Cal Club Autocross Forums

http://forums.bimmerforums.com/forum/sh ... ?t=1264885

Theo Osifeso wrote:http://forums.bimmerforums.com/forum/sh ... ?t=1264885

That's what BMW's do in DSP, something to do with that combo of mods and grip. They all seem to do it to some extent. Don't stick your arm out if it starts to roll over.

Generally its caused by big anti-roll bars. More spring, less bar.

Toby and I are already commenting in that thread, but I'll add it here: neither of us believe this is a function of grip. Something is interfering to cause those angles. Either the tire is hitting the fender (as it did for Toby), they're bottoming out the spring, they have bind in a bar or some other mechanical interference is going on.

If I recall correctly, when Toby had the problem he showed me the fender where it was interfering. I looked at it and thought there's no way that such a minor looking rub could cause that problem, but he was correct. The easy test for the owner of that car is to raise his ride height 3/4 inch. I'd be willing to be any amount of money the problem goes away.

Mike

John Coffey wrote:Generally its caused by big anti-roll bars. More spring, less bar.

'splain, please. Seems to me it's a factor of the combined force vector intersecting the ground outside the wheel plan, no matter the method of generating the roll couple.

Looks like Peru. Grippy concrete like Forbes.

Mike Simanyi wrote:Toby and I are already commenting in that thread, but I'll add it here: neither of us believe this is a function of grip. Something is interfering to cause those angles. Either the tire is hitting the fender (as it did for Toby), they're bottoming out the spring, they have bind in a bar or some other mechanical interference is going on.

As in interference causing a braking force transferred into the chassis that wants to throw the car over?

John Coffey wrote:Generally its caused by big anti-roll bars. More spring, less bar.

uh that's a big negative.

weight transfer > inside wheels normal force.

cg too high, track width too narrow, lateral accel too high.

you could go screw with the suspension and decrease the grip and it will stop doing that.

or pull 800lbs out of it (reduced weight transfer).

Karl A in Toby's DSP BMW @ grippy Atwater '05 ... problem went away with stiffer springs if I recall correctly.

Did the stiffer springs prevent a bottoming out or bind?
Or did the stiffer springs just reduce lateral grip?

Maybe window nets should be required on DSP BMW's.

Weight transfer is a function of cg height, track width, and lateral g's. To keep the inside tires down, the BMWs would need to get lower, wider and stiffer or just go slower around the corners. Tall and narrow + grip = a tip over combo.

E36 Street Mod cars have been doing that for years. I saw Jim Ochi's car go up like that back when SM was a supplemental class. Ryan Carag's car went up on 2 wheels in a slalom at Atwater in 2006 (I believe) and Scott Fraser needed new pants.

John Stimson wrote:...and Scott Fraser needed new pants.

Nothing unusual about that...

Mako Koiwai wrote:Karl A in Toby's DSP BMW @ grippy Atwater '05 ... problem went away with stiffer springs if I recall correctly.

Going up in springrate was a small part, the big part was to cut open the wheel wells to avoid tire contact. Once the suspension could move without interference the problem went away. I think Steve O's RX7 used to do the same thing until he fixed his suspension.

Will Kalman wrote:

Mike Simanyi wrote:Toby and I are already commenting in that thread, but I'll add it here: neither of us believe this is a function of grip. Something is interfering to cause those angles. Either the tire is hitting the fender (as it did for Toby), they're bottoming out the spring, they have bind in a bar or some other mechanical interference is going on.

As in interference causing a braking force transferred into the chassis that wants to throw the car over?

Funny things happens when suspension movement come to a sudden stop, could cause oversteer, understeer or two wheeling. When d something as simple as a front swaybar bind issue we saw our braking G's go from high 1.1 down to mid 0.9

I was told the bottom of the inside tires were waist level. I lifted off the throttle and straightened the steering wheel to put it back on the ground.

We changed everything in the car in a week and without cutting the fenders we could only lower it so far. The car ended up getting its fenders chopped and the car was able to be lowered. So for us it wasn't bottoming out or suspension bind.

and no I didnt need to change my pants after the run. I didn't realize I was that high ( )

Scott Fraser

Large ARBs make the suspension more rigid - like a go kart. There's less "suspension" to absorb roll so the car tends to stand up on the outside wheels once the insides come off the ground. The extreme example is Toby's case where the rear suspension bound with the tire rubbing the wheel well. At that point you don't have a suspension, just a long lever that's directly related to the standard weight transfer equation. Add in a bump or a sudden driver input and you get what's pictured above.

John Coffey wrote:Large ARBs make the suspension more rigid - like a go kart. There's less "suspension" to absorb roll so the car tends to stand up on the outside wheels once the insides come off the ground. The extreme example is Toby's case where the rear suspension bound with the tire rubbing the wheel well. At that point you don't have a suspension, just a long lever that's directly related to the standard weight transfer equation. Add in a bump or a sudden driver input and you get what's pictured above.

Large SPRINGS make the suspension more rigid - like a go kart. [insert the rest of that example above]

Bob Beamesderfer wrote:Maybe window nets should be required on DSP BMW's.

Or maybe wheels on the roof.

If I remember right that picture of Karl was only the second highest he got that right side up that day.

Jay W

I went around to get a shot from the front to make the lift angle more obvious. By that time Beverly was driving and she wouldn't oblige me. I chided her on that later and she said darn right she wasn't going to have any off that. Something about having all four tires sharing the load?

Large SPRINGS make the suspension more rigid - like a go kart.

You're right. I played with a simple example in Mitchell and it really doesn't matter whether the rate is from bars or spring. I guess the examples I made my deduction from were basically stock ride height street cars that had big bars on the car for handling.