Q factor: shimano vs sram?

[Climb01742]

does anyone know the difference between the Q factor for shimano 7800 and sram (standard BB)? i found this over on WW:

http://weightweenies.starbike.com/forum/viewtopic.php?f=3&t=63682

the numbers posted are a bit over the map. does anyone know/have an 'authoritative' answer?

much appreciated.

[Kontact]

The thread you posted is for BB30, which is narrower than Shimano or the SRAM GXP you're asking about.

I kinda doubt there is much variation anymore, since chainline has become standardized and carbon or hollow crank arms aren't very thin.

[retrogrouchy]

I would guess around 160 mm for doubles, in both cases. Way too large....

[6mt]

i thought Q factor meant the flex coming from the pedals' spindle??

[bart998]

i thought Q factor meant the flex coming from the pedals' spindle??

Distance between the pedals...

[6mt]

then why is BB in question in terms of Q factor??

[Kontact]

Because if the BB is really wide, the cranks can't be any narrower.

[Climb01742]

which poses another question: do all the new BB standards effect Q factors?

FWIW, in measuring two of my bikes, both with standard BBs i found:

shimano: 146
sram: 153

and my body can really tell (unpleasantly) the difference.

[Kontact]

The slimmer BBs, like BB30 and English internal, definitely make for more heel clearance. They could help provide less Q, but as I said earlier, Q is more a result of the standard chainline and the design of the arm end. To make a low Q crank the arm thickness has to be pretty minimal to bring the pedal in by still clear the chain.

External BBs or BB86 make it harder because the no one is going to build a crank with the center sticking out farther than the pedal end, and those cranks sometimes really stick out.