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Discussion Starter #1
This weekend I was showing a friend how the DSC worked on the M5. First I turned the DSC off and took off from a dead stop burning rubber through first gear. Then I turned the DSC back on and did another start and of course it prevented any wheelspin and the car accererated up to 30mph or so in first. Afterwords the clutch felt mushy for several miles with a strong burning clutch smell coming from the car. The clutch worked fine but just smelled. The next day the smell was gone and the clutch seems to be fine. Do you think the clutch was damaged or has some sort of problem ?

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hey park

man, you had the same EXACT thing happen to you that happened to me. i dont think you messed up the clutch, but i think you can only do that a couple of times before you hace have a completely burned clutch. It really sucks that this happens, you cannot exploit the M5, and be hard on the clutch, you have to baby it. Granted that you get the best startoffs without wheelspin, and very little clutch burning, but that shouldnt prohibit you from doing it. do it a couple of times, and your clutch is toast.

fas
 

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Parkerfe,

I've posted at least three times that if your going to do the drag race thing to turn DSC off, otherwise all the torque and 400 hp has no place to go except burn the clutch. With DSC braking to avoid wheel spin the load has to go somewhere. Let the rear wheels spin, burned rubber's better than burned clutch.

I've heard a number of posts of burned clutches and the only thing new in this car is the DSC, and that IMHO is whats causing the problem.

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Jim
07/00 M5 Titanium over Red



[This message has been edited by Jim Dolan (edited 30 October 2000).]
 

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That's how my clutch problems started. I *STRONGLY* advise you against performing such activities.

Honestly, you really don't want to go through the whole clutch-replacement hassle with BMW. It's a waste of more than a month's waiting and multiple replacements, dealers being confused, etc.

I used to burn it a bit too, but then i found that it's far more entertaining to realize that you're driving your average freeway speed (which feels very comfortable) and see that everyone else is so slow. What do you know, you're doing about 95-100...AVERAGE.


One of these days...it's gonna get impounded. Enjoying it until then!

--Dan
 

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I think this makes sense...

As I understand it, the DSC system has hooks into engine management. Under high acceleration conditions (such as drag racing), DSC may detect the rear wheels begin to lose traction. DSC initially tries to remedy the situation first by backing off power delivered from the engine. It does this first by retarding the timing... if that's not enough, then it'll completely cut the ignition to the engine. Considering the way it must be programmed, under acceleration conditions, it won’t apply brakes to the individual rear wheels until the engine power has been properly “managed”.

One can imagine that when we drag race, the timing of all of this tends to blur together… Between shifts there's a heck of a lot of inertia in the upwardly spooling engine. We load it into the next gear (or, for some us, dump the clutch from a stand-still.) …DSC may try to slow (or cut-off) the engine, but the RPMs are still very high. There’s probably dynamics in the DSC system where it goes ahead and applies brakes while there's still tons of horsepower/torque running through the driveshaft. I think when that happens, the clutch quickly becomes the weakest link... foul smell… worn disk… burnt flywheel…repair…

Originally posted by Jim Dolan:
...otherwise all the torque and 400 hp has no place to go except burn the clutch. With DSC braking to avoid wheel spin the load has to go somewhere.
 

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Guys - this comes up so often and it just kills me. Sorry for ranting, but here goes:

Jim - with all due respect, the DSC doesn't work that way - it does not brake when the rear wheels slip under acceleration. It reduces the throttle. DSC DOES brake, but only to correct the PATH of the car if you're pointing somewhere other than where it is going.

SO - DSC will NOT affect the clutch. Poor technique will. DSC will keep the wheels from slipping, but by reducing the throttle. DSC does NOT know when the clutch is slipping and it won't rescue you from this!

Note that wheel slip IS OK - it only wears out your tires.

