Right BB length for shimano fc-m730 cranks?

[Miyata-san]

Hello!
Just wanted to ask íf anyone could help me with this one. My bike has 135mm rear hub and fc-m730 cranks. I read earlier that 122,5mm bottom brackets are recommended for the FC-m730 crankset but does the 135mm rear hub mean that I should get a 127,5mm BB instead of 122,5mm to achieve the right chainline?

 

[Piotr]

for 68 and 73 mm
D-EL - 127,5
D-NL - 122,5

 

[Miyata-san]

for 68 and 73 mm
D-EL - 127,5
D-NL - 122,5

Forgot to mention that my bb shell is 68mm. So I should go with 122,5 correct?

 

[Tootyred]

but does the 135mm rear hub mean that I should get a 127,5mm BB instead of 122,5mm to achieve the right chainline?

No. 122.5 in standard form on a 68mm is just fine. In fact 122 or 120 will be great too. Its 1mm each side....

 

[Miyata-san]

No. 122.5 in standard form on a 68mm is just fine. In fact 122 or 120 will be great too. Its 1mm each side....

Thanks for clearing that out! Huge help!

 

[Piotr]

Shimano recomended 122,5
Another firms do 122,5 and 122, are ok.
120 maybe so short for proper operation

 

[Miyata-san]

Shimano recomended 122,5
Another firms do 122,5 and 122, are ok.
120 maybe so short for proper operation

I am specifically looking for shimano bb-m730 and I have noticed it comes in both 122,5 and 122. Maybe one of these is asymmetrical and one isn't?

 

[doctor-bond]

Must resist … cannot resist … the difference is 0.5 mm, divided by two. Any pictures?

 

[FluffyChicken]

Must resist … cannot resist … the difference is 0.5 mm, divided by two. Any pictures?

No no. It might not be divided by two, it may all be on one side, or even shifted all over the place...

To know properly you need the axle code.
D-NL (or D-3NL / D-7NL) is 122.5
D-N is 122, has the same positioning on the drive side, oddly the axle

It could easily just be someone rounding down.
Though it could be a round up of 121.5 aka 3SS / D-3SS and comes from the Shimano nut type (then bolt) in the 80s I think.
However this should actually place the crank 1.5 mm further out on the Drive Side and 2.5 mm further in on the NDS.
This is the same position as the D-EL (aka 3Y) 127.5mm spec for 50mm for the Drive Side.

 

[FluffyChicken]

AND unfortunately Shimano change a bit along the way as originally it was a 117.5/122.5 spec at the start of the M732 era, no idea why but I think MTBs hadn't got so wide? The cranks didn't change as far as I know.
Probably 6speed era chain line length, and not quite caught up with 7speed era fully. Assuming crank hasn't actually changed then chain line is 45.0mm with the 3A axle.

That 117.5 is slightly long (.5mm, yes longer!) drive side than a modern XL118



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