Double fixed hubs.
 

It seems like a comical idea because even if you flip your wheel over to a different size cog, you will have too much or too little chain to run the other side, so whats the point. Or perhaps I am missing something here?

I believe with true track ends, you do have enough lateral freedom to accommodate a one- or two-tooth cog difference.

The whole point is that you can run more than one ratio by flipping.

Or, bugger up the threads on one side and not have to buy a new hub.

If you keep it within 1-2 teeth, you shouldn't have any problems. I have 16t and 17t on mine.

i have a 4 tooth spread on my flip flop with room to spare on my iro's dropouts.

ride to the velodrome on 48/17 then flip the wheel around and ride 48/14 ;)

Exactly. 14t and 15t for me. Ride hard one day on the flats and the next ya wanna make it easier or it might be really windy, just flip it and make it easier. No chain problems for me. Of course if you are at the end of the track end with the bigger cog and flip it then it might be a problem.

Track ends allow you to move the wheel back or forth for good chain tension. So if the cogs are within 2 or so teeth, you will be able to get correct tension with no problem.

i'm running a 13-17 flip flop

Also gives you the most versatility for a flip-flop since you can run a freewheel or a fixed cog on either side. (Sure, you can run a fixed cog on both sides of a fixed/free hub, but you're sans lockring on one side.)

yes

A fixed cog, sans lockring is not a good idea, especially since the aim is versatility. I'd get a fixed/fixed, if they were cheaply available.

u dont need a lockring if u are riding at the track.

wat?

yeah i run a 17t on the street and a 15t on the track with no need for additional/fewer chain links

if you show up as a new track racer and you don't have a lockring on your hub, i'd advise keeping your mouth shut about it.

as to what does and does not fit, let's be quantitative about it.

1 tooth on a cog engages 1/2 link of a chain or 1/2 inch of chain. but the chain only wraps around half the cog, so 1 less tooth on the cog means 1/4 inch of extra chain.

that extra 1/4 inch, though, is shared between the sections of chain above and below the chainstay, so the axle only moves 1/8 inch back in the dropout.

the axle is about .4 in thick, so if your dropouts are 1.4 inches long you can theoretically deal with an 8 tooth difference. in practice you have to account for space for the track nuts to grip and things don't always line up as you'd like them to, but a 4 tooth spread is usually no problem.

just remember that if you have a rear brake it'll probably not line up and may rub the tire or spokes because the rim's position will have changed along with the axle.

Fixed/free flip flop hubs are a foolish buy IMO seeing as how you can run a freewheel on the fixed threads.

I beg to differ. A fixed/fixed hub will yield the greatest versatility as one could use 2 fixed cogs, 2 freewheels, or a fixed cog and freewheel. From what I understand, the few threads not utilized by a freewheel on the 'fixed' side aren't a problem.

I've run a 48/18 and 48/15 on my fuji, with just enough space in the drop outs.

A fixed/fixed will also allow all those combination, and you will be able to have a lockring for your fixed cogs. Fixed/free, if you run double fixed cogs, one side won't have a lockring.

I like the double fixed hub. 15t for flats and low wind conditions or 14t for towing a trailer or strong headwinds.:D

Provided you're not running a "magic ratio" on a bike with vertical dropouts there should be enough room to swap one side or the other without chain problems.

Croscoe is talking about fixed/fixed.

And Plecks, it is ill advised to run fixed on the freewheel side. No threads at all for a lockring so too much back pressure and you've got yourself a mess of trouble.

For some reason I read it as "fixed/free will offer the greatest versatility". D'oh!

I love my Shimano DA fixed/fixed, merely because of the security if I accidentally strip the threads on one side. But I rarely keep a cog on the non-drive side because of the added rotational weight.

^ Really? The weight of a cog that close to the point of rotation is insignificant.