The simplest form of the modern safety bicycle is the trusty fixed gear. On the velodrome, the bikes are pared down to the absolute essentials: no brakes, no amenities. Out in the wild, a wise rider chooses lower gearing, a front brake for a little extra security, and fenders for the crappy wet weather in which the simple vehicle excels. I also recommend a two-sided rear hub, to allow for at least two gear options. Your choices are limited by how many cogs you might securely stack on either side of the hub, and by the length of the dropout to accommodate the difference in chain length.
I see people referring to any single-speed bike as a fixed gear. A fixed gear is a single speed (even with multiple cogs you can't switch quickly), but not every single speed is a fixed gear. It's only fixed if it threads directly to the hub with no ratcheting freewheel mechanism. The difference is critical, because a single-speed freewheel is the worst of both worlds. You only have one gear, but you lose critical advantages of a fixed gear.
With a fixed gear, you can't stop pedaling. This commitment scares some riders. You can get thrown if you forget and stop your feet when you have reached a good cruising speed or you're wailing down a hill. You can get launched if you dive into a corner too tightly and really dig a pedal in. One time on a rainy training ride I had a good line in the corner, but didn't know that the puddle I was aiming to ride through had a pothole under it that was about six inches deep. The front tire dropped into that as the crank came around, driving the pedal into the chunked-up pavement. I hit the road several feet from the bike. I pulled my face up from another puddle in time to watch the rear tire crawl off of the tacoed rim, allowing the tube to bulge out and explode. Lesson learned: never dive into water if you don't know how deep it is and what might be under the surface. Swimmin' hole 101 applies to bike riding too.
That crash was before the Maryland/Delaware district championships that year. The time trial that nasty summer was run in 50-degree weather and a stiff wind, with light to moderate rain. Real nice. The 108-mile road race a week later started under cloudy skies that eventually gave way to another saturating downpour. My elbow was still bandaged from the pothole encounter. And I flatted out of the road race with a couple of laps left.
A couple of weeks after that I slammed an obstacle while bombing around on a warm July night and was out of work for 10 weeks. How does that relate? I actually tried riding on the fixed gear while my right arm was strapped to hold my collarbone onto the top of my shoulder joint and my left hand was in a cast, because I was an idiot who couldn't be inactive. I reasoned -- if you could call it that -- that I didn't need to be able to grab the brake strongly because I could control speed through the pedals. True as it was, I did start to feel like I might dump it and slow my recovery even further. I was also unable to bathe myself because of the combination of medical devices attached to my broken parts, so I didn't want to have to wear congealed sweat for another couple of months. No one available to give me a sponge bath fit the fantasies one might have of such ministrations.
Once I was cleared to return to training -- I mean work -- in September, the fixed gear provided steady pedaling to rehab the lungs and legs. The atrophied, twiglike arms required more carefully selected weights and exercises.
Another nice thing about the fixed gear was that the rear rim didn't have to be dead straight for a good braking surface. I just stomped it basically flat and re-tightened the spokes. I don't remember when I finally got around to rebuilding it, but it was probably years later.
Now here, 41 years after all that, my present fixed gear is not the same bike, but it has some of the same parts. From late 1979, I always had a fixed gear for commuting and bad weather training. Here in New Hampshire, winter and its fringes last longer than in Maryland. If the winter isn't snowy, that means I get out on the bike during those months as well as in late autumn and early spring. With short daylight and cold air, the fixed gear provides continuous pedaling, which helps you stay as warm as you can when you're generating your own 10-20 mph wind chill. Winter riding is one of the trickiest activities to dress for because of that wind chill aspect. I much prefer cross-country skiing for a fitness activity. I would use it for winter transportation if I could.
This fall has presented many obstacles to regular riding. The darkness and winding roads stop my commuting among bulky vehicles that blind each other with their ridiculous headlights, so I have to carve out time from my days off work, when I'm doing every other thing I can't do on a workday. At best I get three consecutive days of riding. Staving off the muscle loss of age, I have to watch how hard I push, but also how much I slack off in between. It seems like there's about a two-day window between good rest and the onset of incurable sloth. A few weeks ago, I blasted out on the fixed gear for twentyish miles, feeling pretty good. The next day I felt a little worn down, so I chose a multi-gear bike. It was the heavy commuter, but it felt heavier than usual. The next day was cold and showery, so I reverted to the fixed gear, expecting to feel even more sluggish. Instead, the direct drive and considerably lighter weight combined to help me drive the bike and the bike to drive me. And there is a critical advantage of the fixed gear: the bike drives the rider. On a freewheel bike, you have to push the crank around. Sure, one crank arm brings the other crank arm around, but only your legs are doing the work. On the fixed gear, the motion of the bike itself keeps the chain moving. The wheel brings the crank around even if the crank isn't bringing the wheel around.
I know the effect well. I wasn't being reminded of its existence when I enjoyed its effects that day. I was only reminded to share it again for anyone who hasn't experienced it. Even my lightweight road bike is more fatiguing to ride than a fixed gear when I'm already tired. If I can ride downhill with a tailwind, or on a route with no climbs and no adverse winds, the road bike is great because I don't have to pedal at all where the going is good. But wherever I have to put forth effort, particularly on a climb, the fixed gear can feel better because of the free lift that my off leg gets on its way back to the top of the pedal stroke.
Coasting on the fixed gear consists of loosening up the legs while maintaining a smooth, precise pedaling circle. You get smooth or you don't last. For a sustained descent, I will stop and flip the wheel to the high gear side. I also have the brake to help, although resistance pedaling and turning like a skier can help scrub speed while maintaining flow. You can't do skier turns with motorists or other riders around, but on an empty road it's a great way to control momentum while keeping a smooth pedaling rhythm. Riding the bike on freestanding rollers will teach you smoothness in a hurry. Just don't try to practice your turns there. If you even imagine turning while you're riding rollers you will end up on the floor.
Twenty (or so) years ago, after my younger colleague Ralph had been fixed gear riding for a while, he applied his analytical mind to it and reported his findings. I had said that you have four speeds: Sitting, standing, weaving, and walking. He observed that pushing back on the saddle and grinding at a low cadence could be a more effective way of climbing than standing on the pedals. While this technically falls under the category of sitting, it's different enough from staying in a more neutral saddle position and trying to keep a higher cadence that it qualifies as its own thing. And you can combine some of these, sitting and weaving, for instance, or even standing and weaving, to surmount steeper grades. As long as you can get the pedals around you're still moving forward. So fixed gear riding expands your power range, making you (possibly) less dependent on shifting as frequently on your multi-geared bike. This applies particularly to grunting in a low gear more than ultra-spin. Diving down a steep descent with a freewheeling system, just coast.
Decades of riding take their toll. Cycling of any kind is not complete exercise. It does not build bone density, and it uses your legs in one plane and a limited range, regardless of the gear. I notice now as I get closer to 70 than 60, that my hips don't like too much high intensity cycling without mixing it up or at least taking more rest days to break up long stretches of riding. Way back in my 40s I noticed that the end of the commuting week left me feeling a bit ragged. And the arc of the whole season built nicely to a peak in July that felt like it would never end, but in late September I felt like a dragonfly, still fierce but now tattered from the constant flight. This effect has only become more pronounced with age. The continuous pedaling on the fixed gear allows no rest en route except for "fixed-gear coasting": relaxing the legs and letting the bike drive. That still requires a little input to keep everything aligned and smooth, compared to freewheel coasting, where you can actually backpedal a half revolution to drop your heel on one side and then the other, stretching the back of the leg.
Sometimes you just have to hop off and admire the view. Savor each ride.