Friday, 15 October 2010

Bike Porn Friday: Size Matters

Mine goes from 36.4 inches to 111.5, Dr Doug's (aka Voxpops) goes from 35.7 inches to 109.5, and Dr Matt's goes from 30.8 inches to a measly 101.1 inches.

'Poor Dr Matt', you might be thinking.

You also might be thinking 'what on earth is he on about?'

Well dear reader (I am assuming our readership has dwindled due to our pathetic lack of posts and failure (as yet) to relate tale of our Welsh adventures), I was ill this week, and spent some of the time on the sofa refreshing my memory on the subject of bicycle gears.

My mind had been troubling as to how the two Doctors had managed to leap, gazelle-like, up the Welsh alps, whilst I wheezed and walked my way up like a rhino.

Partly (mostly) the answer is that they are much fitter and thinner than me, but partly, at least in the case of Dr Matt, it was down to what he was packing at the back.

Of his bike.

In terms of gears.

Bicycle gears are often measured in inches, with the inches standing for the diameter of the direct-drive wheel you would need to have to give the same distance of travel for one revolution of the pedals. Or, slighty more simply,think of a penny farthing, with the cranks attached to the big wheel at the front and the wheel turns at the same speed as the cranks. The gear inches measurement tells you, for any gear your bicycle happens to be in, how big a front wheel it would have to have were it magically transformed into a penny farthing. The bigger the size of the wheel, the harder it is to pedal.

You calculate gear inches like this:

Gear inches = Diameter of drive wheel in inches × number of teeth in front chainring / number of teeth in rear cog.

Or you do what I did and turn to the late, great, Sheldon Brown and use his completely marvellous online calculator.

But anyway, as you'll see from the measurements at the top of the page, Dr Matt's smallest gear (i.e. when the chain is on his small chainring and biggest cog, giving him the greatest possible mechanical advantage) is a walloping 6 inches of diameter smaller than mine, meaning he goes up hills faster for the same amount of effort. His tyres are narrower too (as are Doug's) which makes a small difference.

I always said that six inches was significant.

Ahem.

Flotsnoo!

TroisV!




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