SimpleSimon
Active Member
I just scanned through this thread, and I'd like to address the issue of "freewheeling" and friction drives. Setting up a friction drive to be engageable/disengageable "on the fly" is simplicity itself, and it is equally simple to make it a "two-speed" drive. Front or rear, either way is pretty easy to do. A few months ago we had a discussion of variable "gearing" for friction drives, and I posted a pic of my idea for anyone who wants to take it and run with it.
Basically, two rollers mounted front and rear in a piece of 2"H x 4"W U-channel, with their shafts sticking through one side of the channel and identical small sprockets on each. Engine mounts to a fixed plate above this, with "ears" sticking down and a pivot through the channel from ear to ear. Output sprocket for the engine co-planar with the two roller sprockets, a loop of chain around all three. Put a strong compression spring between the fixed plate and the U-channel at one end, run a cable through the fixed plate to a mounting point on the channel centered in the spring circle. Set it up so that one roller is your default (spring expanded) setting, neutral is centered, spring compressed puts the other roller on the wheel. Use a small T-handle lever "shifter" mounted on the top bar just behind the front forks, and a cable to the friction drive assembly. Set up with positive lock detents on the T-handle shifter (sinple side loading on the T-handle by a small compression spring and a notched mount that a pin on the T-handle engages for each position, and you have a two-speed friction roller drive with a neutral position. Centrifigual clutch on the engine, pedal to 8-10 mph and engage low gear, engine pop starts. Coasting just stick it in neutral and back off the throttle - drive is stationary.
edit: As pictured, the red translucency represents the plane of the drive chain loop around the rollers and the engine sprocket, the blue translucent cylinder represents the compression spring. The solid black line rising above the engine mount plate represents the cable.
Sizing depends on wheel size and roller size. The engine mount plate is fixed to the bike like a rack, front or rear, and the rest of it is pretty darned simple geometry. Don't know why no one builds it - if I had a shop and a few bucks, I'd build a prototype just to have real pictures to show you. Keep in mind, the pivot point needs to be centered, but the engine can go wherever works best on the mount - the chain loop is a fixed circumference which will not change as the U-channel pivots.
Basically, two rollers mounted front and rear in a piece of 2"H x 4"W U-channel, with their shafts sticking through one side of the channel and identical small sprockets on each. Engine mounts to a fixed plate above this, with "ears" sticking down and a pivot through the channel from ear to ear. Output sprocket for the engine co-planar with the two roller sprockets, a loop of chain around all three. Put a strong compression spring between the fixed plate and the U-channel at one end, run a cable through the fixed plate to a mounting point on the channel centered in the spring circle. Set it up so that one roller is your default (spring expanded) setting, neutral is centered, spring compressed puts the other roller on the wheel. Use a small T-handle lever "shifter" mounted on the top bar just behind the front forks, and a cable to the friction drive assembly. Set up with positive lock detents on the T-handle shifter (sinple side loading on the T-handle by a small compression spring and a notched mount that a pin on the T-handle engages for each position, and you have a two-speed friction roller drive with a neutral position. Centrifigual clutch on the engine, pedal to 8-10 mph and engage low gear, engine pop starts. Coasting just stick it in neutral and back off the throttle - drive is stationary.
edit: As pictured, the red translucency represents the plane of the drive chain loop around the rollers and the engine sprocket, the blue translucent cylinder represents the compression spring. The solid black line rising above the engine mount plate represents the cable.
Sizing depends on wheel size and roller size. The engine mount plate is fixed to the bike like a rack, front or rear, and the rest of it is pretty darned simple geometry. Don't know why no one builds it - if I had a shop and a few bucks, I'd build a prototype just to have real pictures to show you. Keep in mind, the pivot point needs to be centered, but the engine can go wherever works best on the mount - the chain loop is a fixed circumference which will not change as the U-channel pivots.
Attachments
Last edited:
