Minor point here. Riding fix gear and changing gears are mutually exclusive only if you choose to go that route (or just default to the bikes that are out there). Riding hills fixed and changing gear ratios has been done for at least the past 120 years. Double sided "flip-flp: hubs with cogs on both sides. It used to be standard practice in races for everyone to come to a stop before the mountain and flip their wheels around. I built my avatar photo bike to do just that. And rode the week long Crater Lake Cycle Oregon on it, bringing every cog between 23 and 12 teeth and taking three each day. (Carried a lightweight chainwhip so I could screw on a 12 or 12 tooth cog for the descents.)

I love riding fix gears. But I knew as I approached my 60th year that I had no business doing mountain climbs on flat ground compromises. Also that my soft areas were way past wanting to do 200 RPM 8 mile descents. The bike I dreamed up to do this and had made is basically a late '80s road race bike except - velodrome standard drive train, a road inspired forward facing dropout that can handle any of those cogs with no change in chain length and a decently high BB so I can pedal fast corners on the 175 cranks my knees love.

22k miles later, I can report that the concept works. It's my favorite bike of all time. Its ridden 4 Cycle Oregons. Done at least two 130 mile days. Using dropouts, NOT track ends, means that two minute wheel flips and 5 minute cog changes is not hard when I am in practice. You are welcome to say I cheat. That's been said about gears right from the beginning. But every inch that bike has gone has been as a true fix gear - meaning I've pedaled down down hills in the 42-23 and up a few on the 12. The little mind game of "do I stop, waste 2 minutes and cool off or do I muscle the next hill" is quite real. I also look carefully at elevation profiles. Whether to do that year's Cycle Oregon fixed is, for me, decided by the number of hills more than the total elevation. Crater Lake is much more fun that Oregon's coast highway!

If somebody wants to do this, the secret is the dropout. I can go look if someone want the number but mine has a slot roughly 2 inches long and takes a 90 degree turn down at the front - so I can run the biggest cog with the tire nearly touching the seat tube and still pull the wheel out easily, like a vertical dropout, to do the flip. A pure joy to use. (You want to keep the chainstays relatively short since the wheelbase gets a lot longer as you push the wheel back for the 12 tooth.) Uphill you want that wheel close so you don't spin the tire on wet or sand and waste that precious energy that, trust me, you need much more than those geared folk. I designed my dropout to sit at an angle of 11 degrees; roughly half the angle of a traditional road dropout. Yes, my brake pads ride up and down the rear rim but the tail end of the bike stays roughly the same height and the bottom bracket doesn't change much running that tiny cog. (And high speeds on the 12 is when you REALLY want all the bottom bracket height you can get!). I solve the pad travel by running deepish rims. Yes, on the big cogs, the pad is well below the intended braking surface but just how fast do you ever go on a 42-23 fix gear? Anybody who wants to do this is welcome to contact me and I'll dig up the drawing. (My invention but I learned after the bike was built that there was a production version of this made in Europe (pretty sure) and perhaps in the 1930s. Shaw, a builder in the Bay area also has built fix gears with a similar (but shorter) dropouts. I've seen that bike in person.