Front mech won't reach

[astrocan]

I got a good deal on a complete drive set (BB, crank, chain, cassette) but it did involve changing from a cartridge BB to Hollowtech external bearing. All fitted well but the crank is longer than the old set up and now the front mech won't reach the outer ring.
The installation instructions for the BB indicated that I should fit one spacer on the left and two on the right. Has anyone tried putting two on the left and one on the right to bring the whole thing across 2.5mm and would this work? Clearly the tubular spacer that sits between the two crank arms dictates the overall distance so is it possible to get a marginally shorter one, though I suspect that would leave an ankle shredding stub stcking out the left side.
The other remedy would be to fit a mech with a longer reach but I have no idea how to differentiate between options that would and wouldn't work so any advice in this direction would be very helpful.
I'm a little bit tightwad about getting a new mech partly because the current one works fine plus on my twelve year old bike I have only the frame, bars and mech left from the original so I'm a bit moist eyed about binning it while its still OK.

 

[Smokin Joe]

The front mech has two stop screws that prevent it from overshooting the chain off either ring. Slacken the High adjuster till the mech moves far enough. If the screws are not marked H & L just look to see which one is butting against it's stop when you are trying to get on the outer.

One other thing to check is that the cable has no slack in it when you are on the small ring as that will also prevent the mech moving far enough.

 

[RecordAceFromNew]

I agree with Smoking Joe those are the first things to check. If they aren't the solution since you are installing a different set of chainset and bb I think you might have a compatibility issue. If you could provide the make AND model code of:
1) bike
2) old chainset and old BB
3) new chainset and new BB
4) front mech
we might be able to figure out why.

 

[Cubist]

Also, you add two spacers to the driveside and one to the non-drive side on a 68mm BB shell, but only one driveside spacer on a 73mm. Make sure you have the right number of spacers on. Have you measured the width of the BB shell?

Next question, what was the width of the original chainline, and what is it now with the new crankset?

Have you changed the sizes of the chainrings, and have you moved the clamping position of the front mech to match the new chainset?

If it brings it all in line then I can't see why you shouldn't play with the spacers as log as the length of the fitted BB doesn't alter I can't see why it should suffer.

In the meantime start afresh with a slack cable and the low position screw holding the mech cage 0.5mm from the inside of the chain on the smallest ring, then pull the slack out of the cable, clamp it and shift up. Use the upper limit screw to allow the cage to sit in line with the big ring, and again the cage 0.5mm away from the outside of the chain when it's on the big ring.

Nothing will leave an ankle shredding stub out anywhere, because no matter how you fit it, the crank arms will always be on the outside of the bearing cups.

 

[astrocan]

Thanks all, I have tried adjusting the stop screws but the possibility of coming to the limit of the cable had not occurred to me so I will start there.

The shell is 68mm and I have two shims on the driveside (right) but I didn't measure the chainline before I took the old kit off (note to self) so without putting it all back on again I don't know how much if at all I have changed it, unless I take a measurement from the old cartridge BB, would the chainline be half the length of that?

Once I have this reach issue sorted I can start on trimming the mech and I will need to take account of any change in the ring size for that.

You are right about the stub though, there is a cap on the non-drive side that pulls the two sides together which always leaves a smooth end.

Can I take it from these answers that most mechs have the same reach?
I would have thought that a narrow framed road bike would need less reach than a wider MTB for instance.

 

[Cubist]

Can I take it from these answers that most mechs have the same reach?
I would have thought that a narrow framed road bike would need less reach than a wider MTB for instance.

Chainline differs but needs to be within limits. The diameter of the seat tube, the method of mounting the front mech, and as you rightly point out, the width of the chainset. Chainline is measured from the centre of the seat tube to a point between the chainrings on a double, or on the middle chainring of a triple, and on a MTB tend to be somewhere between 47mm and 50mm. A fat seat tube triple will be about 50mm. Front mechs can be specific on chainline and most specify minimum and maximum in the tech spec. This can affect reach, which means you must have roughly compatible front mech to chainline. I asked just in case you had somehow managed to source a road bike mech and were trying to fit it to a 50mm monster.