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crank length!

In the process of upgrading/building new bike and have decided to buy a new chainset. I have never really considered crank length, but on inspection of the cranks on my size 56 trek they are 170mm in length. I am 6'1 and have an inseam of 90cm, i think my legs are long for my height, according to the equation for proportionate crank length to inseam I should have cranks of 190! which is a bit barmy to say the least!

So this is a bit of revelation as I could have had an increase in power with a longer crank me thinks, but i've hit a problem. It seems that the chain sets that are in my price range, i.e ultegra etc. only seem to go up to 175, and even though I dont plan on a 190 crank length I would like to get a little closer like 177.5 or 180.

But these only seem to be available in dura-ace etc, which could cost me upwards of £150.

So is it worth me paying a good amount extra for a longer crank? Is the difference that great? do other people have crank lengths proportionate to their inseams?


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    gunforhiregunforhire Posts: 457
    Crank length puzzles me.

    At 5'7" I should be looking at a 170mm crank, but I want to change my older Campaq Veloce chainset for a compact.

    Thing is there aren't many of the old kind about...

    I did find one but it's got a 175mm crank.

    Which got me thinking. Can 5mm really make that much of a difference?[8|]

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    treefrogtreefrog Posts: 1,242
    Lots of opinions on this one. The standard crank size is 172.5. Generally smaller guys and women tend to 170 and very large guys go for 175. However there are many exceptions. Always remember that the extra leverage gained by longer cranks is offset by the extra energy required to turn them and vice versa. Modern cycling tends to high revolutions and therefore many larger guys are using 172.5's, a notable exception is Tom Boonen who uses 177's hence his alloy chainset when he uses Capagnolo. Personally I think that your gearing is of greater priority, but a simple albeit inaccurate trial to test cranks is to put your bike into its highest gear (generally 11 teeth on a tri bike) and pedal over a slight drag, if you feel you want more .... and are a big guy ..... then you probably should go for it, if you want to change down then don't change. Otherwise ask at your LBS for their take(s).
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    as an ex bmxer crank length was a big issue for gearing. A longer crank will give you more torque, but will effectively make any given gear ratio significantly lower. E.g turning a 53-14 with 175mm cranks will need more effort than a 53-14 with 180mm cranks. However, with the longer crank length your top speed will be lower before your legs over spin and you need to change up. This might influence your choice of cassette.

    Treefrog sums it up when he says gearing is of a higher priority. I`d save me money, and ensure my cassette had the right gears for me

    Hope that makes sense.
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