Clutch and brake problems following track event

coglin · 07-30-2007, 10:04 PM

This Sunday, I put over 120 miles of track into my '79 911 sc at Thunderhill. Now, a myriad of problems are emerging. First, there is a clutch problem. When getting going from a stop, there is severe clutch chatter, the whole car shakes violently. And, the clutch pedal is quite rough when depressed - although the clutch is not slipping, and the shifts are fine. Second, the braking is no longer smooth, the pedal gives a grinding feel - not quite a vibration, but a rather severe grind (pads were brand new, and rotors are (were) rather new). The car has 280,000 miles, and did great by the way.

So, anybody care to do a diagnosis of these symptoms? I have not lifted it yet, but plan to do so soon.

Many thanks.

beepbeep · 07-31-2007, 04:19 AM

Clutch chatter:

do you have rubber-damped clutch disc? If yes, rubber has probably cracked. You might have leaked oil on disc friction surface too. This layer of oil, when burnt, will make all kind of noises.

Brakes:

Check your pads for wear. You might have glazed them severely so surface melted and is hard, which gives grinding feel. Take out the pads and check for wear, grind away glazed surface if such is present, replace if too thin. Check disc for signs of overheating, cracks and wear.

tonythetarga · 07-31-2007, 06:30 AM

If your pads were stock you probably wore them out completely. Not hard to do, which is why most go to a more aggressive pad when on the track. If you hear metal to metal when you step on the brakes you need to pull the wheel and have a look if there is any pad left.

mca · 07-31-2007, 07:02 AM

subscribed ... i too want more info about the clutch shudder.

Mine is doing the same thing (not too bad though) that seems to have resulted from lifting the rear via the sump plate. I am thinking that the tranny / engine mounts may have crumbled under the stress ... which can apparently cause the shudder.

Superman · 07-31-2007, 07:46 AM

I doubt if the sump plate lifting did anything. Check the (four?) engine-to-tranny nuts for tightness. Clutch chatter very frequently means a disk that has become contaminated. A trusted 911 mechanic insists that this is nearly always tranny oil crawling up the shaft to the disk. This happened to me. I placed the new seal in a location a couple of millimeters from its normal seat.

Things like new motor mounts, new clutch cable, etc., help......but chattering is usually oil contamination. And of course, nobody should still have one of those rubber-centered disks in their car. That's just a problem waiting to happen.

mca · 07-31-2007, 08:01 AM

Should coglin and I first make sure that the clutch is properly adjusted?

I would imagine that we would:

1) check clutch adjustment
2) check torque specs on tranny to engine bolts
3) throw up b/c the clutch disc is contaminated (which means replacement too right?)

coglin · 07-31-2007, 01:07 PM

Thanks you for the replies.
Clutch:
The clutch is not rubber centered. Would contamination also explain the grindy feel of the clutch pedal? - the pedal only feels grindy when the engine is running, not stopped. Lastly, is there a way to clean the clutch disc without taking everything apart? - maybe spray some brake cleaner in there?

brakes:
I have not looked at them yet, but I am curious if I might have warped the rotors, is this usually obvious? I do feel slight vibration only when braking very hard.

Thank you,
Coglin

tonythetarga · 07-31-2007, 01:19 PM

These rotors don't easily warp. It is probably pad material that has transfered to the rotor that is causing the vibration. Very common mis-diagnosis. Vibration could also be front wheel bearings, you might check the tightness of the center bolt on the spindle. (remove dust cap, check washer movement with screwdriver. It should be firm but moveable slightly either side to side and/or top to bottom. If it needs to be tightened, loosen allen head bolt on axle nut and tighten nut. Keep re-checking the washer movement. When you have it snug, tighten allen head bolt, replace dust cap.)

Superman · 07-31-2007, 01:36 PM

Yes, it does make sense to make sure everything outside the tranny bellhousing is functioning properly. A new clutch cable, installed properly and adjusted properly, is essential. Pedal cluster bushings also wear out, and that could be part of the problem. With a grindy feel, I would suspect this. But once you have freshened and adjusted everything reachable, then you would know that the problem requires removing the engine in order to fix. No, brake cleaner is not going to clean a contaminated clutch disk. With a new disk and new cable, etc., the clutches in these car work quite beautifully. As you would expect.

coglin · 08-01-2007, 09:13 AM

Well, I finally lifted the car and took off the front wheels. Everything was cooked. The rotors have purplish scorch marks. The pads - just installed - were tough to get out because some kind of crispy crud built up around them, and were glazed and had chunks missing. To top it off, the bearing grease melted and poured out of the bearings all around. Haven't checked the back yet, but I am in fear.

What puzzles me, is that the brakes gave no indication of failure. Everything was fine (no fade/boiling) until the event ended and I drove home - adrenaline?

In either event, I am puzzled as to what to do now. I am a graduate student, so I don't have money to throw around. I can drive the car like this, but would like to get the car back into condition so it can be taken to the track without melting the brakes again. Is there an affordable way to do this? or is my only solution to take it easy (I did run the car for about an hour straight and maybe more). Will ducting help? I have no problem cutting holes in the front valence because the bodywork on the car is trashed - but then is there space for tubing on the inside of the wheels?

All advice is very welcome, apologies for the lengthy post.

best.

Karb · 08-01-2007, 09:41 AM

I had a buddy with the teeth shuddering chatter that you thought would pop the front windshield . It was insane. Only from a dead stop in first did this happen. Did all the checks
that were mentioned above, yet to no avail. Eventually we tracked it down to a broken
clutch spring plate. Only two of the leaves had horizontal cracks in them, and had not let
completely go. This was a kennedy clutch package too, so we were surprised, but that's
what it was.

JeremyD · JeremyD's Garage

280K is quite a few miles - for the brake pads - if they were stock - it is not uncommon to go through a set of stock brakes (especially SC brakes) at a session. Maybe look for a set of carrera fronts that will bolt on.

As far as the clutch goes, my guess is that you either cracked the pressure plate - or as superman says - you have contaminated the surface. You didn't report how many miles the clutch has on it.

915 clutch package isn't out of this world, engine drop is actually pretty easy - brake pads - (I've had good luck with porterfields r4's and R4s) I'd start surfing ebay and pelican parts for sale if I were you.

tonythetarga · 08-01-2007, 05:17 PM

If the wheel bearings leaked grease you should remove the bearings, clean and inspect them and if they aren't cooked, repack with high temp synthetic grease (redline or similar). If your backing plates are behind the rotors, remove them while the hub is off to give the rotors extra cooling. If you cut some holes for ducting you need to make sure you have the AJ USA or similar back plate and you will need block off plates and a way to route the duct around the a-arm. For a few DE's a year, I would NOT do that expense. You can get the 993 scoops or make your own. ANY air you can direct towards the rotors will help plus get yourself good brake pads that work when the rotors are hot, like Hawk Blue or Pagid Orange. (don't forget the right brake fluid either)