Belts
 

Apart from racing or complete stupidity has anyone actually had one of the new belts break?
The newer ones are stronger and i believe the "R" belts are wider as well so i was wondering if this potential disaster has gone away as a threat.

This is a difficult question to adress as belts snap for a variety of reasons.
In automotive appications toothed belts fail for the following reasons.
1, exceeding design life in mileage or time
2, damage due to contamination (oil dirt nuts bolts etc)
3, incorrect tension
4, wear in pulleys, tenioners etc
5, manufacturing fault

I run a car workshop for a living and as a result of my experiences I always change belts at 1 year/12000 miles whichever is sooner, But I am aware that even camshaft drive chains have snapped for no apparent reason. Ducati now use toothed belts which are good quality and very strong but they chose to reduce the width of the belt compared to normal practice as the loads are lower when the belt is not having to compress valve springs.
Ducati made a cost/weight/life compromise and specify a replacement period based on that decision. The reccomended replacement mileage is different for some models this relates to type of use and rpm/load variations.
998 testastretta and 749/999 models have slightly different routing of the belts which should have extended the life of the belts(or reduced the risk of premature belt failure).

I know this does not adress the initial post but hope it sheds some light on the need to stick to the reccomended intervals remembering it is mileage or time WHICHEVER IS SOONER

dave

I think that the threat of belt failures is still with us.

I think it’s interesting to note that the Ducati tool for checking the belt tension on the 4V motors has changed for the testastretta. The testastretta engines, as well as all new dual spark engines, use a harmonic tester that can only be used with the Mathesis diagnostic tool. Also, Ducati now specifies different tension specs for the vertical and horizontal cylinders.

The main reason for tensioning in the first place is to prevent the drive belt from jumping a tooth or walking off it’s pulleys. I would think that the minimum tension to prevent this is well below Ducat’s specs. So why are they so picky about it?

My guess, for what its worth, is that they can’t control the manufacturing specs close enough on their supplier’s belts. The suppliers are able to supply belts with adequate tensile strength and at the dimensional tolerances needed, but there’s probably a significant variation between suppliers and production lots regarding belt stiffness.

So you just stretch a less-stiff belt a little more to get the proper tension, you say? Well, yes and no. In terms of linear stiffness, yes - it makes little difference. But in terms of bending stiffness, it changes the way the belt vibrates between pulleys, and in particular, the frequency that it vibrates at. It certainly changes the natural frequency of the belt so as to resonate at different engine rpm. The different specs for the horizontal and vertical cylinder belts is the clue here.

So what? I think that Ducati has traced belt fatigue failures to this phenomena, so they’re trying to control the belts harmonic vibrations instead of just tension.

One more point bears mentioning here. There’s been a lot written about how to accurately set belt tension. No matter what method you use, what's critically important with any toothed belt is the need to rotate the crankshaft (and the belt) after you set the tension, to assure that the specified tension is the same everywhere along the belt, not just in the part of the belt where you first placed the tension gauge.

Weeksy, My thoughts exactly. Thats why i asked the question.

As an add on to those fascinating posts then why is it often said that the belts are often most vunerable to failure in the first few seconds when the engine has been started from cold - hence the often quoted comment of do not rev the ba**s of the engine when it's cold.

Yes I know about all the cold oil, lack of lubrication etc reasons but I mean specifically relating to the belts when cold

Not sure that the belts are more vulnerable when the engine is cold apart from the thick oil factor and the engine perhaps being a little tighter than when its hot. They are certainly under more load when the engine is running slowly though.