2/15/14 1:58 AM

Does anyone know whether the chainline that Shimano quote for their two chainring MTB cranksets is for the outer chainring, or for the midpoint between the two chainrings? I want to use one of these cranksets on a touring bike. The Shimano web site at http://bike.shimano.com/publish/content/global_cycle/en/us/index/products/mountain/New_Deore_XT/product.-code-FC-M785-S.-type-.fc_mountain.html gives the chainline as 48.8mm, but doesn't say how it's measured. That chainline would be ok for my purposes if that was where the outer chainring sits, but too wide if it's taken at the midpoint between the chainrings.

With the old square taper cranksets you could purchase a BB length to suit, and some brands such as Phil Wood give a bit of lateral adjustment on top of that, but with the modern setups you're stuck with whatever comes from the manufacturer.

2/15/14 6:56 AM

...at the center point between the rings for double rings. That's the industry standard measurement method.

MTB cranks will not have an ideal chainline on a bike with a 130mm rear hub, as they are designed for 135mm rear ends. I ran into this when I was considering using a 2x MTB crank on my 'cross bike. Some bottom brackets have spacers that will allow you to tweak the chainline slightly, but not enough to make it perfect.

That said, the difference may not matter all that much, as long as you're aware of the limitation it imposes. Primarily, it means that there's likely one more of the larger rear cogs that you can't use with the big ring. Beyond that, it probably won't have any significant effect.

Middleburn makes a MTB crank with a 47.5mm chainline:
http://www.middleburn.co.uk/cranks_rs8_x_type_duo.php
They were supposed to come out with a road crank with interchangeable spiders a couple of years ago, but apparently they never did.

Cannondale, Specialized and some others offer road cranks with interchangeable spiders, that will accept road or MTB chainrings, single, double or triple.

The Race Face Next SL crank offers a variety of interchangeable spider configurations, including a double with a bashguard that has a 47mm chainline. Just take the bashguard off if you don't need it for the road.

Functionally there does seem to be considerable flexibility when it comes to chainlines. For example, on my 'cross bike, I ended up using a triple crank with the outer ring removed. The new "outer" ring - formerly the middle ring - aligns perfectly with the cassette. However, the inner ring is offset considerably inboard compared to a standard double, yet it works just fine. Because the 39/30 ring combination is small and there's only a 9 tooth difference, I don't have any issues with the chain rubbing the outer ring in the small/small combinations. The potential exists for increased chain wear due to greater chain angles if I use those combinations, but on a 'cross bike, it would be hard to detect a chain wear difference. Besides, I generally only use the inner ring for steep climbs, which means I'm on the larger cogs anyway, making the chainline a non issue.

2/15/14 4:39 PM

Thanks. However, after a bit more research, I think I'll go with one of the Sugino OX-series cranks, which use the road chainline of 43.5mm, but use 74mm *and* 110mm BCDs for the double, so as well as conventional compact choices such as 50-34, you can also use a 74BCD chainring for the inner and have a combination such as 44-30. In fact, given that the crankset uses the bog standard 110 and 74 BCDs, you can put pretty much any chainring combination on it. They're also available in crank lengths with 2.5mm increments from 160mm to 175mm, not that I want anything other than the normal 170mm.

http://www.suginoltd.co.jp/products/road/ox.html