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.