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I think VW recommend a check every 20k miles and replacement every 60k miles. It about a 3 hour job. I have done one of these and they are a bit awkward, so its money well spent to get someone else to do it. Why on Earth we still have timing belts, Lord only knows! I dearly love timing chains, invariably less trouble (if they are properly designed - there are some bad ones about).
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Aprilia, It is good to be back on this forum after a break. Like you, I spent some considerable time in the field of automotive design, but with very much heavier equipment. I think you are perhaps being a little hard on the concept of belt driven camshafts. Although the large engines I worked on never used a chain due to its short life (they all had gear driven camshafts and fuel pumps), the car of today is at the opposite end of the scale. To obtain the performance of a modern car at a realistic price, a belt drive is almost mandatory. Towards the end of life of a car with a chain driven camshaft, an uneconomic repair is usually needed; the mighty Daimler Benz amongst others has fallen foul of this. With a belt driven system, the drive can be restored to "as new" condition at a price which is worth paying, even on a banger. The trick is to understand the statistical nature of belt failure. Some time ago, I wrote a thread entitled "when to change a cambelt". Some drive designs are far more prone to premature failure than others and I would now seriously question the wisdom of buying any engine equipped with a 19 tooth drive. Conversely, a 21 tooth drive with a wide belt has a low probability of failure. The wise man finds the limitations of his equipment and acts accordingly. 659.
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Is this a certain Fireblade riding Yorkeshireman?. if so, welcome back.
Even if it isn't, welcome back anyway.
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To obtain the performance of a modern car at a realistic price, a belt drive is almost mandatory. Towards the end of life of a car with a chain driven camshaft, an uneconomic repair is usually needed; the mighty Daimler Benz amongst others has fallen foul of this. With a belt driven system, the drive can be restored to "as new" condition at a price which is worth paying, even on a banger.
I would take issue with the above. I have worked on many engines over the years and seen very few problems with chains as compared with belts (even allowing for the greater prevalence of belts). BMW have used chains on many engines and they routinely clock up 200k+ mileages without problem. MB have used chains for many many years and I have seen few problems. Some of the pre-'87 models with sngle row chains used to stretch and rattle around 150k miles, but the duplex ones seem to run to 250k or more - in fact I've never changed one. Even swapping out a chain on an MB V8 is not a difficult job and compares well with the cost of a cambelt change.
One of the sweetest and most often-tuned modern engines is the Nissan SR20DE - these are incredibly reliable and I know of one with 500k miles that has never been worked on and doesn't rattle. The VQ-series engine is a quad-cam V6 (2.0, 3.0 or 3.5, some with turbo) again the chains almost never give trouble on these, even at many 100,000's of miles.
I agree that, statistically, a properly designed belt system will rarely fail. But the chance is there and they can go completely without warning - so people stump up the cash and have them swapped. Also the ancilliary components (like waterpumps and tensioners) that are driven by the belt can go bad and damage the belt, which then wrecks the engine. In fact this is probably the most common reason for belt failure. I saw this happen on a lovely 40k miles Honda legend (waterpump bearing siezed, belt jumped and wrote off the engine). When chains start to go bad you normally gets lots of warning (rattle at start up etc.) and its difficult to ignore.
Many belts these days can cost a lot of money to change, and I wouldn't necessarily agree that it is worth doing on a banger. There are some belts around that cost £400 or so to change.
Of course there are bad chain designs (the older Micra engine was not great; VW V6 - with chain at the back; and the big GM/Vaux 3.2 etc is a bit dodgy I gather), but on the whole I feel a lot more comfortable with chains.