The way you screw up the clutch is to launch or shift and allow the clutch to slip while transmitting a lot of power. (IMPORTANT: <font color=red size=+2>YOU, AND ONLY YOU</FONT> ALLOW THE CLUTCH TO SLIP. IT DOES NOT SLIP BY ITSELF UNTIL AFTER YOU BURN IT UP.)

On launch the clutch pedal should be fully out before you reach 1800 RPM AND before your foot hits the floor. If you WANT to burn rubber you can, but let the WHEELS slip, not the clutch. For an agressive 1-2 shift, the clutch should NOT be feathered AT ALL. Side-step it if you want.

Your 1-2 shift should sound like 2 distinct notes: 1st is a higher note, 2nd is a lower one. It should NOT sound like sliding from one note to the other on a trombone.

Once you've burned the clutch to the point you can smell it, you have created a hard surface glaze on the clutch liner. It will now slip easier than it used to, so you have to be even MORE careful.

The clutch is not designed to absorb 400HP in slip - it is designed to TRANSMIT 400hp when fully engaged. Big difference.

Note that if you really spin the wheels in 1st, you'll hit redline at, say, 20mph instead of 40. So now when you shift to second, you're asking the engine to drop from 7000RPM to whatever 20mph is in second - what, 2300RPM? That's a huge drop. If you had let the tires hook up fully in 1st, then when you shifted the drop would be much less (what is is supposed to be.) The side-effect of spinning the tires to redline is that it makes the 1-2 upshift much more demanding and it is much more likely you WILL slip the clutch excessively to absorb all that difference.

OK< I'm tired of this now.
 

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Thanks for the post Greg. I for one wouldn't mind hearing more on shifting and clutch technique for the M5. I don't have one but hope to get an E46 M3 in March and I think much of the same will apply, given it's the same trans / clutch combination.

So here are the questions:

- Best "race start" clutch technique. what RPM, and how fast to let out the clutch? Anything else?

- Best "smooth start", ie in traffic but still nice to the clutch. What rpm and how fast to let out the clutch?

- Best 1-2 "race start" ... are you saying just slip the foot sideways and let the clutch pop back up? Is that side step?

- best 1-2 "smooth start" What rpm and how fast to let out the clutch?

Sorry for all the questions. I'm expecting to go back to driving standard even though it's been over 10 years since I last drove one. I managed OK but my selection of cars was kinda limited... 1958 Dodge tow truck, 1973 Chevelle 3 on the tree, and a motorcycle. I did burn the clutch on one of them (Chevelle) although the fact that the rear seal was leaking into the clutch housing probably did not help.

Hope to spend some time at a driving school first once allocations are decided. Thanks again. Suggestions appreciated.
 

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Discussion Starter #8
Originally posted by greg:

Guys - this comes up so often and it just kills me. Sorry for ranting, but here goes:

Jim - with all due respect, the DSC doesn't work that way - it does not brake when the rear wheels slip under acceleration. It reduces the throttle. DSC DOES brake, but only to correct the PATH of the car if you're pointing somewhere other than where it is going.

SO - DSC will NOT affect the clutch. Poor technique will. DSC will keep the wheels from slipping, but by reducing the throttle. DSC does NOT know when the clutch is slipping and it won't rescue you from this!

Note that wheel slip IS OK - it only wears out your tires.

The way you screw up the clutch is to launch or shift and allow the clutch to slip while transmitting a lot of power. (IMPORTANT: <font color=red size=+2>YOU, AND ONLY YOU</FONT> ALLOW THE CLUTCH TO SLIP. IT DOES NOT SLIP BY ITSELF UNTIL AFTER YOU BURN IT UP.)

On launch the clutch pedal should be fully out before you reach 1800 RPM AND before your foot hits the floor. If you WANT to burn rubber you can, but let the WHEELS slip, not the clutch. For an agressive 1-2 shift, the clutch should NOT be feathered AT ALL. Side-step it if you want.

