Fabricating a large rear pulley

Nashville Geoff

New Member
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9:42 PM
Sep 7, 2008
I'm trying to either purchase or fabricate a large pulley that can be attached to a 26" rear wheel. I'm not using any gear reducing box- Just a centrifugal pulley clutch on a rear-mounted Lawn Boy two-stroke. I had seen a motorized bike, I believe from India, that was using what I thought was a smaller wheel rim as a pulley... That's appealing, but I wonder how well a V-belt would grab without a proper channel, or if it would get twisted, etc. I'm also wondering if anyone has "done the math" as to what size pulley, or wheel, might be the best choice. I want to let the engine operate within it's torque band. I think that with some very well engineered mounts that this could be a very quick & dependable motorized bicycle. Comments, suggestions, part resources? Thanks, Nashville Geoff
research GEBE here... similar to what you are talking about, and can figure out the correct ratio's for your gearing/belt length
rim pulley

I had a motored bike years ago that I spot welded an rim hoop of the same size to the rear wheel. Had to bend the frame slightly for clearance. The front pulley size can be changed cheaply to affect gearing. It can be tensioned to minimize slipping. Can even add a brake lever operated tensioner. Some folks have added 'gription' to the rim pulley (skateboard grip tape, rubber cement glue, spray v-belt dressing etc.) I personally didn't use 'gription' & ran mine with no clutch (direct drive) utilizing engine braking...
Alex, I hadn't thought of welding two rims together... I like that idea; it would strengthen the driven rim. I'm thinking of running a centrifugal v-belt clutch. Any ideas about a v-belt gripping & tracking well on a rim? I suppose that grip shouldn't be much of an issue, with all of that contact area- But making it stay straight might take some engineering.

Nashville Geoff
I tried the centrifugal v-belt clutch without much success. It didn't function as good as I thought it should. The engine I'm receiving soon in the mail has a built in 78mm clutch. I just need to attach the clutch drum w/shaft to mount the fixed pulley on. The rear rim pulley works well & without any problems of the belt wanting to ride up the rim as long as you get the belt line fairly straight with the driving pulley. By not driving the spokes, the wheel is much more durable. Many people I read about break spokes w/the sheaves that attach to the spokes. The belts are very cheap & available in most lengths at any auto parts store. I never had to replace the one I used for that direct drive application.
I'm interested in this conversation.

Attaching a same-size rim to the wheel pretty much eliminates any rim brakes on the rear, no? Also, wouldn't attaching a second 26" rim to the existing 26" rim result in very low gearing? I seem to recall the gebe sheave being somewhere around 17-18".

I've got a brand new Robin 35 sitting in a box just waiting for a drive set-up and I'm thinking hard about a flat, toothed belt driven rig.
Rim pulley

The rim welded on pulley does require the use of either a disc brake, coaster brake or internal drum. The gearing using a larger v-belt driven pulley gets close to the 18/19:1 ratio ideal for higher revving <49cc engines.
I'm a firm believer in the K.I.S.S. method of building a MB & as a DIY project this solves many issues associated with gearing & one-off expensive parts. The biggest challenge is building a narrow rim for the tire & welding a narrow rim pulley to fit inside the seatstay/chainstay junctions (without bending). Perhaps redishing the spoked rim to offset it closer to the pedal driven side would allow a wider rim for the tire (less pinch flats).
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K.I.S.S. is crutial in my book... I'm going to look into fabbing a rim pulley from scratch that's wide enough to handle a belt, and narrow enough to not have interference issues. It might even be set away from the wheel/tire itself. Strength vs. weight, and ease of build... Gotta put on my McGuyver hat.
I finished building my simple drivetrain in this fashion & it is posted now on both the new & old Picture Galleries searchable under ' Rocky Mountain RM7 -Super Motard- '