New crankset not compatible with front mech

alex100a

Hi Guys,

I am in the final stages of refurbishing my old knackered Gary Fisher Wahoo and have discovered a compatibility issue with the retained Shimano FD-C050 front mech and the new Shimano Alivio triple crankset FC-M410.

Basically the front mech has not got enough travel to enable it to reach the outer chainring. To begin with I was confused but then I figured out that the inner most chain ring on the old set up actually cranks inwards towards the frame hence the whole crankset sits nearer the frame allowing the mech could reach all the rings. I've tried to attach a picture with both cranksets side by side (hopefully it works!).

I've popped the old crankset on short term but need to sort this as the new chain slips on it.

Do I need to replace the crankset with an identical unit (cranked) or can I purchase a new front mech instead? This really needs upgrading anyway from a Tourney unit.

nicklouse

It seems that your old BB. is not compatible with your new cranks.

What is the length of the BB axel fitted?

alex100a

Hi nicklouse,

Thanks for your reply. I actually replaced the BB. Below are details of the original and replacement BBs:

Original BB spec -VP Components - BC-73, 73L, BC 1.37x24T, 122.5mm (all codes shown)

Replacement BB spec (CRC Recommended) - Shimano UN55 Square Taper Bottom Bracket 73x122.5mm

nicklouse

Pandora the new shimano layout of the tech docs is poor.so I can't find what I am looking for but I think a 113 would be suited.

But need to find the info.

Yep 113mm

http://cycle.shimano.co.kr/media/techdocs/content/cycle/SI/Alivio-Acera/SI_0094A/SI-0094A-001-ENG_v1_m56577569830667768.PDF

alex100a

Hi Nick,

Thanks for the info. I had not thought about replacing the BB instead of the crankset.

I've looked at my 2 cranksets and wonder whether I needs 110mm BB. The reason being that I've measured from the face of both inner chainrings to the face of the tapered section and the difference is approximately 6mm. If I double this I end up with a dimension of 110mm from the current 122.5mm unit installed. Does this make sense?

Also I note on your link it suggests a UN26 BB rather than the UN55. Do you know if the later is simply a better quality unit? Having said that the UN26 the is so cheap (£6) I could take the gamble and try the 113 as you've suggested and it would not be the end of the world if I could not return it.

Thanks again for your help it much appreciated.

The Rookie

It makes no sense at all, you don't know how far the taper will go into the crankset, that is why you use the manufacturers quoted size to get the right chainline, if the manufacturer says 113, then 113 it is unless you have a particular reason to want to change the chainline.

alex100a

Well that's me told!

I'll order the 113mm BB.

Thanks