Z4 Coupe high speed stability?

[dre99gsx]

How are you guys with high speed stability on these coupes? I'm talking 140mph+ on power (not cruising)? Any comments on alignment setup , wings (that actually prove to work, not just cosmetic)?

I'm less concerned about steering feel and more about the rear end staying firm planted and straight. I noticed my coupe likes to "hunt" a bit in the rear at 140mph+, making me rather nervous.

Could this just be a design flaw: short wheel base, no rear aero, lightweight, no LSD, not geared for anything more than 170 (if wind isn't a factor) ?

[dc_wright]

I can't speak for the coupe but my roadster has been pretty solid at those speeds on the track at Daytona. Even with gusty winds it hasn't felt unstable. Make certain the headlight switch isn't in the auto position and that the windshield wiper switch is in the fully off position. If they're not off, the brake pad wipe function is enabled which makes the pads just touch the rotors occasionally to keep them dry in the rain. They still do this even if it's not raining and all 4 corners don't necessarily make contact simultaneously. At normal highway speed you'd not notice it. At 140 you'd definitely feel it.
It could also be DSC/DTC thinking it needs to do something at those speeds and just touching the rear brakes alternately. I've experienced this at Daytona, but the banking also contributes to confusing DSC/DTC. I turn it off at that track as does everyone else I know that runs an E46 as they exhibit the same behavior.

[WeekendWarriorz]

I take my Z4m over 140+ on my daily commute and it's stable on the SoCal freeways, all with traction off and sport mode on. I'm on 19 inch wheels if that makes a difference but the car feels like she's squatting down.

[dre99gsx]

Maybe it's just me... I thought these cars were very unstable past 140.

My alignment specs are -2 camber rear, 0 total toe rear (can't go any more negative), 0 toe front, -1.6 camber front.

Have my BC coilovers on middle setting all four, and 32psi all four tires. All undertrays in place.

All new suspension bushings (upgraded front and all rears), all new wheel bearings. Car feels solid on the road in any other condition.

Do you guys feel a wing is necessary?

Keep in mind, I'm probably pushing 500whp right now, which may be some part of the issue She's solid if I'm feathering at 140, but soon as I step on it, it begins...

On a side note - I find it funny I'm about 30mph shy of running out of gear She can definitely reach it in 6th that's for sure!

[The HACK]

Originally Posted by dre99gsx

My alignment specs are -2 camber rear, 0 total toe rear (can't go any more negative), 0 toe front, -1.6 camber front.

:confused

Don't know how you would expect any different when there's zero toe all around.

[dc_wright]

Originally Posted by The HACK

:confused

Don't know how you would expect any different when there's zero toe all around.

^This^

Zero toe in the rear certainly isn't helping your cause. Rear toe in increases stability, toe out decreases it.

On my 3.0si to get past 140 in the space available on the track it's floored the whole time right up to the braking point to turn into the infield at Daytona. It's very stable under those conditions, but I'd guess on a public highway with a crowned road and other pavement variations you're getting a lot more movement inputs to the suspension than I've seen. Plus I certainly don't have 500 rwhp as a variable.

[The HACK]

Especially for a "high powered" car. As you tip in throttle, and weight shifts to the rear, rear camber increases. A charming side-effect is the toe INCREASES (toes out). If your goal is any sort of high speed stability for a high horsepower car, i certainly wouldn't align it with zero toe (since at speed, it will toe OUT as you tip in throttle).

[dre99gsx]

Good points. Interestingly, I can't apply any more toe in. I tried to go -1/16 each side, but it is maxed at 0. Car actually had +1/8" toe OUT on each side when I had gotten it a while ago.

[The HACK]

Originally Posted by dre99gsx

Good points. Interestingly, I can't apply any more toe in. I tried to go -1/16 each side, but it is maxed at 0. Car actually had +1/8" toe OUT on each side when I had gotten it a while ago.

Check for the rear trailing arm bushings. If someone replaced it and didn't put it back right (say, offset instead of centered on the bushing?) it may allow the rear trailing arm to sit offset in the bracket, thus leading to some odd toe settings.

http://www.pelicanparts.com/BMW/tech...rm_Bushing.htm

Assemble press tool and new bushing to install bushing into trailing arm. Then install bushing into trailing arm following directions supplied with tool. Check that the bushing is at same installation depth as the one you removed, as per your earlier measurement.

I couldn't find the picture to illustrate, but when you press the bushing back into the trailing arm, the bushing will stick out of either side of the actual arm itself. If the bushing is pressed in flush, then you will either have too much toe-in or not enough depending on which side you push it in from.

Actually, while you're in there, probably better just to replace the rear toe bushings with urethan or spherical bushings, especially for high powered cars.

