Single Speed mountain bikes are quite possibly my favorite kind of bicycle. The eastern woodland trails in my area have climbs that are mostly shorter ups and downs with lots of twists and turns and plenty of exposed tree roots. Over time I learned that the granny gear (22 tooth chain wheel on a standard mtb crankset.) is not really necessary. The 32 tooth "middle ring" up front and and a 19 tooth cog on the back has worked great for me since 2004 when I first converted my bike to single speed.
I have found that the advantages to running a single gear outweigh the negatives for me.
1. No fussy derailleurs/ shifters to adjust and clean.
2. My first off road ride on a mtb was 24 years ago. To this day I still forget to downshift before I get to the begining of a climb and end up forcing the derailleurs to shift the chain under power. On the single speed just pedal. "Keep It Simple Stupid"
3. Tight chain- No chain slap.
4. Lighter overall bike weight.
The only disadvantage is only one gear. Sure it's nice to bail to the granny gear and spin your way up a challenging climb, but you will still suffer. I like to power up the climb and get it over with and go on the fun part- downhill! One season I even put derailleurs and shifters back on my bike. After a couple rides grinding gears and listening to the chain slap I converted it all back to single speed and the bike has been that way ever since.
Back in 2004 I found the Endless Bike Company located in Asheville, NC. Endless, a rider owned company sells bicycle frames, a line of single speed cogs called "Kick Ass Cogs" and a spacer kit used to convert a standard cassette freehub body to single speed. Endless employs a local machine shop to fabricate their cogs and they do a fine job. As an experiment I decided to order a 20 tooth cog to replace the 19 tooth that I have been using. I want to see if I can notice the difference of one tooth while climbing. When I received the new cog I noticed that it had been processed in a vibratory bowl and the information was mechanically stamped into the metal instead of the fine bead blasted finish and laser etching of my original two cogs. Not a big deal it's a sprocket not a piece of jewelry. What I like best about the Endless cogs is that the metal at the splines where the cog engages the free hub body is a full quarter inch thick. The splines are machined to very tight tolerance and the cog fits to the hub tight with absolutely no slop or play. I have used stamped flat steel cogs that have deformed the splines on the freehub which are steel also. Inspecting the splines on my hub after the Endless cog has been in place for years shows no sign of damage.
Kick Ass Cogs are machined from 7075 T6 Aluminum alloy.
I find metallurgy interesting so here is the skinny on this alloy from one of my favorite reference books:
"Corrosion-Resistant Aircraft-Grade Aluminum Alloy 7075"
"-Exceptionally strong but still light weight, This aircraft alloy is one of the hardest and strongest aluminum alloys while maintaining better corrosion resistance than alloy 2024 due to the addition of zinc. Originally developed for aircraft frames, It is also used for keys, gears and other high stress parts. Non magnetic, Temperature range to maintain strength is -320 F to +212 F."
-McMaster-Carr catalog #114 page 3,548