Single Speed Conversions: Gearing, How many teeth?
High or Low Gear?
A key choice when riding a single speed bicycle is which gear you want to ride in. You want a low gear if you want to accelerate fast from junctions, have not too much difficulty on the hills you find but on odd occasions be rotating your legs faster than you'd like.
You'll prefer a higher gear if you prefer to take off from junctions a little slower in order to be able to reach high speeds at a more gentle cadence, and if you don't mind pushing yourself hard to get up steeper inclines. Gear choice will impact on your riding a lot.
Three Factors Influencing Gear?
The three factors which influence gearing are the number of teeth on your chainset, the number of teeth on your rear sprocket, and the size (most specifically the circumference) of your wheels (most specifically of your rear tyre).
Here's how the three factors influence your gear: the more teeth on the chainset you have the higher the gearing will be; the more teeth there are on the sprocket the lower the gearing; the larger the wheel, the higher the gearing.
But What is the Reference Point?
The way we introduce people to this topic is to look at the most readily used single speed bicycles on the market and discuss the gearing from there. (If you rather use the more traditional concept of ‘gear inches’ please see the table at the bottom of the page for a tables for these. For more discussion on the subject see Sheldon Brown on Gain).
Most off the peg single speed bicycles have the following specifications:
Chainset = 42 teeth
Sprocket = 16 teeth
Rear wheel = 700c = 622mm diameter
So if you are running a 700c bicycle and you have a chainset with 42 teeth and want to choose a sprocket, go for 16 teeth for a standard ride, 17 or 18 if you want it to be easy and quick off the blocks, or fewer than 16 (not possible with a single speed freewheel) if you want to push yourself.
Likewise if you are looking at a mountain bike wheel - smaller than a 700c, if you want the same level of gearing as on an off the peg bike and you have a 42 tooth chainset you need a smaller rear sprocket to compensate for the small wheel.
To get exactly the same gearing you'd need a 14.38 tooth sprocket.
So you have a choice, a bit higher gearing with a 14 tooth, or a bit lower with a 15 tooth. The closest is the 14.
So, 42/16/700c is pretty close to 42/14/26"MTB
Have a look at the graphics lower down this page to help you choose appropriate gearing for the bicycle you have. I have put the gear ratio of an off the peg bike as a horizontal line across the chart as a reference and set it to 100 in value. The table shows different configurations of components and the gearing they create relative to that bike.
Multiple solutions, our advice:
If you are replacing your chainset and your rear cog at the same time you may have several options of what to purchase to get the gearing you require:
For example you may want to have the gearing obtained by a 48T chainset with a 16T freewheel. If this is the case you can obtain identical gearing if you use a 42T chainset with a 14T rear cog (as in the charts above).
In making the choice consider three things. Future proofing. If you think in future you might want to have a higher gear then choose large cogs – this will mean you can get a smaller rear cog in future to increase the gearing – cheaper than replacing your chainset! Likewise if you think you might want to reduce the gearing in future, go for small cogs so you can get a bigger rear cog later on.
Two other factors give counter-acting advice. Weight: to save weight you may want to go for the smallest items which give you the gearing you want. Wear: but, acting the other way, if you have larger cogs the wear they will experience will be lesser as all the teeth share the pulling load.
Different Gearing Options and Comparison to a Standard Single Speed Bike Gear:
Gear Inch Tables For Each Wheel Size
Note that a 42/16 700c bicycle has a 69.2” gain