Teeth, Ratios and Inches

Whether you want to build your own fixed gear bike, or buy one new, you'll have to consider what gearing you want to ride as I believe it's crucial to getting the most enjoyment out of your bike.

Teeth, Ratios & Inches

This is one of those subjects that causes people to go cold on the spot, it involved numbers, maths and even slide rules if you want it to. Let's see if we can de-mystify it all enough to make sure you choose your gearing wisely.

In the last article, I shared the fact that my fixie ran on a gear ratio of 2.70 thanks to it's 17T sprocket and 46T chain ring. If you live somewhere hilly you may well want to adjust this ratio to a lower figure and in this case, we may choose to fit a 20T sprocket; this would then give a gear ratio of 46/20 = 2.3. This would make it easier to ride uphill, but you wouldn't travel as far per revolution of the wheel.

Of course, if you live somewhere flat and you're reasonably cycle fit then a gear ratio of 2.3 is going to be way too low and you might be looking to increase the gearing to around 3.0; using a 15T sprocket would give a ratio of 3.06 with a 46T chain ring.

There's quite a useful concept of gear inches, dating back to the days of penny farthings and still used today to compare the effective gearing of bikes, you can also use it to find out how far you travel for each revolution of the pedals.

To calculate gear inches you need to know the diameter of your wheel in inches (including tyre), the number of teeth on the chain ring as well as the sprocket. There are various tables on the web to help with this and the easiest is probably at BikeCalc.com http://www.bikecalc.com/gear_inches - just put in the relevant numbers for your bike and it will calculate the gear inches for you.

For my fixie, running 700c wheels and 23 tyres, plus 46T chain ring with 17T sprocket, the gear inches come to 71.3 (multiplying this figure by 3.14 will tell you how far I travel in one revolution of the pedals: 223.8 inches / 5.66 metres).

What gearing you choose to select on your bike is a personal matter, there's no right and wrong here. Many fixies are actually fitted with flip-flop hubs and these at least give you the option of fitting different gearing to your bike... just like the early days of the Tour de France when they only way of changing gears was to get off of the bike, spin the rear wheel around to the other side and carry on,

which isn't exactly a quick operation (that was why Campagnolo created the quick release skewer after losing a race due to the time it took to get the wheel off).

What is cool however - in a very retro way - is that I can use a slide rule to compare different bikes, with different setups quickly and easily... but that may be for a later article!

Whatever your view on fixies, dangerous or awesome (or if there's something about them that just appeals), I'd love to hear your views and experiences with them.

This article was written by DC. Vickers

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