Chainline Part 23

[Valbrona]

Road bike with 135mm OLD (disc brakes) at rear.

When the chain is on the small chainring it rubs on the inside of the big chainring when on the three smallest sprockets. This is very problematic as the pins in the big chainring keep picking the chain up as I pedal forwards - borderline dangerous, in fact.

This is with my favoured but 'non-permitted' 34/46 combination, but the effect could be worse with 34/50 (which Mr FSA says I should be using).

Would everyone say I need to be increasing crankset spindle length?

Thanks.

[Brucey]

I'd try spacing the small chainring out with some thin shim washers.

BTW the setup you have might work fine under other circumstances, eg longer chainstays or something...

cheers

[recordacefromnew]

The fundamental problem you have is road chainline (likely 43.5mm), and perhaps more than usual slop between chain and rings/sprockets.

Given rear sprocket spacing is c4mm, if you are rubbing on the 3rd smallest sprocket, and given road chainstay length and 50T ring radius are c400mm and 100mm respectively, you need a minimum extra of 2mm spacing between front rings to avoid chain rub altogether.

Alternatively you can "recreate" that 2mm spacing by increasing chainline by c8mm (i.e. getting close to mtb chainline). Or you can achieve same by a combination of the above.

[Brucey]

Given rear sprocket spacing is c4mm, if you are rubbing on the 3rd smallest sprocket, and given road chainstay length and 50T ring radius are c400mm and 100mm respectively, you need a minimum extra of 2mm spacing between front rings to avoid chain rub altogether...

except that the big ring is actually 46T and the chain will be running past it at some (smaller) proportion of the full radius. It isn't as bad as you think.

In addition I'd suggest that with that setup the last two sprockets ought to be verboten anyway (for reasons of execrable chainline), so really clearing the #3 sprocket ought to be a realistic (and easily attainable) objective.

cheers

[Samuel D]

For what it’s worth, my SunXCD cranks came with chainring spacers, which I have used for my 8-speed setup. Are you running an 8-speed chain by any chance? Since they are wider than even a 9-speed chain, they are more likely to rub in the way you describe.

[Valbrona]

Would typical chainline for 135mm OLD and double crankset be 47,5mm?

[recordacefromnew]

Would typical chainline for 135mm OLD and double crankset be 47,5mm?

Typical Shimano double mtb chainsets have 48.8mm chainline.

[BigG]

The centre line of a 9 speed Shimano cassette is by my calculation at about 43 mm from the centre of a 135 mm oln mtb hub. I had always understood that the "chainline" referred to for chainsets was based on the middle ring for triples and midway between the rings for doubles. If this is so, the Shimano chainset is about 5 mm outside the centre of the cassette - or am I wrong about the definition of "chainline"?

[Brucey]

I think BigG has it about right, and I also think that very many bikes (inc. most MTBS and many road bikes) have a 'bad chainline' as a result.

I try to set my bikes up so that if the chainline is 'wrong' it is 'wrong' the other way; this lowers Q and keeps a better chainline in the very lowest gears. But on MTBs this usually just isn't possible; the chainstays get in the way or the front mech limits the chainline being moved leftwards.

cheers

[Valbrona]

I try to set my bikes up so that if the chainline is 'wrong' it is 'wrong' the other way; this lowers Q and keeps a better chainline in the very lowest gears.

I like the sound of that. The limiting factor usually being rub of the chain against the inside of the big chainring when the chain is on the small ring. And with some modern carbon frames you could perhaps also get contact between the FD and seat tube when in the lowest gear the bike offers.

I notice the recent emergence of double road cranksets for bikes with disc brakes. FSA denote these as 'DB' cranksets.