944 clutch judder
Discussion
You probably won't need a complete clutch kit, your pressure plate and release bearing should be fine, you might not even need a new clutch plate - you don't really know till you take it apart. Wait until this evening and you might find that Stewart comes onto the thread because he's interested and I've told him to have a look at it. If he doesn't then tomorrow I'll give you his number. He's a good bloke, and if you tell him what you've done then he'll tell you what you should do now. You could come and visit some pals in Manchester for a couple of days while he fixes it for you - it's worth the trip to have it done right, and for the right price!
This clutch judder is not quite such an easy problem to solve as most would expect because three main problems combine in different ratios to exacerbate clutch judder.
(1) Imagine the engine, torque tube and gearbox being one lump all bolted together - engine at one end and gearbox at the other (torque tube in the middle). The square dash 944 had 2 engine mounts at the front and 2 at the back in a similar wide position - resisiting twist in the teansmission quite nicely. Unlike the older 944's the S2 (turbo, later 944's and 968) used one hanging bracket at the rear allowing the transmission to sway from side to side quite freely at the rear - so as power/torque increased and engine mounts got tired - the whole transmission was able to sway from side to side more - putting more load on the front mounts. Now anything that sets up a sudden twist would allow more twisting of the whole unit until it reaches its limit when it will twist back again - hence an oscillating motion can be set up. The car basically only drives along because the engine is stopped from twisting round (rotating) and instead the same torque is transmitted to the wheels via the gearbox - without engine/gearbox mountings the engine would simply spin round instead in the opposite direction. It seems a bit strange then to fit a rear mounting that provides little or no resistance to twisting = potential problem number 1.
(2) So what can set up an oscillating motion - several things can - (a) hard spots that grow on the iron flywheel face (as reported before and fully explained), tired torsional shock absorbers (either in the clutch (rubber cush) or in the flywheel (dual mass) or worn/broken springs (in the clutch plate). Even the strength of the diaphragm spring can alter the grab of the plate.
All of these items (except the torsional clutch spring) will be weaker as the parts get hot as rubber in the cush or mounts gets softer with heat and load (having a similar affect) and hard spots on the flywheel get harder the more they are heated and cooled - and stick out more as the remaining softer face material still wears down.
(3) The clutch face material can also deteriorate (both with age and type of use) and change with heat and trapped dust - setting up more of a grabbing engagement than a smooth take up - and thus increasing sudden twist in the transmission. Different clutches and pressure plate diaphragm spring rates (sports applications etc) can also "grab" badly in domestic use.
So the whole design is - if you like - prone to a twisting (torsional) snatch due to its design and then as the whole thing wears, several different possibilites all combine in different degrees, to push the system beyond its original design limits.
The problem for garages is that if a customer comes in needing a new clutch and has clutch judder - it is very difficult to tell how bad all the other parts are and how many parts will have to be replaced to put the car right. Furthermore - nor everyone drives the same way - the worse judder tending to be those that drop into second having let the engine revs drop down and try to slip the clutch to slow down the car. Some may rev the engine mid way in the gear change, others will change into the lower gear quicker and similarly some drive off with mild throttle while others give it more stick.
Then - not everyone can afford everything to be changed - even if they experience the problem - especially if it is only on the off chance that it all really needs doing.
It is difficult to see hard spots on the flywheel face with out starting to machine them off - by which time a decision has already been made to re-face the flywheel or replace it if the hard spots are too deep.
All new engine/gearbox mounts, new clutch and flywheel (incl pressure plate etc) should fix the problem but any other solution may not.
If our road test shows up what we think is hard spots on the flywheel we will recomend fixing it as part of the job - but we would rarely anticipate the need for engine/gearbox mounts as well - tending to suck it and see (which is usually OK) and only delve further if problems still persist.
So we discuss the customers needs first (did they just ask for a new clutch - but everything else was OK) then we road test (do we think that the flywheel is OK) then we seek approval for what we want to do and take it from there depending upon the customers reaction, needs and budget.
It usually works out OK.
Broken rubber or spring cush drives are however very easy to spot - either by driving on and off the throttle in a high gear and seeing if there is a delay in the take up of the drive - or by putting your hand in the inspection hole and trying to twist the torque tube in both directions.
