Chainline: double and triple chainsets

[frank9755]

This is more me coming to terms with having wasted half an hour on an annoying job I will have to undo then do again.

The question is how critical is my chainline being 2-3mm off?
It's a 2x10 speed. It's my audax bike so I don't race on it but I do do long rides and use all the gears.

Background is that I want to try shorter cranks (actually on my TT bike but it is a lot easier and cheaper to experiment on this one). I've got a double (Stonglight Impact). I called the people who sell them and it transpired they didn't have a double with 165mm (which I wanted) but they could give me a triple, which I didn't want - but I was assured that it was the same chainset only with inner ring mounts and with the same BB it would be fine (as long as the mounts didn't foul the chainstays)

So I put it on. Dirty hands, a few scratches from the chainring, generally an unpleasant start to the evening

And it is clearly not the same. Probably 3mm too far out.

I know I'll have to change it back, get filthy again, skin my knuckles. Then find a 165 double (if there is one in existence) and do the whole thing again. Unless everyone rushes up to say that a few mm is no big deal...(please!)

[gaz]

Having never measured a chainline in my life I feel unqualified to comment. However I'd be happy to join the orderly queue for an unused 165 triple chainset.

[meic]

The chainline is picked to suit a particular gear range for the front chainrings.
There is a standard but it may not actually be the best for you!

In my case I tend to favour lower gears so I will deliberately have a smaller chainline distance.

You can work this the other way by calculating from your cadence and "favourite" speed which gear you want to spend most time in then see how your chainrings are lined up for that.

So in my case middle chainring is lined up with the fifth sprocket on the cassette which is just right for 16-18mph. I assume this is on a standard chainline.

I can never get the inner ring far enough to the left.

As you mention time trialling, you may be better off with your crankset further out anyway.

[frank9755]

Having never measured a chainline in my life I feel unqualified to comment. However I'd be happy to join the orderly queue for an unused 165 triple chainset.

Thanks, Gaz! I'll get back to you if it comes to that.
The full horror, of course, was that it was just the cranks rather than a chainset - I changed the rings over (and am looking at having to change them back)!
I didn't have to measure the chainline as it fouls the front mech by a good margin.

[meic]

And it is clearly not the same. Probably 3mm too far out.

I can see how they made this mistake, both chainsets will have the same chainline.
This is measured along the middle ring on a triple and halfway between the rings on a double.
So your chainrings will be 3mm further out than they would be on a double.

[Brucey]

if you want to experiment with crank length cheaply, try spa cycles' triple chainset. Again can be run as a 'compact double'.

I don't know how it will offer up vs the stronglight tho.

BTW the issue you had may have been because of the difference between an ISO and JIS taper; stronglight make both....

cheers

[frank9755]

I can see how they made this mistake, both chainsets will have the same chainline.
This is measured along the middle ring on a triple and halfway between the rings on a double.
So your chainrings will be 3mm further out than they would be on a double.

Thanks Meic, I'm sure that's it - at least I understand why I've got the wrong chainset!

I could try it, I suppose. But I don't TT on this bike - it's for audax / long distance rides, which are sometimes hilly. I do use both ends of the cassette but perhaps the bottom gear is more important than the 11T. I'll sleep on it!

[frank9755]

if you want to experiment with crank length cheaply, try spa cycles' triple chainset. Again can be run as a 'compact double'.

I don't know how it will offer up vs the stronglight tho.

BTW the issue you had may have been because of the difference between an ISO and JIS taper; stronglight make both....

cheers

That is exactly what I did do! Spa's own one is exactly the same as the Stonglight / Sugino (both are JIS), but the triple isn't the same as the double.

[Brucey]

ah, I see..... makes sense I 'spose.... of a kind....

cheers

[reohn2]

Providing the granny ring mounts don't foul the chainstay then the answer has to be a shorter BB axle.
Or am I missing something?

[531colin]

well, I'm certainly missing something! (but in my case, its the ability to absorb information, that I had when I was younger)
Spa's chainset and the "Impact" are both really Sugino XD2...
trouble is, there's more than one XD2, there's one where you can see 5 arms, and there's one where you see 4 arms, and the other bolt is in the back of the crank
and there's one which takes a different length BB, but I can't remember why....
But mostly, you need 107mm axle for a double, 110mm for a triple....thats where the difference is made up, not in the cranks.....

[frank9755]

Providing the granny ring mounts don't foul the chainstay then the answer has to be a shorter BB axle.
Or am I missing something?

Yes! A shorter (symmetrical) axle would put my chainset in the right place but pull my left crank too far in. The LHS cranks seem to be the same on each chainset (which they should be).

[CREPELLO]

Why does nobody take into account the width of the rear dropouts? For any given chainset, a modern road hub will need a BB 5mm shorter than that for a MTB hub by my reckoning. But do you know which hub the shop or CS maker are quoting BB length for?

[frank9755]

well, I'm certainly missing something! (but in my case, its the ability to absorb information, that I had when I was younger)
Spa's chainset and the "Impact" are both really Sugino XD2...
trouble is, there's more than one XD2, there's one where you can see 5 arms, and there's one where you see 4 arms, and the other bolt is in the back of the crank
and there's one which takes a different length BB, but I can't remember why....
But mostly, you need 107mm axle for a double, 110mm for a triple....thats where the difference is made up, not in the cranks.....

I think what Meic says is correct; that there is a difference in the drive side crank because you measure the chainline to a different place on double vs triple. Anyway, there is, because my chain is fouling the front mech! And you can see it if you put them side by side; the triple one sticks out further in the centre to meet the BB. That enables it to put its middle (second outermost) ring in the same place that the double puts the space between the outer and second outermost rings.
Mine are both the same model (Spa own-brand XD2s with 4-arms visible).

[meic]

Crepello,
I think that is self-correcting, going back to what I said earlier about choosing a chainline to suit your riding.
You could happily assume that a bike with an MTB rear hub will actually want their chain aligned towards lower gears.

In my case the BB length and chainline is normally restricted by the front deraileur and often I would like it lower than that allows for.