Well there I was sitting back all smug about the condition of my cambelt until I read a recent post similar to this one on low mileage cars. I have a three and a half year old Polo 1.4 tdi pd that has only done 10,000 miles, nearly all in semi rural areas and hardly ever taken above 3000 rpm except for the occasional blast for a clean out.I know age deterioration and wear determine the life of a cambelt.The recommended change for mine was six years or 60,000 miles, now it is apparently down to four and forty. At 6 years old my car will in all probability have only done 16,000 miles and I was happy to leave the change till that age. What with new VW replacement changes and a train of thought that very low mileage use of cambelts can make them deteriorate quicker it has become a bit of a puzzle to me.If this was your car at what mileage would you change it.
Just curious, does any one know an answer to the following
The 1.4 tdi pd is a three cylinder version of the four cylinder tdi pd and I know the cambelt is slightly wider than normal to take up the load of extra pressure on the camshaft (because of the unit injectors).
Is the same cambelt used for the three and four cylinder engines ?.
Thanks
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If VW say four years change it at four years, I had my wife's Clio done at 5 years as Renault recommend though it had only done 18k miles, only £200 at Renault Minute, peace of mind.
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Even low milage use will include a fair number of starts which is when the belt is under the greatest strain;long continuous runs put much less strain on the belt.As an aside,I had a well- qualified colleague who maintained that the camshaft went on the overun when the engine was on overun-he just would not agree that the shaft was always being driven.
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the car is taken out twice a week = four starts a week I should say this is very low number of starts.
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jc2, Your friend was well qualified in what exactly ??? Regards Peter
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BS.
Business Strategies of course.
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If two shafts are coupled together and the power is cut off to the main one, then which one experiences "overrun" surely depends on which one wants to slow down the faster? True, the crankshaft and pistons have more inertia, but they also have the dampening effect of the compression when the throttle is closed.
I could believe that in certain circumstances the camshaft is overrunning. Am I missing something?
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what amount of pressure is put on the cambelt if you stall your engine
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Why is there more strain on the cambelt when starting?
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its surely all down to momentum........
if something is moving then the energy required is less than trying to move an unmoving object,thats how i see it anyway and have had more belts go from starting cars than actually going when engine running
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\"a little man in a big world/\"
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>>if something is moving then the energy required is less than trying to move an unmoving object
Valve springs are not moving all the time,the diameter of the drive pulley would affect the stress the cambelt is under.Ie if the diameter was smaller then more stress would be put on it to turn the camshaft to overcome spring pressure,larger the pulley is the less strain put on belt,a point made about belts is that age is a problem and deteriorate with age,so even an engine thats only done 12k in four years will if man specify need changing due to age
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Steve
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If age didn't matter, manufacturers would just quote a mileage so if VW recommend 40,000 or 4 years, stick to it.
If you want to gamble just look at the cost differential between a new engine and a cambelt replacement!
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>>its surely all down to momentum........
I'd argue that belts failing at start up is perhaps more to do with static friction being higher than dynamic friction, and certainly much higher than the well lubricated quasi-hydrodynamic regime in operation during normal running. i.e. the torque you need to apply to the camshaft for 'breakout' is more than that required to keep it turning.
Superimposed on top of this friction is a varying torque requirement as the camshaft turns depending upon whether more valves are being opened or being shut at any particular time.
Despite this complex varying drive torque, friction withing the valve train will generally make the camshaft a driven part rather than a driving part. As the camshaft also has little rotary inertia (or flywheel effect), it cannot overrun in any significant way.
Number_Cruncher
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>>its surely all down to momentum........ I'd argue that belts failing at start up is perhaps more to do with static friction being higher than dynamic friction, Number_Cruncher
absolutely :)
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"a little man in a big world/"
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As the camshaft also has little rotary inertia(or flywheel effect), it cannot overrun in any significant way.
There was a thread some time ago on this subject. Someone convincingly argued that at speed the camshaft had considerable rotary momentum, so the individual effect of having to lift the valves up and down was small. However at slow speed with no momentum to carry it over, the load on the belt as the valve springs are compressed is considerable. Hence the propensity to break at start up or idling.
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Hi Cliff,
>>Someone convincingly argued...
I hope it wasn't me!
If one considers the camshaft to be torsionally rigid, one can write a rotary form of Newton's scond law, for the shaft, as below;
The sum of the torques acting on the camshaft = The mass moment of inertia of the camshaft * the angular acceleration of the shaft
The torques acting are;
a) The difference in cam-belt tension on each side of the pulley multipled by the pulley radius
b) The drag force from the bearings and friction from the cam lobes - this gets complicated when the cam is static, because of the difference between static and dynamic friction.
c) The varying force from opening and closing the valves - which itself is composed of a spring force *and* the force required to accelerate the valve from rest. The spring does the job of acceleration the closing valve.
The mass moment off inertia of the camshaft is a physical property of the shaft layout (it's a function of the camshaft mass, and how far this mass is from the shaft centre - it doesn't vary with speed)
So, the forces acting on the shaft do vary with speed, and I would argue that the one most likely to become dominant at high speed is that require to accelerate the valve (proportional to speed squared), while the shaft's mass moment of inertia doesn't change.
The equation I gave above is how the camshaft's motion is governed, to obtain camshaft angular velocity, and angular position, it is necessary to integrate the equation with respect to time, once to obtain velocity, twice, for position.
Number_Cruncher
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Thanks, NC. I think that just about winds up the argument.
