Question about front mech capacity...?

[Loola]

Afternoon all.

I'm swapping a 42 tooth front chain set to a Sugino/Middleburn chain set.
The Sugino rings are big/granny 48/22.
Which I believe requires a front mech capacity of 26? (Based upon 48-22=26)

The cable routing requires a top pull mech.
The seat tube is 31.8mm and I'd need a clamp style mech.

The old front mech is an Shimano Altus affair and based on testing it, I don't think it can cope with this change in range?

I'm looking for a new front mech and would like to keep it Shimano, preferably LX or similar?

I'm not sure what to look for?

Any help/advise would be greatly appreciated!

 

[mattr]

Most shimano triple mechs are 22 tooth capacity from what i can remember. You might struggle to get that range with any remotely modern mech. At least, you'll have rubbing in some gears.
Best option is to either look for a road triple mech, some of those are sized for a slightly bigger range (24 maybe) but you might not get the swing due to the chainline. Or a trekking MTB mech, they have in the past been sized for 48/36/26 so you *might* get away with it. Cheapest are around 15 quid (and seem to have similar/same geometry across the range). Worth it for an experiment. I mean, if its a choice of having the range of gears i want and a bit of rubbing in a handful of the ones i don't use, i'll just be careful about where i shift!

Otherwise, see what they use on road tandems. They have massive ranges and quite often odd (mtb sized) chainlines. I know my old MTB tandem ran 48/36/24 with no issues, but that was back in the days of 8 speed........

 

[Loola]

Re:

That's brilliant thank you.
The original mech that serviced it was a SACH'S one. But looking at it, it's hard to see whether it had enough range?
The bike it came off was un-ridable when I got it.
It's taking some head scratching.

Thanks for your help!

 

[ishaw]

Re:

Fairly sure I've done the reverse, running an xtr m953 front mech designed for 46/8t on a compact chainset which was probably running 44/34/24. I don't remember having any shifting issues.

 

[Loola]

Re:

Yeah, it's quite close. To clear the bottom of the derailment arm in the granny, there just isnt quite enough height when trying to get onto the big 48 tooth big ring.
Gonna try going up from a 20 tooth granny to a 26, see if that helps.
Ta.

 

[mattr]

26/3x/48 should work fine with any mech that'll take a ring that big (phnarr).

 

[hamster]

I've happily run 24-48 with mid-90s Shimano front mechs. The problem is that in the end the chain will drag on the tail of the mech with smaller sprockets on the cassette. This is not much of a problem as you won't be running 22-12 or similar gears - however the quoted capaciites are designed to prevent such occurrences.

 

[Loola]

Re:

Just waiting to get hands on a 26 tooth. In the mean time I'll set it up to work as smooth as possible in big and middle and see how much chain drag I'm getting in bottom.
Thanks again.