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Old 04-04-2011, 04:12 PM
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hankmanpei hankmanpei is offline
Motorized Bicycle Newbie
Join Date: Mar 2011
Location: Maple Ridge B.C. Canada
Posts: 6
Default Re: Bottom crank jack shaft.

Originally Posted by ferball View Post
I am making a recumbent, so the I have the engine mounted forward of the my rear bottom crank. (I left it intact on the donor bike for the rear half of the build). So I am able to use the crank as a jackshaft. Here is how I am doing it:

1. From a donor single crank remove the chain ring and weld the key way spacer from the left side into the cent of the chain ring. On single cranks the left side thread is smaller than the right side thread. The by welding the key way spacer into the chain ring it gives it the right diameter and spins the shaft via the key way.

2. I chopped off the pedal arm so I had a straight shaft. I had to weld the right side sprocket to the shaft, because the peddle arm is what used to spin the sprocket.

3. I installed the sprocket from step one on the left side, just pulled off the big nut and put it on and put the nut back on.

4. I discovered that the crank covers from a three piece crank that I had lying around was the same thread as a shimano free wheel, so I welded the crank cover with threads facing out to the right side of my shaft so I my pedal chain will now hook directly to the jack shaft on a free wheel.

I will post some pics, but it is a cheap easy why to do a jack shaft. It is also super easy to change the left side gear, so I could make different sized sprockets to change gearing ratios.
Nice plan and I like it, do you have a formula for gear ratio's? what I'm trying to figure out is coming off the motor on the left is a 2" pully inline to a 6 1/2" pully on the j shaft, to the right of the 6" is ???? inline to a 20 tooth sprocket fixed to a 9 speed internal geared hub (SRAM I Motion) to the right of ????? tooth sprocket is a rear BMX freewheel inline with my pedal sprocket, I only have one sealed crank set ( my j shaft) and plan on welding the pully and sprockets on this piece so it's pretty much a one shot deal,
my previous set-up was the two" pully weldwed to the motor crank shaft, to the 6 1/2
pully welded straigt to the steel rear wheel hub, this worked great with top speeds of 45-50 mph, and a perfectly smooth running crusing speed 35-40 mph without the motor screaming but a little slugish taking off (this is why I purchessed the 9 speed hub,
if I used a 20 tooth sprocket inline to the 20 tooth hub would this keep my previous 3:1 ratio and torque the same, or should I change this sprocket to a 14 tooth??? bigger or smaller, j shafts convert horse power to torque t? or f? I really am lst in this area hankmanpei( bodyman/painter extordanaire)
Hankmanpei (Bodyman/painter extrodanaire)

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