I’m normally reasonably au fait with bike mechanics, a few things I’m unfamiliar with, but one issue has me stumped. I recently replaced a damaged front mech on my girlfriend’s bike with a Deore multi-fit mech. but for some weird reason, it now won’t shift to the largest ring from the middle one. I’ve fiddled and fiddled with it, but to no avail. Tried to compare it with the damaged one, but it was impossible really as the cage was so damaged. I’m wondering if it’s the wrong type of mech. One issue i think might affect things, is tha the largest chainring is a 48t, as it’s more a ‘touring’ ratio type. Are Deore mechs designed to deal just with smaller 44t chainrings max? I replaced the front mech on her other bike a few years ago, and that was fine, with 48t chainring. The middle chainring on the problem bike is a 38t, the smallest a 28t.

Can it be made to work, or is it a waste of time and a different type of front mech is required?

Any advice gratefully received, as she’s giving me grief over it. Apparently it’s my fault her bike doesn’t work, not the fact that she dropped it whilst trying to lift it over a gate.

mike,

Check on Shimano Tech pages, but I think the Deore (and most MTB) mech needs a minimum of 12T difference to work properly and is made for a 44T.
Shimano used to sell a different part number mech for 48T trekking sets.
I had this issue with a 20 – 32 – 42 chainset and in the end fitted an Alivio that was right for the 10T shift.
You also need the mech at exactly the correct height (just clearing the 48T ring).
I see the XT M761A works with 44 or 48 but needs the 12T difference.

PaulD

Thanks PaulD. I did try adjusting the height of how it was clamped, but then it was ridiculously high and the chain was catching on the rear of the mech in higher gear ratios and when in the smallest front ring. \And when looked at from the front, it seemed like the angle the mech moved at was too shallow to clear the chainring. That would make sense if it’s the wrong type of mech actually. I’ll have a look for a touring type one. alternatively, I may fit smaller rings if I can find any to fit in my spares box. It’s a mtb for gentle riding anyway so doesn’t need the higher ratios. i’m mystified as to why it was supplied with such a chainset actually.

So, I’ve bought a new front mech but unfortunately, and frustratingly, it’s the same type. It appears that the correct model is a FD-M591 (this fits the 48t front chainring, the M590 is for the 44t). Ok. So I can go and excvhange it. but then there’s another issue; they come in two angle types, a 63-66º and a 66-69º. It’s for a Cannondale mountain bike, a F300 I think, from about 1999ish.

What a nightmare. Any advice on this is more than gratefully appreciated.

Further investigation revealed a problem more complex and confusing then i’d imagined. Even a phone call to Madison left me non the wiser.

I’m thinking that maybe a road type front mech may be better? Something like a Shimano sora, which has a maximum 50t capacity?

alternatively i may just change the chainset to a 44t one. It’s cheaper than replacing all the rings, plus i could use the existing mech. And it would be more suitable for off road use.