Power To The Wheels
Drive train, Sprocket, and Rear Assembly for Mini Choppers
The drive train is a simple system on karts and mini bikes. The cheapest option to get the power from the engine is the centrifugal clutch. For a much better option, a torque converter can be used which effectively gives the project an automatic transmission.
[Read more about torque converters and centrifugal clutches].
Our design incorporates a centrifugal clutch with direct drive to the rear sprocket — however a jackshaft could also have been installed to increase speed, increase torque, or to simply have two runs of shorter chain instead of one long run.
Our clutch is an 11 tooth Max-Torque centrifugal clutch. The #35 chain was very long (Fig. 1), and we've since added a chain guard to protect the rider if there is a breakage.
Even for a short chain, a guard is a good idea. If the chain breaks, the guide will ensure that it doesn't wrap around the rear sprocket and lock up the back wheel.
Our chain guard was made from 16 gauge material found at a scrap yard. Two 3" pieces were welded together, and then the profile was cut with the grinder. Finally, another 3" piece was tacked to the profile and finish welded after the shape was acceptable.
After some finish grinding, and a few holes drilled for heat dispersion, it looks pretty good!
Though you can purchase small chain guards from parts suppliers, the positioning of the mini-chopper's clutch is dangerously close to pants legs. An enclosed guard is much preferred.
Sprocket is a 60 tooth #35 pitch. This size sprocket needs a 5" wheel for proper clearance. The example photo shows a 4" wheel. Before selling the chopper we switched it to a 5" wheel. The ground clearance on a 4" wheel is not adequate because on a two wheeled chopper you make sharp leans when cornering. When doing this, the sprocket will scrape the ground, damaging it and the chain.
During the test drive, we over-inflated the 4" tire to compensate for the lack of clearance; it blew out! Do not do this, get a 5" or larger rear tire for this sized sprocket.