Building a single speed

Here's one last post with some more technical information, kind of documenting what I did to convert a geared cyclocross bike into a (hopefully) reliable single speed. 

Frame
If your frame has vertical dropouts (like 99% of the cyclocross bikes sold today), you'll need to take some extra measures in regards to chain tension, which I'll get into in a minute.

This frame I converted had cable stop braze-ons on the down tube.  I didn't want those things sticking out in the open, fearful they may tag me in the face during an awkward shouldering of the bike.  Plus, they're kinda ugly just sitting there like that.  I removed all my shifter related braze-ons with a rotary tool and file.  You could leave them on if you wanted, maybe drill and glue a pair of plastic dice on them instead? 

I definitely did leave on my derailleur hanger.  I contemplated having a frame guy braze on some track ends, but I chose not to for a couple reasons (cost and reliability).  With horizontal dropouts, it's quite possible to pull a wheel sideways.  There are things you can do to avoid that, but it takes away from the simplicity of it all.  Also, it seemed to me I'd have to adjust the brakes whenever I changed the gear (due to changing the horizontal position of the wheel), so the added convenience appeared to be a wash.

Short story - leave on the derailleur hanger.

Levers
If you're converting an existing bike, you can just keep your existing levers on if you want.   That's kind of a waste, though.  It might be better to use them on another bike, or hock them on ebay, and buy a nice pair of regular brake levers instead.

Tektro makes very affordable, high quality brake levers in two styles.  One kind of mimics Shimano's aero lever shape, and the other closely matches Campagnolo's ergo shifter shape.  There's even options for small handed people, though I certainly don't need that.  I chose the campy shape, since that's what I've got on my cyclocross bike.  Switching between bikes will be more comfortable by making this choice.

Added bonus -- You'll obviously only need brake cables now, so your front end will be a little more tidy.

Cranks
You actually don't have to do anything to your cranks, but most people only run one chain ring. You'll need to either use some sort of washer to take up the space in your chain ring bolts, or just buy a single chainring bolt set to do this (they're quite cheap, so it's no biggie).  You can use the inside or outside position of your cranks, whatever gives you the best chainline to your liking.

Chain
First of all, you don't need a 10 speed chain on a single speed.  Don't.  Do.  It.  Please.  A 7/8 speed chain will work great, and it's stronger and cheaper.  A 7/8 speed chain is 3/32", and should work more silently with a 3/32" drivetrain.  You can also get a BMX chain, which are usually 1/8".  In either case, the plates are thick enough so you don't need to use those stupid replacement pins (introduced with 9 speed) when you adjust the length of your chain.  If you don't know what I'm talking about with 3/32 and 1/8, just read this.

Getting the chain tension (tightness) right is kind of a pain.  There is no horizontal play in the wheel, so it will not always be possible to get the right amount of chain tension for a specific gear you want to ride. 

There are online calculators that help calculate how many links you'd (theoretically) need for the right tension, and they'd be helpful in helping you find different gear combinations that would use the (almost) perfect amount of chain. I personally didn't find that very useful, though, as my approach is find a gear I have, and make it work.

The main problem is you actually need to remove/add two links at a time whenever you adjust chain length.  Sometimes you will get stuck where one length is a way too long, and the next size down is just a little too short.  Half links can help here.  They're a single chainlink that can take the place of the two normal chainlinks.  You might want to buy one.

I went all out and bought a chain made entirely of half links.  Gusset makes it, and it looks cool.  Unfortunately, when I got it, I found is was way too short, even for a 42x17 gear!  I'll add some links to it later, so I can use it.

Cogs
There's a lot of "single speed conversion" kits available out there, for transforming a rear wheel with a freehub body into a single speed wheel.  They usually comprise of a steel cog, and some spacers to fill in the remaining space of the freehub body.

The steel cogs you can buy for single speed use are probably a lot stronger than the cogs on a multi-speed cassette, so consider getting a couple.

I decided to just use an old 8spd XT cassette I had lying around.  I took the 17 cog off and used that, and I combined the spacers from that cassette with those of an old campy 8spd cassette I also had lying around.  Works great, but I don't know how long that cog will last.  We'll see.

Chain Tensioning Device
If you kept your vertical dropouts, you'll need some way to take up the slack in your chain to get to a proper tension.  Too loose, and it will fall off.  You don't want that!

See my previous post for chain tensioning options.  There's a lot of them.  White Industries probably has the best solution - an eccentric hub.  This allows you to adjust the horizontal position of the rear axle with vertical dropouts.  However, you'll be tied to that one wheel for all use on the bike.  I kinda wanted something were I could use any of the wheels available in my garage.

I personally decided the best choice for me was a fixed position roller type solution.  Rennen makes a really nice one.  Gusset makes one, too, but from experience, it's not quite as good for this purpose.  There are also spring loaded versions out there, like the Surly singulator.  However, if it's spring loaded, it seems like it can loosen the slack in the chain (maybe from a bump, maybe from hitting something...who knows), which could lead to the chain falling off.  No thank you.  The fixed position options are cool - one anchor point goes through your wheel axle (i.e. quick release skewer), and the other bolts on to your derailleur hanger.  Once tightened down, it does not move.

These chain tensioners can either push the chain up, or down.  It's better to set it to push upwards, so more teeth on the rear cog get a bite of the chain.  However, sometimes, the device doesn't travel upwards enough, and you have to push downwards.  Ah well.

Putting it all together
When you put this thing all together, your main concern is chain tension, and chain line.  Make sure the chain follows a straight path from the chain ring to the cog.  If you close one eye and look from the back of the bike, you can easily see it.  The cog/chain line photo above shows a good point of view for eyeing it (this particular photo shows the rear cog is slightly off to the outside).  If you have no skill for eyeballing straight lines, you can measure from the center of your bottom bracket to the center of your chain ring, and adjust the distance from the rear cog to center of the hub that same distance.

When I tension a single speed chain (e.g. this bike, or maybe my fixed gear), I make the chain just tight enough so the chain can't get over the first tooth on the chain ring it contacts.  Just try to push the chain up with your thumb over the first tooth (e.g. left most tooth on the top that the chain grabs) of the chain ring.  If you can easily do that, the chain is too loose.  The picture at right shows an example - this chain is just a TINY bit loose, almost perfect.  It took a lot of force to get that chain up there, so I might even ride it like this.

Too tight is not good, as it could result in a broken chain, extra resistance, and increased wear.  The chain should definitely have some give to it while pedaling.  Give the rear wheel a backwards spin, which will make the pedals spin backwards as well.  Tap the top of the chain (in the middle) with a wrench (please don't use your fingers), and make sure it gives.  Check it several times as the wheel spins, so you know there's no tight spots.  If it's too tight, loosen it up!  Just make sure it's not TOO loose.

It's kind of a pain at first, but it's made easier with a chain tensioning device.  If you're using a bolt-on axle with horizontal dropouts, just slowly tighten each side a little bit at a time to avoid common frustration of mis-tensioning the chain over and over again.

Good luck!