Your 1-2 shift should sound like 2 distinct notes: 1st is a higher note, 2nd is a lower one. It should NOT sound like sliding from one note to the other on a trombone.

Once you've burned the clutch to the point you can smell it, you have created a hard surface glaze on the clutch liner. It will now slip easier than it used to, so you have to be even MORE careful.

The clutch is not designed to absorb 400HP in slip - it is designed to TRANSMIT 400hp when fully engaged. Big difference.

Note that if you really spin the wheels in 1st, you'll hit redline at, say, 20mph instead of 40. So now when you shift to second, you're asking the engine to drop from 7000RPM to whatever 20mph is in second - what, 2300RPM? That's a huge drop. If you had let the tires hook up fully in 1st, then when you shifted the drop would be much less (what is is supposed to be.) The side-effect of spinning the tires to redline is that it makes the 1-2 upshift much more demanding and it is much more likely you WILL slip the clutch excessively to absorb all that difference.

OK< I'm tired of this now.
Thanks. Should I go ahead and have the clutch replaced due to the glazing or wait til it starts slipping?



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hey greg
thanks for the info. unfortunately anytime i do anything half aggressive i get the "trombone" effect, and not the distinct noise. I think this is due to the glazing you reffered to. The unfortunate part about this is that i have only slipped the clutch hard once, maybe twice, and even then it is due to the spinning wheel effect, and not dropping the clutch at high revs. But dont you agree it is a shame that once or twice, and that's it, your clutch is slipping. As is said, i now cant do any hard shifting, just get the trombone.

fas
 

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FAIS - You need a new clutch. Sorry you had to learn the hard way, but you do.

Yes, it's a shame, but at these HP levels it is a fact of life. (My 40hp Volkswagen would tolerate a lot of slip-clutch shifts!)

I am not completely sure I understand what you mean by "...and even then it is due to the spinning wheel effect, and not dropping the clutch at high revs." - but let me say it again - the fact that your wheels are spinning is independent of whether the clutch is slipping at the same time. You can have a fully engaged clutch and still have wheel spin.

/Greg
 

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Parker -

If your clutch doesn't slip when your foot is OFF the pedal and you floor it, you don't HAVE to get a new clutch - however you need to avoid drag launches and slip shifts because it WILL slip more easily now and it can only get worse. It won't do any damage to other parts of the car. (Once it starts to slip with the pedal out, though, you can create a lot of heat on the flywheel and damage it by warping or heat-treating it.)

You may also find that it is harder to make a normal launch smoothly - sometimes once the clutch is glazed it will buck on normal engagement.

Cheers -
 

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is it the high torque, high power or high revs that destroy the clutch or they do it altogether?

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SL
 

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Originally posted by Mr. Grinch:


So here are the questions:

- Best "race start" clutch technique. what RPM, and how fast to let out the clutch? Anything else?


I won't say I know how to get the absolute best accel out - 4.7 0-60 like Motor Trend - but you can get a damn acceptable launch as follows:

1) Throttle to about 1500 RPM
2) On "GO", let the clutch out at about the same speed as you put the throttle pedal down - but make sure the clutch starts to engage before you add throttle. Whole thing should happen in about 1/2 second.

Shift 1-2 at about 6500RPM (to be sure you don't run into the limiter) and bang the shift. Clutch should be in and out, no feathering. The wheels should chirp.

- Best "smooth start", ie in traffic but still nice to the clutch. What rpm and how fast to let out the clutch?

Same as above, but start from about 800 RPM, lengthen the "feathering/feeding" to about 1-2 seconds. You CAN feel when the clutch is fully engaged - be sure to use very small amount of throttle until you're hooked up.

- Best 1-2 "race start" ... are you saying just slip the foot sideways and let the clutch pop back up? Is that side step?

I don't slip my foot sideways only because I just don't like to beat the car quite that hard - but I sure do lift my clutch foot up in a hurry.

best 1-2 "smooth start" What rpm and how fast to let out the clutch?

