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Discussion Starter #1 (Edited)
Had to happen I guess. Took the car to the local 1/8mi track this evening with a couple buddies. 1st time the car has been to the drag strip.

1st run: Bunny-hopped 1st & 2nd gear so bad I had to just let off the gas.
2nd run: good launch, smooth, but when I floored-it at about 30ft it bunny-hopped again and A LOUD POP. I limped off the track and inspected the drivers side rear axle and it was twisted in the outer boot (closest to the wheel).

My buddy trailored me & the M5 home. I'll get a tow to my local shop in the morning to have the axle (or just the CV joint?) replaced.

My question to you that have a similar setup to mine (ie. stiff suspension, strong clutch). Is there a beefier/stronger axle for us? Is that the solution, or once the axle is upgraded does that just mean the drive shaft is next to twist?

I am going to be talking to a few of the board sponsors and getting their opinion, but appreciate any input from those of you that have had similar problems (ie, lots of torque, but due to the upgraded clutch and suspension...not ANY "give" or flex).

Robert
 

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I'm afraid that you just demonstrated one of the reasons that BMW puts a weak clutch in the E39 M5. You have also demonstrated why heavy, high-powered drag cars do not have IRS. C2-C4 Corvette drag cars are retrofitted with solid rear axles (with Ford 9 inch center sections). A solid rear axle does not have this problem, but its street and road course handling leaves a lot to be desired.

Regards, Dick Roberts
 

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Paul,

He is speaking of the dreaded "axle hop" where the tires loose traction and the back end gets a bit hoppy.

1bad, you might want to contact Shadowman on this one...only person i know of that would have contacts to get beefier parts. God knows we talked about it on my car.
 

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1badm5 said:
...stiff suspension, strong clutch).
Man, I was about to laugh and then I read your sig and saw the UUC stage 3. Ok, strong clutch is covered in spades. :biggrin:
As others have stated, something has to give. Usually it is the tires, but when they hook up and the clutch doesnt go either the subframe or smaller componets take a beating. Not really a car designed for drag racing. Heck, for $5000 you can find an old F-body that will be just as fast.
:cheers:
 

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BmwNut said:
Usually it is the tires, but when they hook up and the clutch doesnt go either the subframe or smaller componets take a beating. Not really a car designed for drag racing. Heck, for $5000 you can find an old F-body that will be just as fast.
:cheers:
Absolutely. Much like the Z3 that notoriously busts frames from wheelhop, the M5 driver needs to carefully modulate the throttle and clutch pedal to avoid wheelspin and wheelhop on the M5.

A strong clutch is always advantageous for hard launching, since it is capable of withstanding heat caused by slip. A driver can safely hold a heavy duty clutch in a sustained slip condition in order to maintain RPM as the car accelerates, without tireslip or wheelhop. This can not be done with an under-rated clutch. Drivers with weak clutches learn to "sidestep" (immediately engage) a weak clutch to minimize slip-induced heat. The hap-hazard clutch-dropping technique results in unopredictable wheelspin depending upon grip. Under condition of good traction, the clutch slip time is automatically extended because the clutch is incapable of transmitting enough torque to induce wheelspin. This "saves" the drivetrain, even when the driver is not modulating the clutch.

With a strong clutch, sudden "clutch-dropping" techniques must be quickly broken...or the drivetrain and/or frame may become broken.

Drivers with strong clutches who find themselves using too much throttle or engage the clutch too quickly (causing wheelspin/wheelhop) should seriously consider adding a CDV to the slave cylinder. Since wheelhop is difficult to predict under wheelspin conditions (varies depending upon road surface), wheelspin should always be avoided.

In 5 years of ownership, I never once churned the tires on my Corvette from a standing start. IMO, aluminum halfshafts & lightweight suspension componentry (optimized for handling) do not deserve such treatment.

With a strong clutch, churning tires at launch is purely the driver's option. Aside from glare ice conditions, the root cause of tireslip is -> clutch and throttle modulation techniques used by the driver.

I agree with the recommendation above...a $5K solid axle ponycar is the proper tool/toy for such duty. Anybody who installs a strong clutch in their M5 thinking they can go around churning fresh sticky tires (like kids in ponycars) is taking a financial risk.

The real beauty of a strong clutch is: 1) it's ability to absorb heat from sustained slip and engage upon request 2) able to quickly engage on upshifts and 3) long life.
 

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Discussion Starter #7
Good news. I have a new rear axle coming to me from BMW ($240). No other damage found. I spoke to various board sponsors and after much discussion the concensus is it may very well had been a faulty CV joint that found a great time to "blow-out". Showdowman was quite surprised and had not seen that happen on even the high HP M5's w/ stage III 6-puck ceramic clutch. I will have the new CV joint installed Monday and be back in "business" again!

Thanks LSCMAN for your insight. I you know your stuff! I believe I need many more launches with this clutch before I get as effecient as I need to be with it.

Paul, a previous post pretty much explained "bunny-hop". When I accellerated hard (from dead stop) the track was so sticky it allowed for NO wheel spin. The rear-end "hopped" violently. Because I had no: 1) clutch slipage 2) wheel spin, something somewhere had to "give". Hence the broken rear axle.

Next on the mods: Custom built 3.45 diff with modified slip differential...that stops this!!

