I've just realised there are no timing belt threads yet, you can't have a tech forum without one so...
Had a sporty petrol thingy in a couple of days ago. Customer asked for the timing belt to be changed at 36K (genuine mileage and original belt) because the car was 6 years old.
While getting the bits for the job someone was saying to me it was a complete waste of money and only mileage could affect a belt.
Well forget the technical for/against but the slack on the old belt was amazing. With the camshaft locking pin inserted a massive movement of the flywheel existed (as seen on the timing marks).
Made me quite worried I had agreed to give the customer a lift back home in his own car and ended up putting a good few miles on it.
David W [Moderator]
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mailto:david_moderator@honestjohn.co.uk
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David,
No-one's following up your initiative in getting a timing belt thread going (do they have threads, or strands?) so I'll give you a hand.
I would have thought likewise to yourself, that there is more than pure mileage affecting belt life. Surely rapid acceleration of the engine puts a strain on it which could shorten its life, especially when it is already getting old. And, like tyres, natural aging degradation must take place.
My local very-expert garage man says that Vauxhall and especially Ford belts need changing at much shorter mileage intervals than recommended by the makers but he has never known a Peugeot/Citroen belt fail before the recommended change point.
I don't like the things anyway, which is one reason why I went for a 1992 BMW with the M50 chain-drive engine rather than an earlier one with the M20 belt-drive engine (and certainly rather than the later M52 engine which was basically the M50 but with an unlinered aluminium cylinder block instead of foolproof cast-iron).
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Another very good reason as to why I hate driving cars of which I don't know the history of.
It wasn't that long ago that I turned the key of a p/x 1.8TD Mundano in our car park to shift it round to the compound and the engine shuddered and went bang, Yep, you guessed it, the belt sheared.
Not clever at all.
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I agree there is more than mileage afecting the life of a timing belt. The essential ingredients are: fibres to take the strain, and a rubberised coating to hold them in place and protect them.
The outer sheath will suffer atmospheric and heat/oil mist attack, and eventually lose its effectiveness. As it does this it will become progressively cracked in the stress areas of the teeth.
The strain members, probably polyester or kevlar, are designed to work in one direction - tension. They will need to bend gently over the sprockets within a minimum bend radius, and this will give rise to flexing. This will degrade the life of the belt. Once an engine is up and running the tension in the belt will be reasonably uniform, but it will be at its greatest when the belt is subjected to the inertia resistance of start up.
David W's comment about the belt being slack is worrying. I would not expect such a belt to stretch - if it did, then the belt teeth would eventually be out of kilter with the sprocket teeth, and lead to accelerated tooth wear. This happens with chains, but the sprockets also wear to keep pace with the chain - up to a point. I've a feeling that the belt was never fitted correctly, or that a tensioner had slipped.
I would expect that belt manufacturers have carried out some sort of testing programme to arrive at a recommended replacement interval,based on cycles (or mileage). Just as in the case of other rubbers (hoses etc) the casing will perish, which is why they put a time limit on the belt. They will have based it on "worst case" scenarios, and this will fit in nicely with car manufacturers wanting to offload maintenance responsibility onto the second owner - it increases their dealers' service revenues.
You can't rely on visual inspection - change belts early and be damned, I say! You'll be damned if you don't.
Ian Cook
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Ian,
I only claim to be a mechanic++ on this, not a tech materials engineer.......but..
Most of the timing belts I see are "slack to very slack" at change time. Of course I assumed it was due to stretching.
Thinking about the material I see if one is torn or cut up it does seem the inner "thread" could stretch.
Time for tensile tests in the garage???
David
David W [Moderator]
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mailto:david_moderator@honestjohn.co.uk
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Timing belts (toothed belts) are designed not to stretch, even over time. However, in reality they do stretch over time and under load.
I have designed machines (not cars) in my dim and distant past, that used toothed belts to transmit drive. On these machines, whether a tensioner was fitted or not (because one can design a two pully arrangement without a tensioner pulley) over time a degree of additional slack appeared.
However, from tooth to tooth the amount is relatively small, so the belt still runs correctly around the pulley. Also, a small change in length is observed as a much 'larger' slack - the overall length change is small and so the timing is still accurate.
Hope this helps!
El Dingo (Martin).
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Time for tensile tests in the garage???
Thats a good one, as a tensile test is a destructive test.
