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Honda GX31 Power Assist

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Power assist bikes are slowly gaining popularity these days, usually in the form of electric power. Two stroke weed eater engines have been poplular for power assists too, but the noise, smoke and unreliability from these short lived engines never appealed to me. When I found Hugh Currin's Recumbent Bicycle Power Assist page and Ross' Honda GX31 page I became interested in Honda's small 4 stroke engines, the GX22 and GX31. I found the engine from Roy Padget's Small Engine Warehouse online for only $189 and ordered one. I also bought a clutch housing, drum and shaft from the Tulsa Engine Warehouse online.

The GX31.

The spiffy red anodized alloy clutch housing manufactured by Staton Inc. Attached is a 1 1/2" v belt pulley.

While I really wanted to mount the engine on my recently finished home built short wheelbase recumbent, I thought that it'd be easier to mount it to a mountain bike I recently aquired for free (!) from the nearby landfill transfer station. To facilitate a power assist on this bike I swapped the rear hub on it with a steel hub, and brazed a piece of a coaster brake hub on for a left side drive cog

The transfer station bike. All alloy components, TIG welded chrome moly, free!

The hub with the brazed on flange for a left side driving cog.

Photos on the "Power Fork for Any Bike" page convinced me to use v belts for the primary gear reduction, so I bought various new and used v belts and pulleys. Then I discovered go kart parts and ditched the v belts for toothed belts.

A 14" x 1/2" v belt pulley.

90 tooth aluminum pulley, 14 tooth steel pulley, toothed belt.

Then I discovered Staton Inc, your one stop shop for gas engine kits and parts for bicycles. They have a good selection of speed reducing gear boxes which negated the need for any other gear reduction parts. Given this, I revisited my decision to not retrofit my recumbent.

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