on the tires... also has observed understeer behavior on street tires.

Bimmers arent the only cars that like big front bars. I have camber plates as well and have close to 3* with a 35mm front bar with 900lb-in springs on my mustang. This choice was advised by and sold to me by Sam Strano. It is not an uncommon thing. Mac-p strut cars loose camber in roll and you compensate by limiting roll.

I think gimpy's problem is that he drastically made the front stiffer with only a mild increase in the rear, a bit due to springs, but largely due to bar. Deviating from a neutral handling car the stiffer end will loose traction first (more is being asked of it because of the force that the stiffer spring is transferring, traction has limits)

The solution (IMHO) is to either stiffen the rear or soften the front to equalize the forces front to rear. Either bar or spring on either end can do this (with different ramifications for each to other characteristics)

As a quick 'n dirty test, try some of those spring rubber inserts from the latest issue in the rear. If you like what you see, consider buying a rear bar. Or soften the front more if you have the parts on hand to do it. I wouldn't mess with the shocks too much until you're happy with the way the car handles in the middle of the turn.

Dead front tires WILL change balance, as will temperature variances.

I wouldn't change a thing until I ran the car on fresh(er) fronts.