motobecane69

So i just got a touring bike that came with a road triple square taper crankset (FSA VERO) I'm planning to swap this crank for an FSA Dyna Drive MTB triple and I'm a bit concerned about the chainline. I'm not sure I fully understand why there are so many differences in chainline between different types of cranks but I suspect that in the case of road vs MTB, the 135mm rear hub spacing is part of why it needs to be spaced wider. In this instance, its a MTB crank going on a road frame with a 130mm rear hub so in reality, I should just keep the same bottom bracket and not get one with a longer spindle correct?

FBinNY 

Ideally the centerline of the chainrings - the middle ring of a triple, or the center of the space between doubles - should line up with the middle of the cassette. This ensures the least chainline deflection in the various combinations.

Starting with the published chainline recommendation for a 135mm rear axle, switching to a 130mm would bring the cassette inboard 2.5mm, therefore the cranks should do likewise.

I prefer not to rely on tables, and so measure the width of the cassette, divide by 2 for the center, add 4mm for the cassette to Rt dropout clearance, subtract that from the half the axle width find the chainline to the center of the cassette, which is my target crankset chainline. ie a 40mm cassette on a 130mm hub calls for 65 - (20+4) or 51mm.

Or, with the rear wheel on, use a ruler to measure from the right side dropout to the center of the cassette, and subtract from the OLD.

Usually I try to stay outboard of this slightly because the vast majority of my riding is with the outer ring and outer half of the cassette, so I prefer to optimize that at the expense of the inside of the cassette which sees less use. That's a personal bias, you might fudge your centerline differently or not at all based on the combinations you use the most.

In any case a minor chainline error of 1-2mm is rarely an issue, except that some braze-on FDs on 1-1/8" steel tubes may not have the travel needed to reach the outer chainring of a triple with high chainline.

JiveTurkey

Each crank has a specified BB spindle to achieve the correct chainline. E.g., two road triples will specify the same chainline but may use totally different spindle lengths depending on the shape of the crank arms.

Therefore, you can't just keep the original road BB and mate it to a new MTB triple and assume you'll have a road chainline.

You need to look up what BB spindle length is specified for the new crank (as well as what chainline that would achieve). Then, you can use a shorter spindle to achieve a narrower chainline. You'd shorten the spindle by double the amount of chainline you're trying to lose (e.g., 8mm shorter spindle to bring the chainline from 47.5mm to 43.5mm). But, you have to make sure the cranks won't hit the frame.

motobecane69

What I think i have going for me is that both cranks are FSA so they are probably built pretty similarly so using the existing BB is probably going to be fine, but I'll double check it.

What if i decide to go with a new outboard bearing MTB crank? arent the bb shells of MTB's 73mm while road bikes are 68?

JiveTurkey

Same brand is no guarantee. You should try to find the specified BB of each.

What's the shell of the current bike? 68mm?

MTB external bottom brackets are designed for 73mm shells; spacers are included for 68mm shells. The spindle length for external cranks are fixed. You can move spacers around, but then the left and right cranks won't be equidistant from the frame. So, you're pretty much stuck with the chainline it has.

bradtx

motobecane69, Matching a square tape BB to a crankset can be a bit frustrating. Main factors to consider are chainline, chainring clearance and crankarm clearance to the RHS chainstay. Some may also want to consider Q factor.

This differs because some cranksets have more negative chainring back spacing (chainrings are inboard of the BB axle boss) than others, BB shell width and rear dropout width. Most modern road (race) frames have a 68 mm BB shell and 130 mm rear dropout spacing. Most modern mountain bikes have a 73 mm BB shell width and 135 mm rear dropout spacing. Then there are the frames, often touring frames, that have a 68 mm BB shell width and 135 mm rear dropout width.

Sheldon Brown has a chainline article in his stickies for a starting point for more details. If I were you I'd measure your current chainline, mount the new crankset and re measure.

Brad

motobecane69

UPDATE: This is exactly where i'm at, I ended up getting a used deore m590 crankset with external bottom bracket. My rear chainline is 45.5 which is essentially road triple. Seems like I need to do one of the following

1. Install bottom bracket with 5mm of spacers on the NON Drive side. This will give me a 45mm chainline up front. However, this will mean that my cranks aren't centered. I don't know if 5mm is a difference that I can feel or not.

2. Install a 2.5mm spacer on each side. This will push the chainline up fromt to 47.5 while it will be 45.5 in the rear. This bike will be used for commuting in NYC where normally on a 42 tooth road triple I almost never shift out of the middle ring. If i move the chainline out a bit, doesn't that mean I should just favor using the smaller cogs on my cassette? That probably won't be too too bad right? If i'm in the 32 tooth middle ring on the crank i'll almost always be in the middle to small cogs in the rear. If I'm on the 42 tooth big ring up front I may want to go as low as probably the third largest cog, i don't think that would be too severe of a cross chain right? I imagine on a triple the chain rubbing on the front Der cage would be enough notice that I shouldn't be in that gear.

bradtx

motobecane69, The chain is going to be moving in and out on the rear cassette, 2.5 mm of chainline difference isn't going to mean a whole lot. I'd rather have equal pedal spacing anyway.

Brad

JiveTurkey

Why did you buy the crankset if you knew the problem you'd have, but not the solution yet? Edit: Also, if you were already near-exclusively using the 42T middle chainring on the old crank, why switch out at all? There are external BB road triples with 42T middles.