buy an SS conversion kit from performance or wherever...should be cog(s?)/spacers/spring tensioner.
remove derailleurs/cassette/shifters/chainrings, install above...done. be sure to position the spacers so that the rear cog is directly inline with the middle chainring position and use that ring.
for true XC use, you will have chaindrops like crazy with this setup...but you said cheap/simple and on fire roads, and i think this will work well. if you see XC in your future, you will want a dedicated SS (no shift ramps) chainring and a fixed tensioner (or eno hub). i run an 1/8" drivetrain with an eno and matching freewheel.
you may need to adjust gearing. for XC i use 2:1 (34/17), but that is certainly too low for your purposes. what is your current middle ring? before you strip the gears, ride it locked in the middle ring and find the rear cog that feels 'right' (too high uphill and too low down), count the teeth, add 1, and that's where I'd start for the SS cog. a one tooth change in the rear is much more significant than in front and cogs are cheaper, so typically you replace the cog to adjust ratios. ideally, your conversion kit includes a couple cogs...not all do.
Last edited by dookie; 06-06-11 at 03:46 PM.