Clutch failure - looking for possible causes

[NigelStu]

Had our first ChumpCar race of the year at Road America in April, and unfortunately clutch issues plagued us all weekend, taking us out of contention both days. I am looking for possible reasons why the clutches (yes, plural, 2 of them) failed, so I can correct it before the next race.


The car started the race with a NEW Sachs clutch and pressure plate, throwout bearing, spring and pilot bearing. These were installed into a engine/trans/driveshaft/LSD that all came from a running/driving '89 325is. Trans received new seals and filled with Redline MTL. A new CSB was fitted, the original clutch master/slave from the '87 es were used. Everything went back together OK, and I noticed no issues when driving the car around the block when loading the car on the trailer.


The first driver had no issues with the car until about 1.5 hours into his stint, he came on the radio and said the clutch pedal was stuck on the floor; driver indicated hearing a loud clank noise when this happened. He was able to pull it back up but couldn't get into any gear and ultimately had to be towed in.

Upon removing the trans, we found this:



We found a spare used clutch disc/pressure plate, installed it, bleed the slave and ran a few more hours on Saturday, along with ~4.5 hour on Sunday until the spare had what we are expecting the same issue: pedal went to the floor, driver was able to get it back up and find gear, but clutch was slipping and he again had to be towed in. I haven't pulled the trans yet, but expecting to find bits and pieces of clutch material all over the place.


So - knowledgeable people, what would be likely causes? What should I be looking for? What do I need to check/measure for runout/broken stuff/etc.? Keep in mind this setup came out of a car that seemed to be working just fine on the street and the first clutch had less than 2 hours of run time. Data acquisition showed that none of the drivers were abusive to the car/clutch.



List of possible causes:
-Pilot bearing
-flywheel uneven install / warped
-PP warped
-excessive heat in disc (got this one from a Sachs clutch engineer)


TIA

[LJ851]

Is there a pilot bearing in the end of the crank?

[Dominic49]

The clutch disc was over-reved and caused the facing to burst.

This was caused by prematurely placing the vehicle in too low of a gear when the wheel speed was still high. ie traveling into a 2nd gear corner after a long straight and putting it into 2nd gear with out first slowing the vehicle.

100% driver error


source: I am the warranty/tech dude at a international clutch company.

p.s. if you want to send stuff to me to have it checked out shoot me a PM I can test it all out and make sure everything is 100%

[NigelStu]

LJ - yes - replaced with a new one. Although, it is on my list of things to check to make sure it is still in the right position and actually holding up the end of the trans input shaft.


Dom - Don't think that is it. Drivers were all rev-matching downshifts (per instruction, be kind to the car for the 14 hrs of racing), data did not show any over-revs. When it happened the 2nd day, the driver did a 5-4 downshift, slowing from ~115 -> ~85 before gear shift and then a 4-3 downshift at ~45 mph, which is when the pedal stayed on the floor.


Keep 'em coming.

[TurboJake]

Possibly the flywheel has been machined unevenly, flexing the disk in oscillation directly related to the engine RPM as it will never be correctly centered to the transmissions input shaft.

Enough of that and bam, she gives. Everything will look and function normally, until shit breaks.

Uneven mating and overall clutch alignment gets my vote. It could also be the transmission itself being misaligned somehow.

[NigelStu]

Turbo - are you thinking like the fly wheel face isn't perpendicular to the crank/transmission shaft? I could see that as a possibility if it would create uneven loads on the clutch disc. Flywheel itself had not been machined prior to re-install (it looked OK, no obvious warps, cracks or uneven wear) and had been on a car with a used clutch with no issues - granted that was street driven so RPM range was generally at the low end vs. at 4k and above on the race car...


Anyone else have some good ideas on what to check/look for? Would the front bearing on the trans shaft be a possible/likely failure item? Or something else internal to the trans that decided to show up at constant high RPMs?

[Dominic49]

LJ - yes - replaced with a new one. Although, it is on my list of things to check to make sure it is still in the right position and actually holding up the end of the trans input shaft.


Dom - Don't think that is it. Drivers were all rev-matching downshifts (per instruction, be kind to the car for the 14 hrs of racing), data did not show any over-revs. When it happened the 2nd day, the driver did a 5-4 downshift, slowing from ~115 -> ~85 before gear shift and then a 4-3 downshift at ~45 mph, which is when the pedal stayed on the floor.


Keep 'em coming.

just because the engine wasn't over reved does not mean that the input shaft wasn't you can put the car in 1st gear at 100 mph and even if you dont let go of the clutch pedal, the disc will still be traveling over 20,000rpm with a stock tire of 195/65-14.

http://www.schaeffler.com/remotemedien/media/_shared_media/library/automotiveaftermarket/brochure_1/downloads_5/luk_schadensdiagnose_pkw_en.pdf

[NigelStu]

Thanks for the link to that PDF - interesting information. Although, it looks like the results of the failure are more like issue #18, drive plate segments sheared, not #19, burst facing. Take a closer look at the picture in the first post, facing material is basically whole, no separation at the rivits like in the PDF picture. Some of it still attached to backing plates, which are all separated from the hub. Granted a lot of it probably got chewed up more when the driver tried to get the car moving again.


Also -additional information from the driver.

"...it happened while the clutch was on the way down.

The feel was as if I were stepping on a metal rod lengthwise, and it snapped in half. Sudden significant decrease in resistance on the clutch pedal, along with an audible metallic snap / failure noise that I also felt through the pedal."

Driver also confirmed the order of action was - fully depress clutch pedal, then move shift lever. Failure happened with clutch release, not changing gears.


Any more ideas with these details?

[NigelStu]

2nd busted clutch disc out

Finally got around to pulling the trans. I'm even more perplexed than before since what I found was a disc that has no material left on the pressure plate side, and full material that still looks good on the flywheel side...


Pics:
Flywheel side


And pressure plate side


The offending paired pressure plate is the one in these pics.



One item to note, the input shaft on the trans has some movement perpendicular to the shaft (up down left right). How much is an allowable amount?