Clutch disc

[manny]

hello gentlemen, can some one explain to me why a clutch disc would burst/xplode/come apart when it still has plenty of disc material there?

[Rabbit07]

hello gentlemen, can some one explain to me why a clutch disc would burst/xplode/come apart when it still has plenty of disc material there?

You exceeded the burst speed of the disc. That means you likely mec.hanically over reved it.

[joeg]

Not matching revs on a downshift (failure to heel and toe or not showing some mechanical sympathy, i.e, banging the downshift).

[manny]

thanks guys, i dont i over rev'ed it but to be honest i think joeg hit it on the head. i guess i need to learn the heel toe method. thats tough, i get so excited when racing. just an old guy loving the sport :D
thank you again

[Gary L]

You need to remember that on a downshift, clutch disk rpm is determined by the ground speed at which you engage the lower gear... period. Engine speed may or may not match at that point; in other words, regardless of your heel-and-toe technique, a clutch disk over-rev may not show up as an over-rev on the tach.

[billf]

Manny,

The problem is in the construction of the disc, and its severe usage. First, the material is attached to the marchal spring (the wavy surface that the clutch material is attached to) with rivits. The rivits have limited contact (it's only a fastener, holding the material to the metal disc). They don't even have full thickness, as the rivits are recessed!!

Second, when the clutch is engaged/disengaged, the rivits work on the holes in the material, weakening them. Eventually, the failure is at the rivit holes, cracks appear, and the material disintegrates. Examine the fragments from the blown disc, and I'll bet you can see the curved surfaces from the rivits holes!!!

A simple, long lasting solution (better than simply replacing the disc with a new one, and waiting for the next failure), is to take a used disc to a clutch rebuilder (find them in the yellow pages, too). Ask the reabuilder to Bond the disc to the marchal spring. Doing this will give the clutch material much, much greater surface adhesion to the metal spring, resisting any movement from the engagement forces, or very high RPM.

An added bonus is that you can request stronger springs be installed at the center of the disc...won't loosen up as easily as stock, AND....

The Bonding method requires the epoxy be baked to fasten the material to the machal. The consequence of this is to collapse the spring. This is good for racing, as it makes the shifting interval shorter...faster engagement of the clutch. (Marchal springs are installed to soften the engagement to make take-off from a dead stop smooth. You don't need this for racing.)

I've done this many, many times with the cars we have built. The clutches will last the lifetime of the material...which should be forever if you co-ordinate your shifting. You can even remove the rivits after bonding, for even longer life.

Hope this helps.

Good racing,

Bill

[Tom Blaney]

I've never toe & healed in my life, not needed with a modern car gearbox. The clutch (as billf explained) got beat up from use. You can take billf's suggestion if your lucky to have one near by, or buy high quality clutch's and a much stronger pressure plate to limit the slip when you reengage the clutch.
Also don't be lazy with your shifts, develop and rhythm between your hand and foot so it's a single fluid motion. This can be a daily practice when you drive a street car, so on the track your gear change is quick enough that you don't have to worry about the clutch dragging the motor back up to speed, since your foot will automatically be on the gas (or brake) to maintain the appropriate momentum. In my very first SCCA drivers school, Mark Donahue told us that brake pads are cheaper than a clutch disk.

[manny]

Thanks guys. I knew i would get answers that make sense. Yes bill that is exactly what all of the pieces looked like when i got the bellhousing off, broken off at the rivets. Yes Tom brake pads are cheaper and much easier to change:023:.

[billf]

Sorry guys,

I mis-spelled the name of the spring...Marcel, not Marcal. I couldn't look it up this morning, but I finally found it with the correct spelling.

Found this, as explaination:

http://photos.motoiq.com/MotoIQ/Tech/Clutch-Tech/9-marcel-spring-metal-disc/759866019_b69dd-L.jpgJim Wolf Technology has put a Marcel spring in some of their full metal clutch discs to help make them relatively streetable. This is an unusual feature of JWT's engineering to make a race type clutch that can hold lots of power more streetable.
Clutch discs have a few different features that control their operating properties. Most stock and HD clutches have what is called a Marcel spring. This is a wavy flat spring that is bonded to the friction material, and then riveted to the disc itself. You can see it when looking at a clutch disc on its edge. The marcel acts like a cushion, smoothing out the final engagement of the friction material by allowing the clamping forces to build up a little slower. When the clutch is fully engaged, the marcel smashes flat and has no effect.
http://photos.motoiq.com/MotoIQ/Tech/Clutch-Tech/8-marcel-spring-semi-mettalic/759865866_NLNur-L.jpgThe Marcel spring on this stock Nissan disc is this wavy piece of sheet metal. Its job is to make the clutch engagement more gradual and smooth.
The marcel is removed from racing-type and metallic clutches for two reasons. The marcel prevents the friction material from being 100% bonded to the disc, reducing burst strength and it also increases engagement travel, possibly slowing shifts which is a little harder on the syncros. Some companies do innovative things to incorporate Marcel into semi-metallic and Kevlar metallic discs by bonding the friction material to a steel substrate first then riveting it to the marcel spring to increase burst strength.

Locally, we have a clutch re-builder that was used for this during the years, and the next one is in a neighboring city. I'm sure you can find someone for this job, but it requires some phoning/web inquiries. The places local to me do heavy truck clutch repair and replacement, if that helps.

Good racing,

Bill