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It's good to be back for a lively debate. I can see both points of view in the chain vs. belt discussion, but a chain drive is generally the more expensive to make, and consequently to buy. In my experience, the only chain drives which will run to the life of a good engine are either duplex or single with hydraulically damped tensioning where the oil pump is not driven by the chain. There are plenty of single chain drives around which also drive the oil pump, and these just don't last. I had one of these on a Mercedes 190 - not their best model. In the real world of cheap motoring, cars nearing the end of their economic life are generally fixed by the owner or friend, and this is where the belt drive can give a substantial saving over a chain. Twin cam engines have often been a problem with belt drives. In order to preserve a low bonnet line, the temptation is to use a 19t crank wheel which is not sound. Plastic faced idlers are another poor detail. The latest PSA twin cam diesels in which the second camshaft is chain driven from the belt driven cam are an example of how this should be done. However, I would not let the drive run to 150k, which is their recommendation. Caveat emptor - use the forum. 659.
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Take a look in this latest thread:
www.honestjohn.co.uk/forum/post/index.htm?f=2&t=30...6
Three people with failed belt-drive in one thread!
The early 190E (M102 engine) had a single-row chain up to about '87. MB acknowledged this as a problem and went to a duplex chain thereafter. Very few problems with those.
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Aprilia, I wonder, in a good humoured way, if you skipped your statistics lectures years ago? The number of light automotive vehicles with belt timing drives vastly exceeds the number with chain or gear drives. If anything, your thread reference supports a point I made earlier, ie that PSA generally design their belt drives well. If the salient points of a drive design are taken into account, bad products ruled out and a change interval adopted which gives a high probability of survival, problems will be very few indeed, assuming good workmanship.
When the vehicle is old, that high survival probability can be maintained for comparatively little money. This is beginning to apply to motorcycle final drives too, at long last.
I rest my case, with good wishes. 659.
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Aprilia, I wonder, in a good humoured way, if you skipped your statistics lectures years ago? The number of light automotive vehicles with belt timing drives vastly exceeds the number with chain or gear drives.
Oh, I agree. But there are not an insignificant number of chains out there. Think about it - loads of BMW's, all the MB's; most Nissans; loads of Fords since 2000 ( all the Mondeo Duratec and Duratorque Diesels); the Vauxall Diesels, lots of Mazdas, etc etc - not as many as belts, but still a lot of cars. I have heard of almost no chain breakages other than with the VW V6 and the big Vauxhall - which are both poor designs IMHO.
As I said before, the problem with belts there days is not so much the belt, but failure of the bits and pieces that are driven off them.
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If the salient points of a drive design are taken into account, bad products ruled out and a change interval adopted which gives a high probability of survival, problems will be very few indeed, assuming good workmanship.
Lots of ifs and buts here. The "assuming good workmanship" is a particular problem for those of use not capable of diy. The problem with belts isn't just the hassle and expense of changing them, it's that it's easy (apparently, from reports on this forum) to fit them incorrectly. So you're caught between a rock and a hard place - keep the belt longer and worry about it wearing out, or have it changed and worry about incorrect fitting. Either way, the bill when things go wrong can be enormous - I was out £1500 when the belt went on my Seat diesel.
So I'm with Aprilla. Of course I reserve the right to change my mind if my chain-driven Yaris ever gives me any grief .. :-)
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659FBE,
I'm being inquisitive, now. I can see the logic of using a belt of decent proportions, but the inquisitive part of me is asking what is the significance of a 21 tooth drive against a 19 tooth drive? Assuming a similar tooth pitch, the bigger pulley will give less distortion, but the speed of the belt(feet per minute) will be higher. Am I on the right track, or has old age set in.
PS I am thoroughly enjoying the debate.
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Briefly, damage to toothed belts is generally due to cracking at the roots of the teeth, degradation of the bulk material or occasionally, failure of the reinforcing fibres. A 19t drive will exert a higher shear stress on the teeth than one with larger wheels because there will be a higher force at the smaller radius for a given driving torque. As there will also be fewer teeth engaged for a given wrap angle, this represents a compound problem. The bending will be greater with a smaller wheel, which causes the teeth to bed less precisely into the roots of the wheels, further increasing tooth stress. A smaller bending radius also causes the belt to run hotter, due to hysteresis in the compounds used as the belt flexes, shortening the life of the materials. High linear belt speeds will certainly give reduced tension at speed due to centrifugal forces acting on the belt in regions of bending, but in practice this is not usually a limiting factor on engines, where the drive centres move apart and the tension consequently increases as the engine heats up. Aluminium engines can sometimes be difficult in this respect. A longer belt has more material for a given duty, will run cooler and last longer. Caveats: belts are no different from cars or anything else, choose wisely and take good care of them to get the best results. Belt drives have helped to make affordable some very useful engines, especially diesels. 659.
