ok, so the bike I bought came fitted with 2x10 but to save weight and keep one type of component (9 speed instead of 9 and 10 speed) I fitted 9 speed gear set.

I am looking to fit smaller rings on the front (currently 42/28) to run with 11-34 cassette.

Now the thing is, I can get a lightweight 3 speed chainset that is lighter than the SRAM XX plus could go for a lighter cassette 11-32. (obvioulsy I need to change the front mech to a 3 spd and long cage rear mech).

I mostly ride in middle ring, needing the granny ring for steep hills, the big ring seeing use on the road and TBH the 2 rides I have had on 2x9 system I found I could not decide which of the front rings suited me. I am either in little ring with smalllish rear sprockets or big ring with largish sprockets so the chain line is not as good as it would be with a middle ring.

So any reason why I should not go back to 3x9?

You are experiencing the main problem for some people using doubles on mountain bikes. For some terrains (and for some physical abilities), a double crank is not ideal. Although Shimano offers double cranks, they actually really promote their triples. They are intended to be used mostly in the middle ring. And now that the small ring is a little larger than before, and the big ring is a little smaller, they are more usable than they used to be, and shifting to them won't kill your momentum as much.

Besides maybe the q-factor increasing & chainline changing no, no reason at all not to switch back to 3x9 if it saves you a bit of weight and more importantly makes your riding more comfortable/enjoyable/smoother/whatever.... nothing wrong with 3x9 at all!

... well I've splashed out on a FSA K-force crank - on spesh offer at Chain Reaction.

should see me save 50 g over the SRAM XX plus whatever the GXP adapters weigh - guessing around 40 g plus 20 g by running 32 T instread of 34 T cassette.

.... of course it assumes that the crank weighs the same as my other one.

downside is I had to buy 3 speed SLX mech since XTR nor XT E2 type mechs either do not exist (not found XTR one) or not available for sale - a 2 speed mech ain't gonna work right?

anyways, means all bikes will have same set up so it is a non brainer as to which gear to be in.... still feels kinda wrong going against the trend of 2x9 or 2x10 but the pros outweigh the cons.

I was using the 2x9 setup about 3 years,but from the July I using 3x8 setup and for me it's great.

E2 is direct mount if i'm not mistaken... so... I think the XT version is the M771 direct mount, as for XTR I want to say M981 direct mount but I think thats for 3x10.

..... well I made a mistake ..... I did not realise the E-type mech fitted with the bottom bracket clamp are the same as those without .... hence could only find SLX. (picture shows it without yet it came with the clamp hence how I twigged!)

now I see that there are XTR and XT e-type mechs - all you do is remove the clamp - doh ....

so need to send back the SLX and either get an XT for same price or splash out on an XTR..... decisions deciosions ...

..... hence my question about XTR M980 and M970 e-type front mechs.

You should try to make your 2x FD work with 3x before buying a 3x FD. Back out the limit screws all the way and see if its got enough movement range to cover the 3 cogs. If not, you can probably file the inner stops. Seeing as though plenty of people have used dura-ace FD's in 3x9 setups, and 2x MTB FD should work 3x with a bit of adjustment.

not possible - remember these are e-mount so there is hardly any adjustment. In fact I took the 3x9 out the box, compared it to the 2x9 and thought damn, I bought an unnecssary part as everything looked identical.

I then proceeded to fit the chainset, mech was in middle ring that allowed enough cllearance for the large ring to get past. I changed mech to big ring and the teeth of the large ring are inside the cage (3x9 is 44 T whereas 2x9 is42 T max) hmmmm...... I then get the new mech out the box, take a very very careful look and can now see the alloy mount is slightly longer meaning the cage sits a bit higher.

Now ofcourse I have the probem of how to remove the mech - the chainring prevents access to the mech mounting bolts and the cage prevents the chainset from being removed. .... but somehow my putting the mech into a smaller gear and a bit of force to move the cage up I managed to squeeze the chainring. (otherwise only solution would have been to remove the chainrng to undo the mech bolts).

So now that I know the e-type mechs shown fitted to the bottom bracket clamp are the same as those that are shown without one, I am looking to get XTR or at very least an XT, the XT being same price as SLX. Which leaves me an unrideable bike this weekend .... but the weather is crap and has been so best keep this one out the mud for now.... and I still need to tart up the spokes. ........ but the WW build is coming along - my spreadsheet shows I should get below 9.9g even though my target is 9.0 kg. .... there may even be photos .....

Shimano actually developed Dynasis to work better as a 3 ring setup. They now use a bigger small ring and a smaller big ring in order to aid in front shifting. For most people a 3 ring front crank is ideal. 2 ring setups came about so that companies could sell people a "new" product.

_________________
Light. Strong. Cheap. Pick one.

^^^ oooh you are being cynical

a 2 ring front set up may work out better than 3 if the rings were "correctly", much like you describe, and you would get less of an overlap in gears - pretty sure Bontrager back in the mid to late 90s came up with 2 ring chainset.

Dunno about bontrager. Ritchey was the first that I remember. He offered a custom twist shifter, a 29t inner ring, and a large rear cog that went behind the cassette. I think it was a 33tooth? It may have been 9speed before there were 9 speeds.

yep - I meant Lionel Ritchey - got him mixed up with Keith.

Well what did you end up with? Personnal I am thinking of going from 2x9 to 1x9 and really simplify my ride and loose a few more grams. Now if I could just loose the las few kilos myself and get back to my old racing weight!

me?
SRAM XX 42/28 with BB30 adapter, XT 10 speed 11-36 cassette, SLX 3x10 shifters, XT 10 speed R mech, SLX 2 spd F mech
to
FSA K-Force with XTR 11-32 cassette, XTR 3x9 shifters, XTR 9 speed R mech, XTR 3 spd F mech

Saved well over 200 g*, mostly on chainset, cassette - the GXP to BB30 adapter was 50 g. as you can imagine, my wallet lost some weight too

By the way, Chain Reaction are virtually giving away the K Force chainset - bargain price.

If theres is any interest, I can post the exact weight for each component.