How can I correct oversteer?

[UntzUntzUntz]

Hello everyone,

Some of you probably saw the bar I installed at the bottom rear of the Z (couple threads below this one). After I installed it the car handled amazing. Fast forward a few weeks of driving the car, I began to learn the car would have an on and off switch when being pushed to it's limits. On certain turns and on rare occasions the car would decide to oversteer. By oversteer I mean if traction was off, I'm flying off the canyon road; that bad. I always wanted oversteer, but now that I have it, I hang on for dear life.

How can I correct this?
And just out of curiosity, can a car be too stiff to the point it becomes unstable?

My setup: KW coilovers (all close to full stiffness), Strong Strut brace in the front, Ultra Racing bar in the back.

Thank you in advance

[dc_wright]

If you talk to the folks at Dinan or in particular TC Kline, they'll absolutely state too stiff is just as bad as too soft. For your suspension to work it has to be able to act like a suspension and has to be able to move to some extent to accommodate the road forces. If your coilovers have damping adjustment you might try backing off on rebound just a bit and see if that calms the oversteer.

[pokeybritches]

The stiffer your suspension, the more it will skip over bumps. Start by softening the end of the car that needs grip.

The stiffer your overall setup, the more it will be prone to snap movements. A softer setup is easier to manage.

TC Kline runs no sway bar in the rear, IIRC.

[Frank851]

Pokey:
What suspension are you running on your Z4m?

[The HACK]

It's all psychosomatic. That bar you installed does absolutely nothing to the structural stiffness nor does it actually tie into any part of your suspension. You installed something expecting a result, and the result is you are now actively seeking to validate that the part you installed made the rear of the car "stiffer."

What you described, the fact that when push comes to shove, the car has a propensity to oversteer quickly, is all part of the charm and allure of the E85/E86 chassis. It's because it's got a relatively short wheel base with the driver literally sitting right over the rear axel. Meaning that arm, or polar moment of inertia, is much shorter than your typical BMW, and since you as a driver sits behind the actual center of gravity on this car, it feels far more dramatic when it DOES break that rear end grip and the pendulum starts to swing.

In a not quite so pleasant way, this is actually what makes the chassis so special. It's quick to react to input, either via throttle or hands, and it is very easy to induce understeer and oversteer at will. Of ALL the BMW chassis I've ever driven, there are two that are like this. The E30, and the new M235. The E30 because the rear trailing arm design goes from steady state grip to losing camber curve quickly, and the M235 because of the gobs of torque available at low RPM allowing easy breaking of traction on a, you guessed it, relatively short chassis. The E85/E86 wheelbase is shorter than both, but it's got the fancy 4 link rear rather than the single link semi-trailing arm of the E30 (thus the rear end is a lot more predictable), but not the torque monster the M235i is, so despite the short polar moment of inertia, it is actually relatively easy to reign in (another benefit of a short polar moment of inertia, is that it does not require a ton of force for it to want to go back straight again).

I'm going to relay my experience at the BMW M Performance school when they came out west to Auto Club Speedway long time ago. They did their "rat race" wet skid pad exercise with a couple of MZ4 Coupes, and there's a good reason why. With low traction conditions, the rear end of the MZ4 comes around very quickly with a judicious application of throttle, but since MOST of the audience at the event have zero high performance driving training, to stick them in an M5 or M6, probably would have ended up in a comedy of errors with everyone basically looping themselves around repeatedly. But with the MZ4 Coupe, a quick flick of the hand brings the car right back to its original vector, and you can keep your foot buried in it the whole time.

But this is probably not going to address your concerns. What I would do is soften the rear rebound on your coil-overs if you are concerned for your safety, and shift the grip back to the rear of the car.

[UntzUntzUntz]

Originally Posted by dc_wright

If you talk to the folks at Dinan or in particular TC Kline, they'll absolutely state too stiff is just as bad as too soft. For your suspension to work it has to be able to act like a suspension and has to be able to move to some extent to accommodate the road forces. If your coilovers have damping adjustment you might try backing off on rebound just a bit and see if that calms the oversteer.

