Lew,
I agree that an AT20 might hold. Our bikes have a torque of about 60Nm at 3500 rpm. Taken the total ratio of the primary, secondary and first gear of 11.73 and a 70% efficiency would give us a belt width of 85 mm. There are room for about 30 mm, which leave us below the 3x overload capacity you state in your post. Plus we have to consider that we use this high torque only on rare occations, at normal driving conditions we probably won't even overload the belt. My major concern is the tension. A steel reinforced belt just can't stretch. Our swing is mounted behind and below the front sprocket which means that when our suspension works, the chain streches and slacks. The way we would have to set up our transmission would be with the belt tight with no weight on the bike. Then, as we load the bike, the belt will slack. Go sit on your bike and keep a finger on the chain and you can feel it slacking as the load increases. Even though the tooth on an AT20 belt is 5 mm high, they still can't take a lot of slack without wearing down quickly. The obvious solution would be a tensioner, but it would have to be a spring loaded one, and they are pretty hard to find a good place for. I can't really give you an option that I know will work, except for the unbelievably expensive Harley solution. Those belts can take it, but if you, for instance, bend them the wrong way just once when mounting them, it will render them useless. The ATP-belt would give us a smoother and quiter drive, plus it can take more torque than the AT-belt, but the stretch problem will remain. I will look deeper into this issue next winter, when I plan on converting my own bike to belt drive. Please let me know what you settled on, and how it works out.
