clutch autopsy

[hoveringuy]

Took the car to the dyno a few months ago and one of the new employees insisted he drive the car onto the dyno. "I get paid to do that..."

Ass-clown has all 4 wheels snug against the rollers and he's trying to ease it up on top of them. Instead of backing up a few feet and getting a running start, he rides the clutch mercilessly until it's smoking..

Fast forward to this month and the car is shifting like crap, the clutch engagement is weird and it sounds like canvas rubbing on canvas.

Removal shows that all rivets have quit. Melted???

Flywheel and pressure plate are both fine, although I will replace them. I thought a lot of S50's were running the OEM Sachs clutch?

Edit: Consensus seems to be that the rivets failed from mechanical stress, not from heat..

[jlevie]

I see no evidence of the heat damage that it would take to melt out the rivets. It looks to me more like the rivets tore out of the clutch discs and the spinning discs ate off the flanges of the rivets. The usual cause of that is repetitive damage from too weak of a clutch for the engine and/or driving style. It can also be caused by a weak or hanging pressure plate. It certainly would not have happened from one instance of slipping the clutch.

[hoveringuy]

The rivets are aluminum, I figured that they annealed from the heat or something.

You're saying repeated mechanical stress.

Anyone else ever seen this?

[John T]

How thin are the two friction disks? If they are supper thin; normal wear (perhaps accelerated by tq/clamp load ratio) thinned the clutch into range of the rivet heads. Once one rivet goes it's over - snowball.

Or

Engagement shock broke a rivet with the same end result, once the disks free from the hub; wear multiplies due to the introduction of the inside faces wearing against the hub.

Chicken/Egg?

Most of the time "riding" a clutch or insufficient clamp load will cause severe bluing and grooving on the steel surfaces - flywheel/press plate.

Appropriate clamp load but insufficient friction rating on the clutch material will cause it to "shred". Tq/clamp load ratio exponentially affects the "shredding" rate. In the event of a lightly over loaded combo; the clutch just appears to wear quicker. On the other side of the spectrum, I've removed clutches that are shredded so bad the clutch spins freely inside the clamped fly/press-plate assembly. As a free gift of this scenario you also get a baseball sized dusty hairball of fibrous material.

Some of this may sound like I'm shooting my self in the foot;
A properly matched clutch assy to power output senario is basically a slowly "shredding" set up due to the ability to efficiently sync flywheel/input shaft speeds.
The inability to sync those two efficiently basically ends up in the wear of the weaker part
Clamp load does not equate to holding capacity
The calculation of clamp load + friction rating = holding capacity

Any holding capacity is attainable; it's the operation/engagement characteristics that need to be determined.

For instance in a front wheel drive high horsepower drag car I will use an extremely heavy plate with a soft friction material resulting in a stiff but slip(shred) able clutch, in this case the friction material is to be sacrificed.

Still to be taken into account is drivetrain wieght, wheel diameter, engine isolators, and everything else that contributes to engagement shock; a topic still to be touched

I fear I've begun to rant about this topic, I am OCD
I've also professionally and personally been through a lot of clutches.

[John T]

After looking at the pics closer;
the larger than normal rivet holes that are torn all toward the same direction indicate engagement shock breaking rivets and tearing them out.
The first rivet could have broken a year ago for all we know, a few more on the dyno, and the last one a few days ago.


I'm broke so this is the closest thing to 2 cents i have.

[hoveringuy]

The disc appears to be worn only slightly from an OEM disc I compared it to, maybe 1/32" per disc.

The holding power seemed to be sufficient, I never had a slipping clutch. This car saw 100+ passes on the dyno and 8 passes on the 1/4 mile strip. Plus, even though it's a daily driver, the torque just makes me giddy and I can't help myself sometimes. Ok, often times.

I thought a lot of S50's were running stock, OEM clutches?

Another factor is that I had switched from 195 to fatter 245 rear tires. I am now back on 225. Where I used to get wheelspin I converted that to stress on the clutch, I guess...

[John T]

Why you gotta break stuff??

[John T]

At least the rivets held in this one (not my pic)

[hoveringuy]

Why you gotta break stuff??

Because it's fun??

I have this on order: SPEC stage 2. This is either an awesome clutch or a pile of crap, depending on who you ask. $399. Salesman said it's a Sachs PP stamping, but their own straps and clamping mechanism.

Edit... I changed the order to a Clutchmaster FX100. SPEC clutches have a reputation for inconsistency and the Clutchmaster appears to me more Blue Chip.

[hoveringuy]

Clutchmaster FX100 installed.

Pedal is not stiff; silky smooth. Good grab, not jerky.

Good stuff!