Bicycle Mechanics - fixed gear question

is it possible to build a fixed gear in back with a front deriallieur?
would chain length be a problem?

Well, chain length would most certainly be a problem. You could use one of those goofy chain tensioners, but I have no experience with such things so you'll have to wait for other to chime in.

You cannot use a chain tensioner with fixed gear. Only with a freewheel.

If you have a flip-flop hub that you can put two different size cogs on, you could run two different size chain rings giving you two gears: big ring/little cog and little ring/big cog. As long as the number of teeth for the two combos add up to the same (+-1 or 2 depending on the length of the horizontal dropouts or the adjustability of the eccentric rear hub if that's what you're using), you can use the same chain length. An example would be 40/20 and 45/15. But since you would need to stop and take out the wheel to change gears, you wouldn't need a front derailer.

If you have some loooong dropouts you could go the route of the QuickBeam
http://www.rivendellbicycles.com/html/bikes_quickbeam.html

They use a flip flop and a dual chainring with a quick release on the back. You may want to consider carrying a popsicle stick to keep the fingers clean.

I love that green.

You cannot use a chain tensioner with fixed gear. Only with a freewheel.

Does anyone actually have any support for this claim? I hear it over and over and my understanding is basically that the reverse tension could over-extend and trash a tensioner (and rip apart the old deralleur body as tensioner). It seems though that as long as you set it up so that the chain can hit a straight line without wrecking the tensioner, it should be ok.

^But then the extra chain slacks out on top, and is then possibly ripped around by the wheel.

Does anyone actually have any support for this claim? I hear it over and over and my understanding is basically that the reverse tension could over-extend and trash a tensioner (and rip apart the old deralleur body as tensioner). It seems though that as long as you set it up so that the chain can hit a straight line without wrecking the tensioner, it should be ok.

If you did this you would need a chain tensioner on both the top and bottom chain runs. Otherwise, when you backpedal, the top run of chain would go slack and derail off of the rear cog.

Fine point. So double tensioners (ick, but we're not talking aesthetics). Now back to the question at hand: can anyone substantiate the claim that one cannot use a tensioner on a fixed gear under any circumstances?

I believe the reason you can't do it is that when the chain on the underside went taut, you'd a) have a period of no control as the slack in the underside run is taken up (tensioner pulling straight with chainline); b) have a shock transferred to the crank and to your legs when that slack is taken up and the chain snaps straight; and c) have a lot of slack chain on the topside run between chainring and cog- this would likely cause the chain to come off, or at least potentially cause some nasty chainsuck. That's how it seems to me, anyway- anybody else have any thoughts?

edit: Weird, when I replied, BostonTrevor's question was the last post, but once I posted it, several others from last night showed up. Must be asleep still :)

Nah, you don't need much slack to get it right (basically no more than a half-link's worth, that's going to be the limits of your precision on vertical dropouts). That's not much.