Why can’t you position the mech lower and still get it to shift? Both my bikes have double & bash + standard front mech and they’re sitting low

as long as you position it so that it clears the biggest chainring by a couple of mm you should be fine.

The quick answer is yes, yes, and yes.

When I first converted from 3×9 to 2×9 I stayed with the standard triple FD (SLX M660 in my case). It worked, but it wasn’t optimal. I had a few issues with chain drop and chain rubbing in various gear combinations. I changed to the M665 double specific FD and it solved those problems, and is a much smaller/neater package too.

However, I was running 36/22 and the problems were mainly due to the large difference between the two rings, and the chain running higher on the 36T than it would on a 32T. The M665 is designed for this combination, and to solve these issues. With 32/22 you may not get the same issues in the first place, and therefore won’t be so much benefit going to the double specific FD.

My advice – if you’re not getting any shifting issues with the triple FD, stay with it. If you’re getting issues, or want to go 36/22, then yes, worth getting a double specific FD.

do the 2 ring specific 10 speed front shifters have the same pull ratio and thus will still work with your shifter?

Yes, you’re correct on this point. The standard triple FD has a wide part of the cage where the chain runs on the middle 32T chainring, to allow use of the whole rear cassette without the chain rubbing on the FD cage. If you lower it more than a few mm the chain will now run in a narrower part of the cage and you’ll get chain rubbing issues at one extreme of the cassette, or other, or both. If you raise the FD you risk chain rubbing on bottom of cage when in granny ring and small end of cassette.

So, if you’re going to run a standard triple FD with a double chainring setup, the optimum position is the same as for the triple, but you just dial in the high screw to stop it being able to change up to where the big ring would have been.

Note: if you also change your 32T middle for a 36T middle, then the chain is running higher, so you actually have to raise your FD a bit, so the chain is still running in the widest part of the cage – or, as per my post above, you get a double specific FD.

You’re welcome.

I used a standard mech initially and found that the chain regularly dropped irrespective of how I tried to adjust it. An SLX double front mech fixed the problem immediately.

My SLX double mech is often a bit sticky, doesnt like moving to granny sometimes, cable is slack but it stays where it is so I give it a boot, then changes fine.

Guess that goes to show that FD tuning is a bit of a black art rather than a science!

How much you can lower it before you get into the narrower part of the cage and rubbing issues will depend on a number of things – how high it is to start with, shape of cage (deores are wider than SLX – don’t have an XT to measure), wheelbase (shorter distance BB to rear axle = increased chain angle), size of largest sprocket on the back & chainwheel on the front (chain will run higher on larger diameter sprockets), and suspension travel on a FS bike (FD is fixed whilst cassette & thus chain moves up and down with suspension travel).

For example, I just measured my triple setup, and all I can say definitively is that for me and my setup the chain currently runs in the section of the cage that is 13mm wide, which is enough to use the whole range of the the cassette without chain rubbing on the FD cage. But, if I move the FD down by just 3mm the chain is now in a part of the cage that’s only 10mm wide, and no matter how much tweaking I do, it’ll rub at one end of the cassette or ‘tother. Now some people accept a little rubbing in 1 & 9 as normal, but I’m a bit anal about my gears and expect to use all 27 (or 18 in a double) without any annoying noises!

Can only suggest that the OP tries lowering his FD until he starts to get rubbing, and then he’ll know that’s as far as he should go with his setup.

Surely if your chain didn’t drop when going between the middle and granny on your triple, then there’s no reason it should if you just take off the big ring. Unless … you changed something else in the process, like changed height of FD, chainline, or put on a different size middle ring – any of these things could introduce a new element that causes the chain to drop, and then the double FD with its different geometry would indeed be the solution.