BB spindle length question

ChrisEyles

Veteran
Location
Devon
Getting pretty confused about BB spindle lengths today!

I bought a replacement crankset for my commuter to bring it back into active service and tried to fit it this weekend, only to find it requires a different spindle length on the (square taper) bottom bracket.

Old set up:

Shimano altus 48/38/28 triple crankset ca 2014
122.5mm shimano square taper BB
Good chain line

New set up:

NOS Shimano STX (? not sure but it looks a lot like it to me) 42/34/24 triple crankset ca 1997 (? again guess work)
Bad chain line with the 122.5mm BB

The drive side crank is sitting well outboard of where it needs to be. The offset is tricky to accurately measure, but my best guess is around 8mm. Basically when the FD is in the top ring position it's currently roughly aligned over the middle ring of the new crankset.

In a sensible world, I'd say to move the crankset in by the required ca 8mm I'd need a 16mm shorter spindle, so a UN55 107mm BB should do the job. There is enough clearance between the chain stays and the crank arms for the cranks on both sides to move inwards by the required 8mm.

But a bit of googling has left me confused - sometimes more of the difference in length comes off the drive side, sometimes the non drive side.

Does anyone know:

1) How to determine what BB spindle length you need when you can't look it up (as I'm not sure of the crank model)

2) Where I can find a good diagram showing the length of the drive side and non-drive side extension of the spindle past the BB shell. I'd be happy with UN55 or UN26 BBs, or anything else recommended here.

3) I guess there's a chance I'm wrong about it being NOS shimano and the new crankset could have an ISO taper interface rather than JIS (which would also place it outboard on the BB) - anyone know of a way to check this?

Thanks for the help, as always!
 
OP
ChrisEyles

ChrisEyles

Veteran
Location
Devon
ps - as an additional complication it seems most modern BBs have equal amounts of spindle protruding either side of the BB shell, whereas some of the old (late 90s) BBs I've had have been asymmetric, which may or may not be the case for this crank.

What do they say, "a little knowledge is a dangerous thing"!
 

DCBassman

Going up hills, very slowly...
I tried exactly this with a Deore chainset donated by @Milkfloat. The Deore rings were dished differently in just the manner you describe. No workable solution presented itself without expense that I couldn't afford at the time. I'd still like to do this to the Trek, and that needs a new BB, so I'll be watching this with great interest!
 

C R

Veteran
Location
Worcester
Have checked the Shimano website? They give the info on which BB to use with a given chainset. I guess the problem may be if the model is too old, or you can't definitely identify it.
 
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ChrisEyles

ChrisEyles

Veteran
Location
Devon
I can't find a model number on the cranks, which makes it impossible to look up.

If I'm right in my guess of a late 90s STX model (it's quite a distinctive spider, with the middle ring mounted on the same bolts as the granny) , then Sheldon Brown's website suggests 110mm - 113mm, so I'm tempted to go for a 110mm BB. Might see if the lbs has any I can try for fit first.
 
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ChrisEyles

ChrisEyles

Veteran
Location
Devon
Just taken the old BB out and it's an asymmetric 117.5mm UN26 rather than a symmetric 122.5mm one as I misremembered.

I'm thinking I need to take it into a shop to directly compare the spindle lengths either side, as I don't want to end up with an unusable online purchase.
 
OP
ChrisEyles

ChrisEyles

Veteran
Location
Devon
Dropped off the BB to my LBS, who are hopefully going to source a suitable one for me. Will report back when I've made more progress!
 
OP
ChrisEyles

ChrisEyles

Veteran
Location
Devon
Got a 107.5mm BB yesterday. Dry fitted the cranks and looks like it should work

The BB threads felt a bit dodgy on the drive side when fitting the unit. Spent ages checking it wasn't cross threaded but it still felt stiff. Maybe a bit of rust on the BB shell? Anyway, seemed to tighten up ok in the end.

Will fit a chain and check the shifting all works this week / next weekend.
 
Location
London
Trust you didn't have to use what seemed like excessive force to get it to "tighten up".
 
OP
ChrisEyles

ChrisEyles

Veteran
Location
Devon
Just used the regular (still fairly large) amount of force to tighten the BB into the shell, and it felt OK. The extra resistance was coming threading the unit in before this - usually you can do this with two light-touch fingers, but this time it required the BB tool and a very firm finger grip or a light touch on a spanner.

I'm not worried about it this time around but I am thinking I don't want to be replacing the BB too many times on this frame!
 
Location
London
Ah, sounds ok then. I feared you had done what I did once. I now know that that one I possibly did misthread. Got it in but with a suspiciously large amount of effort needed.
 
OP
ChrisEyles

ChrisEyles

Veteran
Location
Devon
Set up the FD and the chain line is now good with the new BB.

Unfortunately, with the smaller 42t big chain ring the FD won't go quite low enough for perfect shifting (the bottom of the derailleur cage contacting the chain stay limits how low it's possible to set it).

Shifting is a bit clunky going from the granny to middle ring (would be fine on friction, but with indexed brifters you have to overshift two clicks up and then trim one click back) - but I can live with that quite happily as the granny will only really get used when occasionally (when carrying camping gear or up very steep hills).

When shifting into the big ring, the chain dropped a couple of times on the test run I did. Apparently this is common if the derailleur cage is set too high, but there's not much I can do to fix this (at least without getting a new FD). I added on a plastic chain ring guard and that seems to be sufficient to stop this happening (no dropped chains in 15 miles with lots of shifting up front).

So I think that counts as a success!
 
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ChrisEyles

ChrisEyles

Veteran
Location
Devon
Very pleased with the gearing now too.

42 x 11 is amply high enough as a top gear for me, the 24 x 30 bottom gear should come in handy for camping trips, and the shifting pattern works well.

I don't understand why modern road bikes come specced with a 53 x 11 top gear! The 52 x 14 high gear on my old ten speed seems plenty high enough to me...
 
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