On 9/1/2019 9:12 PM, Tom Kunich wrote:
On Friday, August 30, 2019 at 7:49:45 PM UTC-7, AMuzi wrote:
On 8/30/2019 9:07 PM, Don Gillies wrote:
If a vintage crank needs an offset on one side, you can just add the offset to the other side and substitute a symmetric crankset, so a 116 R+5 bottom bracket can be substituted with a 121 symmetric bottom bracket (120 or 122 would work fine.)

- Don Gillies
Palo Alto, CA, USA


Which makes your tread ( aka "Q") wider.
Deal breaker for some riders.

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971

The iliac width is why the BB50 has become so common. People's legs do not have to spread so wide and the length of the BB shaft is narrower and consequently stiffer. Average hip breadth is 13" or 33 cm. So the narrower the BB the more angularly correct a pedal spacing would be.

Time for Frank to tell us all that I don't know what I'm talking about.

No, I'll point out that you're correct on one point. If the length of a
bottom bracket shaft (as you call it) is a bit less, then other things
being equal it will be a bit stiffer.

The deflection is proportional to the length. So if you moved from a 120
mm shaft to a 116 mm shaft, you'd increase the stiffness about 3%. With
laboratory equipment that was sensitive enough, you'd be able to detect
that.

--
- Frank Krygowski