M730 Cranks arms, which BB length?

[Andy B]

I have a KHS (see link in sig) and have just fitted some M730 cranks to it, the BB is too short giving a chainline of ~45mm

I think I may need a new BB that's longer, Sheldon reckons it's a 122.5mm BB for a M730, no idea what's in the frame at the mo, haven't checked it out yet!

Anyone used a 122.5mmBB with M730 crank arms? Does it work as it should & give the right chainline?

 

[Tad]

I use a 122.5 with my M730 cranks.

 

[Andy B]

 

[legrandefromage]

I thought 122mm was fine, just found that I needed 127mm

 

[onzadog]

Bottom bracket

122.5 rings a bell with me. I seem to remember that when I killed off my Topline cranks, I needed a shorter axle to fit the XTs rather than the 127 they needed.

 

[FluffyChicken]

Andy, I PM'd you and yes you are correct for standard 47.5 chainline and 127.5 for the longer 50mm chainline. Same length as used by DX and LX (and in fact the whole range) of that era.
I couldn't tell you what the 121.5mm is for though :s
Also it mentions the length in the back of the 1991 catalogue as well as the 1992

 

[onzadog]

old chainlines

Didn't chainlines used to be 45 and 47.5mm?

I thought the modern idea of 47.5 and 50mm came about with the introduction of HTII bottom brackets and nine speed drives.

I might be remembering things incorrectly though.

 

[FluffyChicken]

Re: old chainlines

Didn't chainlines used to be 45 and 47.5mm?

I thought the modern idea of 47.5 and 50mm came about with the introduction of HTII bottom brackets and nine speed drives.

I might be remembering things incorrectly though.

Road bikes maybe ?

Nope 47.5 Shimano measurement has been around since the turn of then 90's at least (which would also mean the late 80's)

47.5 being the default, 50 being the maximum alternative if it was needed for the mech or frame (usually went hand in hand)

NOW it is 50mm standard chainline and pretty much nothing else in Shimano land.
Mechs are all the widest band size and shimmed down.
HTII's are allset for 50mm and 'shimmed/spacers' to fit the frame.
Makes everything cheaper to make when you just make one size rather than three.

 

[mfh126]

There's a bottom bracket/crankset table on bikepro.com that specifies the recommended BB spindle length for most cranksets availble in the early 90's. It lists 122.5mm for the M730 cranks. I've posted the link a few times before but I can repost it again when I find it.

 

[Andy B]

I've been looking through my newly acquired Shimano catalogues and found this info coding about M730 BB sizes

70-T-122.5, 68-T-122.5, 73-T-121.5, 68-T-127.5

I assume this means 'Shell width'-T-'Axle length'

So a 68mm shell would require either a 122.5 or a 127.5mm BB axle, that 5mm makes the difference between the 47.5mm chainline and the 50mm chain line

Basically:

68 shell + 122.5mm BB = 47.5mm chainline
68 shell + 127.5mm BB = 50mm chainline

Though I didn't know that you could get a 70mm BB shell :?