Baz Hartech
(1) Imagine the engine, torque tube and gearbox being one lump all bolted together - engine at one end and gearbox at the other (torque tube in the middle). The square dash 944 had 2 engine mounts at the front and 2 at the back in a similar wide position - resisiting twist in the teansmission quite nicely. Unlike the older 944's the S2 (turbo, later 944's and 968) used one hanging bracket at the rear allowing the transmission to sway from side to side quite freely at the rear - so as power/torque increased and engine mounts got tired - the whole transmission was able to sway from side to side more - putting more load on the front mounts. Now anything that sets up a sudden twist would allow more twisting of the whole unit until it reaches its limit when it will twist back again - hence an oscillating motion can be set up. The car basically only drives along because the engine is stopped from twisting round (rotating) and instead the same torque is transmitted to the wheels via the gearbox - without engine/gearbox mountings the engine would simply spin round instead in the opposite direction. It seems a bit strange then to fit a rear mounting that provides little or no resistance to twisting = potential problem number 1.
(2) So what can set up an oscillating motion - several things can - (a) hard spots that grow on the iron flywheel face (as reported before and fully explained), tired torsional shock absorbers (either in the clutch (rubber cush) or in the flywheel (dual mass) or worn/broken springs (in the clutch plate). Even the strength of the diaphragm spring can alter the grab of the plate.
All of these items (except the torsional clutch spring) will be weaker as the parts get hot as rubber in the cush or mounts gets softer with heat and load (having a similar affect) and hard spots on the flywheel get harder the more they are heated and cooled - and stick out more as the remaining softer face material still wears down.
(3) The clutch face material can also deteriorate (both with age and type of use) and change with heat and trapped dust - setting up more of a grabbing engagement than a smooth take up - and thus increasing sudden twist in the transmission. Different clutches and pressure plate diaphragm spring rates (sports applications etc) can also "grab" badly in domestic use.
So the whole design is - if you like - prone to a twisting (torsional) snatch due to its design and then as the whole thing wears, several different possibilites all combine in different degrees, to push the system beyond its original design limits.
The problem for garages is that if a customer comes in needing a new clutch and has clutch judder - it is very difficult to tell how bad all the other parts are and how many parts will have to be replaced to put the car right. Furthermore - nor everyone drives the same way - the worse judder tending to be those that drop into second having let the engine revs drop down and try to slip the clutch to slow down the car. Some may rev the engine mid way in the gear change, others will change into the lower gear quicker and similarly some drive off with mild throttle while others give it more stick.
Then - not everyone can afford everything to be changed - even if they experience the problem - especially if it is only on the off chance that it all really needs doing.
It is difficult to see hard spots on the flywheel face with out starting to machine them off - by which time a decision has already been made to re-face the flywheel or replace it if the hard spots are too deep.
All new engine/gearbox mounts, new clutch and flywheel (incl pressure plate etc) should fix the problem but any other solution may not.
If our road test shows up what we think is hard spots on the flywheel we will recomend fixing it as part of the job - but we would rarely anticipate the need for engine/gearbox mounts as well - tending to suck it and see (which is usually OK) and only delve further if problems still persist.
So we discuss the customers needs first (did they just ask for a new clutch - but everything else was OK) then we road test (do we think that the flywheel is OK) then we seek approval for what we want to do and take it from there depending upon the customers reaction, needs and budget.
It usually works out OK.
Broken rubber or spring cush drives are however very easy to spot - either by driving on and off the throttle in a high gear and seeing if there is a delay in the take up of the drive - or by putting your hand in the inspection hole and trying to twist the torque tube in both directions.
Baz Hartech
Baz,
As usual, a helpful, well-written and informative post. Thank you.
A couple of questions arising from it.
1. You say that the 'hard spots on the flywheel' have been discussed elsewhere - can you point me in the direction of the discussion? Thanks.
2. You say that "If our road test shows up what we think is hard spots on the flywheel we will recomend fixing it as part of the job". What are the symptoms of hard spots on the flywheel as opposed to tired engine/gearbox mounts?
3. Machining the front face of the flywheel is presumably cheaper than a new item; how much are new flywheels and how hard are they to change?
