Mounting a big sprocket on a little one.

Particularly useful with Sram, Shimano and Sturmey (spit) gear hubs. This pic shows one way

The big sprocket is bored out to allow the little one to sit in then it is drilled and tapped to allow the two to be bolted together.

If anyone has other methods, feel free to post.


  • You are a genius
    so, the actual small sproket uses the teeth to lock the bigger sproket

    Pure engineering
    What ratio is that set up in the picture. I think it might be about 5 : 1 ?
    correct me if wrong
  • Standing on the shoulders of giants 13 :-)

    Almost everything has been done before.

  • There's not so much room with the shimanos - the changer mech. is also between the dropouts & sprocket room is further inhibited by the cassette and cable. We have always blted as above to ensure concentricity & then MIG welded & taken the bolts out. The M8 bolts we used did go round OK but were within a mm of the cable & it was a disaster waiting to happen.
    Weld a bit at a time & quench lots or the cog will warp.
    The shimano sprocket teeth give you nice pockets to put "rosette" welds into....
  • TSR-2 has used this method since it was build in 2003. Using 8 mm platewheels, the centre is bored our until the sprocket is a tight press fit inside - using a big vice. This takes advantage of the inherrent springiness of the bike sprocket. The 8 mm platewheel is also skimmed to be the same thickness as the bike sprocket for about 10 mm extra diameter, then a number of bolts (M5, with cheap squashy washers each side are simply tightened up through the holes made by the bike sprocket teeth. NEVER had a single problem with this method,


  • We are hopefully going to attempt this method of gearing on our Shimano Alfine Hub (which is built into a 20" wheel).

    Does it matter if the centre of the teeth on the new sprocket are not directly aligned over the centre of the teeth on the original hub. I have some basic physics knowledge but not enough to deduce whether the slight off setting might cause major torsional stresses upon the hub.
  • Shouldn't be a problem.
  • Wooo. I guess I now have the fun task of going round all the local motorbike shops and making a complete fool of myself & trying not to spend to much money + remember to get the chain and another sproket for the motor.

    Arr. Multi-tasking arrr.
  • We have reliable used a similar method, but have turned a hole the size of the small sprocket, in the centre of the large sprocket using the lathe. We then lay the small sprocket in the hole (on a flat surface) and mig weld the tip of each small sprocket tooth to the big sprocket hole.
  • Sounds similar to BobC's method, unofficial Greenpower Guru! I'll need to take the shell off Blazer 2 and see exactly how much space there isn't. Hoping that it'll take a 5/8" pitch sprocket with a nice solid chain so I can get rid of the bicylce half link chain we have just now. Very easy to fit and shorten to suit, but a bit too much lateral movement on the teeth for my liking.
  • Interestingly, did you have to make a homemade faceplate to mount the sprocket in the chuck for counterboring?
  • We are using 8mm chain and sprockets which are considerable smaller than 1/8" bicycle sprockets, this allowed us to just used the external jaws on a Colchester Bantam for counter boring.....
  • I used my 4jaw chuck to hold the big sprockets - takes a while to centralise it properly but I'm getting better at that ;^)
    5/8" pitch chain sounds a bit enormous!!! We actually use 1/4" on the motor (smallest chain in greenpower I suspect - unless someone is using 6mm..) The tiny motor sprocket seems to last about a season.
  • Likewise, external four jaw chuck. I actually go to another school and use theirs as ours is pants.
  • Alternative way to get a big cog....
    you don't have to buy one from HPC or the like. You can make one. And it's easy.
    Here's one Rich made for his bike.
    That's a 72 tooth bike crank chainwheel; try to find one of them on the web!
    What Rich did was....
    1) use a "free off the web" autocad type drawing package to design his chainwheel. Took about 30 minutes.
    2) email the .dxf drawing (must be 1:1 scale) to our local laser cutting firm. Asked for it to be cut out of 2mm stainless steel.
    3) 2 days later, took the man £25 and went home with an unobtainable (outrageous) chainwheel.
    4) tooth edges needed slight chamfrering to work with the deraillieur.

    I really think this is THE way to make large wheel chainrings - you cheaply get any number of teeth you want, total accuracy (literally within 0.02mm), holes where you want 'em, the right shape in the middle to go straight on a gear hub for example, and absolutely minimal fettling to make it work well.

    We're lucky enough to have our local laser cutting firm at the end of our street.
  • This freeware looks as if it will do the job, (as long as you don't want to go below 12 teeth).
  • edited May 2010
    Good one Brian, but that seems to be set up for doing the hard work yourself, sourcing material and lots of drilling & filing - all very worthy but not very 21st century ;^). Autocad file to the laser cutters; they source a finished item for little or no more than the cost of the raw material!
    I've been converted in a big way to trying to use the laser cutters for as much as possible. I think it's a good angle for the kids too - get them to do the CAD - it's much more like the REAL world of engineering in the 21st century.
    PS can you imagine trying to make a stainless cog that way - how many 5/16" drills would you need.....
  • Brian's Australian link no longer seems to work. Here's a useful page with a good method for creating a tooth form for your own sprocket designs.
  • Hi Peter, Do you use and/or have access to Solid Works? ;-)
  • Yes I do and yes I do.
  • Check your inbox on Monday!
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