So I got the Homebrewed cogs in the mail awhile back - and they are every bit as beautiful as people say they are.

15, 16, 33

Nice and thick to prevent damaging your  cassette body

On the bike

Choosing a Cog

Caution: I'm not a mechanic or any kind of bike expert. I hardly know anything at all. But I will try to share what little I do know. What you read here is just based on my own personal experience and the research I've done. It might not be adequate or even accurate.


So don't blame me if you ruin your bike, get hurt, maimed, or die after following my instructions. If you have any corrections or thoughts please let me know - anything to make these instructions clearer or more accurate for others.


Besides the number of teeth (that's a much bigger discussion here's one of my posts on choosing a ratio) you need to look at the following:

1. Width of the base
2. Chain size
3. Material

There are a few other things to consider, like color, one piece or two, brand, and uh, yeah, price. But those are more subjective and I'll leave them up to you to decide!

I'm assuming you know to buy one that fits your hub, and that your hub isn't a fixie or BMX one that would require a threaded cog. The cogs here are not threaded and will not work on a singlespeed bike.

Width
If you buy a cheap singlespeed conversion kit, it may come with an equally-cheap stamped stainless steel cog. This should be a one-piece skinny one as seen here.

Look how skinny this cheap cog is

So what's wrong with cog with a thin base? It will dig into the freehub body of your hub, especially if your freehub body is of the more common, and softer, aluminum variant (as opposed to stainless steel or titanium).

When a cog (or even cassette) digs into your hub like this it can be difficult to remove, and it destroys part of your hub. You may have to file it down to remove the ridges in the freehub body if this happens.

A higher-quality cog, like a Surly or a Chris King, will be a machined piece with a fatter base. Check it:

See how thick the base is: 4.35mm. By the way, the wear you see is from 428 km of off-road use.

Chain Size
There are two main chain sizes you should know about: 3/32 and 1/8. The first is the standard that you will probably be familiar with if you ride a geared mountain or road bike. The latter is a BMX standard. Either can be used on singlespeeds, but your cogs will have to be compatible.

There are plenty of arguments for both sides, but that is another discussion. If you are converting from geared to singlespeed, it'll be simpler to opt for the more conventional 3/32.

Just ensure that your chainring and your cog match your chain. Notice on the Surly image above it says "3/32"".

Material
Cogs are made from stainless steel, aluminum, titanium, or in the case of the Surly SCM415 ChroMoly. (Turns out motorcycle cogs are made of the same material as Surly's cogs). And in some cases, they're a combination of titanium and stainless steel, as in the case of one example from Homebrewed Components. See image below.

The black center is stainless steel, the teeth titanium

So which material should you choose? Stainless steel is the strongest, but also the heaviest. According to the Homebrewed site, it's two times the weight of titanium, and more than two times the weight of aluminum.

Aluminum is the lightest, but naturally the weakest. It's best for racing and those that don't mind changing their parts more often.

Aluminum can break easily

While not a cog, you can see that aluminum can break easily. This happened under normal conditions, just climbing over a log. It's unlikely that a cog, with a much smaller diameter, would fail so catastrophically, but after having witnessed this absurd breakage, I won't be buying aluminum cogs.

Titanium is by many considered the best of both: It's almost as strong as steel and almost as light as aluminum. But it's expensive.

Choosing a Singlespeed Ratio

All first-time singlespeeders wonder what gear ratio to start out with. How big or small should the chainring and rear cog be? Can existing chainrings or gears be reused? What's best for a beginner?

It's often said that for mountainbikers, 2:1 is the best to start. This means that if your front chainring is 32 your rear should be 16. So for every revolution of your pedals, the rear wheel makes two. By the way, a 34:17 ratio is still 2:1.

For road riding the ratio needs to be much more aggressive - a bigger chainring and a smaller cog for faster road speeds.

Back to mountianbiking - while 2:1 may be a good rule of thumb, there are a number of factors you need to consider:

How strong are you?
If you're the fastest and fittest rider around, you may do well with something more challenging than 32:16 - you may try 32:15*...but that's highly dependent on the next point.

How hilly or flat are your trails?
If you live in the Himalayas or Tetons 32:16 may be crazy hard, meaning you won't be able to climb many hills no matter how fit you are. But if you live in Kansas you may want to look up some roadie ratios!

How much road riding will you do?
I have to do a few kilometers of riding to get to my trails, and anything beyond 32:18 makes me spin out really fast on the road - pedaling like crazy but getting nowhere fast.

How much pain are you willing to endure?
If you think you can train up fast, 32:15* could be good - but be ready to suffer. I started on 32:18 but then moved to 32:16 and never looked back.

What components are you using?
Are you on a 26" bike or 29er? This changes the ratio. How long are your cranks? 175 or other? This will affect your ratio too. Check out Sheldon Brown's online gear calculator to get into the dirty math.

Also, if you can manage to find the 'magic gear' or the ratio that gives you a tight chain based on your chainstay length, meaning you won't need a tensioner, you may be inclined to go with that. But will that ratio be ideal for you? This could dictate your ratio. See the Fixed Innovations magic gear calculator.

If you opt for a cog less than 16T, you will have fewer teeth interacting with the chain (less chainwrap) which can lead to chain skipping. Even with a 16, you only have 8 teeth biting into the chain, and on a tough climb you could blast your chain over the cog.

In short, the more teeth on your cog (for mountainbiking) the better. But your 30 speed bike has an 11T cog you say? That's because gearies will only use the 11T on the fast flats or downhills - when they hit a hill they always shift to an easier gear!! ALWAYS!

Plus, because you are never shifting gears, that rear cog is constantly subjected to wear, and the smaller it is, the fewer teeth there are to absorb all this brutal chain pulling. Smaller cogs wear out faster.

The workaround, if you need a 15 or smaller in back, is simply to get a larger front chainring. 32:16 is the same as 38:19, for example, except with the 38:19 you have a lot more chainwrap. Less skipping, less wear.

A new Surly 16T cog

Once you have considered these factors and understand the variables, simply ask other singlespeeders in your area. Another approach is to ride a multi-geared bike in a certain gear for a while and don't shift. It will be hard to resist the urge to shift but this is a good way to find out what you are capable of.

You will be surprised.

*The 15T idea is just for illustration purposes - it's not recommended to use such a small cog - see the last section on components