4. Are the spring cush drives and rubbers part of the new clutch assembly? (All previous clutches I have fitted have had the cush drive as part of the clutch plate - I presume this is the same on a 944 clutch?)
I too suffer from a jerky drive when the car is hot, in traffic. To be honest, I have learned to drive around it, but other people struggle when they drive the car for the first time.
Oli.
As usual, a helpful, well-written and informative post. Thank you.
A couple of questions arising from it.
1. You say that the 'hard spots on the flywheel' have been discussed elsewhere - can you point me in the direction of the discussion? Thanks.
2. You say that "If our road test shows up what we think is hard spots on the flywheel we will recomend fixing it as part of the job". What are the symptoms of hard spots on the flywheel as opposed to tired engine/gearbox mounts?
3. Machining the front face of the flywheel is presumably cheaper than a new item; how much are new flywheels and how hard are they to change?
4. Are the spring cush drives and rubbers part of the new clutch assembly? (All previous clutches I have fitted have had the cush drive as part of the clutch plate - I presume this is the same on a 944 clutch?)
I too suffer from a jerky drive when the car is hot, in traffic. To be honest, I have learned to drive around it, but other people struggle when they drive the car for the first time.
Oli.
Thaks for this Barry, it's really interesting and useful. It does two things, gives a layman's explanation as to how it works and the associated problems, and subtley answers / defends the queries as to why the other parts are not checked at the same time - i.e. you do check them, but that doesn't mean they all do.
I have to say that we have met when you changed my 1989 944 2.7 Lux clutch about eight years ago, you showed me then how the rubber had just broken up - hence the clutch just failing, but I never suffered judder. It was a top job, £700 well spent, although I think you still charge the same price now - so it's a bargain for anybody else reading this.
I also e-mailed you last October before buying my current S2 when I was concerned about a 139500 one clutch car. You were kind enough to e-mail back and said that you have seen clutches last from 5000 miles (driven by a boy racer wan**r) to 150000 (driven like an old lady). Fortunately my car was one owner and the previous driver drove like me - like a vicar! Now on 153000 and still the original clutch. However it does judder from time to time when very hot from being in traffic jams, no other time. It doesn't judder half as much if I pull away in 2nd as it does in 1st (is there a reason for this and does it give any clues?), and it's just a pig in reverse. I know it's a long shot ..... but any initial ideas? I think it's got to be something rubber because it's affected by the heat which, like you say, will weaken and become more flexible as it warms up - but what do I know! Can the engine / gearbox mountings be easily checked with the car cold?
Last question - does the water pump make any sort of whining noise if it is not happy after a run?
When considering the relevance of my questions, please take into consideration that I am not a motor technician like yourself.
Thank you.
I have to say that we have met when you changed my 1989 944 2.7 Lux clutch about eight years ago, you showed me then how the rubber had just broken up - hence the clutch just failing, but I never suffered judder. It was a top job, £700 well spent, although I think you still charge the same price now - so it's a bargain for anybody else reading this.
I also e-mailed you last October before buying my current S2 when I was concerned about a 139500 one clutch car. You were kind enough to e-mail back and said that you have seen clutches last from 5000 miles (driven by a boy racer wan**r) to 150000 (driven like an old lady). Fortunately my car was one owner and the previous driver drove like me - like a vicar! Now on 153000 and still the original clutch. However it does judder from time to time when very hot from being in traffic jams, no other time. It doesn't judder half as much if I pull away in 2nd as it does in 1st (is there a reason for this and does it give any clues?), and it's just a pig in reverse. I know it's a long shot ..... but any initial ideas? I think it's got to be something rubber because it's affected by the heat which, like you say, will weaken and become more flexible as it warms up - but what do I know! Can the engine / gearbox mountings be easily checked with the car cold?
Last question - does the water pump make any sort of whining noise if it is not happy after a run?
When considering the relevance of my questions, please take into consideration that I am not a motor technician like yourself.
Thank you.
zcarcogp - hard spots occur when the cast iron flywheel face is a little less flat than when new so when you drive off - the clutch plate initially touches one small spot on the flywheel as you release the pedal and it transmitss all the torque and gets red hot for a few seconds and cools quickly when the plate eventually rests on it.
