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Is there a proper way to space crank arms in relation to the bottom bracket? Ive noticed most builds are generally +10mm on the right side in relation to the bottom bracket, but i feel like maybe they should be even for leg ergonomics? Is there a standard here from edge of bb to the outer edge of where the crankarm mounts to the bottom bracket?

  • You have chainrings to fit on the right side, so it would be not unreasonable to expect certain degree of assymetry in how the cranks "look". But if you properly measure everything from the bicycle's centerline, distances of cranks should be equal. – Grigory Rechistov Mar 31 at 20:02
  • i am using a caliper, and i have compared many factory and pro builds. They are often +10-15mm to the right, but youre saying im correct in they should be symmetrical? Can you confirm a width standard if you need to correct on one side? Or is it whatever width, just symmetrical – polar Mar 31 at 20:04
  • There is no width standard either, because different bicycles have different Q-factors. A fat bike will inevitably have a longer 100 mm BB width to fit 5" tires, compared to a track bicycle where a 68 mm bottom bracket and proportionally narrower cranks are appropriate. Mountain bikes have BBs of 68, 73, and 83 mm widths, and who knows what other "standards" will be, are or were in use. – Grigory Rechistov Mar 31 at 20:10
  • Most cranks have a recommendation for chainline, which is the distance from the bike’s centerline to the middle chainring on a triple crank or between the chainrings of a double. – Andrew Mar 31 at 20:42
  • BB spindles are often/normally longer on one side, because chainrings. But If 10mm was important for leg ergonomics, the important measurement would be where the pedals interface each crank, relative to the bike centreline - does this offset you have measured come through to the pedals? – Swifty Mar 31 at 21:29
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The gap you're talking about is more or less random and does not itself affect how symmetrical the Q-factor winds up being.

The quick visual check for Q symmetry is to eyeball against the chainstays, which is close enough in almost all cases to know whether there's a problem unless the frame either has an asymmetrical rear end or is smashed. To get an exact measurement, one uses the depth gauge part of a vernier against either the seat tube of a round-tubed bike or the back rim (after checking dish) for bikes with asymmetrical frames.

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  • q factor is off by 5mm – polar Mar 31 at 21:33
  • Is it possible you've got an older crank that wants an asymmetrical spindle and you're trying to put it on a modern symmetrical BB cartridge? What are the cranks? – Nathan Knutson Mar 31 at 21:35
  • no, its an older square taper bb that came with crank arms specific for it. i cant change the this builds bb spacing but im thinking of adding two 1.5mm pedal spacers to the negative side and calling it good. – polar Mar 31 at 21:36

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