Robert
 

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Discussion Starter #11
Re: Situation change

The situation has gotten a little more comlicated. Upon furthur inspection we found that the rear (upper) rubber mount pulled THROUGH THE BOTTOM. That's right...(due to the violent bunny hopps) the (OEM) upper rubber mount was pulled all the way through the bottom), leaving the strut simply "dangling" loose. And we think that is what caused the CV joint failure. That would certainly make since. Due to the wheel/shaft NOT being supported (by the strut), the CV joint was put in an awkward position and "twisted-out". Upon consultation we have decided the OEM CV joint (rear outer axle) is very suffecient to to the job.

Another result of the torque presented to the driveshaft with "dropping" the clutch at 7k rpm with the 2nd to 3rd shift (and no clutch slippage) is it puts the rear portion of the MAIN drive shaft (where it meets the rear differential) CV joint out of position and I lost the boot and hence the grease in that CV joint.

I'm on the phone with the "specialist" (shadowman) this morning. I plan on:
1) replacing the (rear) main drive-shaft CV joint. Replacing with OEM or one of shadowman's I won't know until I consult with shadowman.
2) replacing the rear axle CV joint (drivers side). Will replace with OEM
3) securing the upper rubber mount on the rear (drivers side), preventing the rubber mount from pulling through. This upper (rubber) mount is the only part of the KW Coilover system that remains OEM (per KW).

I will update later today after I talk to my mechanics and shadowman...

Robert
jeeperjohn said:
Did the CV joint fail or the axle shaft?
 

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Re: Situation change

With all due respect, why would you launch and shift an M5 this hard? If you read past threads, many folks, myself included can get a 12.9 1/4 mile with tender use of the stock clutch. Beating the hell out of your car will, at best, gain just a bit of time. My fear is that as you keep strengthening each part of your car, you will just reach the next "weak" point and another more costly repair. JMHO, but as wonderful as these cars are to drive, they are not 1/4 champs and certainly not suited for an 1/8 mi strip.
 

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Discussion Starter #13
Re: Situation change

Duelly noted. I had already made up my mind that this car IS NOT A DRAG CAR...hence it will NEVER...EVER see another drag strip! This car is being modified to my particular driving style and taste. I will also enjoy the auto-x track tremendously.

Robert

TexasM5 said:
With all due respect, why would you launch and shift an M5 this hard? If you read past threads, many folks, myself included can get a 12.9 1/4 mile with tender use of the stock clutch. Beating the hell out of your car will, at best, gain just a bit of time. My fear is that as you keep strengthening each part of your car, you will just reach the next "weak" point and another more costly repair. JMHO, but as wonderful as these cars are to drive, they are not 1/4 champs and certainly not suited for an 1/8 mi strip.
 

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Re: Situation change

1badm5 said:
The situation has gotten a little more comlicated. Upon furthur inspection we found that the rear (upper) rubber mount pulled THROUGH THE BOTTOM. That's right...(due to the violent bunny hopps) the (OEM) upper rubber mount was pulled all the way through the bottom), leaving the strut simply "dangling" loose. And we think that is what caused the CV joint failure. That would certainly make since. Due to the wheel/shaft NOT being supported (by the strut), the CV joint was put in an awkward position and "twisted-out". Upon consultation we have decided the OEM CV joint (rear outer axle) is very suffecient to to the job.

Another result of the torque presented to the driveshaft with "dropping" the clutch at 7k rpm with the 2nd to 3rd shift (and no clutch slippage) is it puts the rear portion of the MAIN drive shaft (where it meets the rear differential) CV joint out of position and I lost the boot and hence the grease in that CV joint.
Robert
The condition you describe may be what happens just before the frame fails. Once the strut pulls through the bushing, I suspect many components see unintended loads. I wonder if it might be a good idea for folks to check the condition of these bushing mounts on a regular basis. I am not exactly clear about which strut pulled through a rubber bushing.
 

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Re: Situation change

Lscman said:
The condition you describe may be what happens just before the frame fails. Once the strut pulls through the bushing, I suspect many components see unintended loads. I wonder if it might be a good idea for folks to check the condition of these bushing mounts on a regular basis. I am not exactly clear about which strut pulled through a rubber bushing.
Sounds like an upper shock mount.

Regards, R. Roberts
 

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On my old E28 I had ripped the center section mounts out of the car.
The thing was hanging there, till it broke that last inch of steel.

I found it funny on the tread about valet parking a car, that no one wanted to do it but we abuse or own cars even harder.
I'm glad we drive our cars un like the guy in the CL65 AMG biturbo that can't do greater then speed limit.
 

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Painful lesson learned, I guess ;) Autocrossing is particularly interesting in a M5. I think you and I are just about the only ones to try it.
 

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Discussion Starter #18
Yep...nothing ventured nothing gained! This is going to cost me about $400 in parts, and $200 in labor...I've definately paid more for lessons! Hey, you live on the edge, sometimes you get cut a bit....part of it! I wouldn't change a thing!~

Robert

TexasM5 said:
Painful lesson learned, I guess ;) Autocrossing is particularly interesting in a M5. I think you and I are just about the only ones to try it.
 

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TexasM5 said:
Painful lesson learned, I guess ;) Autocrossing is particularly interesting in a M5. I think you and I are just about the only ones to try it.
I've autocrossed junkers & luxury cars hundreds of times. I won FS class more than once in these oversized, oddball V8 vehicles and never broke. For minimum lap times, wheelspin & resulting wheelhop should always be avoided. I find this to be true for drag racing, road racing and autocross. "Looking fast" (high slip rate/severe traction loss) is generally what breaks hardware.
 

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1badm5,

I have a couple of questions:

How often do you launch your car hard?

What condition are you tires in?

How wide are you tires?

Was there anything sticky on the track?

What rpm's did you launch at?

What tires are you running?
 
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