Tried road crankset on MTB, close but no cigar!

[Manc33]

This thread isn't to ask a question really, it is for anyone else that might want to try this.

The short answer is don't bother lol.

I had a standard 22-32-42 crankset on my MTB with a 110mm square taper bottom bracket. That works perfectly, it was a stock setup from a well known LBS. The chain never dropped once and I used it that way for about 2 years.

I just tried putting a 50-39-24 crankset on with a 127mm bottom bracket and found a few problems...

Because the front mech needs to be quite a lot higher up than if it only had a 44t outer (instead this is a 50t) it means the chain rubs on the small metal part at the back of the cage, that little connecting bit, on any sprocket except the lowest one. This is I guess because the mech is "so" high up, but this FD-M770 claims to be usable up to 48t in the Shimano PDF - using it with a 50t outer was the whole reason I got it. That's only 2t bigger than the spec sheet.

Also, because this crank is a road crank, it needs to have a 10mm longer bottom bracket spindle, but in that case the outer 50t would be touching the chainstay, so I used the 127mm bottom bracket, but even with this at 17mm longer than the 110mm that came off originally, my chainline (that should be 50mm) ended up at 45mm. Despite this, the chainrings are visibly just too far out, I can tell when on the granny with the lowest sprocket it is at a bad angle (like the chainrings are too far out) and putting it 5mm out more (to the correct chainline) would make this even worse.

Another problem is the crank arm on the drive side is probably 1.5mm away from the front mech when on the outer but I am OK with that.

The main problem is the chain dropping off the granny when shifting from the middle (about 50% of the time) and this is the reason why I will need to swap back to my normal MTB chainset... spinning out on 42x11 down every hill.

Maybe shifting down from the 39t middle to the 24t granny is just too big of a jump, since no MTB crankset would ever have such a big difference there. Most are 42-32-22 or 44-32-22 or 48-36-26 and so on, well all of those examples only have 10t difference between the middle and granny rings, I have a 15t difference with my 39t middle and 24t granny.

One annoying thing is why does a chain slip so easily down a gap which it then will not come out of?!

[andrewjoseph]

i think you need a road frame to fit that crankset.

[Brucey]

I just tried putting a 50-39-24 crankset on with a 127mm bottom bracket and found a few problems...

Because the front mech needs to be quite a lot higher up than if it only had a 44t outer (instead this is a 50t) it means the chain rubs on the small metal part at the back of the cage, that little connecting bit, on any sprocket except the lowest one. This is I guess because the mech is "so" high up, but this FD-M770 claims to be usable up to 48t in the Shimano PDF - using it with a 50t outer was the whole reason I got it. That's only 2t bigger than the spec sheet.

if you look in the specs the chainstay angle is also mentioned. If you are using all the chainstay angle allowance (as you might do on a typical MTB frame) you shouldn't expect to get any extra capacity. BTW if the front mech clears the rings during the shift at all, that is 'enough clearance'.

...... my chainline (that should be 50mm) ended up at 45mm. Despite this, the chainrings are visibly just too far out, I can tell when on the granny with the lowest sprocket it is at a bad angle (like the chainrings are too far out) and putting it 5mm out more (to the correct chainline) would make this even worse.

are you sure you are measuring correctly? A 45mm chainline (which should be measured to the middle ring on a triple) should not be 'visibly too far out' on a typical MTB.

Another problem is the crank arm on the drive side is probably 1.5mm away from the front mech when on the outer but I am OK with that.

bags of room. You should try running some older French stuff or pre CSC campag.

The main problem is the chain dropping off the granny when shifting from the middle (about 50% of the time) and this is the reason why I will need to swap back to my normal MTB chainset... spinning out on 42x11 down every hill.

Maybe shifting down from the 39t middle to the 24t granny is just too big of a jump, since no MTB crankset would ever have such a big difference there.

MTB doubles are somewhat like that, but the mechs they use are designed for that range, and the mech you have is not. Most folk fix this with a chain catcher of some kind.

One annoying thing is why does a chain slip so easily down a gap which it then will not come out of?!

Sod's law? But actually, even very gentle pedalling can exert tremendous force on that small diameter, so if it can jam, it will do.

cheers

[Manc33]

Cheers Brucey, I was going to try the "cable tie chain catcher" trick but it is an awkward squared off frame in most places, including there. I also don't know how one might be fitted to this with a clamp-on FD (last chain catcher was part of the braze on of my road mech).

