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I picked up a cheap frame recently which came with bottom bracket, 3x speed chainring, crank arms and pedals, handlebars, forks, and front & rear rim brakes.

I've removed the chainring and right crank arm and I want to buy a new crankset but I'm not sure what specs I need to make note of when shopping for it. I know it's a square taper BB, but other than I don't know what else I should be looking out for.

What measurements and/or fittings should I have my eyes open for?

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You'll obviously want to select the appropriate chainring size and crank arm length.

Cranks have a specific chainline (the nominal distance from the chain to the mid-line of the frame). The required chainline is affected by the spacing of the rear wheel dropouts, i.e. a 148mm spaced disc brake MTB frame has a wider chainline than a 130mm spaced rim brake road frame.

If you want to re-use the bottom bracket you'll need a crank with a square taper interface, however there is an annoying issue with matching cranks where the axle is built into the bottom bracket (i.e. '3-piece' crank/BBs: square taper, Octalink etc.): different crank models require a specific bottom bracket axle length to achieve the correct chainline.

Normally, if you can't find a crank that requires the same length axle you have, you'll need to replace the bottom bracket along with the crank. Often, replacing the bottom bracket opens up the possibility of upgrading to a '2-piece' crank with external threaded bottom bracket bearings such as the Shimano Hollowtech II system.

If you are converting a derailleur bike to a single speed (your question is a bit ambiguous as to whether you mean a proper single speed or 'one-by' with a single chainring a derailleur/cassette in the rear) then you can play around with the position of the sprocket on the freehub body to get good chain alignment and chainline isn't important.

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  • The middle ring in a triple set-up might have the correct or at least a fitting teeth count if paired with an adequate rear cog. – Carel Jul 24 at 18:02
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    @Carel problem is chain retention with ramped and pinned chainrings. 1x-specific rings do such a good job of holding on the chains now. – Paul H Jul 24 at 18:30
  • The front derailleur correctly adjusted with the LO-HI screws in a fixed position makes an excellent chain guide. In fact any recycled FD does. – Carel Jul 25 at 19:54
  • Or see if it's easy to get a new chainring for the existing crank arm. Narrow wide chain rings can be found for very little money on eBay or similar, especially for 4 bolt crank arms if this one happens to be of this type. – Superman.Lopez Jul 26 at 14:27

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