clutch postmortem

[JGood]

I'd love for this to be driver error problem. I thought taking my foot off the gas was an acceptable way to slow down. Is that not the case at 7500?

I can't think of any instance where I'd want to wind my motor above 7k, full throttle, and then just engine brake. If you want to wind out a certain gear, and not the next gear, why not throw in the clutch and shift into the next gear, then leave off? You'll still be up at 5k or so, depending on gearing.

Isn't it really jerky when leaving off abruptly at those RPM's? I'd think that would create excessive wear on almost everything... CSB, guibo, mounts, clutch, etc...

[nando]

I suppose I might let off at a high RPM during an autox, but still, it wouldn't be more than ~6500rpm. If I'm at 7500rpm in an autox, it's during a straight where I've decided upshifting wasn't worth it.

However I still can't see that causing the PP to grenade itself.

[jasapp]

I can't think of any instance where I'd want to wind my motor above 7k, full throttle, and then just engine brake. If you want to wind out a certain gear, and not the next gear, why not throw in the clutch and shift into the next gear, then leave off? You'll still be up at 5k or so, depending on gearing.

Isn't it really jerky when leaving off abruptly at those RPM's? I'd think that would create excessive wear on almost everything... CSB, guibo, mounts, clutch, etc...

Yeah, it is jerky. I think you're definitely right about it causing excess wear.

I suppose I might let off at a high RPM during an autox, but still, it wouldn't be more than ~6500rpm. If I'm at 7500rpm in an autox, it's during a straight where I've decided upshifting wasn't worth it.

However I still can't see that causing the PP to grenade itself.

Two of the straps on the last PP were bent. Bent straps, according to that link on bimmerforums I posted earlier, are caused by reverse loading of the clutch which is what happens when I engine brake. Is that it? I'm not sure. Can I explain why one strap was broken, or which happened first? Nope.

On the positive side of things, I can pull the transmission in about 45 minutes now. :)

[jasapp]

what specific pressure plate did you have? or do you not know? Just whatever IE felt like sending you?

http://store.nexternal.com/shared/St...unt2=845817208

That's a link to the pressure plate I bought. It says sachs on it. Other than that, I don't know.

[nando]

direct links to IE doesn't work, is it the "Clutch Pressure Plate - M20 heavy duty (228mm)"?

It doesn't look like they sell the Sachs HD pp anymore. Which means what you got was probably some made in china junk, but with a sachs clutch disk, or perhaps a stock 325i PP. a real sachs HD PP costs twice what they are charging.

[jasapp]

direct links to IE doesn't work, is it the "Clutch Pressure Plate - M20 heavy duty (228mm)"?

Yes.

It doesn't look like they sell the Sachs HD pp anymore. Which means what you got was probably some made in china junk, but with a sachs clutch disk, or perhaps a stock 325i PP. a real sachs HD PP costs twice what they are charging.

http://www.pelicanparts.com/cgi-bin/...h%2011%2F88%29

Is that a real Sachs 618? If not, can you point me towards one? I'll order one now. I'm tired of this shit.

[nando]

yes

notice the price :p

also, it probably would be a good idea to avoid letting off the throttle above ~6500rpm. Engine braking is fine, but with such short gearing and so much intertia at 7500 it's probably not good for it.

[nando]

BTW, the 618pp will hold 300ft/lbs with the stock M20 clutch disk, so it's more than up to the task of an S52.

[jlevie]

The bent straps on the pressure plate are from reverse loading of the drivetrain. That happens when you use the engine to deccelerate rather than the brakes. If you have the car in a lower gear and the engine at high rpm (especially with a 4.10 diff) and completely release the accelerator it is possible to generate enough reverse loading to bend the straps. The more commonly encountered case occurs when downshifting and you fail to properly rev match.

[jasapp]

The bent straps on the pressure plate are from reverse loading of the drivetrain. That happens when you use the engine to deccelerate rather than the brakes. If you have the car in a lower gear and the engine at high rpm (especially with a 4.10 diff) and completely release the accelerator it is possible to generate enough reverse loading to bend the straps. The more commonly encountered case occurs when downshifting and you fail to properly rev match.

