Monday, 2 May 2011

A Beginners Guide to Road Bike Gear Ratios

Gears.  They're just there aren't they?  You buy your bike, sit on it, give it the "do I look good on it" moment, wheel it out the shop and start your cycling career.

For a new road cyclist, whether to buy a bike with a compact or triple chainset is often a big question.  So, what are the differences?

A triple chain ring, give you more gears to choose from within it's low gear range, mostly to give you more flexibility on making the pedal turns easier, as you build you fitness or if going up a hill.

It has three front rings, one with 50 teeth, one with 39 teeth and one with 30 teeth.  Depending on what cassette sits on the rear wheel, dictates what gear ratio your bike will have.  Most bikes tend to come with a 12-25 rear cassette.

A compact chainset, has two rings on the front.  A 50 tooth and a 34 tooth.  So, you've quickly figured out that you have slightly less range in gears, with one less ring.  I went for a triple on my first bike as I was worried I'd need the low gear range for climbing hills. 

My first bike, had a 12-25 rear cassette.  If you look at the table below, this shows what gear ratios that delivers.  You get a gear ratio by dividing the front chainset number by the rear cassette number.  So, ratio for 50/12 would be 4.167.  So, the easiest gear on my triple using the 30 front ring and the 25 rear, is 1.2.

On my second road bike which has a compact chainset, I'm running a 12-27 rear cassette.  So, as you can see, the gear ratio for climbing (how easy it is) is 1.259 (lower the number, the easier it is to push the pedals, but your distance of travel is less).  I could make this a little easier by putting on a 12-28 cassette, which would give me a little more capacity to 1.214, which is very close to the 1.2 that I'm able to achieve in the lowest gear on my triple at present.

So, when choosing a road bike.  You don't always have to choose a triple front chainset, to get the desired gear ratio for making hills easier.  You could change your rear cassette instead.

With the forthcoming Fred Whitton challenge, I'm leaving nothing to chance.  I'm putting a 12-28 cassette on my triple, which means that my gear ratio will be 1.071, think I might need it!

If you want to get really fancy, you could always visit this Bicycle Gear Calculator.  This takes into account, things like crank lengths, which is another variable.  But for the average beginner, this should get you started.  There's also an excel chart here from the CTC which you can enter a couple of variables into, which gives you the gear ratios in inches.

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