Unfortunately cast iron has enough carbon in it to go almost glass hard in this scenario and once it is of course - it doesn't wear anymore - so the hard spot gets bigger as it is even more often than not the first contact spot and the softer cast iron around it continues to wear out making the spot get bigger and bigger - a vicious circle with no get out. Once the spots are there - the take up when driving off - all drives through the little lump - which causes the worn transmission to snatch
and set up jerky torque reaction from all the rubber bits in the cush drive and the engine/gearbox mounts.
However when you inspect the flywheel face - it just looks normal (unless you have experienced these spots before) and the hard spots only really show up when machining the face. This has to be done in a large lathe with a carbide tipped cutter on a very slow speed and feed and you cannot tell how deep the spot (or spots) are until you have machinined them all off. They can be ground off in a facing grinder - but then it is more difficult to see when the hard spots have all been removed and if even a tiny patch is left the whole sceario will simply continue. I can send a picture demonstrating the machining here at Hartech - if you give me your e-mail address.
We usually limit the amount of removal to 1mm max and do it in cuts of .15mm. Prices vary from about £60.00 depending on the number of cuts and how true it was to set up but are always much less than a new flywheel and for the age of cars - probably a better solution - if there is enough soft surface left after machining.
Removing the flywheel is not too difficult once the gearbox and torque tube are removed.
Symptoms are hard judder changing down into second slowly while slowing down. You need to rev it in third then let the revs completely drop while you change into second and then let out the clutch pedal slowly to slow the car. Most cars by now have lumps and spots on the flywheel.
New clutches have new rubbers in them or an alternative has the older type springs as shock absorbers.
We were brought an S2 that had had a new clutch fitted by a competitor - but the customer was slagging them off because it still juddered. We found that the whole clutch had been correctly replaced and a good job was done but they simply had not noticed the need to reface the flywheel, had not tested the car, or perhaps the customer was advised of the need but decided not to replace the parts. Anyway we explained that it was not really their fault - that they had done everything they were paid for and anyway they then replaced the then damaged new clutch plate FOC which we fitted with a new flywheel and it was then all OK. It is therefore easy to criticise but not easy to always get right - but easy to go over the top to make sure it will be OK at great expense to the customer.
Z3944 Thanks you for your kind comments - I think that the rubber has partly gone (they tend to crack at 45 degrees (typical torsional stress distribution) or in a flat circle if just very old age from careful driving.
Don't forget though that the faces can also wear down to the rivetts - it's just that we usually find that the rubber cush has gone before the clutch material has worn down - but with careful use it can still happen and then a similar scenario occurs with the metal rivetts hitting the hard spots.
Baz Hartech
Unfortunately cast iron has enough carbon in it to go almost glass hard in this scenario and once it is of course - it doesn't wear anymore - so the hard spot gets bigger as it is even more often than not the first contact spot and the softer cast iron around it continues to wear out making the spot get bigger and bigger - a vicious circle with no get out. Once the spots are there - the take up when driving off - all drives through the little lump - which causes the worn transmission to snatch
and set up jerky torque reaction from all the rubber bits in the cush drive and the engine/gearbox mounts.
However when you inspect the flywheel face - it just looks normal (unless you have experienced these spots before) and the hard spots only really show up when machining the face. This has to be done in a large lathe with a carbide tipped cutter on a very slow speed and feed and you cannot tell how deep the spot (or spots) are until you have machinined them all off. They can be ground off in a facing grinder - but then it is more difficult to see when the hard spots have all been removed and if even a tiny patch is left the whole sceario will simply continue. I can send a picture demonstrating the machining here at Hartech - if you give me your e-mail address.
We usually limit the amount of removal to 1mm max and do it in cuts of .15mm. Prices vary from about £60.00 depending on the number of cuts and how true it was to set up but are always much less than a new flywheel and for the age of cars - probably a better solution - if there is enough soft surface left after machining.
Removing the flywheel is not too difficult once the gearbox and torque tube are removed.
Symptoms are hard judder changing down into second slowly while slowing down. You need to rev it in third then let the revs completely drop while you change into second and then let out the clutch pedal slowly to slow the car. Most cars by now have lumps and spots on the flywheel.