[dre99gsx]

I upgraded the rear bushings to rogue engineering RTABs, installed it myself. They were universal (could be installed in either direction). The rear upper bushings were replace with new OEM units. Lowers were untouched (they looked Ok).

[The HACK]

I think sometimes aftermarket companies assume that the toe bushings are installed CENTERED on the trailing arm. It is entirely possible that the factory position is not 100% centered leading to the aftermarket toe bushings to be just out of the adjustment range. If that is not the case, I would check alignment with another shop and see if they can add some toe-in for rear stability.

Another thing you can try to physically change the geometry in the rear, and if you haven't done so, is to use adjustable camber arms to push the bottom of the king-pin out. Go up to -2.5º of camber in the rear may add enough toe-in.

[3002 tii]

Tires would be another factor... what kind are you running?

[NickyC]

Good grief, all you people doing 140 on public roads! Careful out there!

[dc_wright]

Originally Posted by NickyC

Good grief, all you people doing 140 on public roads! Careful out there!

Not me! Only on the track for me, no way I'd do that on the road.....It's risky enough in a controlled environment.

[dre99gsx]

Originally Posted by 3002 tii

Tires would be another factor... what kind are you running?

Nitto NT555 G2. Got up to almost 160 at Pocono this weekend, squirmi-ness seemed to have not been to big of an issue. I did stiffen up the rear and front suspension, but it seems the +1/8" toe each side was definitely making it worse. Going to 0 was an improvement. I may raise the rear a tad which should then give me just the right toe in needed.

[wdb]

Jiminy. Subscribed just for the education. Thanks all for sharing your knowledge and experience.

[joenationwide]

Originally Posted by dre99gsx

Good points. Interestingly, I can't apply any more toe in. I tried to go -1/16 each side, but it is maxed at 0. Car actually had +1/8" toe OUT on each side when I had gotten it a while ago.

FYI

- toe is toe out
+ toe is toe in

If you had +1/8" toe, you had toe in.
Maybe have the shop check again, you should have plenty of adjustment range in toe.

You should target 1/8" total toe in for the rear.

[dre99gsx]

Actually, I do my own alignment, and I had toe out. Since negative camber points in at the top, I've always used negative toe as "pointing in at the front", although nomenclature is +toe = point outward (yes, a bad habit!)

As stated, I can't go with any toe-in in the rear as I'm maxed at my adjustments, thus 0 toe is all she can do for now. I bet the RCAB upgrade is what affected my limited adjustment unfortunately.

[3002 tii]

Originally Posted by dre99gsx

Actually, I do my own alignment, and I had toe out. Since negative camber points in at the top, I've always used negative toe as "pointing in at the front", although nomenclature is +toe = point outward (yes, a bad habit!)

As stated, I can't go with any toe-in in the rear as I'm maxed at my adjustments, thus 0 toe is all she can do for now. I bet the RCAB upgrade is what affected my limited adjustment unfortunately.

I'm considering Poconos this Saturday with NASA, any chance you'll be attending? Would love to see this beast in person.

[dre99gsx]

Would love to go, just realized NASA holds local events...

I've only been on Motorsportsreg and will be at Pocono next weekend. Looks like HPDE1 is full for this weekend (I've been written off to Solo at a PCA event, but don't think NASA would start me at HPDE2)

Would like to meet you as well! I'll see if NASA has some additional events between now and October!

Originally Posted by 3002 tii

I'm considering Poconos this Saturday with NASA,
any chance you'll be attending? Would love to see this beast in person.

[dc_wright]

Originally Posted by dre99gsx

Would love to go, just realized NASA holds local events...

I've only been on Motorsportsreg and will be at Pocono next weekend. Looks like HPDE1 is full for this weekend (I've been written off to Solo at a PCA event, but don't think NASA would start me at HPDE2)

Would like to meet you as well! I'll see if NASA has some additional events between now and October!

Typically NASA starts you in HPDE1 your first event with them unless you've got an actual competition license. The other exception is if you're lucky enough to have an instructor in the group you're signed off in that will vouch for you with NASA.

[3002 tii]

Originally Posted by dre99gsx

Would love to go, just realized NASA holds local events...

I've only been on Motorsportsreg and will be at Pocono next weekend. Looks like HPDE1 is full for this weekend (I've been written off to Solo at a PCA event, but don't think NASA would start me at HPDE2)

Would like to meet you as well! I'll see if NASA has some additional events between now and October!

You could go 2 if you write down your history during registration. Else go 1 and ask for a checkout ride. Look into some other tracks as well, we're lucky in that we have 3 pretty awesome tracks nearby (Lime Rock, Watkins and NJMP). And tbh Poconos doesn't compare to any of those 3...