Double vs triple crank - chainline?

Discuss light weight issues concerning road bikes & parts.
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alcatraz
Posts: 1132
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Hi WW's

To get some steep climbing gearing on my bike I've been looking for a mountainbike crank to put on my road bike (bb30). With my current cassette and RD (maxed out) I can get away with an 18t difference in chainrings so optimally I'm looking for a 44/26t or 44/28t crank.

I haven't decided if I should go with a triple or a double. The triple has the benefit of already having a good large chainring for my purpose. (With a triple you replace the chainring bolts and put the large chainring in the middle ring position. Bit ugly but functional.)

My question is about chainline. The smallest ring on a triple, will it touch the frame? Will it have a screwed up chainline? I'm hoping to use it with at least the 1-8 easiest cogs on my 10s cassette.

Maybe a double would be better but the large chainring is too small and needs replacing (expensive).

(I already checked and my FD can take quite small chainrings if I lower it. Smaller than I will need.)

Thank you for your input.

/a

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kdawg
Posts: 104
Joined: Wed Nov 20, 2013 9:10 pm

by kdawg

Road double chainline is 44.5mm - but that measures to the middle of the space - so the rings are at 41 and 46mm.

MTB is about 47.5 - to the middle ring. So, in this case you're only pushing things out about 1.5mm.

It would be easy enough to add a small spacer to both chainrings to push them in a bit too - as long as they're the same that would keep the shifting good.

It feels like the biggest issue would be the chain rubbing on the big ring when you're in the small one.

26 or 28/32 is a crazy gear! Even I could climb a wall with that.
I'm left handed, if that matters.

by Weenie


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kdawg
Posts: 104
Joined: Wed Nov 20, 2013 9:10 pm

by kdawg

As the inner ring will be small it should be away from the frame too I think.
I'm left handed, if that matters.

alcatraz
Posts: 1132
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

kdawg wrote:Road double chainline is 44.5mm - but that measures to the middle of the space - so the rings are at 41 and 46mm.

MTB is about 47.5 - to the middle ring. So, in this case you're only pushing things out about 1.5mm.

It would be easy enough to add a small spacer to both chainrings to push them in a bit too - as long as they're the same that would keep the shifting good.

It feels like the biggest issue would be the chain rubbing on the big ring when you're in the small one.

26 or 28/32 is a crazy gear! Even I could climb a wall with that.


Wow thank you so much. I didn't think someone just had these numbers in memory.

It sounds like the triple will have the best chainline for the purpose but 3mm doesn't sound like that much. Sure I'd crosschain easier in the big ring and in the largest cog.

Yes 26/28t rings is crazy but the goal here is not to just make the climb. I can do that out of saddle at low cadence but this is no way of doing it efficiently. The strava climbing segments could be all taken wih such a crank and 100 cadence seated on a 25% grade I think. :lol: A used triple crank is just like 20-30usd here in China.

Also regardless of setting good times, when was the last time you made it up a 28% climb without breaking a sweat? This could be possible... :D

Great info. Cheers /a

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