Of course one can see that NC. But surely during the development stage the device must have been checked for its capacity to survive normal levels of clutch use and abuse?
I think the problem may be the number of drivers who think it's all right to hold the car on the clutch at a steep traffic intersection, not for the couple of seconds that might sometimes be necessary, but for half a minute or longer at a time. Even otherwise good drivers who have never had to replace a clutch with their own hands are sometimes to be seen doing this. It really sets my teeth on edge in any car.
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Of course one can see that NC. But surely during the development stage the device must have been checked for its capacity to survive normal levels of clutch use and abuse?
If it's so simple Lud, perhaps you should give design engineering a go!
The specification of development tests which can uncover problems, but yet not lead to rampant over-design is difficult; especially when you are considering a new technlogy.
The company involved in my example is well known in the industry for having *extremely* tightly specified acceptance tests for components. Among the problems is that their normal development tests aren't too worried about putting heat into the flywheel, and the tests were primarily designed to give the friction surfaces a rough ride.
There's quite an important interplay between the duration, the frequency of clutch applications and the level of heat build up in a flywheel, so, you can easily have a test which does give the friction surfaces a really hard time, but allows the flywheel heat time to dissipate, and a company with a history of standard flywheel use would have to be *really* on the ball to pick this problem with DMFs up in design, analysis, or development.
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If it's so simple Lud perhaps you should give design engineering a go!
Clearly it isn't simple at all, and I am not a design engineer nor do I think myself qualified to become one or wish to become so qualified.
As I suggested, and as you confirmed, manufacturers do test these things for their capacity to survive ordinary clutch use and abuse. The problem seems to arise with extraordinary clutch abuse.
Now, either the culprits have made such a clamour that they have created the false impression that DMFs are fragile, or there are more of these incredibly gross and somehow brainless clutch abusers than one might expect, which would mean that DMFs as they are at present are not quite fit for purpose.
As I said above, I think the real blame can be laid on driving instructors. And utterly moronic drivers of course.
Personally I love a bit of NVH if it is going to save me a grand or two after some monkey has driven my car.
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>>manufacturers do test these things for their capacity to survive ordinary clutch use and abuse. The problem seems to arise with extraordinary clutch abuse.
Yes, each manufacturer develops their own suite of tests for each major component, based upon their experience. In some cases, petitioning to change these internal company standards is a proposition comparable with flying to Mars!
If these design standards, and development test suites covered the worst possible cases, the most incompetent, or the most enthusiastic drivers, cars would be so heavy they would barely move! Some small number of premature failures in service must therefore be expected for any component or system. However, I think there's something more systematic going on with DMFs.
For example, clutches in mk II Astras would either last about 100,000 miles, or the life of the car, but, I've replaced them on cars with less than 10,000 on the clock. If the clutches were designed to account for this abuse, they would be vastly over-specified for an average, sensible motorist. Which, I think is Lud's point exemplified.
In a way, I'm quite lucky in that in my line of work, the loadings and service life that the parts I design is very well known and characterised. This has led to a certain rigidity of approach which is a bit difficult in some circumstances, but, there is (necessarily!) a low risk of failure.
>>DMFs as they are at present are not quite fit for purpose.
I think that there is a lot of truth in this, but I don't think there has necessarily been any negligence or incompetence on the behalf of the manufacturers or thier engineers. As the manufacturers can't know in advance exactly how their vehicles are to be used or driven, it's difficult to always have conservative design and development procedures in place - some degree of using the early customers for development is always, despite the preponderance of computer based design and analysis, inevitable.
I have complete confidence that DMFs and common rail diesels will become much more robust very quickly, but, I am also confident that the pace of automotive evelopment isn't going to slow, and so it won't be long before the new "problem" emerges.
>>Personally I love a bit of NVH
You're weird!
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Yes NC, I take the point that some drivers can destroy anything in very short order, and quite often have very limited understanding of their own role in the destruction. I take the point too that these components usually need further development after they have gone into production, and tend to get it even when they don't need it. So DMFs and common rail will soon be as reliable as (say) hydraulic tappets or the electric self-starter.
But they aren't yet. HJ comments in his reply to the reader that the DMF problem is getting 'very common'.
A driving instructor, Robin Reliant I think, denies that instructors encourage clutch abuse. I am glad to hear that he doesn't. I think it works like this: the learner is taught to hold the car on the clutch at uphill road junctions when waiting to move off. To avoid complication - rocking the boat as it were when the learner is still nervous - the instructor doesn't point out that the technique causes wear and if used too freely further sorts of damage from overheating. So the average candidate, a mechanical ignoramus, acquires the habit of clutch slipping. Do it three times for thirty seconds in the same five minutes on a hot day, and serious overheating may result. But the newly qualified driver doesn't know that, and probably never will. A bit like bears trying to play the piano.
Edited by Lud on 01/06/2008 at 13:35
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