Yet another front mech query!

[compo]

I have been trying to get the front mech on the Barracuda MTB to shift properly. It is perfect in the high and low settings but the middle ring just wont line up and the chain rubs on the inside plate of the mech. I did fit a semi-sealed bottom bracket just to get everything working and make sure the bike was actually worth doing up a bit before spending any money. The semi-sealed BB is 124.5mm, but the old one I took out was 122mm. Would the extra 2.5mm be sufficient to prevent correct adjustment. I guess given what is happening I may have answered my own question.

I usually start with no cable connected. Set up the high/low stops, connect the cable, take out the slack and Bob's your whatsit. Done it many times on many bikes without issue.

 

[RecordAceFromNew]

122mm (or more precisely 122.5mm, Shimano code NL) bb's are very common, but 124.5mm is rare in terms of modern specification, but was common as adjustable cup and cone bb's (see 3NN or perhaps 3U here) in the old days, perhaps yours is for their replacement?

In any case it does not matter that much as long as the chainline is correct and there is no clash. If you have a MTB chainline should usually be around 47.5mm or 50mm. Is it?

If it is, and your front mech is a typical modern mtb unit and so is the front shifter, given you say you have no issue with high and low, the issue may be that the cable is attached at the clamp wrong, which will affect indexing. There is also a small chance that the shifter is faulty, but that is quite unlikely. If the cable is correctly clamped (worth finding out mech model code and checking with the tech doc of the mech), then what are the symptoms if you adjust the cable tension to make the mech perfectly centred on the middle ring when you look directly from above at the 2nd gear (assuming the L and H limit screws have been set where they should be)?

 

[compo]

122mm (or more precisely 122.5mm, Shimano code NL) bb's are very common, but 124.5mm is rare in terms of modern specification, but was common as adjustable cup and cone bb's (see 3NN or perhaps 3U here) in the old days, perhaps yours is for their replacement?

Perhaps I should have explained better. The 124.5 semi-sealed BB I used was originally bought for a Raleigh road bike, but I never used it so it has been kicking about in my spares box for a couple of years. It had no relationship with the mountain bike at all until last week when I needed something to put into the MTB just to see it all together. The old cup and cone I took out of the MTB was almost rusted solid, and the cages that held the bearings were almost dissolved in rust, literally! It was that spindle I measured at 122mm.

I have put the bike away for today but will follow up your advice tomorrow and check the chainline and recheck the cable attachment. I expect I will take one look at it and suddenly realise where I have gone wrong. That's what usually happens when I have a problem.

 

[RecordAceFromNew]

Yeah. I guess the way bb's go is you could have two spindles differing by 0.5mm in length, but make a world of difference in chainline - that 3NN and 3U are case in point - the difference in chainline would be 4.5mm - enough to cause a clash or mech out-of-range. If chainline is the issue (probably not in your case, if you can get high and low set satisfactorily), and if the 122mm was the original spindle, then the best way to id a replacement is to find if it has a code, and if not what the 3 measurements are, from which I might be able to help you figure out if e.g. which Shimano square taper "might" work.

 

[compo]

I have been trying to get the front mech on the Barracuda MTB to shift properly. It is perfect in the high and low settings but the middle ring just wont line up and the chain rubs on the inside plate of the mech. I did fit a semi-sealed bottom bracket just to get everything working and make sure the bike was actually worth doing up a bit before spending any money. The semi-sealed BB is 124.5mm, but the old one I took out was 122mm. Would the extra 2.5mm be sufficient to prevent correct adjustment. I guess given what is happening I may have answered my own question.

Thinking about it, if the chain is rubbing on the inside plate (frame side) of the mech then using a shorter spindle to bring the chainwheels inwards wont solve anything, in fact it will make it worse. So, following RecordAce's suggestions it is down to checking cable routing and tension tomorrow.

 

[raleighnut]

I have been trying to get the front mech on the Barracuda MTB to shift properly. It is perfect in the high and low settings but the middle ring just wont line up and the chain rubs on the inside plate of the mech. I did fit a semi-sealed bottom bracket just to get everything working and make sure the bike was actually worth doing up a bit before spending any money. The semi-sealed BB is 124.5mm, but the old one I took out was 122mm. Would the extra 2.5mm be sufficient to prevent correct adjustment. I guess given what is happening I may have answered my own question.

I usually start with no cable connected. Set up the high/low stops, connect the cable, take out the slack and Bob's your whatsit. Done it many times on many bikes without issue.

The indexing on the centre cog of a triple is set by cable tension alone changing the BB for one slightly longer could not affect this it could only be an issue on the outer or inner cog if you were to exceed the mechs travel limit. Chain rubbing on inner plate too much tension- chain rubbing on outer plate not enough tension. You may find that the cable is slightly slack when on the "granny ring" but do not worry about that as long as the limit stops are adjusted correctly so that the chain cannot move in any further.

 

[compo]

Sorted. I just went out and stuck the bike on the workstand and disconnected the cable. Checked everything again, reconnected the cable and it's working perfectly. I have no idea what I have done or why it was playing up before. Only thing I can think of is that as it is a new cable and housing with new cable ferrules things have compressed slightly and seated better allowing slightly more slack in the cable.

 

[gwhite]

The indexing on the centre cog of a triple is set by cable tension alone changing the BB for one slightly longer could not affect this it could only be an issue on the outer or inner cog if you were to exceed the mechs travel limit. Chain rubbing on inner plate too much tension- chain rubbing on outer plate not enough tension. You may find that the cable is slightly slack when on the "granny ring" but do not worry about that as long as the limit stops are adjusted correctly so that the chain cannot move in any further.

^^^^^^ This.