New clutches have new rubbers in them or an alternative has the older type springs as shock absorbers.
We were brought an S2 that had had a new clutch fitted by a competitor - but the customer was slagging them off because it still juddered. We found that the whole clutch had been correctly replaced and a good job was done but they simply had not noticed the need to reface the flywheel, had not tested the car, or perhaps the customer was advised of the need but decided not to replace the parts. Anyway we explained that it was not really their fault - that they had done everything they were paid for and anyway they then replaced the then damaged new clutch plate FOC which we fitted with a new flywheel and it was then all OK. It is therefore easy to criticise but not easy to always get right - but easy to go over the top to make sure it will be OK at great expense to the customer.
Z3944 Thanks you for your kind comments - I think that the rubber has partly gone (they tend to crack at 45 degrees (typical torsional stress distribution) or in a flat circle if just very old age from careful driving.
Don't forget though that the faces can also wear down to the rivetts - it's just that we usually find that the rubber cush has gone before the clutch material has worn down - but with careful use it can still happen and then a similar scenario occurs with the metal rivetts hitting the hard spots.
Baz Hartech
z3944 - sorry I missed the water pump question.
Yes you raise a very important point - which we found out about years ago when trying to trace a noise from the front of an S2 - only when it was hot (remember 997GT3?).
We always remove belts etc to test rollers and the water pump - but obviously always from a cold starting point.
After replacing many parts like rollers etc and running the engine without power steering and alternator belts etc the only thing we could think of was that the water pump went from feeling perfectly OK when cold to being lumpy when hot - but usually we awaited the engine cooling down before strippiong it and then could only test it cold.
So we stripped off as much as possible and then when for a drive - upon returning from which we quickly stripped off the remainder while it was hot and - yes - there it was - all lumpy, noisy and horrible - which we recorded on video before and after (with sound) to prove to the customer - who would be entitled to otherwise be sceptical.
We learned from this how to identify the problem next time and so yes water pumps can go bad when hot but seem OK cold.
Bearing in mind that the cam belt goes round the water pump - if the pump fails there are potentially 16 valves to bend - head off - new gasket etc etc - so it was an important discovery.
Similarly we also found that low mileage cars (or those that were not used for 6 months of the year)-despite very low mileages covered - often suffered premature water pump failure. Rollers similarly had reduced lives.
This is because even inside a sealed grease filled bearing, moisture can come out of the air gap when it is very cold and condenses onto the metal. If the grease has gone hard at this point - rust pockets can develop and then when the car is eventually started up - initially the grease is hard and the bearings have surface damage - so life is shortened.
Also - we find now that the gear profile on the driving pulleys etc - wears down over the years resulting in the diameter and distance between the teeth reducing (often leaving a hollow in the centre) - so a new belt actually doesn't fit as nicely as it used to. In fact the new belt wants to sit at a slightly bigger diameter so as it goes round the pulley it is continually rubbing in and out of the root of the gear profile rather than simply sitting in it. This often requires re-setting of the tension more often - or they are found to be too slack after a short mileage. Put all this together and we find it neccessary to reduce the inspection times between checking belts, water pumps and rollers - to ensure as the cars age that they do not fail prematurely.
It can however - be hard to convince a new customer - about the need to inspect and perhaps change parts that we know will shortly fail but that at the time are still working OK - but we do build in to all our prices - discounts when combining such work as an incentive to get it done in time and before it eventually costs more to fix when the parts completely fails.
The 968 has a similar problem because the automatic tensioner can be almost (if not) fully extended even when fitting a new belt - due to the wear on the pulley gear teeth and therefore no longer tensions the belt as it stretches, wears and settles in. We make and fit an extension piece to correct this potentially expensive problem that afflicts most 968's over 100K - but I sometimes think that the customers do not fully appreciate how much work goes into our determination to solve such problems on their behalf.
I think it is this kind of engineering analysis and our genuine interest in doing a good job for our customers that has influenced everything we do here over the years and our whole approach to preventative maintenance - looking into reasons for problems and identifying the cause and how to minimise the consequences. It is all this that enables us to run such a low cost maintenance scheme because it enables us to avoid expensive problems more often than not. It is unfortunate that with fixed price servicing price comparisons - being so competitively Internet driven - such care and eventual cost savings do not show potential new customers - exactly what goes into such good service provision - when they are simply comparing prices but cannot experience what they are getting for their money in advance!
