the timing belts on my bike were replaced for the first time last August...that would mean they were at least 18 yrs old over the years I gave them a couple of visual inspections and adjustments...according to a lot of posts here, this must be some sort of "timing belt longevity record"
I do think though...every two years might be too often, has anybody on this forum had broken belts, and if so how old were they

It's easy enough to do and the price isn't too bad so it's cheap insurance. I know of at least 3 bikes that have had broken belts. all were about 3-4 years old; 1 ridden regularly and 2 not so much. It's an ugly event that leaves behind damamged engines. On 1 bike we were out in the hillside and to limp home, decided to ride on one cylinder. The bike made it home and luckily didn't do extensive damamge. Change them when you should and you can sleep well at night.

thanks for all the replys, first of all I would like to mention that I didnt know any better, I didnt do it to save money or because I was lazy, but I was convinced that they should last at least 20K km (as per 907 workshop manual) now I know better (thanks to this forum )
Jon...the broken belts were they on two valve or four valve heads, I think twin cam, the belt has to work twice as hard and may be more prone to breakages...when I did finally replace the belts I had a good look at them and they still looked in good shape, but I agree its cheap insurance and peace of mind...which now makes me think, that chain that I replaced about 8yrs ago may not be the proper rating and I will soon replace that as well with rivet linkage (not clip which I have now)
then the rubber...the tyres were replaced around 2003, nearly eight years old...the rubber must be hard and should be replaced...not before a "burnout photo shoot"

What you can`t see is that the belts as they continuously dry loose their elasticity. Check the belts tension with cold and hot engine, there will be a difference. An old belt could not be able to compensate this and start cracking.
Regarding changing intervals it`s 20K km or every 2 years, whatever comes first.

G.

As well as the belts, do like in the car industry and change the tensioner as well. The tensioner on my VW golf starts to moan just about when the belts are due for changing. Giving that bike engines run on average a higher RPM then it is worth changing tensioners also.

Something worth noting is that as belts stretch with use they drag the timing out a little (maybe 5 degrees) which is enough to make the engine not run perfectly. My 750 was noticably smoother after fitting new belts having retimed the valves.

Dont forget to tension the belts regularly. A belt slipping is far more common than one breaking, but with potentially similar consequences.

higgy, can you please explain...I thaught it was a good practice to keep the belts reasonably tight, but not to overtighten them too much...

Hmmm,seems I am going to have to back pedal on this one a bit. Normally with this kind of belt mainly due to the size of the pulleys and the type of tensioning used. No spring type automatic tension control,it would be a very bad idea. Seems Ducati sees things a little differently. Big surprise Eh? This goes a long way to explain one of the shortest replacement intervals across the entire drive belt industry. In the automotive world 60,000 miles(96560Km) would be a typical belt replacement.

more later,I owe I owe its off to work I go

The underlying reason for the short time interval is the bend radius that the pulleys have. The fiber GLASS is strength material that makes it very non-stretchy also hates to be bent and fractures over time. This why the time limit vs mileage. Change them, on a 2 valve engine, Stevie Wonder can do it!!!

If you want to break a bealt, kink it and you can hear the fiber cracking.

Belts like tyres have aramid fibres in them. The aramid is elastic and will stretch somewhat with changes in acceleration. Over tightening the belt will mean the aramid is already stretched, hence no damping effect and breakage. Same as any other material really, except aramid stretches more than most so there is more leeway than with steel for instance.

Aramid fibres are thermotropic liquid crystals. With repeated heating/stressing can result in changes in the alignemt of the molecules in the material (they tend to coil up rather than remain aligned), and hence a loss of strength and elasticity.

Also, energy depends on velocity squared, and the momentum depends on acceleration. Both are much higher (on average, and all other things being equal) on a bike than a car hence the need to change belts more often on a bike.

The trick is dont stress the system by heating/accelerating. Industrial drives tend to run continuously hence stable temperature and no acceleration forces. At university we had a vacuum pump that ran at 60,000 rpm 24/7 x 51 weeks a year for 5 years before the ceramic bearings wore out. It was down once per year to change the oil. Not a belt drive system but the illustration of the benefits of continuous running are illustrated.

A bike is a hostile environment for materials. Probably only a boat being more hostile as far as consumer systems go.

Changing /adjusting belts is a small effort for the enjoyment the bike delivers overall.

The belts on my 1992 907ie were changed - for the first time - a couple of months ago. The engine was run up a couple of times before their change without complaint - but she did no road miles with them before they were changed. (I didn't keep them)

It would seem to support the idea that time doesn't really effect them but use/stress does. My bike had been sitting in a museum for all its life and had only ever done 1900 miles - so no real stress on the belts - and nowhere near the 20,000km life span BUT way past the two years. The danger being natural degeneration over time.

Don't fully understand many of the above statements but totally comprehend their meaning: I will change the belts every two years like it was the 11th Commandment!