I’m building up a bike for my son and have managed to find some rings for an old 165mm crank arm. I only have the drive side, so need to get a left hand arm. I can buy the matching Thorn one for £25, but there are others around for half that. I imagine they’re all pretty much identical, but if not (ie if the Q factor differs by a few mm) does it matter at all in real life?

assuming squaretaper

make sure you have square or diamond set taper…

ie your cranks will be 45degrees out if you get that wrong 😀

then there is ISO vs JIS taper but – look on sheldon browns site for diagrams and explainations.

Thanks. Yep, square taper. Both cranks will be the same length and fitting, I just wonder if it matters whether they are identical or potentially differ slightly in terms of their offset

Thanks for the Sheldon tip – I had in fact already looked :-). He seems to have been unusually quiet on this point…

I once managed to install my cranks 180 degrees out of synch, something didn’t look right but I couldn’t see what was wrong as hard as I looked and the more I did the more convinced I became I was being paranoid, it became very apparent when i tried to ride the bike.

<Edit> With regard to the q factor etc I can’t help directly but having ridden a bike with the BB spacers incorrect ds/nds the wrong way around (essentially throwing the q factor) the only noticeable thing was getting the indexing right

As trailrat says, watch our for the orientation of the square for the square taper. Some companies have the square in line with the crank (Shimano) some offset it by 45 degrees (Suntour, which I suspect your cranks are).

KF (Fisher Outdoor’s own brand parts) do very cheap crank arms in either square square taper or diamond square taper, have a google and they should come up.

ah, now I get it – apologies trailrat, I didn’t pick up on what you meant about diamond/square but I see now. That’s really important, and very helpful to know! I’ll check the cranks when I get home for their orientation an take it from there.

Won’t cause any problems.

Just been riding a bike with different length cranks!

Thanks all 🙂

Just been riding a bike with different length cranks!

I had this on my winter/hack bike for a (long) while. Mind you it happened to Cav in the Tour according to his autobio.

All sorted; he seems very happy with it!