I have built a jackshaft from scratch. It was cheap and it works great.
Here's what I did:
Bracket: fabbed a rectangle out of 3/16" mild steel. Carefully bent it around of pipe the same size as my seat post. From the edge, it looks like the letter "U".
Drilled two 1 1/2" holes properly aligned to hold the jackshaft body.
From the Lowe's fencing area, I got one of those fencepost couplings. ID is precisely 1 3/8", OD exactly 1 1/2". You'll need to calculate and trim to working length. This pipe will hold the bearings.
The jackshaft itself was 1/2" stainless steel. You will have to calculate proper size and cut to length as needed.
Bought two high quality sealed high speed bearings size 1/2" ID 1 3/8" OD. Ordered two nice 9 teeth sprockets from Staton Inc. None of this cost a lot.
To make it all work, you'll have to dig in key slots in the proper places to hold the two sprockets. You'll also need to file in a few lines on the shaft for steel e-clips to hold the bearings in place. Think it through carefully.
I drilled a hole through the bracket so a 3/8" bolt would pass just behind the seat post. Even with the big bolt tight, that's not good enough. I made a strong clamp below the bracket to let it from moving vertically. And I tied in a short piece of aluminum flat stock to the seat post bolt to fight any twisting forces.
Nice jackshaft design Timbone.
I'm planning on double pulleys on the motor and jackshaft for 2 gear ratios.
Chain drive from jackshaft to rag joint sprocket on rear wheel on left side of bike.
This is the plan, have not built yet. Considering a wide tired bike where the jackshaft will bean asset.
Find a piece of 3/16” mild steel and cut it into a long rectangle with the short side being 3” to 4”. Length of the rectangle will depend on how far you want to place the jackshaft axle from the seat tube.
The idea is to wrap the piece around the seat tube. Use another tube as your working model away from the bike. Apply heat and bend the piece into a symmetrical U shape.
Once the piece wraps around the seat tube perfectly, figure out where you want to center the axle. At that point, use a 1 1/2” hole saw to set the axle placement. The idea, and it will require precise measurement, is to place a 1 3/8” ID x 1 1/2” OD tube (I used fence couplings this sized that found at Lowe’s - I use them as hub bodies for my wheels, too!) through the two “ears” of the metal U piece. Within the tube, you can use two or three sealed bearings (about $6 a piece) to hold a 1/2” rod axle.
You have to get that jackshaft square and true - the best way to lock it down is to weld it to the seat tube. You can also use carefully placed 3/8” threaded rod to squeeze tightly to the seat tube. Would be best to strengthen that seat tube first.
A million ways to do this. Weld the sprockets to the axle or grind out a key slot.
No trans will be very annoying for stop and go. You'll regret it trust me. I'd advise setting up a cheap belt drive using old lawnmower pulleys/belts. you can even put together a hand/foot clutch by using a tensioner pulley on a pivot. Check out Oliver motorized on youtube he has several builds with belt drive. I need to invest in the budget build way I've spent alot on stuff. I'm guessing it is indeed a bike frame you're using?