"Do you want your timing belt tested free, all as part of the service sir?"
Later phone call.
"We did the test and we really must fit a new one as a result" Not a fib been spoken. ;-)
Also Dizzy's comment about Pug/Citroen belts was interesting.
Quite prepared to be shot down here , oh Fenmeister, but I always thought Citroen belts were quite heavily stressed as they have the frequent on off loading of all those power pumps. Or does it vary model to model?
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FIF,
I was thinking of tensile testing a removed belt hung from the workshop roof then measuring extension vs loading. There are plenty of heavy things to hand here...shetland pony for one.
As to Cit/Pug belts well I am pushy about changing them at/before interval but no I've never had one snap yet. It is just the cost of the engines (diesel in particular) that stops you taking a chance. No it's the Ford 1800 diesels and Ecotecs that have given grief here.
You are right that some Citroen models have a camshaft driven hydraulic pump so the timing belt is carrying this load as well.
David W [Moderator]
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mailto:david_moderator@honestjohn.co.uk
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FiF, you said: "Quite prepared to be shot down here , oh Fenmeister, but I always thought Citroen belts were quite heavily stressed as they have the frequent on off loading of all those power pumps. Or does it vary model to model?"
My friend the garageman only said that he has seen no failures in Peugeot belts. I added the 'Citroen' bit in the belief that the engines in both are basically identical, but I forgot about the power-take-off differences. Please ignore my reference to Citroen!
By the way, thanks for your useful elaboration on my stainless exhaust posting. Interesting about the huge increase in machining speeds for stainless steel, though I assume that stainless is still 'relatively' more difficult to machine than mild steel as I was suggesting.
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Dizzy you said
"though I assume that stainless is still 'relatively' more difficult to machine than mild steel as I was suggesting."
Yes totally correct in that assumption, just that the gap has narrowed significantly.
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Dizzy,
My local very-expert garage man says that Vauxhall and especially Ford belts need changing at much shorter mileage intervals than recommended by the makers but he has never known a Peugeot/Citroen belt fail before the recommended change point.
I've just had a similar discussion about changing the timing belt (at 40000) on my Pug 406 with my local independent specialist. He also reckons that it's overkill on Peugeot/Citroens *as long as the car has not been mainly used for short start/stop journeys* and that the recommended 72K change is sufficient. Mind you, he won't have to fork out for a new engine if it does snap will he? Deliberations, deliberations...
Question for David [Moderator] - If I do go ahead and have it changed (£99+VAT), is it false economy not to change the tensioner as well (£35). I'm aware that HJ recommends this, but does this apply to all vehicles or just GM & Ford?
Thanks
Rich Mixture
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RM,
You have to trust your mecahnic on the tensioner issue or specify a new one anyway. My thinking is....
If you change the belt at half interval around 40-50K then the tensioner should be fine up to the next change. If you are going the full 72K then the tensioner can be in poor condition by the next change at 144K.
The Cit/Pug tensioner is mostly a better made steel item compared with the plastic Ford/Fiat/Vauxhall ones.
David W [Moderator]
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mailto:david_moderator@honestjohn.co.uk
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Actually I think I've thought of another explanation.
Customer attempts to change belt himself.
Customer realises that job is beyond his ability
Customer puts back together and passes to someone who knows what they are doing (David).
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I think that there was a thread on the old forum about this.
It got quite involved and referred to the aviation cycles/hours wear syndrome. Maybe it bears repeating in paraphrase:
Put simply,(and -v e r y- generally) "cycles" cause more wear than "hours".
A 747 doing transatlantic and similar trips will have massive hours but disproportionatly less cycles (take offs, pressurisations, depressurisations, landings.)
Conversely, an aircraft doing short haul stages of around one hour will have almost the number of cycles as hours.
(the 737 that lost it's roof ("737 cabriolet") and mercifully still landed O.K. some years ago had around 69 000 hours and about 60 000 cycles. Phew! It had been "island-hopping" in the Hawaii area for all of it's life)
Yes, hang on, there is a valid automotive point somewhere in this stuff! Honest!
I would change a taxi/school run/shopping cart vehicle belt way before the recommended interval. Conversely, I was not too worried about my Pug XUD9 running to 46,000+ on the same belt recently, even with mild oil contamination;- it spends most of it's life at 70+ on long trips.
Ramble over. Time for Horlicks.
rg
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