Baz Hartech
Yes you raise a very important point - which we found out about years ago when trying to trace a noise from the front of an S2 - only when it was hot (remember 997GT3?).
We always remove belts etc to test rollers and the water pump - but obviously always from a cold starting point.
After replacing many parts like rollers etc and running the engine without power steering and alternator belts etc the only thing we could think of was that the water pump went from feeling perfectly OK when cold to being lumpy when hot - but usually we awaited the engine cooling down before strippiong it and then could only test it cold.
So we stripped off as much as possible and then when for a drive - upon returning from which we quickly stripped off the remainder while it was hot and - yes - there it was - all lumpy, noisy and horrible - which we recorded on video before and after (with sound) to prove to the customer - who would be entitled to otherwise be sceptical.
We learned from this how to identify the problem next time and so yes water pumps can go bad when hot but seem OK cold.
Bearing in mind that the cam belt goes round the water pump - if the pump fails there are potentially 16 valves to bend - head off - new gasket etc etc - so it was an important discovery.
Similarly we also found that low mileage cars (or those that were not used for 6 months of the year)-despite very low mileages covered - often suffered premature water pump failure. Rollers similarly had reduced lives.
This is because even inside a sealed grease filled bearing, moisture can come out of the air gap when it is very cold and condenses onto the metal. If the grease has gone hard at this point - rust pockets can develop and then when the car is eventually started up - initially the grease is hard and the bearings have surface damage - so life is shortened.
Also - we find now that the gear profile on the driving pulleys etc - wears down over the years resulting in the diameter and distance between the teeth reducing (often leaving a hollow in the centre) - so a new belt actually doesn't fit as nicely as it used to. In fact the new belt wants to sit at a slightly bigger diameter so as it goes round the pulley it is continually rubbing in and out of the root of the gear profile rather than simply sitting in it. This often requires re-setting of the tension more often - or they are found to be too slack after a short mileage. Put all this together and we find it neccessary to reduce the inspection times between checking belts, water pumps and rollers - to ensure as the cars age that they do not fail prematurely.
It can however - be hard to convince a new customer - about the need to inspect and perhaps change parts that we know will shortly fail but that at the time are still working OK - but we do build in to all our prices - discounts when combining such work as an incentive to get it done in time and before it eventually costs more to fix when the parts completely fails.
The 968 has a similar problem because the automatic tensioner can be almost (if not) fully extended even when fitting a new belt - due to the wear on the pulley gear teeth and therefore no longer tensions the belt as it stretches, wears and settles in. We make and fit an extension piece to correct this potentially expensive problem that afflicts most 968's over 100K - but I sometimes think that the customers do not fully appreciate how much work goes into our determination to solve such problems on their behalf.
I think it is this kind of engineering analysis and our genuine interest in doing a good job for our customers that has influenced everything we do here over the years and our whole approach to preventative maintenance - looking into reasons for problems and identifying the cause and how to minimise the consequences. It is all this that enables us to run such a low cost maintenance scheme because it enables us to avoid expensive problems more often than not. It is unfortunate that with fixed price servicing price comparisons - being so competitively Internet driven - such care and eventual cost savings do not show potential new customers - exactly what goes into such good service provision - when they are simply comparing prices but cannot experience what they are getting for their money in advance!
Baz Hartech
Since returning from France three weeks ago and posting notes about clutch judder I have suffered no clutch judder at all. I think the main reason for this is that when in traffic I have started putting the car in neutral and letting the clutch out - every time I stop moving. Even when the car has been really hot it has not juddered following this routine, and it's not inconvenient to do. This must keep the clutch plate away from the direct heat and therefore alieviate the problem.
If you try it you might be pleasantly surprised, I have been! What a difference!
If you try it you might be pleasantly surprised, I have been! What a difference!
Z3944, thanks, will try it.
