Can't shift to smallest chainring
 

Hello forum.

Recently, I rebuilt my old 2000 Specialized Hardrock and fitted a completely new Shimano Acera groupset to it. I adjusted the gear components according to Shimano's instructions, but the front derailleur can not shift to the smallest chain ring.

It doesn't appear to be a problem that can be fixed with the limit screws. The derailleur is hitting its "hard" travel limit and simply can not move any closer to the seat tube. So, it looks as if the chain rings might be too close in to the bottom bracket.

The bottom bracket is a Shimano BB-ES25, which comes in two spindle lengths, 118mm and 126mm. If it turns out I have the 118mm version, will replacing it with the 126mm version be the correct solution to this problem?

Thanks.
JM

If your front derailleur is actually hitting the seat tube before shifting to the smallest chainring then a longer bb spindle is needed.

Hi. I believe you may have a problem with your chainline in the front.

Did you change the crank, but not the BB?
The BB length is determined by the crank.

+1 Your old BB may not necessarily be the right length for your new crankset.

The b.b. and cranks were supplied together with the new groupset. I've just checked the original order and confirmed it is a 68x118mm b.b.

I've measured the chainline (from centre of seat tube to middle chainring) and it's about 44mm. Replacing the b.b. with a 126mm one would add 4mm to each side and take it up to 48mm which seems more appropriate for a MTB.

Thanks for the replies so far.

Which model Acera is the crank?
Looking at the Tec docs, it appears all but one Acera crank takes longer than a 118mm BB. (123mm for the majority)

FC-M391, the Service Instructions for which suggest I need a BB-UN26, but that seems to be a square taper type.

The "Catalog" shows a 123mm BB.
That crank IS a square taper.

So it is! I must have the FC-M391-8 then, because it's definitely octalink.I stupidly assumed the '-8' referred to a part for 8-speed gear train and dismissed it, forgetting for a moment that the number of speeds at the back end has little bearing on the chainrings at the front.I'll double check the part number by looking at the actual crankset, but if I do have the FC-M391-8, then the BB-ES25 (126mm) seems to be the right part.

I replaced the BB-ES25 with the 126mm spindle version and I am able to use all gears now.