Sunday, July 31, 2011

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


Philip Joshua Sinohin said...

thanks for this blog. tempting to try 22x11. i have, 11,13,16,18 cogs lying around anyway

Andrew Patterson said...

But like I say, do you really want to use an 11T cog in the rear? You'll only have half those teeth engaged (5.5) which will likely cause considerable chain slippage (dangerous when you're mashing up a hill!). Now this is worse on a conversion with vertical dropouts as the chain isn't as tight as it should be, even with a tensioner.

Are you on a SS frame or a conversion?

You would be way better off with a 32 x 16 or 36 x 18.

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