Shimano M739 cranks vs M737, chainline & Q-factor question

[vshuvalov]

The question is about FC-M739 model (1998 model year) compared with older FC-M737 model.

For FC-M739 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 FC-M737 model) to 110mm? Note once more that all difference goes to non-drive side!

Thanks in advance for your answers.

 

[FluffyChicken]

Re: Shimano M739 cranks vs M737, chainline & Q-factor questi

Probably just setting the length and distance of the none-drive to a standard for frame builders,so they can design the arm around that. It's probably hidden in the Shimano build a bike specs somewhere.

Then its only the drive side to worry about where chain line and a move to 9speed compact chainset is the standard and full sus designs.
737 had its designs build with a much older atb/CD 8sp era in mind and of course they are used through the ranges.

 

[vshuvalov]

Re: Shimano M739 cranks vs M737, chainline & Q-factor questi

Once again, all the difference between 107 and 110 bb is on NON-DRIVE side. Newer cranksets need more space on drive side (to enable chain guards), which is more or less clear. But when moving from M737 to M739 Shimano decided either to change the shape of non-drive arm or to move it further from the frame. WHY? This is the main question.

If they just changed the shape, then why do they recommend 113mm bb for 50mm chainline? There should also be longer asymmetric version (115mm? not sure).

 

[FluffyChicken]

Re: Shimano M739 cranks vs M737, chainline & Q-factor questi

I understand, I tried to explain
By doing away with the 107 you now only have to worry about one crank arm
110 giving the drive side ~47
113 giving the drive side ~50

The frame designer and Shimano do not need to worry about differences to the none drive side. It is in a set position.

 

[vshuvalov]

Re: Shimano M739 cranks vs M737, chainline & Q-factor questi

So you mean that the non-drive crank is now (with M739) in the same position, no matter what chainline you choose? Sounds reasonable.

But if this is the case, it means that you can still use older 107mm instead of 110mm and get more symmetric pedals if the frame allows you to do so, right?

 

[FluffyChicken]

Re: Shimano M739 cranks vs M737, chainline & Q-factor questi

So you mean that the non-drive crank is now (with M739) in the same position, no matter what chainline you choose? Sounds reasonable.

But if this is the case, it means that you can still use older 107mm instead of 110mm and get more symmetric pedals if the frame allows you to do so, right?

That's my theory anyway. Its cheaper for a builder, its cheaper for Shimano.
More symmetric would depend on how the arm was designed.