Twin Chainring Fixie

This seems so obvious that it seems strange no-one has already done it.
I am looking to fit a twin chainring to my fixie with a front cage shifter and a friction changer (none of this indexing - keep things basic, as is the fixie ethos).
I can't see that the lack of a freewheel will have any bearing on front shifting and if the C/L is set for dead center between the rings, there should only be a few mm variance to line on either ring.
I can see that there will be slight extra wear and tear on the chain and cogs - but apart from this I can't see any reason this setup wouldn't work..
Before I go out and spend £100-or so on components, has anyone tried it already - or any heads-up as to whether it could work?

How do you propose to maintain chain tension?

http://www.lfgss.com/conversations/225289/

Here's a good one from a while back

Not that I have the slightest how they work but I've seen a bike - made by a guy in east- who builds fixed but with a two speed hub at the rear. If it's clean lines you're after I'd say it's not a bad thing to investigate!

How do you propose to maintain chain tension?

Well duuuh, by using a spring like mechanism that keeps tension as well as allowing a possible gear like system to be....oh wait....

The only reason I can see it not working is that chain length is going to be different between the rings, so you'll have no chain tension on one ring unless you move the wheel in the dropouts.
A chain tensioner doesn't work with fixed, only single speed.

^ this.

what you want is impossible to do well. Chain tension needs all parts to stay in their place. Chain tensioners don't work on fixed gears because they cannot handle the forces on the chain being applied 'in reverse' i.e. when skidding/slowing down.

As you probably know, a slack chain can jump off the cog/chainring. When this happens it easily gets caught between the cog and the chain stay, resulting in a crash/very sudden halt.

Two-speed hub might be the way to go :)

edit: some more thoughts on this here. You could go for the 'ghost ring' option but when you're also gonna shift the chain and thus change the chainline and have that awkward moment when its between rings, I just wouldn't...

you want retro-direct...

This seems so obvious that it seems strange no-one has already done it.

Because it's dangerous.

The only reason I can see it not working is that chain length is going to be different between the rings

No. You can match axle position (therefore chain tension) with careful selection of gears. Add chainring and cog together, both sets equating to the same total. In my experience you might need to tweek with an added tooth but it is a fine starting point.

If the chainstays aren't in the way...

https://www.youtube.com/watch?v=q_BG-Amaq-w

you're thinking of a fixed dingle speed. the OP wanted just a single cog in the rear and two rings with a derailleur up front.

with a dingle speed you'd also have to stop and get off the bike to change gear.

like above, you'd have to put in the ghost sprocket every time you changed to the smaller gear. and take it out every time you changed up.

you're thinking of a fixed dingle speed.

Yep.

the OP wanted just a single cog in the rear and two rings with a derailleur up front.

I see, you're right. That wouldn't be very useful even if it was possible. Perhaps @GrimlyFeendish should look into Sturmey Archer fixed hubs, or embrace fixed substituting a selection of gears with #HTFU ?

Excellent idea, no one has ever thought of it before, go for it and be the pioneer.

Before I go out and spend £100-or so on components, has anyone tried it already - or any heads-up as to whether it could work?

I believe only top of the range Dura Ace chainsets (standard, NOT compact) are compatible with this. Buy one of those and then try it out. Someone should be able to confirm this.

What would happen if you were to weld up the freehub in a normal geared setup?

I have read that a Rotor setup with Q rings is the best, you offset the rings to peak in different places and this compensates for the chain tension issues.

I seem to think that 11 speed Ultegra works as well. Something to do with ring width.

Chain would come off every time you stop pedaling.
Or you would just rip the deraileur off.
Anyway: "bad things"

Not on Dura Ace Di2 derailleurs it wont.

such a pessimist, murphys_law

It might work if the chainrings are exactly the same size.

is this inverse shim mk. ll ?

tynan ?