xt m770 triple to double crank

rickbst170

Sorry to ask, I should be able to work it out but can't and can't find the right answer anywhere.

If I take off the three rings from an m770 triple chainset, and buy a 24 and a 38 ring, will the spacing be the same as an m785 double? Basically, will an m785 double front mech do the job? I know chainline is different for a double vs triple, so will the two rings left on the m770 have the same chainline as before, just with no big ring, or do the rings bought affect the chainline?

Cheers
Rick

cooldad

It'll work fine - just keep your current mech and use the high limit screw to block out the top position.

rickbst170

Cheers cooldad, already got an m785 double mech but no cranks. It's a build and I got the shifters and both mechs as a package. I therefore want double chainset but can get my hands on an m770 crankset cheap. Wondering about converting that to a double.

Either convert that to double (needs rings anyway), buy a ready made double, or replace mech for a triple. Whatcha think?

cooldad

It'll work fine either way.

The Rookie

rickbst170 wrote:

If I take off the three rings from an m770 triple chainset, and buy a 24 and a 38 ring, will the spacing be the same as an m785 double? Basically, will an m785 double front mech do the job? I know chainline is different for a double vs triple, so will the two rings left on the m770 have the same chainline as before, just with no big ring, or do the rings bought affect the chainline?

It won't be the same as the double which has the nominal chainline between the 2 rings where a triple has it inline with the middle ring, that means you should try and use the options as if it were a triple.

The M785 mech will do the job though, spacing between rings will be the same triple and double.

rickbst170

Is the big ring the same distance from the bb on both triple and double, but triple rings are closer? Basically, are the arms the same so with new rings and spacers the chainline can be made to match? Ta

Chunkers1980

No.

rickbst170

Crap. Ah well. I'll fab something.

cooldad

The difference in chainline will be a mm or so - it will work.

rickbst170

I'm thinking I can just stick a washer or two in between the big ring and crank arm, then use a double chainring spacer to the middle ring to get the right chainline?

Chunkers1980

No.

If you're using 1 DS 2.5mm BB spacer, then use 2, to make 5mm - that's about it really.

You can run a shorter chain so only really rules out the bottom 2 cogs on the rear when in the inner ring.

The Rookie

rickbst170 wrote:

I'm thinking I can just stick a washer or two in between the big ring and crank arm, then use a double chainring spacer to the middle ring to get the right chainline?

You said 24 and 38, the 24 will have to be on the granny tabs which means the 38 has to be in the mid position, not sure where you'll be fitting the washers as that will move the 38 too close to the 24 surely?

rickbst170

Good point. I guess I want to use big and middle rings, buy have 38 and 24 teeth somehow.

Might just measure my rear mech and see if I can run triple. Got an lx front mech in the garage. Shame to miss out on an xt m770 chainset for a tenner!

Chunkers1980

The min ring size on a 104 BCD is 32 - 30 can fit but is touch and go, so can't be done as the outer 2 rings.

rickbst170

Ok. Think I got too hung up on 'perfect chainline' earlier (got my reasons but over it now). I've got 2x10 Mechs already so don't want to go triple really.

Front mech is m785 which shifts 14 teeth. Might as well make use of that range. Next question...does 22/36 with 11-32 make more sense than 26/40 with 11-36? Or in the middle? Range is better in option 2, but sounds like a lot to ask of the chain when on 40 and 36. Will be slack on 26-11.

What is a sensible set up? I guess 24/38 with 11-34 seems a good middle ground. I can work out overlap ratios, but unsure of chain length limitations. I think the 10 speed m770 only came in 24/32/42 so I'll already have a 24 granny (won't know ring sizes of the current setup until next week - long story).
Ta

The Rookie

Using the outer 2 locations will give the same chainline error amount as running the inner two locations.

Just fit the 2 rings and use it like everyone else does without issue.

rickbst170

Got hung up on the chainline topic. Basically, why go 2x10? Because it saves 70g weight and offers better chainline. If I can't get the chainline, what's the point in sacrificing 3 or 4 gears?

I was thinking if I use the big rings I could space them with spacers to get line right. Can't space outwards though, so would only work on big rings and not little ones.

Doesn't matter though, over it now. I'll go 2x10 still but only because I have a 2x10 mech and medium cage mech, not for weight or chainline reasons. Not sure that there are any other reasons to do it.

The Rookie

Your chainline will be perfect on the 38T, that is better than either ring on a double specific if you want to be anal about it!

You won't sacrifice 3 or 4 gears, between a 38t and a 42t is one gear using the normal spacing of circa 12%.

Double specific mechs are usually lighter than triples, so you'll save there!

rickbst170

Yeah, I know. Was always told to use the right tool for the job though, so don't like cutting corners but needs must.

Guess 11-36 is most sensible option for 24/38 to get max range (think range will be important while I'm unfit). Suspect I'll be needing that 36 for a while until I'm somewhere near healthy!

The Rookie

The difference in chainline is less than the difference in cassette location on different hubs, would you only buy hubs with a perfect chainline? (When I use a shimano rear hub on my MTB the gears are out by exactly one cog compared to the Superstar Trizoid hub) - you can overthink these things!

rickbst170

Cheers rookie. As always, appreciate the advice. I do overthink everything. That's one of the reasons it takes me ages to get anything done!

The Rookie

No problem, seriously this is a tiny issue, if you want 2 rings, do it, it won't cause an issue!

taffygriffiths

Slx M670 triple (42t removed to make a double)

and XT M786 DM double mech on my recent build.

works fine... theres far more adjustability in the movemement of the mech than the marginal differance in chain line.

Just fit it, Set it, and get on it!

rickbst170

Cheers matey. Good to know about mech adjustability.

Can't wait now. Need wheels, Seatpost and clamp, pedals and gear outers to finish the parts collection, few bits waiting on deliver, then I can get on an build her up!