The rally cry, "One Fu@&!ng Speed" or "One is all you need". It's not just the stuff of cheezy decals, it's a whole mindset. These days there is a whole counter culture of fixed gear and single-speed fanatics that want to be part of the hip and trendy urban crowd. I've been involved with single-speed mountain bikes for a little over ten years now.
My buddy PK and I got into it along with the now defunct Wachussett Puke Boys. Many of us experimented with single-speed conversions of various frames typically using crappy old parts we had lying around. These parts would ultimately fail during inopportune points like a really steep grunt climb, at which point you would usually endo or drive your knee into the stem or fork crown or something like that. Ah the good old days!
I can't say that my reasoning for getting into single-speeds was necessarily more noble than the hipsters. We got into it because it was something new and different and afforded a different perspective all together on some of the same old trails. It also afforded us the luxury of drinking PBR pounders before they were the trendy thing to drink. In terms of providing variety though, the single-speed in it's strict minimalist form has been a recurring, go-to theme for me over the years. It has provided a way to do something that is very similar and familiar, something that I truly love to do, in a slightly different and often more exciting way. Think of it like being similar to your normal spouse or partner with big old implants, making something good just that much different and fresh :)
The way you ride a single-speed in the woods is very different than you ride a geared bike. A geared bike allows you the luxury of being reactionary to what the terrain throws at you, in many ways where as a single-speed requires you to be proactive in anticipating and preparing for what is coming up. Momentum is a key factor and you need to maintain it at all costs, so as not to get bogged down in a climb or technical feature due to your static, and in this case, inappropriate gear choice.
Mitigating that inappropriate gearing is the principle around which all single-speed riding revolves. By inappropriate gearing I refer to the fact that a static gear will never be perfect for varying terrain. Sure, you can choose a gear that is better than another but unless the terrain and conditions are static a single gear ration will never be ideal for all conditions. Around these parts most of us run a sub 2:1 ratio for 26ers mountain bikes and even lower for 29ers. The most common gear ratio for my 26er crowd is 32:18 though I had recently switched to a 32:17 before trading in my 26er for a 29er. For the 29er trail riding crowd a 32:20 is typical though I run a 32:19 and have done both a successful XC race and Kingdom Trails on this gearing. I also regularly ride our local trails on it with no issues. My current MTB is a Soul Cycles Dillenger with a 100mm RockShox Reba up front.
A few years later our interests had turned more toward road riding so once again, PK and I put together converted single-speed road bikes and did a ton of miles on them. It was great in the same simplistic way that single-speed mountain biking was and still is great. Within the last five years, many of us have jumped on the cyclocross bandwagon in a big way. It only made sense that a single-speed cyclocross bike would be just as much fun as it's siblings. I've had a few different versions and I think my current setup is pretty darn good. It's a Cannondale CAAD9 with a Team Beer BB30 EBB. It's light, stiff and works really well so far. I train almost entirely on the single-speed cyclocross bike and I've done some racing on them as well. A couple years ago I did Brockton (bad idea as it was too much road) and Ice Weasels SS. Last year I did the MRC race, Ice Weasels SS and Ice Weasels Elite with one gear. This year I did Porky Gulch cross (bad idea as it was too much high speed road) and will likely do at least Ice Weasels SS again.
In terms of the actual specifics of a single-speed, even though they are far simplified versions of the bicycle, there are actually certain issues that a single-speed drivetrain presents you with. The primarily issue is simply how to tension the chain correctly. There are number of methods to address this issue and I have tried most of them. The simplest approach is to use a normal bicycle frame with vertical dropouts and employ some device to take up the slack in the chain similarly to a multi-geared bicycle. You could use an old derailleur or any one of the numerous tensioners specifically designed for this application. The purist however, will argue that it just isn't right. I'll argue that because bicycle drivetrains are not perfectly round, it would be ideal to have some variable tensioner that can account for the minor imperfections. That said, those imperfections are usually within tolerance of a static system and you just live with the fact that your chain will be tighter in one section and looser in another. Most people start their single-speed career with a tensioner and graduate to a single-speed specific setup due to the fact that tensioners are a royal pain in the butt and require fiddling or just plain fail, frequently.
The most popular approach for single-speed specific bicycle frame makers is to use horizontal, slotted dropouts. This allows you to hand tension the chain with the drivetrain in place and the wheel on the bike and then tighten down the rear axle in the proper position. The downside is that it typically requires nutted axles that require tools to tighten/loosen and they can slip. The fix for that is to use a BMX tensioner, which works well but requires special tools to adjust. Also, the fixed nature of brake position either with road, cantilever or disc brakes come into play when you change the final resting position of the rear axle in order to tension a chain. Bottom line is that I used this method for years. It's a fine approach if you don't mind living with the issues.
The tensioning method that I now employ on almost all of our, yes our because in my house we have two of everything, one for me and one for Cathy, is the Eccentric Bottom Bracket or EBB. The nice thing about this method is that the tensioning is done at the bottom bracket rather than the wheel which means that you can use normal, vertical dropouts, a normal wheel with a quick release skewer and the brakes don't become an issue. The down side is that you change the seat angle, seat height and bottom bracket height by rotating the eccentric to tension the chain. The EBB is also usually heavy and they tend to creak. Personally, I can live with that and actually like this adjustability in most cases. For instance, my Cannondale single-speed cross bike had a lower bottom bracket and longer top-tube than my geared Ridley race bikes. The EBB allowed me to run the bottom bracket higher and forward such that I could slide the seat forward and retain the seat tube angle while effectively shortening the top-tube length.
There are also some other designs that use sliding dropouts and newer super custom designs that use pivoting dropouts. The latter seem like a really cool idea and could easily be the best solution if done right. The former have issues of their own, primarily that they are just cumbersome.
Anyhow, just like in everything, variety can be a really good thing. The same can be said about simplicity. Not having multiple gears to choose from dramatically simplifies the ride and allows you to be more one with the bike. It teaches you lessons about adaptation and overcoming as well as just working with what you have at hand. You learn to anticipate and prepare for an obstacle as well as to value and preserve your momentum. These practical lessons then become invaluable once you get back onto the geared bike.
Bicycles are not only utilitarian tools, they are also toys to be used for fun. I think the single-speed helps accentuate that fact.