Isaiahc72

I've done a lot of research on this and have come up with nothing.
I'm looking into a new build. Buying and old 21 speed mountain bike. What if I were to get a single-speed rear wheel(one rear cog) and a chain tensioner but then keep 3 gears up on the front. Given the fact that I'd have a chain tensioner, would I be able to efficiently shift between the 3 gears on the front?

dabac

Chain tensioners are generally only made to take up the slack needed to cope with not having horizontal dropouts. If you want to use your triple front, you need to keep the derailer, lock it at one chainline and use that.
A 7-speed MTB rear is most probably a 26" wheel with a 135 mm axle spacing.
I don't think it'll be that easy to find a ready made SS wheel in that specification.
7-speed rears can be either freewheel or cassette designs. Quite different. Turning them into SS wheels requires very different methods.
So there's a fair bit of effort for marginal "improvement".
Why not simply stick the rear somewhere mid-stack and leave it there?
90% of the result at 1% of the effort.
If you really feel the urge to do anything, use a short length of cable, run it through with the button at the end lodged in the barrel adjuster of the derailer. Clamp it down to set the derailer at the gear of your choice. Tune with barrel adjuster.
Done.

dsbrantjr

The time-tested approach for folks who want 3 speeds is to get a three-speed rear with a single ring up front.

Since you will likely need a derailleur or something similar to deal with the chain take-up anyway, why not just use it to shift the rear with a single ring up front?

cny-bikeman

The front derailleur has a higher demand on it and is fussier as to adjustment than the rear, so it seems odd to eliminate the rear.

ThermionicScott 

This idea pops up every so often. Sometimes even from a person in SS/FG who believes they've found a brilliant way to make a 3-speed fixie, without quite thinking it all the way through.

As posted above, you'll need to keep a whole rear derailleur (rather than a simple chain tensioner) to deal with chain slack. For example, with a 48/38/28 MTB crank, you're still looking at 20T of capacity even though you won't be shifting the rear anymore. You could switch to a "road" RD, though, for better aesthetics.

The good news is that it ought to work, especially if you use a track/SS cog with tall teeth, and it would be a unique setup.

fietsbob

there was a engineer trying to have a crowd funded start up of a 3 speed crank transmission .. internally geared.

It was not using Kickstarter, which refunds the pledges if the company business scheme does not Go.

But another one that the proponent keeps the funds even if the plan did not bear fruit..

wondering aloud how that worked out?

bikemig 

Go 1 x 7 or get an IGH 3 speed rear wheel but you'll need a tensioner. Personally I'd just go 1 x 7 since it's cheap, easy, and functional. Plus it's hilly in NW AR (where the OP lives) so the gears will come in handy.

MichaelW

The advantages of a 1x7 over 3x1 are:
1. You loose weight in the cranks which you spin.
2. You loose the front mech, cable and shifter
3 You keep a closer selection of inbetweeny gears.

If you really want a derailleur 3 speed, you can replace some rear cogs with spacers.

12strings

I effectively had a 2 speed road bike for several years after the rear derailleur had a few parts break that rendered it unmovable with the shifters...I just set it in a middle cog and left it, using the front double chainring and (downtube) shifters.

Needless to say, hills were not fun...I've since gotten a new bike.