I have no idea how I can have a MTB crank/110mm BB then swap to a road crank/127mm BB and it "only" be a 45mm chainline, but still look so far out.

I am pretty certain that's why the chain drops, the low sprocket is nothing like lined up.

On a MTB chainset the granny is tucked right in, I mean it is nearly overlapping the BB cup!

Now on the road crank, the chain can fit down the gap.

All I can think is this had a crazy chainline like 35mm or 40mm to start with.

I will find out today because I am swapping the 42-32-22 back on.

I'm going to hate the gearing - my highest gear will be about 101" (compared to 124" on my road bike)

I use up to 90 gear inches on the flat, which was obtainable from my 38t middle and 11t sprocket. I am going to be on that outer a great deal I think, maybe even riding in the highest gear on the flat at times. I also have so many more overlapping gears and only the biggest three sprocket on the granny and the smallest two on the outer don't overlap.

A few other problems were the chain getting all jammed shifting from granny to middle and my heel hitting the chainstay because the cranks arms are further in (before it had a FD fitted, testing the bike out).

42x11 @ 100 RPM is about 30 MPH so I should be OK.

Well, I hate pedalling like that, I need more gearing there, but I don't use cleats or toe clips, exaggerating the "problem".

One upside is I now have a steel granny (and middle and outer lol). I have slightly lower gear going from a 24t granny to a 22t.

The bike is about 50% heavier than my road bike was, so going to a 32t middle (from a 39t) it will be a godsend up a lot of hills.

There's a good side and a bad side to it like with most things.

The funny thing is I had only just got that 52-38-24 running perfectly.

[Manc33]

I just setup the MTB chainset with its original 110mm BB and again it has a 45mm chainline - just like with the road crank (Prowheel Ounce) with a 127mm.

The road chainrings were sticking out, the MTB chainrings aren't.

I will leave someone else to figure that out, it doesn't matter to me now I am back on the 42-32-22

There's some "quirk" about it, the way the crank arm itself is designed, I guess.

The chainrings themselves are miles closer in now and I couldn't get it to change off the granny with this 42-32-22, the front shifting on it is glorious now I am using an actual MTB front mech as opposed to that R773 (that I never really could fathom whether it was a match with my MTB shifter but it did work quite well).

However the shifting now with a FD-M770 and SL-M951 is just brilliant. I did get it to shift off the outer once, but I just turned the outer limit in a bit more (which I didn't want to do since that can possibly "flick" the mech and make it shift from outer to granny) and dialled in the cable a little, it was solved, but then I do have to give it a little extra shifting from granny to middle, but I think this is the best setup because it avoids shifting off the outer and shifting between middle and outer is going to be happening far more than I shift from granny to middle.

Who knows why, but taking just 2 links out of my chain (52t outer down to a 42t outer) has made it so I don't need to turn the b-tension screw around anymore or use an allen head bolt, or put Milliput there, but I still might because I want a little leeway on the b-tension screw.

When they say road chainsets don't go on MTB's I get it now. There are a lot of potential problems.

The best bet is probably a 4 arm MTB chainset that has a 48t on it, but its unlikely then to have a 22t. It has to be MTB though, which is a shame because you can get a Tourney one cheap, but that's road. When they have riveted chainrings of course there's no option to swap a 26t granny to a 24t or 22t.

[MartinC]

I just setup the MTB chainset with its original 110mm BB and again it has a 45mm chainline - just like with the road crank (Prowheel Ounce) with a 127mm.

The road chainrings were sticking out, the MTB chainrings aren't.................................

I don't understand how one set of chainrings can "stick out" more than another set with the same chainline. Either "sticking out" or "chainline" don't mean the same to us both.

[Manc33]

On the road chainset the (50t) outer was about 5mm from the chainstay. If my 42t outer of the MTB chainset were made bigger and kept where it is, it would be touching the chainstay, but yes, both have a 45mm chainline.

So unless the gaps between the MTB rings are bigger then I have no idea, but my granny ring is now tucked right in, on the road chainset it had a gap big enough for the chain to drop down there.