Originally Posted by pokeybritches

The stiffer your suspension, the more it will skip over bumps. Start by softening the end of the car that needs grip.

The stiffer your overall setup, the more it will be prone to snap movements. A softer setup is easier to manage.

TC Kline runs no sway bar in the rear, IIRC.

Originally Posted by The HACK

It's all psychosomatic. That bar you installed does absolutely nothing to the structural stiffness nor does it actually tie into any part of your suspension. You installed something expecting a result, and the result is you are now actively seeking to validate that the part you installed made the rear of the car "stiffer."

What you described, the fact that when push comes to shove, the car has a propensity to oversteer quickly, is all part of the charm and allure of the E85/E86 chassis. It's because it's got a relatively short wheel base with the driver literally sitting right over the rear axel. Meaning that arm, or polar moment of inertia, is much shorter than your typical BMW, and since you as a driver sits behind the actual center of gravity on this car, it feels far more dramatic when it DOES break that rear end grip and the pendulum starts to swing.

In a not quite so pleasant way, this is actually what makes the chassis so special. It's quick to react to input, either via throttle or hands, and it is very easy to induce understeer and oversteer at will. Of ALL the BMW chassis I've ever driven, there are two that are like this. The E30, and the new M235. The E30 because the rear trailing arm design goes from steady state grip to losing camber curve quickly, and the M235 because of the gobs of torque available at low RPM allowing easy breaking of traction on a, you guessed it, relatively short chassis. The E85/E86 wheelbase is shorter than both, but it's got the fancy 4 link rear rather than the single link semi-trailing arm of the E30 (thus the rear end is a lot more predictable), but not the torque monster the M235i is, so despite the short polar moment of inertia, it is actually relatively easy to reign in (another benefit of a short polar moment of inertia, is that it does not require a ton of force for it to want to go back straight again).

I'm going to relay my experience at the BMW M Performance school when they came out west to Auto Club Speedway long time ago. They did their "rat race" wet skid pad exercise with a couple of MZ4 Coupes, and there's a good reason why. With low traction conditions, the rear end of the MZ4 comes around very quickly with a judicious application of throttle, but since MOST of the audience at the event have zero high performance driving training, to stick them in an M5 or M6, probably would have ended up in a comedy of errors with everyone basically looping themselves around repeatedly. But with the MZ4 Coupe, a quick flick of the hand brings the car right back to its original vector, and you can keep your foot buried in it the whole time.

But this is probably not going to address your concerns. What I would do is soften the rear rebound on your coil-overs if you are concerned for your safety, and shift the grip back to the rear of the car.

Wow. Very informative as always! Thank you everyone for your input. That's 3 strikes, time to soften the rear.

Hack, thank you for your thorough response. I learn something new every time from your posts.
It's funny the first paragraph you wrote reminded me of those high school days where changing the muffler made the car "faster" and I "felt the power." But this isn't one of those lol. The bar you probably saw in the thread was the one installed in the trunk. That part is completely useless. I keep it because I like to refer to it as the "junk in the trunk." Scroll a little lower you'll see a second one installed beneath the car. This is the bar that made a difference. If you're around Los Angeles you're more than welcome to take it for a spin.

So far it's all pointing to soften up the rear coilovers. And that's what I shall do.

Oh and one more thing I just thought of. I know too stiff isn't the answer but just out of curiosity, can't this be comprised by stiffening the front? Not necessarily through the coilovers but instead getting thicker anti-roll bars?
Don't laugh lol I'm just curious.

Thanks again guys.

[The HACK]

Originally Posted by UntzUntzUntz

So far it's all pointing to soften up the rear coilovers. And that's what I shall do.

Oh and one more thing I just thought of. I know too stiff isn't the answer but just out of curiosity, can't this be comprised by stiffening the front? Not necessarily through the coilovers but instead getting thicker anti-roll bars?
Don't laugh lol I'm just curious.

Thanks again guys.

Stiffening up the front to remove grip so you'd experience less understeer is the @ss backwards way to fix things.

