Matt Jones

Building a Single-speed Mountain Bike

Late last year, I decided to build myself a single-speed mountain bike. There are a number of reasons for this:

  1. I rely on one bike as my main form of transport. If it breaks or I need to do some work on it, I haven’t got another bike to ride.

  2. In my experience, some staff in bike shops aren’t very helpful, give the wrong advice or are too busy to help you. I decided it was time to learn how to build and maintain bikes myself. I’m sure that there are many very good bike shops around, just not in my area.

  3. My other bike, a Charge Tap, is a great street bike but it’s not so great in wintery conditions. I wanted to build a simple, low maintenance bike that I didn’t have to worry about too much while riding in the winter months.

So, having recently finished building it, I thought I’d share what I’ve learned with the hope of helping others thinking about doing the same. I should add I’ve been hugely inspired by the late Sheldon Brown and his excellent online resource for bike maintainers.

Frame & Forks

I wanted a light-weight frame on which to base my bike. There are plenty of frames for sale on eBay, but finding a decent one that’s suitable for a single-speed can be tricky. Single-speed and fixed wheel bikes have horizontal dropouts to allow the back wheel to move backward to keep the chain tensioned. However, unlike vintage road frames popular with fixie and single-speed riders, many newer mountain bike frames have vertical dropouts, meaning that you can’t tension the chain without a tensioning device (essentially a jockey wheel and a spring that hangs below the chain stay).

As I wanted to keep the bike as simple as possible, I decided to look for a frame with horizontal dropouts instead. There are two options here; you either look for a newer low-end frame (many Raleigh and Peugeot frames from the 90s have horizontal dropouts) or an older but better quality frame from the late eighties and early nineties (early frames from Specialized and Trek had horizontal dropouts). If you’ve got a bit of money to spend on a new frame, manufacturers like Surly and On One make new frames designed specifically single-speeds and bikes with hub gears.

I managed to find a 1993 Specialized Rockhopper frame on eBay that came with forks, headset, stem and seatpost, all for £16 (although not including postage!). It’s best to try and find a frameset as complete as possible, mainly because the steerer tube on the forks is cut to fit the headtube and headset, which can be an arbitrary length depending on the size of the frame. Also worth noting is that headtubes either fit a 1” or a 1 18” steerer tube; older frames tend to fit a 1” threaded fork.

Wheels

I bought a pair of second-hand but hardly used 26” Vuelta wheels complete with shimano cassette hub, chunky tyres and tubes, for about £25. To convert to a single-speed bike, you’ll need a conversion kit, which is a single sprocket and a set of spacers which replace the cogs (cassette) on the rear wheel. The conversion is quite an easy job with the right tools: a lock-ring remover, chain whip and adjustable spanner.

Gearing

With a single speed bike, it’s important to get the gear ratio right depending on the terrain you’ll be cycling on. My bike is intended as a street bike with occassional off-road use on bridle paths. If you look up gear ratios for a single-speed mountain bike, many suggest a starting point of 2:1. The sprocket that came with the conversion kit was a 16 tooth, so to get this ratio I would need a 32 tooth chainring. This ratio is rather low; for every rotation of the crank, the rear wheel goes round twice. It’s good for hill climbing, but not so good for riding on the flat as you’d be pedalling like crazy. So I decided to get a 38 tooth chainring with the 16 tooth sprocket, this gives a high enough gear to cruise along at a moderate speed, but low enough to get up those hills… unless it’s really steep.

Bottom Bracket

Some second-hand frames are advertised with an old bottom bracket already installed. While this may be tempting to save installing one yourself, it’s really best to get a frame without a bottom bracket. You don’t know what state it’s in, and worse, it might be seized to the frame leading to potential thread damage when you try and unstick it.

With the right tools, dealing with bottom brackets is quite straightforward, as they’re mostly sealed units that screw into the frame (although do note that the right side of the bottom bracket tends to be reverse threaded). There are a few different types that need to be considered. I decided to go for a standard square tapered bottom bracket, which some now consider to be the old fashioned type, ISIS having become the more popular standard. There’s also Octalink, Hollowtech or X-type bottom brackets, which seem to be an attempts at manufacturer lock-in to BB/Crank combinations.

Square tapered bottom brackets are cheaper but very robust, and fine for my requirements so I went for a Shimano UN54. Most frames take either 68mm or 73mm width bottom bracket depending on the width of the shell. The length of the spindle varies as well; for a mountain bike, you shouldn’t go too far wrong with 118mm. However it’s worth checking which length spindle is most suitable for your choice of crankset.

Crankset

Most right-hand crank arms are either all-in-one, where the crank arm and chainring are one unit, or you can buy the crank arms separately and bolt on a chainring. I recommend the latter option because it allows you to be a bit more flexible about the gearing. Also, this is the one component of the bike where I didn’t scrimp on second-hand parts because they need to be strong and reliable, especially on a single-speed where up hill riding requires a lot of force to be put on them. Most crank arms connect to the chainring with either 4 or 5 bolts at a varying diameter called BCD (Bolt Circle Diameter). You just need to make sure that the BCD matches on the crank arm and chainring. For most mountain bikes with a 4 arm crank, the BCD is 104mm.  I bought some FSA El-toro crank arms for about £25 and bolted on an FSA 38 tooth chainring for about £15.

Chain

I went for a Charge Masher half-link chain which is designed specifically for single speed bikes. The half-links allow a little more flexibility when both adjusting the length of the chain and positioning the rear wheel to keep the chain at the correct tension.

Brakes

Originally the frame would have had cantilever brakes on it, but I decided to fit vee brakes instead. I thought these were a straight replacement for cantilevers because the frame bosses are the same for both. However, while installing them, I had a problem when routing the cable through to the back brakes. Frames designed for cantilevers have braze-ons that ensure the cable drops down between the brakes. This is no good for vee brakes, as the cable enters the brakes from the left side. So, I had to buy a clamp-on cable stop that allowed the cable to enter the brakes at the right angle.

Epilogue

So I’ve glossed over a few things, mainly the outlay on the tools needed to build the bike, and the other bits and bobs like pedals, handlebars and the £2 bell. In total, it probably cost me about £160 to build. You could ask me why I didn’t simply buy an old but complete Specialized Rockhopper for about £50 and convert it to a single speed. Yes I could have done that, but I wouldn’t have learned as much and it wouldn’t have been so much fun!