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Belt drives have helped to make affordable some very useful engines, especially diesels. 659.
Totally agree with that one. The PSA XUD diesel which really started the boom in small european diesels back in the early 80's is a good example of that. If it had been chain driven and therefore more expensive,perhaps it might not have been so enormously popular and paved the way for other designs.
Of course speaking from an engineers standpoint (of which i am not) like Aprilia, the chain is preferable.But finance restrictions so often get in the way of good design and i feel the belt is a good compromise and is reliable if maintained correctly.
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Thank you, reply appreciated. I suppose the bigger the better, but the limit is going to be the space needed to fit the components in, bearing in mind that the cam pulley will be twice the size of the one on the crank.
Cheers,
MJM
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Perhaps I slightly highjacked this thread without helping the originator very much. I have no detailed design knowledge for VW diesels so, here are a few general comments. The VW drives are generally robust, with a 1 inch wide belt and in some older diesels, a 22t crank wheel. Paradoxically, these can sometimes give problems with drive resonance, and using a crank wheel with an odd number of teeth often reduces this effect. However I have no reason to doubt the basic durability of this drive. There is one big practical problem which results from their use of stretch bolts on some models to secure the crank wheel. If these are not renewed and properly tightened when the belt is changed, trouble will ensue. I suspect this may have happened to our friend with the SEAT. The later PD diesels place a high shock loading on the belt drive and should be very carefully assessed for suitability. I believe the belt change interval was shortened for these models. The rival HDi design is far kinder to the drive as the high pressure pump is not timed to the engine, and represents a much smoother load. Your VW is probably due for a belt change, and 50k miles would be a reasonable interval, assuming favourable operating conditions. Go to a good independent garage, and make sure the crank wheel bolts are itemised on the receipt. 659.
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659 do you think its worth waiting another 10,000 miles or is it too risky?
I was thinking of taking it to the local VW dealership as I dont know any good independant garages and the dealership had a very nice showroom and it was very good looking. The servicing area was clean and looked really good so I think they will be good but I dont know because as I am no car engineer.
If I ask them to change the cambelt, should I ask them to change the tensioner and crank wheel bolts as well?
And is there anything I should mention to them to give them the impression I know what I am talking about so that they do a good job?
Thanks for the help :)
Ps. I am getting a annual service done this coming week in which they check the cambelt so they should tell me if it needs changing and I checked my service book in which it appears to say the cambelt should be changed every 60k
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I think there would not be an excessive risk in letting this drive run to 60k miles if all is well. However, who do you think pays for the smart VW showroom? Once, whilst purchasing a part for a PSA vehicle at one of their outlets, I happened to go around the back, and spotted a mechanic dismantling a gearbox directly on a concrete floor. It turned out upon investigation that he was subcontracted by the dealer, and was working out of the back of a van. The poor vehicle owner was paying over £70 per hour for this standard of work. Take a few days to do some homework, find a reputable independent, preferably with an interest in your vehicle type, who has been trading from the same site for a good while and get your service and belt done together. As this will be the second belt change, I would recommend that the tensioner and any idlers are renewed at the same time, together with those infamous bolts. Despite the recommendation of VW to check the belt periodically, this is not really possible in situ unless it is grossly defective, so renewal is the only sensible option. As far as appearing knowledgeable goes, this is a hard one. In your position, I would put my efforts into finding someone trustworthy who runs his own business. You could ask to see the old parts, but this is not fireproof. 659.
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