Shimano M739 cranks, chainline & Q-factor question

[vshuvalov]

The question is about FC-M739 model (1998 model year).

For this model Shimano lists 110mm BB length for 47.5mm chainline and 113mm for 50mm chainline. But is is known that 110mm BB is asymmetrical, the non-drive side being 3mm longer, though 113mm BB is symmetrical. So at least in one case the left and right Q-factors will be different - or, in other words, left and right pedals will have dirrerent distances from the center of the frame.

1. Does anybody know in which case (110 or 113) pedals will be more symmetrical?

2. Any idea about the reason Shimano changed symmetrical 107mm BB (which is listed for older M737 model) to 110mm? Note once more that all difference goes to non-drive side!

Thanks in advance for your answers.

 

[lewisfoto]

My recollection about Q-factor is that the total distance is more of a concern than the symmetry. At any rate we are talking just a few millimeters?

 

[vshuvalov]

At any rate we are talking just a few millimeters?

Well, 3mm shift is not that much, but still noticeable (imho). Of course it can be corrected by adjusting the cleat.
Anyway, the main question is the reason WHY Shimano decided to go with non-symmetrical bb. Newer cranksets are often non-symmetrical to give space for a chain guard on the drive side, but here we are speaking about non-drive side!

 

[Anthony]

I couldn't tell you why Shimano did it that way, but as far as I'm aware, the 107 is symmetrical, 53.5/53.5, and so is the 113, 56.5/56.5, but the 110.5 is asymmetrical, presumably 56.5/54.0(?)

The 110.5 spindle gave a 47.5 chainline and the 113 gave a 50.0. With the way they designed fms in those days - the swing of the mech being the same - the chainline all boiled down to what size seat tube you had. So a 34.9 seat tube needs a 50.0 chainline and 28.6 needs a 47.5, while a 31.8 can manage either.

Once they went over to one size fms, with shims to fit smaller seat tubes, the same chainline would work for any frame.