On Aug 28, 4:05 pm, "Morgans" wrote:

I recall that while it is important to vary ratios in a toothed gear redrive
away from exact ratios like 2:1, so different teeth mate with both gears
(prevents wearing a certain pattern in each other) that is not a necessary
condition for toothed belt redrives. I recall that in fact, it is not
desirable to do that, but again, my recall is incomplete.

Rotax two-strokes are available with a variety of gearbox
ratios, among them 2.0:1, 3.0:1 and 4.0:1. The others are odd, like
2.24:1 and 3.47:1. They've been building these things for a long time
and I expect they've figured out what the problems with even ratios
are and how to fix them.
http://www.rotax-aircraft-engines.co...n.Data_503.pdf
I have a Hummelbird that I want to get finished someday, and
I've considered the Rotax 503 for it. I don't like the rum-rum-rum
sound made by the odd ratios, and would choose an even number,
probably 3:1 or 4:1 so I could swing the largest prop possible for
better takeoff and climb performance.
The Hummelbird is supposed to use the half-VW, but after
Veeduber's advice about the VW's inadequate cylinder head finning and
its resulting propensity to burn its valves regularly, I think maybe
I'll stay away from it, even though I prefer the sound of a four-
stroke over the whine of a two-stroke. My first car was a VW Bug and
it burned its valves on a long uphill pull. It just wasn't made to put
out 100% power for any length of time.

Dan