Baz Hartech, Your description of what causes clutch judder is good, and I follow it. However, why does it start, and why are 944's prone to it? The set-up is not different to any other car, and the process you describe could happen to any clutch, so why do all cars (of all makes) not experience this regularly?
Oli.
Baz Hartech, Your description of what causes clutch judder is good, and I follow it. However, why does it start, and why are 944's prone to it? The set-up is not different to any other car, and the process you describe could happen to any clutch, so why do all cars (of all makes) not experience this regularly?
Oli.
Sorry I do not know all about all the other cars and different makes and so I don't really know thw answer and so can only discuss the Porsche problem.
I think the rubber cush is quite rare and how well it lasts and how it performs with age - are all issues.
Rubber gets softer as it is both heated and also stressed (both of which will happen to a 944 clutch).
This means that it will be more likely to become too soft (or move too far) under load as it gets older.
Also the amount of torque being delivered is relevant to the reaction to the snatch because the shock loading will be more if more torque is trying to be delivered when the plate first comes into contact with the flywheel and 944's are very torquey.
Also the heat generated at the "spot" will be more with more torque and the degree by which the spot will harden will depend upon the metalurgical composition of the flywheel.
It could just be that with a lot of torque delivered into a plate that may be many years old, with a rubber cush that is shock loaded by a hard spot - that might only be there because the composition of the flywheel is a little different to others - the whole problem occurs.
Also the more the rubber is twisted by the sudden snatch on the hard spot - the softer the rubber will become and if it is too soft for the application it will set up an oscillation.
You can easily test this - if you ever cannot fit a rubber pipe over a tube because it is too tight - try hitting it flat with a hammer on a bench a few times and you will find that while it does not become too hot - it does go very soft for a short while.
The metal springs used in most clutches do not suffer reduction in stiffnnes and indeed the stiffness (or more properly deflection) is proportional to the load. This is not the case with rubber.
If the flywheel was flat (not lumps) then everything should be OK but once the tiny spots start they get worse and the rubber cush was never designed to handle that added problem.
Of course a rubber cush that has started to split may have the same problem even if the flywheel is OK as it would then put the same torsional load through a smaller area and effectively would be running with a softer cush that would twist further and get softer quicker.
Baz Hartech
I think the rubber cush is quite rare and how well it lasts and how it performs with age - are all issues.
Rubber gets softer as it is both heated and also stressed (both of which will happen to a 944 clutch).
This means that it will be more likely to become too soft (or move too far) under load as it gets older.
Also the amount of torque being delivered is relevant to the reaction to the snatch because the shock loading will be more if more torque is trying to be delivered when the plate first comes into contact with the flywheel and 944's are very torquey.
Also the heat generated at the "spot" will be more with more torque and the degree by which the spot will harden will depend upon the metalurgical composition of the flywheel.
It could just be that with a lot of torque delivered into a plate that may be many years old, with a rubber cush that is shock loaded by a hard spot - that might only be there because the composition of the flywheel is a little different to others - the whole problem occurs.
Also the more the rubber is twisted by the sudden snatch on the hard spot - the softer the rubber will become and if it is too soft for the application it will set up an oscillation.
You can easily test this - if you ever cannot fit a rubber pipe over a tube because it is too tight - try hitting it flat with a hammer on a bench a few times and you will find that while it does not become too hot - it does go very soft for a short while.
The metal springs used in most clutches do not suffer reduction in stiffnnes and indeed the stiffness (or more properly deflection) is proportional to the load. This is not the case with rubber.
If the flywheel was flat (not lumps) then everything should be OK but once the tiny spots start they get worse and the rubber cush was never designed to handle that added problem.
Of course a rubber cush that has started to split may have the same problem even if the flywheel is OK as it would then put the same torsional load through a smaller area and effectively would be running with a softer cush that would twist further and get softer quicker.
Baz Hartech
968csreading said:
Had a new fly wheel fitted on Friday (cost a small fortune) but it has transformed the car and there is no judder or nasty feedback throught the transmission.
I had some new belts and pulleys and a new rear wheel bearing fitted last week. That absorbed my otherwise spare small fortune, leaving me with none to spend on new flywheels or the like. Looks like I'll be putting up with a bit of judder for a while to come.
Nice to know it solved the problem tho'.
Oli.
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