I can't find what you're referencing to wrt what you did to the rear, but the reason why you're experiencing far more "loss of grip" in the rear now, is because a suspension that is too stiff is preventing the tires in coming into full contact with the road, thus allowing the rear end to lose traction before the front.

Think of grip in relation to stiffness as a bell curve. If you've ever seen a bell curve, there's a top point where your return on investment (grip) will start to decrease past a certain point of investment (stiffness). You've now moved past the top, or apex, of the bell curve.

Not knowing the relationship between the bell curve of the front end of the suspension, there are two possibilities. Either you add more stiffness and it increases front grip, thus EXACERBATING the oversteer issue, or you add stiffness so it's past the optimum grip level and you fix the oversteer by introducing more understeer, which means you're taking away available grip.

Since you already know you've moved past the optimum grip level for the amount of stiffness in the rear, the LOGICAL solution is to soften the rear, not stiffen the front.

Does that make sense?

[UntzUntzUntz]

Originally Posted by The HACK

Stiffening up the front to remove grip so you'd experience less understeer is the @ss backwards way to fix things.

I can't find what you're referencing to wrt what you did to the rear, but the reason why you're experiencing far more "loss of grip" in the rear now, is because a suspension that is too stiff is preventing the tires in coming into full contact with the road, thus allowing the rear end to lose traction before the front.

Think of grip in relation to stiffness as a bell curve. If you've ever seen a bell curve, there's a top point where your return on investment (grip) will start to decrease past a certain point of investment (stiffness). You've now moved past the top, or apex, of the bell curve.

Not knowing the relationship between the bell curve of the front end of the suspension, there are two possibilities. Either you add more stiffness and it increases front grip, thus EXACERBATING the oversteer issue, or you add stiffness so it's past the optimum grip level and you fix the oversteer by introducing more understeer, which means you're taking away available grip.

Since you already know you've moved past the optimum grip level for the amount of stiffness in the rear, the LOGICAL solution is to soften the rear, not stiffen the front.

Does that make sense?

Lol the word exacerbating sounds funny... and dirty.

I'm referencing post #16 in that thread: http://www.zpost.com/forums/showpost...2&postcount=16
Is this what you were referring to?

Also, wouldn't having the front stiffer in this case be better say in a track environment? Canyon roads make sense since it's full of bumps and uneven surfaces where the suspension would need to compress to get optimal grip. On the track, and let me bring back your bell curve example, I can imagine the bell curve peak point having a little more room before it starts to decrease. Am I on the right page here? Or is it still the same case...

It makes complete sense what you're saying. That's the reply I imagined. Thanks again I appreciate it a lot!

[pokeybritches]

Originally Posted by Frank851

Pokey:
What suspension are you running on your Z4m?

On one M I'm stock. Great for the street, but sloppy when changing direction.

The other has TC Kline D/A's and I believe 450/500 springs (been a while since I've messed with it). It's set soft. I'm a fan of soft suspension setups, maybe to mask my driving It's probably the most predictable setup you can get on the Z4M.

Some may consider the stock suspension to be rewarding to master. I found it to be a bit of an animal, and a little frustrating if you're trying to be precise. It's fun to thrash, but if you're a stickler for precision, it's a tough thing to completely own. The TC Klines are a night and day difference.

Fortunately the car is communicative, which I love. IMHO the stock Z4M is so talkative that if you mess it up, it's because you weren't listening. It's not as stable, meaning it requires quicker recognition and reaction, but also responds quickly.

The TC Klines increase feedback and the car's reaction to minor adjustments, but decrease the severity of large inputs. In not so many words, the TC Klines make the car do what I want without punishing me quite as much.

