Shimano STI shifting MTB cranks

RichJS

Likes Dirt
So, I got a cross bike for the better half that needs lower gearing (cause it'll do a little CX, and a lot of towing kids around..)

It's got 105 STI shifters, 105 front der (double), road compact cranks, KMC 10sp chain - IE, not a funny shaped Shimano one. (Also got 105 rear mech and 11-32 cassette.) Problem is, 34-32 just isn't a low enough gear so we want the kind of gearing offered by MTB Double cranks on there. Shimano cranks are nice and cheap, so was thinking SLX 10sp 38/24 - 24-32 is much nicer for spinning up a steep hill (especially carrying stuff ..)

So, anyone tried Shimano* MTB cranks on Road shifting?

I'm guessing there's a few things:
  • ~50mm chainline instead of ~43mm. I am hoping the 105 derailleur will shift outwards the extra few mm to reach the big ring on MTB cranks ..
  • The HG-X construction on new Shimano cranks/rings. Will it shift a regular (non-Dyna-sys) 10sp chain without dramas? Should I look at other-brand 10sp MTB cranks (that cost a lot more)?
  • The 105 derailleur shifting smaller rings. I'm hoping it's OK cause I hear that Shimano Road front vs MTB front derailleurs are different cable pull .. That would rule out changing to a MTB derailleur.
Any other suggestions? (Going 1x10 with a 28t Widgit ring on mtb cranks is the last resort..)

* I got a SRAM Rival (Wi-Fli) groupset on my own CX bike and replaced the cranks with some X7 42/28. Rival shifting the wider chainline was fine, but the cable routing was still annoying: My routing is top pull so the cable had to go down to a pulley below the derailleur and come back up to the bottom-pull only Rival front der. So I replaced the front derailleur + pulley with an X9 (can do top-pull) .. apparently SRAM use the same cable pull for road/mtb front derailleurs - it worked great.
 
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Calvin27

Eats Squid
My concern would be the smaller chainrings and FD drop. If you have a braze on then this might not work, band on and you can move it to suit - assuming other clearances.

For chain line, the existing FD should be fine as there is quite a bit of tolerance (just try it before you go nuts buying stuff haha). If not, you could always snap up a road triple FD (cheap) which would definitely cover the range and will be compatible with your shifters. Saves you working out the sorcery associated with shimano cable pulls between mountain and road (wait until they start a cross groupset!).
 

takai

Eats Squid
My concern would be the smaller chainrings and FD drop. If you have a braze on then this might not work, band on and you can move it to suit - assuming other clearances.

For chain line, the existing FD should be fine as there is quite a bit of tolerance (just try it before you go nuts buying stuff haha). If not, you could always snap up a road triple FD (cheap) which would definitely cover the range and will be compatible with your shifters. Saves you working out the sorcery associated with shimano cable pulls between mountain and road (wait until they start a cross groupset!).
Road triple requires you to have a 3sp shifter though.

I have a top pull MTB mech on my twin ring CX bike (road cranks). But it uses a friction shifter so i dont care about ratios. But it covers enough range.
 

RichJS

Likes Dirt
My concern would be the smaller chainrings and FD drop. If you have a braze on then this might not work, band on and you can move it to suit - assuming other clearances.

For chain line, the existing FD should be fine as there is quite a bit of tolerance (just try it before you go nuts buying stuff haha). If not, you could always snap up a road triple FD (cheap) which would definitely cover the range and will be compatible with your shifters. Saves you working out the sorcery associated with shimano cable pulls between mountain and road (wait until they start a cross groupset!).
Yeah, the existing 105 derailleur is band-clamp and will move down fine. I will take a look and see how much further out it will swing once I completely loosen the H limit screw.

A road triple doesn't go low enough, usually only 30t minimum and as mentioned I'd need a new triple derailleur and shifter to go with it. I'm trying to keep costs at a minimum. (Mind you, I have my suspicions that some Double and Triple shifters might be identical .. I mean, with double rings the limit screws are just going to stop you going to the "third" ring anyway, right?)

Also, Shimano now have (partial) Cross groupset: CX70 / CX50. The cranks are 110BCD just like road compact, so no good.
 
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Calvin27

Eats Squid
I've been playing with the idea of a 1x10 and it always leads to sram. Everything just works with everything else. If only they actually got more of their stuff on low end OEM all these customisations would be a hell of a lot cheaper!

Another crazy unfounded (and possibly useless idea) is to stack spacers on the non drive side to take up some of the chainline.
 

RichJS

Likes Dirt
Another crazy unfounded (and possibly useless idea) is to stack spacers on the non drive side to take up some of the chainline.
That would indeed move the chainline in a bit, but wouldn't having off-centre cranks affect the rider?

