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Old 02-26-19, 04:10 PM
  #13  
T-Mar
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Originally Posted by dddd
I'm not understanding why moving the "hub" of the crankarm inward would increase the Q-factor at all.

Assuming that the newer crankarms were mounted on the appropriately-narrower new bb spindle, the Q-factor would seem to a design variable quite independent of the change to lower the "ankle" profile.


So I am thinking that if the Q-factor of certain of these low-profile cranksets did increase when redesigned, that it would have to do with wanting to add chainstay clearance (possibly for aluminum chainstays?) or to accomodate any increase in the chainline dimension.


Again, I am assuming that the crankarms and spindle dimensions would have been redisigned together as a system, so that changes in spindle width/offset would not force changes in the Q-factor or chainline.

You could increase the heel clearance on the drive side without affecting the Q-factor. The spindle protruded further on the drive side, so you could shorten it and move the crankarm/spindle interface inwards. However, on the non-drive side, the shoulder of the spindle taper was already very close to the outer face of the cup, so the spindle couldn't be shortened any more without decreasing the spindle/crankarm interface and the integrity of crankarm in the immediate vincinity of the spindle. . Consequently, to achieve more heel clearance, the non-drive side crankarm had to have more offset at the pedal end, which increased the Q-factor. If you wanted to keep the Q-factor symmetrical on both sides, you had to go back and offset the drive side crankarm even more, which further increased the Q-factor.


Chain stay clearance was typically not a big issue. Even on racing bicycles, the chain stays were rarely shorter than 40.5cm. Consequently, increasing the rear spacing from 120mm to 126mm only reduced the chain stay clearance by a maximum of ~1.25mm for a 172.5mm crankarm.
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