So it always requires quite a lot of effort to turn the camshaft, it doesn't get easier with speed, so there is no reason for saying the belt is more likely to break at low speed?
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Thanks, NC. I think that just about winds up the argument. So it always requires quite a lot of effort to turn the camshaft, it doesn't get easier with speed, so there is no reason for saying the belt is more likely to break at low speed?
I think that the high breakout torque (because of the large number of mating surfaces being forced together which all have to begin sliding at the same time*) required to start the camshaft from rest is a potential cause for cambelt failures during starting.
* If you begin to consider the camshaft as an elastic part (which of course it is in reality), then breakout from static probably happens in a sequential manner down the shaft.
Number_Cruncher
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Moment of intertia is not a limiting factor in the stressing of cambelt drives.
The main reason why cambelts tend to fail during initial cold cranking is simply that the belt is at its slackest (engine cold, wheel centres closest) and consequently the tooth engagement is incomplete when torque is applied to the driving shaft. If you examine a belt which has failed during cranking, in 90% of cases, the teeth will be stripped where they engage with the crank wheel.
During running, torque reversal stresses the belt far more than inertia. Remember that the stored energy in both the valve springs and the (diesel) fuel pump will apply a cyclic torque reversal to the shafts as they are driven.
I would not leave a cambelt on any engine for longer than 5 years regardless of mileage due to degradation of the materials. If an engine has been left standing for more than a year, I would change the belt before an attempted start, after freeing the various components,
659.
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If this helps - I run a 1999 Polo TDI 90 Estate with nearly 70k on the clock. My mobile mechanic recommended early cambelt change (40k) stating manufacturers extend services intervals on wear and tear components to satisfy demands of fleet users. Us ordinary drivers don't rack up mileage quite so quickly and probably do more start stop driving so early servicing is preferable to premature failure. So, my oil and filter are changed every 5k not 10 and the cambelt every 40k/4 years. All in the intersts of longevity and piece of mind. My mechanic replaced all three belts (cam, alternator and power steering and the cambelt pulley for £141 two years ago).
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It has a cam drive by chain, not belt. Which is good news really:)
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Please bear in mind also, that belts don't usually fail on their own, it could be a failed tensioner, idler or even Water Pump. An oil leak softens the belt & can cause teeth to come away from the belt surface. We supply an independant garage who perform umpteen Cambelt changes a week, if there is a Cambelt Kit available they replace the whole lot, PLUS water pump!!
The professionals take no chances with belts, the average belt kit is roughly £50-70 from a factor, some are even cheaper and this will be OE quality as well (GATES, DAYCO etc). Also, any competent garage will replace the fanbelt as a matter of course, you don't want that snapping and wrapping itself around the crank-pulley!! PS, check all oil seals behind the pulleys.
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One could write a thesis on this. I can't be doing with all the typing this morning, so:
Alan, change the belts.
Yes, the camshafts are being driven all the time, though the load does vary.
Interestingly, Gates who make belts for OEs and aftermarket WILL NOT warrant a belt that has been removed and refitted. So if you ever remove a belt (head repair for example), replace the belt as a matter of course.
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Makes you wonder how some manufacturers manange to produce non interference engines. Like my Fiat Seicento 1.1 - mind you, it can't pull the skin off a rice pudding.
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A friend's Renault Scenic DCi turbo diesel recently suffered a cambelt failure after the alternator seized and cooked - then threw the alt/pwr steering/air-con belt which subsequently got caught around the crank shaft pulley, tore into the cambelt cover then snapped the cambelt. It happened at 25 mph and the damage to the head resulted in a £1700 bill from a renault main dealer!!
There is a known fault with alternators fitted to certain Renault diesels but due to the high mileage of the car they weren't interested in making a good will payment - even though car has full service history.
Apparently, the alternator pulley incorporates a shock absorber/clutch that is supposed to absorb the pulsations characteristic of a modern diesel.
Anyway, back to the original question. Never skimp on cambelt servicing as the cost of a new belt and tensioners will always be far less than an engine rebuild.
On the technical side of it, the highest stressed part of a modern engine is the camshaft. The point loading between the cam face and the tappet/valve assembly is huge albeit for a fraction of a second. I read somewhere the pressure can be around 20,000 pounds a square inch!!
It's amazing a rubber cambelt lasts as long as they do. (or not as the case may be)
I have seen belts snap well within their service life and vice versa. It all depends on the type of engine, how the car is driven and where. Cars that spend all their lives in town/cities will need their belts changed regularly due to stop start conditions.
Personally, I am glad that most manufacturers are switching back to chains.
Spanner
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Yes, chains are the way to go IMHO...
Thats why I like Fiats Multijet.....( 2005 engine of the year)
On these the alternator and waterpump run off the *fan* belt, which itself has an estimated life of 150,000 miles.
Re volkswagens in the past. I serviced a polo for 16 yrs and always thought the bodywork was above average ( for the mid 80s) but it seemed to me the engine was less so.....
I recently owned a new PD VW it was a relief to get rid of it just as the warranty ran out..... multiple faults and always the cambelt worry hanging over you .......
Ask youselves this.
How many people do you know personally who have had a cambelt snap?
I know 6, and 3 of these being Volkswagen....
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Must disagree with previous comment RE waterpumps running off accessory belts. How many engines have been cooked after the driver continued on with the battery light on, thinking everything is OK apart from the battery not charging?
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Sounds like a 1950s Morris Minor David :)
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