Looking at almost all double ring chainsets, the inner ring is usually no more that 30t. However, if you wanted to convert an existing triple chainset to a double, you couldn’t go any smaller than a 32t inner ring (due to bcd/tooth number issues).

So……..

What about if you used the granny ring tabs to mount the inner ring, and then you could have 30t (or less) chaingrings, without any bcd/tooth number issues. You could then mount the outer ring where previously the middle ring was mounted (and use chainring bolt spacers to take up the slack).

The only potential issue which springs to mind, could be the front mech clearing a larger than normal granny ring?

Eh? Just take the big ring off. Ta-daaaa – double ring set-up.

Anyway, there are issues over the spread that a front mech can handle to answer that one.

Isn’t this how everyone with non-off-the-shelf doubles does it anyway? That’s how i’ve run doubles for years.

many frames have limited clearance for granny rings

obviously elivated stay FS bikes are immune but most hardtails cant take a massive ring in granny position and also the chainline would be pish !

isn’t that how they are done anyway?

my xt double has the ‘granny’ (28t) on inner ring style bolts, and the the outer (44t) on the inside of the spider (where the middle would go); there isn’t a mounting place outside of the spider (which is where a triple would put its outer.

Dave

Correct This is how it’s done on dedicated double ring set-ups – but the above applies to converting a triple.

And yes, can just remove the existing granny ring to make the triple in to a double BUT it doesn’t give you a good gear-spread like, for example, a 29t/42t set-up.

what I meant was, if dedicated doubles are just a triple without the mounting point for the outer, then if you want a double setup on a triple crank why on earth would you do anything other than remove your outer and fit rings of the sizes you want?

to summarise – no it isn’t a crazy idea; it is the only sensible way, and it is what everyone does.

Dave

Chainline needs to change too. trail_rat speaks truth.

“what I meant was, if dedicated doubles are just a triple without the mounting point for the outer”

they arnt , the chain ring mounts are both moved outwards from the bb to give good chainlines – other wise it would be a noise fest like when you try to run your granny ring and the 12tooth cog …

Ummm, I’ve been doing this for about two years. TA Specialites for a 28T 64 BCD ring (mounted in granny position) and a 40T middle ring.
It works fine.

Ok – I’ll explain using an example – and when I talk about double ring set-ups here, I’m talking XC-wise – ie inner ring in the high 20’s and and an outer ring in the low 40’s.

Take your typical compact triple ring crank – in my case a RaceFace Deus. I can fit/buy, off the-shelf, 42/44t outer rings. I can also fit/buy, off the-shelf, middle 32-36t rings. But I can’t fit anything under 32t. So I have the option to run it as a double, with 32/44t rings, but I can never run, for example, a 29t/42t set-up, as that chainring size to fit on the middle ring tabs does not exist (and I was also told by, can not be made due to to the bcd).

@trail_rat:

oh, ok then..

presumably though, if you did what the OP was suggesting was the other way he’d done it (i.e. 2 rings in the middle and outer position) you’d have a similar problem from big ring to biggest on the block.

I’ve run loads of double and bash setups, but just followed the standard rules of avoiding the small end of the block when in the granny.

I haven’t actually fitted my xt double chainset – It arrived last week and is sitting looking shiny in the living room, so I haven’t had a chance to notice a difference in chainline.

Dave

SCMIKEN – thanks for your reply! I never new TA did these size rings.

You answered my question : )

Well, if you have a 5-bolt compact drive spider you could have a 29t inner and a, say, 44t outer like a Stronglight Oxale 2 did many years ago and just get the correct length bb to suit chainline.

Or get a Middleburn equivalent still.

Much as I’d like to think my 22/36 setup Hone crank makes me some sort of trail blazer I’m affraid your “idea” has been pressed into service by lots of people over the last 6~8 years or so…

You’ll have to try harder… 😉

This is what you want:
http://www.chickencycles.co.uk/index.php?cat=32&ord=2