Mounting a bike sprocket to a keyed shaft

Joined
Sep 6, 2022
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Hi there, I have this 3 way gearbox that I am trying to mount a bike sprocket to.

it has a 20mm shaft with 6mm keyway.
I’d like it to be fixed so no freewheel.

I live in the UK and I haven’t been able to find anything off the shelf. I would love to make my own adapter but I don’t have access to a lathe.

im currently looking for flanges/ shaft coupler etc to make a mount from but not having a great deal of luck.

what do you guys recommend?
 
Joined
May 31, 2013
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Location
South Benfleet, Essex, England, UK
Needs some pictures or diagrams really.

I needed a bigger sprocket on my SA 8-Spd, so I got a "granny-gear" sprocket and welded it to the standard SA 25Tooth one.

Can you not just cut a 3mm steel plate with a hole and a keyway filed in it and weld your sprocket to it?

:)
 
Joined
Oct 19, 2012
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1,930
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Wakefield, UK
Get a 20mm double split clamp and either cut the keyway into it with a grinder or remove the keyway and rely on the clamp to hold itself. Get a right hand thread bottom bracket cup and weld it to one half only of the clamp. You'll need to centre it around the shaft. By only welding to one half you'll be able to remove it. Any screw on sprocket will then fit. If you want fixed then skip the bb cup and just weld a sprocket to one half of the clamp.
 
Joined
Sep 6, 2022
Messages
3
Needs some pictures or diagrams really.

I needed a bigger sprocket on my SA 8-Spd, so I got a "granny-gear" sprocket and welded it to the standard SA 25Tooth one.

Can you not just cut a 3mm steel plate with a hole and a keyway filed in it and weld your sprocket to it?

:)
Sorry I realised that I needed to provide some pics but I don’t think I can upload them to the forum?
 
Joined
Sep 6, 2022
Messages
3
Get a 20mm double split clamp and either cut the keyway into it with a grinder or remove the keyway and rely on the clamp to hold itself. Get a right hand thread bottom bracket cup and weld it to one half only of the clamp. You'll need to centre it around the shaft. By only welding to one half you'll be able to remove it. Any screw on sprocket will then fit. If you want fixed then skip the bb cup and just weld a sprocket to one half of the clamp.
I like this idea. So is the bottom bracket the right size and thread to spin on a fixed sprocket?
 
Joined
May 31, 2013
Messages
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Location
South Benfleet, Essex, England, UK
Sorry I realised that I needed to provide some pics but I don’t think I can upload them to the forum?
No, no pic uploads tothe forum directly. You need to use an external service and provide the forum with link to the pics.
 
Joined
Oct 19, 2012
Messages
1,930
Location
Wakefield, UK
The r/h threaded bb cup is the same thread as a std screw on sprocket whether fixed or freewheeling. Don't use the l/h threaded one by mistake. Here's one I made earlier. Welded the lock ring and after grinding off the cup end just loctited it into the ring. It's auto tightened in use anyway. I could have used a non lock ring cup but this way I kept the welding spatter away from the threads. Also the cheap ones are steel whereas better ones often aren't so are no use for welding unless you can AC TIG. Make sure to get a steel clamp, not an alloy one.

 
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