I have a problem with fitting a new Shimano LX M581 front derailleur on an MTB with a standard 3 chainring system. If I set the vertical height to give the required vertical clearance between the outer part of the derailleur and the big ring, the inner part of the derailleur fouls the middle ring when trying to engage the big ring. If I raise the unit to enable the inner part to clear the middle ring, the big ring clearance on the outer looks much too big and when I change to the big ring the chain comes off irrespective of end stop setting (presumably because the outer plate is too far away from the ring vertically to stop this happening).

Any suggestions?

I got the item by mail order and wonder if I have been sent an item that is intended for a 2 ring system with a smaller inner ring by mistake? (Checking on the web suggests that the M581 is correct for MTB use which I assume would mean OK for a 3 ring system? The one I have received looks like the pictures of the M581 on the web but are there small fdifferences that arent visually obvious?

What did it replace?  If it was non-shimano then there may be an issue with the shifters & compatability. What chainset/rings are they?

Also, are you using the correct spacer for the clamp?  If your seat tube is (say) 34.9mm but you've attached the mech with the 31.8mm adapter in it the mech will be sitting too proud of the frame as a start point.  This would explain the overshifting despite end stops being adjusted.

The M581 replaced a very old Shimano front mech. I am not sure what type the old mech was but it was original on the bike which is an old MTB with no suspension at all that was ~ £350 about 9 years ago. I guess it would be whatever was bottom end Shimano about 9 years ago. I got a new LX shifter at the same time as the mech and that is what I am now trying to use with it. (I just use the bike for commuting. I am lucky I can commute offroad but it doesn't need a great bike and is very muddy in winter so I use the old bike to save wear and tear on my better ones. )

It has a 31.8 seatpost and I used the right spacer for that size (it was slack with no spacer and wouldn't fit at all with the 28.x spacer. The chainrings are a standard 22 34 44 Middleburn set. The bb is shimano 68mm x 107 mm which matched the original (by measurement, markings were illegible). The chainline looks OK with the 107 mm bb spindle, i.e. middle ring to middle sprocket gives a straight looking chain parallel to bike. Offset gears give a visually slightly s shaped chainline when you look along the chain. I think this is what you would expect.

Just an idea...

I believe that that mech is designed to run with a 22-32-44 setup (see the Shimano website). In other words, with a 12 tooth difference between top and middle rings. I know this isn't much of a difference, but it could mean that the mech is designed to move a greater vertical distance when switching between top and middle rings than you require with a 22-34-44 setup. This would explain the symptoms you are reporting, but it is just speculation on my part.

I think I may have had the same issue with a LX dual pull jobby...it really does seem to pull way too much cable through.

I just played and played with it (not exactly science) to get it indexed right, I'm running I believe 38t upper but i'd have to check.

I have looked at the Shimano website more closely now and the MINIMUM middle to large ring size difference that can be accomodated by the derailleur is 12 teeth so it doesn't accomodate my 34-44 combination even though the old mech did and I just assumed it would be OK (it seems a retrograde step to reduce the range of sizes that is accomodated). All the current Shimano MTB front mechs at least up to XT (I wouldn't put XTR on my old bike) share the same limitation. SRAM kit does seem to be a possibility but the middle ring is quite well worn so the most cost effective option seems to be to get a new 32 tooth middle ring and try again.

 I'll confirm if that works.