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. |