Pedal start for four strokes
Hey, I've been browsing all over the place getting ideas and noticed mention over the years and threads of, basically, how to get around pull starting the four strokes. The last thread I read was discussing electric starting options and petered out in the middle of 2008.
What about this? (bear with me, a drawing would be nice but beyond my computer talents at this point).
The motor is mounted within the triangle of tube (as conventionally) as close to the crankset as is mechanically possible. I haven't done any real experimenting yet so here's some very rough numbers for the sake of visualization. On the motors' pull start side is a 20 tooth sprocket. On the crankset, inboard (or outboard) of the chainring that drives the rear wheel is another chainring with sufficient diameter to come within about an inch of that 20 tooth sprocket on the motor and perfectly aligned with it.
Now the tricky description, but some of you might have already guessed. Take a, I don't know, pair of 14 tooth idler sprockets and a handful of 1/2 inch segments of steel rod of the exact diameter of the chain roller bushings and set the pile on your workbench. Now tig weld (everyone does that right?) the two sprockets together by aligning 14 (or is it 13? my head is starting to ache) of those steel rod segments into the valleys between the teeth and then spot welding (won't take much) each end into it's space. (god I need a picture).
There are a few ways of doing the above, heck, there might even be an off the shelf version of it some where, but I doubt it. But if you get the picture at all, you now have a "gear" to mesh together the 20 tooth sprocket on the motor with the very large sprocket on the crankset that is spinning (you're pedalling right now in your imagination, right?).
The idler "gear" that you've produced by following my very excellent instructions will be attached to a spring loaded arm and the arm attached to a lever at the handlebars such that while the crank is spinning (keep pedalling dammit) you can bring the "gear" down between the sprockets. The torque on your crankshaft sprocket and the resistance of the motor will suffice to capture the "gear" between the sprockets and maintain the mesh as long as there is resistance from the motor (compression, valve train, etc). And here's the beauty! As soon as the engine ignites, it will eject the "gear" from between the two sprockets, returning the "gear" on its arm back to the ready to start the whole shebang over again as soon as you're ready to start the whole shebang over again.
If I had a drawing it would be revealed to be a much much simpler and elegant solution than my tortured prose has suggested to pedal starting a four stroke motor with centrifugal clutch. Maybe I should have just waited until I'd tinkered this out and photographed it before burdening you all with such ravings. But I'd be interested to know if anyone has done this before, or if there is a yawning chasm in my thinking on this. Maybe it wouldn't actually work!
What do ya think?
Last edited by frascati; 08-21-2009 at 10:50 AM.