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Seems to me that the Q-factor for an external bearing bottom bracket will be greater than that of an equal size (say, 68mm road bike BB) cartridge bearing. Is that the case?

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    Q-factor is also a function of the crank design. Furthermore, your realized stance will be a function of Q-factor, pedal axle length, and cleat position. – Rider_X Jan 25 '15 at 18:36
  • You should put that as an answer, @Rider_X. 68mm is just the shell width -- you can get different axle lengths in a regular bottom bracket which will also change the Q-factor. – Batman Jan 25 '15 at 19:44
  • @Batman - external bearings were explicitly stated. With these designs the axle is usually part of the crank, this is why excluded mention of the axle as it is typically not changeable in this configuration. – Rider_X Jan 25 '15 at 21:36
  • Yeah, but for comparison purposes, you can't just compare to a "68 mm road BB" since for a given shell width you can get different Q-factors by changing the axle length. For external BB's you can't do this obviously. – Batman Jan 25 '15 at 22:39
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Not necessarily.

For one thing, many cartridge style bottom brackets will have some spindle exposed when the crank is mounted. This is because even though two bottom brackets might both be 68 mm, one may have a longer spindle than the other. If you mount the same crank on both bottom brackets, it might fit flush against the bottom bracket shell on the shorter spindle, but would have a gap on the longer spindle.

More importantly, most cranks curve outward from the bottom bracket so that the cranks will clear the chainstays. You can see what I mean in this picture:

Deore XT Crankset

That's obviously an external bottom bracket, but a crank that sat flush against the bottom bracket shell would have to have an even more significant curve to clear the chain stays (assuming the crank arms were the same length, thickness, etc).

Technically, Q-factor refers specifically to the distance between the point where the pedals attach to the crank arms.

Sheldon Brown Q-factor diagram

However, many people refer to Q-factor in the broader sense of the distance between your feet when pedaling. Given this broader definition, your Q-factor would also be affected by the length of your pedal spindles combined with the position of your feet on the pedals, whether you're clipped in or riding on platforms.

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