What kind of bike should I convert?
First you have to decide if you want a road or mountain bike single speed. Many road bikes have horizontal dropouts which make it easy for tensioning the chain. Most mountain bikes have vertical dropouts which mean that you have to use a chain tensioner to have the right tension on your chain. This adds a little more complexity to your bike so if your aiming for the lightest and simplest bike, horizontal dropouts make more sense. There are also some road bikes with horizontal dropouts that have clearance for a little larger tire so that could work if you want to ride some gravel trails with your single. As I mentioned earlier, I have converted a few mountain bikes to single with a tensioner and they have been great bikes.
Below are photos of a horizontal dropouts and a vertical dropouts.
Horizontal dropout from my Trek Soho S, you can see that the horizontal dropout allows you to pull the rear wheel back to tighten the chain.
This is what you want to look for in a bike if you want to have a horizontal dropout.
A bike that I converted that had a vertical dropout with a tensioner.
Vertical dropouts on a mountain bike.
One of the first lessons I learned when I started single speeding was to not start with a cheap bike. Cheap bikes have components that are welded together. It is tough to take apart a chainset if it is one large hunk of metal. If you can easily disassemble the chain rings you know you have a higher quality bike.
Below is a photo of a less expensive bike where the chainset can’t come apart. You could always replace the entire chainset and cranks and maybe even the bottom bracket, but that becomes a much more expensive and complicated project.
Now a photo of an classic old Schwinn LeTour with allen screws to take apart the chain rings. This is a great bike to single speed.
If you are shopping for a bike to convert, your best bet is to find a bike with horizontal dropouts and a chainset that can easily come apart so you can re-use one of the chain rings for your project. You will save yourself lots of headaches if you start with a decent quality bike.