small transmissions

[HydroJim]

Anybody know what kind of transmission is used in this set-up?

http://www.youtube.com/watch?v=ORgPqGCkN2A

I'm having trouble finding information on this kind of set-up and for some reason, I can't wrap my mind around how it works.

[mechman600]

Google "Comet CVT". Same thing as snowmobiles and scooters.

The front (drive) clutch is centrifugally operated with a set of weights. More RPM gives more pressure to the sides, causing the belt to move to a larger circumference on the drive clutch (pulling it to a smaller circumference on the driven clutch), deceasing the ratio (increasing the output speed).

The rear (driven) clutch is torque actuated with ramps. More torque applied causes the pulley sides to squeeze together, causing the belt to move to a larger circumference on the driven clutch (smaller on the drive clutch), increasing the ratio (decreasing the output speed). A spring keeps the driven pulley sides initially closed together.

So you take off gradually. The drive clutch speeds up enough to initially grab the belt and the vehicle starts moving. As the engine continues to increase in speed, the weights cause the ratio to decrease. However, if you "floor it" or encounter a hill at WOT (etc.), the driven clutch forces a higher ratio and revs increase for the increased power demand.

Tuning these things is generally a balance between weight size in the driven clutch and spring weight in the driven clutch. Smaller weights/more spring pressure = higher RPM at max power.

Just rent/borrow a scooter and go for an hour ride. It will become clear pretty quickly.

[HydroJim]

Thank you for the explanation.

This is just what I needed for my trike build! a simple transmission system. I take it they are not the most efficient?

[mechman600]

I have heard that they are about 80% efficient. But that's anecdotal, so who knows. I believe it - a V-belt will rob power turning all those corners.

Have you thought of using a small transmission from a two stroke dirt bike? I have heard of guys removing the crankshaft and machining a through shaft to replace it (supported with sealed bearings), driven on the stator side with a short serpentine belt (more efficient than a v-belt). The whole unit can be mounted on end to create space, as these transmissions are not pressure lubed anyway. That's what I would like to do when I get around to making my diesel motorcycle.

[HydroJim]

Problem is I'm not looking to have a clutch or any gear shifting. It has to be automatic.

[mechman600]

A Comet clone (I don't think Comet is in business
anymore) will work well for you then. They are generally tuned for a 3600 rpm peak HP engine, aka any small Honda/Briggs. About $100 on eBay.

Good luck!

[P-hack]

yah, comet (or re-purposed scooter) CVT transmission is your quickest/cheapest option for no shifting at that scale. If your trike is electric you might not need a transmission to get rolling too, though it can still be advantageous to have one.