I thought about how the 4.10 might be effecting things for awhile. Can you double check my reasoning on this?

A 4.10 differential requires 4.10 turns of input for 1.0 turn of output.
A 3.73 requires 3.73 turns of input for 1.0 turn of output.
A 4.10 produces more torque, but less output speed than a 3.73.

As the turns of input decreases, or the turns of output increase, the differential produces less torque and more speed (corollary 1). Now say you had a differential that for every 1.0 turn of input, it made 4.10 turns of output (call this differential A). And another differential that for every 1.0 turn of input made 3.73 turns of output (call this differential B). By corollary 1, differential A would create less torque and more speed than differential B.

Now apply that to my case where the momentum of the car is turning the wheels and applying force backwards through the drive train to the clutch. A 4.10 would actually be applying less torque to the clutch than a 3.73 would be.

Right?

[JGood]

I thought about how the 4.10 might be effecting things for awhile. Can you double check my reasoning on this?

A 4.10 differential requires 4.10 turns of input for 1.0 turn of output.
A 3.73 requires 3.73 turns of input for 1.0 turn of output.
A 4.10 produces more torque, but less output speed than a 3.73.

As the turns of input decreases, or the turns of output increase, the differential produces less torque and more speed (corollary 1). Now say you had a differential that for every 1.0 turn of input, it made 4.10 turns of output (call this differential A). And another differential that for every 1.0 turn of input made 3.73 turns of output (call this differential B). By corollary 1, differential A would create less torque and more speed than differential B.

Now apply that to my case where the momentum of the car is turning the wheels and applying force backwards through the drive train to the clutch. A 4.10 would actually be applying less torque to the clutch than a 3.73 would be.

Right?

I could be wrong here, but I don't think it's the gradually engine braking that is causing the clutch damage. I think it's the abrupt lifting of the throttle, going from one extreme twisting force, to one in the opposite direction (generated by engine compression vs. car momentum). It's that instantaneous 'bucking' that I would think would do damage, if anything.

I don't think the reverse torque load placed on the drivetrain after everything has stabilized, and the engine is braking, should be taken into account. It seems like forward torque which is then reversed by that instant lifting is the problem. And with a 4.10 you have that much more forward torque in any gear.

But, I am no expert... and could be completely wrong.

[jasapp]

I've ordered a 618. It should be here Friday.

[Jason89i]

i would not peg it, running right up to red line and then just lift in second or third gear. I must admit, i do it regularly at 7800..... but im in 4th gear and my car revs to 8300. i dont ever think ive throttle lifted above 6000 in second or 7000 in third.

think of it this way......

engine produces 250rwhp at high rpms. engine also produces -60rwhp (throttle lift) at high rpms. (the higher the rpm, the higher the force.) with a 4.10, (im guessing you were in 2nd or 3rd) the car accellerates faster AND decellerates faster due to gearing. by lifting at 7500rpm (in a low gear) you are creating a delta ~310rwhp. by lifting quickly, you are also requesting inertia forces to change direction QUICKLY.......whiplashing the entire driveline.

with that said. go get a good clutch pp and disk. if your going to be driving like that, get something WITHOUT a sprung hub ment for the track.

i have heard that the sach clutches are worthless now. (hear say..... no experience.)

cheers, jason

[nando]

if he can break a 618, I'll be impressed. Who knows who the manufacturer of that IE kit was.

but yeah, I'd avoid the sudden throttle lifts, and mismatched downshifts, if you want it to hold together longer than 4 days. the clutch aside, it's harder on everything - the crank, transmission, driveshafts, etc. Plus it unsettles car car and could cause a spin.

[jasapp]

The new clutch should be here Friday. I'll avoid taking my foot off the gas suddenly, and I think I'll just avoid taking the engine over 7k rpms period. I imagine I'll also pull the 4.10 and see if I can find a 3.48 locally.

Thanks for the help.