I found Sugino ZX801D's have a couple of mm narrower chainline (still not roadie), 40/26 or even 40/24 options, narrow Q-factor, light .. So what's not to like? At $500, the price!

I'm also having the same thoughts with a Sora 3500 (9sp triple - 50/39/30) eqipped bike. Buying Sugino XD2 600T cranks (46/36/26) with square-taper BB would mean I could choose the right width BB for the correct chain line ... But that's a lot pricier than just buying Deore cranks (M590 48/36/26) for $90 and hoping it works!
 

The Duckmeister

Eats Squid
Pretty sure it'll work.... The chainring spacing is the same for road & MTB, so it'll bridge the gap fine. The pivots are the same for double & triple derailleurs, only the cage differs, and road triple cranks are wider than doubles in order to fit the third ring in. MTB doubles use the inner & middle positions of a triple. so tuned accordingly the current FD should reach fine.

The shifting may be a bit rough, mainly because the derailleur cage is shaped to suit the bigger road rings - you'll have a relatively huge gap between the derailleur & chainrings at the tail of the cage, and that will affect the shifting up to the big ring, as there will be a fair bit more flex in the system when trying to mash the chain up.

......you could always snap up a road triple FD (cheap) which would definitely cover the range and will be compatible with your shifters.
Road triple requires you to have a 3sp shifter though.
I suspect you misread the suggestion.... Calvin only mentioned a triple derailleur, not whole shebang. The shaping of the derailleur may be considerably more suitable to the small rings of the MTB crank. It will work perfectly well with 2x shifters if there are only two rings to shift between.
 

takai

Eats Squid

The Duckmeister

Eats Squid
I did misread it indeed.

The other thing to perhaps consider is the Shimano CX50 cranks: http://www.chainreactioncycles.com/au/en/shimano-105-cx50-cyclocross-double-10sp-chainset/rp-prod70118 Still a double, lower gearing than standard road.

Oh and for Calvin, Shimano do make a CX gearset... thankfully its 99% road with a few minor changes.
Problem with a CX rig is that it's a tighter pairing than either of the road standards. Critically, the OP wants a small ring smaller than the road compact's 34T, and the CX doesn't do that (36T).
 

silentbutdeadly

Eats Squid
I'm with the Duck. If you use a slx double then it'll fly mostly fine. But don't discount the variety of rings that one can get on the aftermarket...
 

takai

Eats Squid
Yah, complete brain fade on this thread. Missed the FD in that first post and now missed that he is already running compacts.

Anyway, i measured up chain line on a road frame that i popped a pair of Hone cranks into for testing and it looks like by running a pair of spacers on the non drive side (and one on drive side) it should tuck the chainline in a bit tighter. On my setup though the 34T fouled on the chainstay with no spacers on the drive side.
 

RichJS

Likes Dirt
Update: SLX M675 38/24 cranks are on the bike. The 105 (5700) front derailleur just swings wide enough to shift up to the 50mm chainline.

The problem now is that I can't slide the derailleur down so that it's only ~2mm above the 38t ring. The cage snags the chainstay (when shifted to the little ring) when I slide it to within ~12mm of the big ring. There's a large gap. Shifting to the big ring is poor - I'm assuming that's the derailleur location and not the KMC 10sp chain on the Shimano "HG-X" chainring.

Will a MTB derailleur be more compact and not go near the chainstay? Maybe I could use a ShiftMate Straight model #7S to cope with the difference between Shimano MTB and Road shifter indexing / cable-pull amounts .. but I am concerned that they keep referring to Triple Road and Triple MTB. Do Shimano use different cable pull amounts on their double vs triple shiftering?

It's a pity this bike didn't come with SRAM like my Dirty Disco. Road shifters + MTB derailleur + MTB cranks = no dramas. Maybe the chainstay is lower on that bike, too.
 

wombat

Lives in a hole
I'm assuming that's the derailleur location and not the KMC 10sp chain on the Shimano "HG-X" chainring.
It won't be the chain, I've run KMC chains on HG-X rings no problem, it's also OEM spec for plenty of bikes.
 

RichJS

Likes Dirt
Do Umlenker do anything that would help?
That was a good thought .. However, it appears that while Speen make versions of the Umlenker for both road and MTB shifters, they only make them for Road derailleurs.

The main aim of their device appears to be to convert from bottom-pull-only to top-pull, and I guess pretty much all MTB derailleurs do top-pull. The ability to use MTB shifters is a bonus.
I was looking at buying a Speen Umlenker for the Dirty Disco, but it was cheaper to change derailleurs to from a Rival to an X9 front (and shifted a bit better.)
 
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