[dc_wright]

If you go back to the basics, you want to maximize the contact of all 4 tires with the road. Too soft a suspension allows body roll which transfers weight to the outside tires while simultaneously unloading the inside tires. To eliminate this your main effort is to reduce or eliminate body roll, and once you've stiffened the suspension (thicker sways, increased rebound/compression shocks) to control body roll, you've hit the peak on the bell curve. When you go further, you reduce the suspensions ability to follow the contour of the road surface, and now you're back to overloading the outside tires again if there are any irregularities in the road surface because the inside tires are off the ground for a period of time. Where you've currently have your suspension set it feels great so long as all 4 tires have road contact but the instant a rear wheel hits a pebble you've lost rear grip and you get the snap over steer.
If you can find a deserted stretch of road or empty parking lot, I'd try backing the coilovers off to full soft, and then run S patterns at 40 or so mph and monitoring the effect on body roll. Increase the firmness both front and rear in small increments and at the point where body roll has flattened out a bit (but not completely gone) I'd stop and then try that setting for a bit as a starting point to see how that feels and if that's reduced the snap over steer. From that point if you make any adjustments I'd go slightly firmer on the front and slightly softer on the rear. Both Dinan and TC Kline claim the Z4 likes the rear softer than the front and so far that's been what I've found on my car. I still have some optimization to do, but front stiffer than rear definitely improves transitional handling like in autocross slaloms.

[Wotfun]

I found this thread after searching for info on e85/e86 oversteer. There's a lot of helpful information here and I appreciate everyone who contributed.

My car is an '07 e86, manual, no sport pkg, stock except 235 width PSS tires.
I've done BMW HPDEs for 3 years. The last three schools were in the Z4. After my last school, my instructor said I should get camber plates and stickier tires if I want to continue to make progress with the Z4.

Since October 2016, I've experienced snap oversteer twice. Once on a public road and once on track. I understand it was my fault since I was running summer tires and the temperature was around 40 Fahrenheit. I'm sure I went into the turn too fast and / or lifted off the throttle, thereby causing the oversteer.

But, the information posted here indicates the Z4 is prone to oversteer and it's a good idea to have the rear suspension a bit softer than the front. It also seems like a car, like an e92 Coupe, might be less prone to snap oversteer, due to having a longer wheelbase. And, it might feel less sudden, due to not sitting toward the back of the car (like in the Z4). Again, thanks for all the helpful information.

I want to continue to do HPDEs and I want to feel confident that I can both drive fast and keep the rear of my car from passing the front.

I'd appreciate your opinions re whether I'd be better off spending money on my Z4 to get the handling more neutral OR just getting another car (e92 maybe) that isn't prone to snap oversteer.

Thanks again!

[dc_wright]

Originally Posted by Wotfun

I found this thread after searching for info on e85/e86 oversteer. There's a lot of helpful information here and I appreciate everyone who contributed.

My car is an '07 e86, manual, no sport pkg, stock except 235 width PSS tires.
I've done BMW HPDEs for 3 years. The last three schools were in the Z4. After my last school, my instructor said I should get camber plates and stickier tires if I want to continue to make progress with the Z4.

Since October 2016, I've experienced snap oversteer twice. Once on a public road and once on track. I understand it was my fault since I was running summer tires and the temperature was around 40 Fahrenheit. I'm sure I went into the turn too fast and / or lifted off the throttle, thereby causing the oversteer.

But, the information posted here indicates the Z4 is prone to oversteer and it's a good idea to have the rear suspension a bit softer than the front. It also seems like a car, like an e92 Coupe, might be less prone to snap oversteer, due to having a longer wheelbase. And, it might feel less sudden, due to not sitting toward the back of the car (like in the Z4). Again, thanks for all the helpful information.

I want to continue to do HPDEs and I want to feel confident that I can both drive fast and keep the rear of my car from passing the front.

I'd appreciate your opinions re whether I'd be better off spending money on my Z4 to get the handling more neutral OR just getting another car (e92 maybe) that isn't prone to snap oversteer.

Thanks again!

No car can overcome driver error, no matter how sophisticated the technology. If this weren't true no F1 car would ever have an "off". The Z4 actually is more prone to understeer, so if you got the rear to snap around I suspect you were already near the adhesion limit and lifting shifted weight off the rear unloading the suspension and the rear came around. If you had DTC turned on it probably felt one of the rear tires losing traction and applied braking to that caliper and made the situation worse.

