Singlespeed & Fixed Gear - anyone ever built a chain tensioner?

I picked up a trek 1400 frame the other day on the cheap. Anyway it has vertical dropouts, so I would need a chain tensioner.

Has anyone ever made one? I figured I a busted RD and some creative welding or something might get the job done.

I know they are like 20$ for a cheap one, but what the hell might as well give it a shot.

probably just the derailleur and liberal use of the limit screws will do ya'.

I "made" one out of an old rear derailer once. It was terribly ugly, but it did work fairly well. I removed the lower pulley and just ran the chain around the upper pulley. I must note that I did have some problems with chain wrap, where the chain would skip under load. I decided to stop messing with it and found the magic ratio instead. Much cleaner, much smoother, but it made gearing changes virtually impossible.

http://i812.photobucket.com/albums/zz48/Jacob54901/My%20bikes/IMG_0864.jpg

I've seen someone make one out of a V brake arm once. It was pretty cool. I think it's best if you can have something which locks into place, preferably in "push-up" mode. Then you don't have the chain bouncing around. I'm thinking about making one this weekend so I will update if I can get something to work well.

I made one with the side of a rear derailer arm once. Very simple, just the pulley and the half arm, IIRC(was years ago). Bolted the top of the arm to the hanger, with a spacer to keep it loose enough to pivot. Bent the arm in two places for the correct chainline spacing. Used a wide innertube rubber band for the spring. Actually worked quite well.

Using a rear derailer as a chain tensioner is only ugly if you start with an ugly derailer. I used an old Campy NR rear derailer as a chain tensioner and it was gorgeous.

Well if you access to the machine tools and an old rear derailleur, you can salvage the pulley wheels and whip up something much compact and simplified for a cleaner look.

One of these (Huret Svelto) ought to work pretty well as a tensioner:
http://www.disraeligears.co.uk/Site/Huret_Svelto_derailleur_%281st_style%29_files/IMGP6096-filtered.jpg