As a BMxer, I love single speed. I spent every spare minute as a kid pedaling around the city on my BMX bike.
Frankly, commuting on a multi-geared bike pisses me off. I only ever leave it in one gear, but if I have a flat or bump the shifters, then I have to think about those damn shifters again.
And I think a single speed bike looks way better than a bike with a bunch of gears.
You guys know I have geared bikes. My touring bike and mountain bikes have a lot of gears. I don’t shift often enough, but I’m not just this universal gear hater.
But, for a city commuting bike, I freaking love the simplicity of a single speed.
Which brings us to chain tensioning. You need the chain to stay on, and it will only stay engaged if you have the proper tension.
Which is why the derailleur is key. It has enough spring tension to keep the chain from hopping off. When you remove the gears, you remove the need for a gear selector When you remove the derailleur (gear selector), you lose that tension, and the chain starts coming off.
When converting bikes, you have a couple of options. Fixie bikes, for example, usually have a tensioner. This device lets you tighten the chain and keep the chain from being able to move the wheel and cause unnecessary slack.
This can also work on a single-speed with a freewheel. The freewheel is nice because it allows you to coast. And many fixie bikes come with a flipflop hub that will allow you to have both options. However, the chain tensioner will work in either mode.
Finally, when you are converting your bike, it makes a lot of sense to use a bolt on chain tensioner. These after-market devices can work well with any geared bike to add a slight amount of tension to the chain and replace the role of the derailleur.
This is probably my favorite option as these tensioners are easy to work around when changing flats.
But all of these systems work. If you are going single speed, just splurge the $30 on a tensioner. It’ll make your life so much easier.