[MartinC]

Chainline isn't the gap between rings and chainstays. It's the distance between the centreline of the frame and the middle ring of a triple (for a double, mid way between the rings). From what you're saying the chainset a BB combinations you're using have different chainlines.

[MartinC]

Just went back to your original post to try a more helpful response.

I had a standard 22-32-42 crankset on my MTB with a 110mm square taper bottom bracket. That works perfectly....................I just tried putting a 50-39-24 crankset on with a 127mm bottom bracket and found a few problems........

If you do this there are 2 things you need to sort out (or you'll have problems).

1. Chainline. You'll need t find a BB with a spindle length that puts the middle chainring on the new chainset the same distance from the seat tube that the old one was i.e. so it's got the same chainline. You'll need to do this empirically - try a BB, measure it, guess what length spindle it needs - then repeat 'til you get a result you like. The shape of you chainstays may prevent you achieving this 'cos the frame is designed for fat tyres and little chainrings

2. Front mech. You now need a mech with a capacity of 26 teeth (compared to the 20 tooth capacity you needed before). If the mech doesn't have this then the chain will catch on the rear of the cage in some gears (smaller rings and smaller sprockets). You say this is happening - that's a bit of a clue. The other thing that will happen is that the chain will come off when dropping to the granny ring 'cos the mech is just going to throw it inwards with nothing to stop it going too far 'cos the chain is below the cage. The downside of having a mech that's profiled for a 42 or 44 big ring is that it'll have to be fitted higher than ideally so that the bottom end of the cage plate clears the big ring. This will exacerbate both of the previous problems.

The biggest part of your problem is that a front mech that works on a 42-32-22 chainsset is unlikely to work on a 50-39-24.

[Manc33]

Yeah the angle is just all wrong on a road chainset even if you get the chainline exactly right.

This FD-M770 is rated up to 48t so I might just (eventually) get some old square taper 4 arm triple MTB crank arms and get something like a 48-34-22 setup and see how that works.

The reason the FD was struggling was I am certain because of the chainrings being too far out.

MTB cranks appear to have a "pushed in center" and flared crank arms, meaning the chainrings are tucked right in as far as possible.

I know the chainline is the middle of the metal on the middle chainring to the outer edge of the seat tube.

I always assumed every MTB front mech under the sun worked on a 50mm chainline, so maybe I do need to swap the 110mm BB for something like a 120mm to bring it out by 5mm but it works OK now as it is.

Its handy having this 42-32-22 on because every single gear is available, every sprocket on the granny - with no rubbing in any gear. Also going right down to a 42t outer (from 52t) for some reason has meant I do not need to turn the b-tension screw around, it just about works with it screwed in fully, but normally.

I quite like it like this but it is annoying how only the smallest two sprockets on the outer don't overlap with the 32t middle chainring. All I know is with only a 10t difference between the granny and the same between the middle and outer, shifting on the front has never been better, but then again I am actually using a MTB shifter with a MTB mech with normal sized chainrings probably for the first time ever.

When I got this MTB it had an Alivio front mech (and this chainset, this BB) so maybe that Alivio (FD-MC21) has a 45mm chainline, I never looked. Its using the FD-M770 now.

Yep, 50mm:



So then my current chainline is about 4mm too far in, for this FD.

That means it ideally "needs" a 118mm BB.

It also explains why the chain was able to shift off the outer but I think I have fixed that.

http://velospec.com:8080/components/shimano/fdmc21

The original FD (that was with this 110mm BB) from what I can tell was also a 50mm chainline so the shop got it wrong then.

[recordacefromnew]

I know the chainline is the middle of the metal on the middle chainring to the outer edge of the seat tube.

Alas, things start to make sense... Chainline is the middle of seat tube to the middle ring of a triple. Now perhaps we can understand why when you have a "45mm chainline" with a 127mm bb yet the 50T ring is "so far out"...

From the OP it also appears you assume:
1) If chainset A with a Xmm bb delivers Ymm chainline, then chainset B would deliver the same chainline with the same bb, and
2) If chainline is Ymm with a Xmm bb, then chainline will be Y+Nmm with a X+2Nmm bb.

Both assumptions are nearly always incorrect.

[Manc33]

No wait I did get it wrong typing that, I was measuring it to the hole in the middle of the BB shell, sorry.

That now is 46mm (and should be 50mm for this FD and the stock FD that came off).

Its all working fine.