[bibbetson]

My 968 has been fairly well relegated to track / autocross duty. The only time it seems to hit the street is when I need to burn some gas, test a mod or take it to the shop. At the last two track events I've developed CV joint noise in very hard right hand turns. The noise comes from the right hand (unloaded) side of the car. At both events (two different tracks) the noise developed in the second day in the same type of corner. Before the most recent event I completely disassembled and relubricated the joints. The joints looked fine with some wear in the cages.



My mechanic (who also races in our club) believes that the transmission and rear components are moving enough to compress the axle into the transmission or hub thus causing the noise. Alternately, it could be pulling the axle and CV joint apart enough to basically hyper-extend it. Either way, he feels stongly that the noise is a symptom of the rear components moving around. I've got to find a solution to this before the next track event. Based on the noise that developed Sunday something is going to break soon.



His proposed cure for this is installing a solid transmission mount and solid bearing for the trailing arms. I've already replaced the transmisson mount with an OE one a couple years ago. Anyone experience the same thing in their track car? Anyone know where to get a solid transmission mount? I could only find one for a 951 from Kokeln and it requires a 951 cross member to fit the 968.

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[flash]

wow - the good news is you are obviously going very fast



the bad news is you have found the weak link in the rear suspension - what is likely happening is indeed a compression of the rubber bushings, most likely the lower control arm to torsion tube bushings - the solid ones will help here, and as long as the car is a track car, the added noise won't bother you - i have the elephant racing ones, and i love them



you could replace the rubber torsion tube bushings with their polybronze ones too, which would also eliminate some lateral compression, but a lot less than the above mentioned ones - it is a whole lot more work too and at that point it would be a good time to consider deleting the torsion bars and going coilover - this could mess with your classification though if you are racing



as for the solid tranny mount, based on the design, i don't think it will do much in the way of reducing your lateral movement - given that it is at the top of that "triangle", the loads are still going to make it move left to right - however, it will eliminate vertical movement, and help make things hook up better - in a race car, it could be helpful - in a street car it will be noisy



hope that helps

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[Eric_K]

I've never experienced what your are talking about. It would seem the transmission mount would have to be falling apart to allow that much movement. In which case a stock replacement would fix the problem. I have a hard time buying that the rear end parts are moving enough to compress the cv's. Wouldn't the loaded side be more compressed anyways?



Did you flip the axles (tranny side cv to the wheel side) when you serviced them? It will change the wear pattern on the cv and may make the noise go away or get worse.

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[bibbetson]

I lost track of which CV went were while I was cleaning, greasing, cleaning myself, the house, the cat and the neighbor - so the CV's went back in randomly. (If you have never greased a set of CV's get ready for a treat!) Due to this, it's very unlikely, but possible, that the same CV went back into the same location.



A contributor to this in my car different than other track cars may be that I'm still running stock rear torsion bars. This could be causing much greater CV angles, the worst of which would be the unloaded side.



I'm seriously contemplating doing a spring upgrade on the car this winter. That and the trailing arm bearings sound like a good first stab at correcting this, looking at the transmission / torsion tub mounts later.

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[bob blackwell]

Bill, I think Lindsey Racing has both solid transmission and motor mounts for our car. www.lindseyracing.com I have reversed the axles on several of my water-cooled Porsches and it really works well to extend the life of the CV joints. Good luck, Bob blackwell.

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[PorscheDude]

[quote name='Eric_K' date='Oct 4 2006, 08:34 AM']I've never experienced what your are talking about. It would seem the transmission mount would have to be falling apart to allow that much movement. In which case a stock replacement would fix the problem. I have a hard time buying that the rear end parts are moving enough to compress the cv's. Wouldn't the loaded side be more compressed anyways?



Did you flip the axles (tranny side cv to the wheel side) when you serviced them? It will change the wear pattern on the cv and may make the noise go away or get worse.

[right][post="26622"]<{POST_SNAPBACK}>[/post][/right][/quote]





I don not think you can flip half shafts. I was under the impression that the 6spd has different length half shafts.

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[Eric_K]

They are indeed different lengths, so you can't swap side to side but you can take the same axle and rotate it 180 degress. That way it is effectively running in the opposite direction.

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[bibbetson]

I replaced the rear trailing arm to torsion tube carrier bushing with the Racer's Edge solid bushing this winter. The first event at Willow Springs did not demonstrate the problem, so hopefully this issue is fixed. Buttonwillow and Spring Mountain will tell for sure, but that's not for months. My time improved 2.6 seconds as well (1:34) at Willow Springs, but I attribute that mostly to this being my second time at that track.



       

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[RS Barn]

Have you checked your motor mounts? I am rebuilding a 968 Firehawk car with bad motor mounts. When pulling CV's to remove torsion tube the left side was fine and the right side CV was very difficult to remove. You could also move trans side to side a bunch. Solid motor mounts are realy bad on these cars. Vibration is unbelieveable and actually sets off knock sensors

I just received some 944 Super motor mounts with high density poly material. I read about a 968 Race car in England using these to reduce the lateral movement in the trans when suspension is side loaded.

They're expensive @$295 each but if you can stop the motor from swaying which transmits movement to the trans mount, it may be the ticket.

Pete

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[bibbetson]

I put in new motor mounts and transmission mount maybe three years ago. Certainly they could be bad after three years of track / AX time, but it wouldn't be at the top of my list. I bought an extra RH axle w/ CV's just in case.

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