doing my head in but i think I may have the answer ..perhaps someone in the know can confirm.
basically after changing the chainset/bb on my bros bike ( fitted an old octalink triple with the bb it came with originally ) I cannot get the deore triple mech to shift across enough to avoid serious chain rub on highest gear ( outer front and rear) . It's not the stops or the cable but the mech just doesn't reach across enough. I know that the chainset and bb were matched and worked OK on a previous bike ( forgotten what ) so assumed it would be OK but am I correct in thinking that it'll be to do with the fact that shimano often specced 2 different BB lengths for the same chainset depending on the seat tube diameter and that the bike it came off probably had a fat seat tube so required the longer bb wheres the tube on the my bros bike is a skinny 27.2 . As a result the chainset is sitting too far out for the mech ? After writing this realised I should go and measure the chainline to confirm . back in a mo!
There certainly used to be various axle lengths, no idea why you needed type a or type b beyond that you did but, yeah my guess is the bb is too long.
Probably that, yeah
looks like the chainline is 52/3mm so seems i'm unlucky ..don't want to buy another bb as it's a cheap build ..wonder if you can space out front mechs!
You can get octalink BBs in 68, 73 and 70mm (IT) shell widths, axles are 109, 113, 118, 121, 126, might be some e types still available as well.
You can measure your chain line, or measure what bb you currently have and guesstimate how much shorter it needs to be.
I had a similar problem with the mech on my road bike after swapping the sq taper crankset with a Sora double.
I posted numerous times on here about it, took it twice to the bike shop who reckoned it should all just work but I could never get it shifting properly after changing the crankset. Mine was the opposite way round though, the front mech wouldn't move far enough inboard.
In the end I bought a 1mm bb spacer from somewhere to push the rings out a bit & it completely resolved the issue.
No good in your case though.
How does the front mech attach? Is it a band?
If you measure up the OD of the seat tube & how large you want it spaced out, I can 3D print you a couple of half cylinder spacers you can put on the seat tube to increase the seat tube diameter in that region.
I'd need to know OD of seat tube, how thick you want the spacer (i.e. how far do you want the mech moving over) and how tall do you want the spacer (i would have thought 12-15mm would be enough.
Let me know & I can print something over the weekend. It won't be an instant fix, but happy to help out if need be.
But aren't Shimano front mechs. designed for fatter tubes & provided with shims. for 28.6mm tubes; as such wouldn't the mech. be positioned the same? I don't recall being able to buy a 28.6mm specific band-on mech. but I could be wrong.
Another variable could be the bottom bracket shell width; how would fitting a 68mm BB into a 73mm shell affect things? I'd assume that the driveside spindle would be too far out, but the non-driveside would also be short.
IIRC 113mm axle gave a 47.5mm chainline and an 118mm a 50mm one.
Feeling dense at the moment in the heat so could be wrong on all counts.
pjay in olden days front mechs were never shimmed, different mech for each tube size.
I don't know if each was designed to factor that in though.
I don’t know if each was designed to factor that in though.
Yeah, the clamp would be differently shaped to put the parallelogram in the same place. All MTBs are supposed to have the same chainline (at least they were when it was all 135mm hubs) so mechs would be designed to that, not to a seat tube diameter.
OP, regardless of how it came about, it sounds like your chainline is off, and a 5mm shorter BB axle should be the solution provided you've got sufficient clearance between the chainstays and the chainrings, and between the chainstays and cranks when they're at the rear of the stroke.
Surely Octalink BBs must be cheap, I can't imagine there are more than about three people in the world who represent any sort of demand for them 😉
If you are using a triple mech with largest shim, then a jtek eccentric front mech shim may help you*
*May also be more than new BB as they are £13 from sjs cycles 😟
E-type BB? Meaning that the B.B. is too far out? Should be marked with an E
P20 E type would have an effectively shorter axle without the mech plate behind the cup.
Best I'd hope so, difference is 3mm. (Didn't some bikes have 50mm chainline?)
Just a thought - is the mech at the right height?
mech is at right height . bottom bracket shell width was 68 as was unit fitted.Now finally sorted after trying numerous combination of old bits and pieces from the spares box. Been interesting though ...good point about newer ( ish) mechs all sitting in same position due to different spacers being available ..the eccentric spacer from SJSC was interesting and the whole experience got the old grey matter working on different options to resolve . Since the advent of H11 I haven't thought about chainlines/spacing cartridge bb's etc . Finally a big thanks to Stumpy for his very kind offer to print me a spacer ..that was very much appreciated indeed.
Ha. No problem. These things can be a right arse to sort out.
Glad you got it sorted...