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this post is a little different to your norm, so I'm not expecting much.

I'm partaking in a competition where we are building a chain driven wheelchair and currently have it set-up using bicycle wheels with threaded 19T fixie sprockets, however we need to slow it down. Commercially, 21T is the max number of teeth I can find but require around 34T. I can get some custom made at a lot of cost, but does anyone here have any other suggestions using items that are already available on the market, if there are any? I've seen bolt on fixie sprockets, but they require the hub to be changed.

Any help would be greatly appreciated!

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  • The nearest thing I can think of to what you want is the Shimano Mega range screw on freewheel sjscycles.co.uk/freewheels/… No idea if it would be compatible with your hub. Sheldon brown has a disassembly - it looks like all the cogs are on a screw on carrier - maybe with a bit of work you could just use the 34t on the carrier?
    – Andy P
    Commented Jan 4, 2019 at 8:41
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    What sprocket is on the other end of the chain ? Can you shrink the tooth count on that ? Or consider a jackshaft with two cogs on it, so that you have two chains on each side ? Space and mounting might be an issue.
    – Criggie ♦
    Commented Jan 4, 2019 at 10:35
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    Do you have access to a 3d printer? You could probably 3D print a workable cog of whatever size you need. An FDM machine printing ABS should probably result in something durable enough to survive your competition.
    – whatsisname
    Commented Jan 4, 2019 at 21:11
  • People used to braze / jam up cassette hubs to act as fixed gear.
    – Batman
    Commented Jan 5, 2019 at 3:10

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This is a total hack and I can't recommend using this in a fixed gear drivetrain, possibly not even on a freewheeled singlespeed bicycle. I have not tried this myself and probably never will, but for the purposes of your project, it might just be suitable.

This page claims that with some minimal filing, one can mount a "22t, 104 mm four bolt" chainring on a 16t cog, using regular chainring bolts. Obviously, the bolt circle diameter (bcd) of the chainring in question is not 104, but the author must be referring to a granny ring of a crankset with 104 mm bcd for the bigger rings. 104/64 mm cranksets used to be a well-adopted standard, so most likely the author of this hack used a 64 mm granny ring that had been filed to accommodate a 65-66 mm bcd.

Unfortunately, 34t chainrings probably don't exist in 64 bcd, as the larger 104 bcd rings used to start from 32t. Large rings with this bcd seem to be quite rare, but few can be found on online marketplaces and apparently SRAM has manufactured a 32t/64 mm ring, which is probably the closest you can get.

As you'll need to replace your existing 19t cog, I recommend getting a thick, 1/8" cog with little to no wear.

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  • Extending the thinking - machinehead-software.co.uk/bike/chain_length/… and sheldonbrown.com/cribsheet-bcd.html might help finding a sprocket to match a larger BCD. For instance a 76mm BCD 34 bolted to a 20 tooth cog. (Tooth count of selected sprocket would have to be divisible by the number of bolts on the chain ring, so 20tooth matched 5 and 4 bolt rings) Failing that, welding steel sprockets could be an option.
    – mattnz
    Commented Jan 4, 2019 at 20:33
  • @mattnz I actually considered this idea, if my calculations are correct, the diameter of a cog only grows approximately 0,4 mm per tooth, thus requiring unrealistically large sprocket to even reach the next common size, which I guess would be a 5-bolt 74mm.
    – Wsal
    Commented Jan 4, 2019 at 21:33

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