Totally depends on conditions and rate of acceleration. Whatever feels natural. Remember, a Ferrari 360 Modena has almost no torque below 4000RPM - you really want to let it wind up and keep it wound up to get performance from it. The M5 has prodigious amounts of torque from 2000RPM, and considerable amounts even at 1500. Try rolling along at 1500RPM in 1st (clutch out) and jump on it. You'll see. The most important thing is to "feel" the full engagement of the clutch before adding additional throttle.

Hope that helps.
 

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Because there is less mechanical advantage provided by the transmission, the rotational forces on the clutch will be highest when "The Beast" is floored in 6th gear. If it doesn't slip there, you should be okay... for a while.

Originally posted by greg:
Parker -

If your clutch doesn't slip when your foot is OFF the pedal and you floor it, you don't HAVE to get a new clutch...
 

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Be careful- get it inspected immediately. I had a problem when I first got my M5- it seemed like this slipping problem, there was smell, etc. Then the clutch pedal was stuck in for a little. Then it popped back out. turned out I had a bad master cylinder. It was fixed under warranty but only when I woke up one morning to find a pool of liquid under the car and the pedal all the way in. They had to flat bed it. So hopefully its just the clutch. But to be safe, get it checked. Btw- Ive never "dropped" the clutch, but if I engage it around 1k rpms and then put the foot down..... wow!

ME
 

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hey greg
when i said wheel spin effect it is when you reffered to the wheels spinning and therefore you are moving slower and so the revs need to drop more, so you end up burning more clutch. I have only once or twice tried a 2k drop that would burn the clutch. But my question is what about the other cars that are torque monsters, maybe like the vette? I dont hear people on the vette forum complaining so much about their clutches burninig up at 9k miles? And i know i dont abuse my clutch out of the norm. I dont think i have perfect clutch technique, but i do know that i dont outwardly and advertantly burn my clutch, and yet still it is blazed and glazed like a donut
. that's damn unfortunate.

fas
 

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FAIS - you're right - other torque monster cars are probably a bit more tolerant - but sooner or later allowing slip to the point of bad smell will ruin any of them.

Cheers
 

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Parkerfe's words

"Then I turned the DSC back on and did
another start and of course it prevented any wheelspin and the car accererated up to 30mph or so in first."

greg's words

"DSC DOES brake, but only to correct the PATH of the car if you're pointing somewhere other than where it is going."

Greg, I agree with virtually everything you say below but offer the following.

Very often when drag racing the rear end yaws to the left and to correct this we steer to the left slightly. Perhaps DSC senses this yaw and corrects with braking.

I don't know everything about this, but there have been a lot of incidences of burning clutches including the one at SC I drove. I never burned a clutch in my life, including this one I have now. I know very well how to drive a stick with a powerful engine and this baby is different and I think its DSC.

I will avoid drag racing with DSC engaged and let the wheels spin, the car yaw and correct it in the usual way.



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Jim
07/00 M5 Titanium over Red




[This message has been edited by Jim Dolan (edited 30 October 2000).]
 

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I believe Jim pegged it: 1) The DSC yaw control asserts rear wheel braking; 2) the rotational inertia of the engine and drive train -- even with throttle attenuation -- is sufficient to cause accelerated clutch fatigue; 3) field stress tests should be performed with the DSC mode de-asserted.
 

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I have to disagree with the thought that DSC is braking under straight line acceleration. Unless you go to the skidpad in Spartanburg, or drive where there is snow or ice, you have to throw your car HARD into a corner before you'll feel DSC apply the brakes,. But when it does, you can HEAR it and you can FEEL it. I am about 98% certain the DSC is NOT applying any braking during straight-line acceleration - it is only reducing the throttle.

I don't believe there is any negative effect (in terms of clutch wear/abuse) in leaving DSC on, though it sure isn't the fastest way out of the hole!

But - I agree to disagree, you might be right.
 
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