[Wotfun]

Originally Posted by dc_wright

No car can overcome driver error, no matter how sophisticated the technology. If this weren't true no F1 car would ever have an "off". The Z4 actually is more prone to understeer, so if you got the rear to snap around I suspect you were already near the adhesion limit and lifting shifted weight off the rear unloading the suspension and the rear came around. If you had DTC turned on it probably felt one of the rear tires losing traction and applied braking to that caliper and made the situation worse.

Wow. I didn't realize DTC would make oversteer worse. But, yes, I had all "nannies" on. I always thought that was the safest way to go.

Aside from whatever else went wrong, my alignment was likely a significant factor. I had an alignment done this week. All wheels except the right front were out of spec. Worst of all, both rears had the toe pointing to the right. Both snap oversteer events were left hand turns.

I also suspect bushings are worn. I replaced the fcabs last summer, but the rest are original as far as I can tell. Additionally, I had my tire pressure set per the door jamb sticker- 39 rear & 33 front. I'm guessing that reduced rear grip relative to front.

Going forward, i plan to keep pressure at about 32 all around. Maybe a pound or two less in back than front.

I appreciate your response. If you have any additional information regarding effects of traction and /or stability control on oversteer, I'd appreciate you sharing.

[dc_wright]

Originally Posted by Wotfun

Wow. I didn't realize DTC would make oversteer worse. But, yes, I had all "nannies" on. I always thought that was the safest way to go.

Aside from whatever else went wrong, my alignment was likely a significant factor. I had an alignment done this week. All wheels except the right front were out of spec. Worst of all, both rears had the toe pointing to the right. Both snap oversteer events were left hand turns.

I also suspect bushings are worn. I replaced the fcabs last summer, but the rest are original as far as I can tell. Additionally, I had my tire pressure set per the door jamb sticker- 39 rear & 33 front. I'm guessing that reduced rear grip relative to front.

Going forward, i plan to keep pressure at about 32 all around. Maybe a pound or two less in back than front.

I appreciate your response. If you have any additional information regarding effects of traction and /or stability control on oversteer, I'd appreciate you sharing.

If you're already turning at the limit of adhesion, the DTC pulsing a rear caliper is like doing a tug on the e-brake handle. On the 3.0si I turn it all the way off as it's turned me completely sideways a couple of times.

[Wotfun]

Thanks dc_wright. Would less grippy tires reduce likelihood of sudden oversteer? For instance, would a 400 wear rating squeal before breaking loose completely (and give me time to correct with counter steer) vs a 200 wear rating that might break loose more abruptly?

[Wotfun]

Also, if I installed a limited slip dif, how would that effect oversteer?

[The HACK]

Originally Posted by Wotfun

Also, if I installed a limited slip dif, how would that effect oversteer?

Depends on how oversteer is initiated.

[Wotfun]

I'm hoping to make oversteer initiation as progressive, predictable, and controllable as possible.

I know a LSD will increase traction. I'm wondering if it will help towards goals listed above.

[joelselman]

WOTFUN: Not 100% sure about your car but based on my experience in my car, go with your instructors recommendations. One step at a time. My car is running square tires (265/35x18) with front camber plates on a stock sport suspension. With the right driver (not me) it's very competitive. Natural tendency of my car was to push, now slight oversteer.

[Wotfun]

Originally Posted by joelselman

WOTFUN: Not 100% sure about your car but based on my experience in my car, go with your instructors recommendations. One step at a time. My car is running square tires (265/35x18) with front camber plates on a stock sport suspension. With the right driver (not me) it's very competitive. Natural tendency of my car is to push.

Joelselman, Thanks for replying. Which model do you have? How did you fit 265s in front without rubbing?

[joelselman]

2005 3.0i sport smg. The 265's fit fine on 9.5" wheels (ET38 or so I believe) with only an occasional rub at extreme lock. BTW, I changed my last post to read car did (past tense) understeer.

[Wotfun]

Wow. Other folks on this forum have suggested 255s front and back, but said it required close to 3 degrees negative camber to prevent rubbing up front. I think that was for an 06-08 3.0si though. Maybe fitment for the 05 is different.