We're not comparing with plastic chain anymore, are we? It's already been determined HTD and plastic chain is unsuitable (And they appear to comparable in the severity of unsuitability).
I thought we were. So, is this about noise, or not?
HTD is unsuitable for what? Give a specific example.
Above 5-6 NM, it breaks the belt. Although I'm sure that's partially attributable due to the non-perfect setup, the fact that it doesn't break at lower torque levels suggests the belts rating is being exceeded (Note, I've since reduced the current level to lower the motor torque so it's not breaking it anymore). This experience is corroborated from the technical figures in the catalog I linked to in the previous sheet, and extrapolating from that to a 20 NM motor suggests general difficulty. There might be a possibly workable combination (Like 25mm-width 5mm pitch,driving pulley teeth of 20, and limiting the motor torque to 12 NM - 4 kW at 30mph only needs 7-8 NM.), but it'd be too close to the "edge" for my comfort given each belt would probably cost at least $30. I'll continue looking into, though. Btw, noise levels on a belt and chain at 4000 RPM really isn't *that* much different according to data-sheets I saw.
And, yes, it's about noise. But it's more specifically about something that'll work with the performance requirements and is the quietest of that group. If it's too weak, fuhgetaboudit.
But... now that I'm thinking about, maybe an 80-100 tooth timing pulley might fit in the crank area (With more difficulty to install, of course.). My 60 tooth timing pulley has a diameter of, maybe, 4 inches.