How do I know what length my crank arm should be on a new Dura Ace double

Question

I am in the market to buy a new crank for my road bike but i am not sure on what size to get. Can anyone give me some tips?

Answer 1

Standard crank lengths are 170, 172.5 and 175mm. The length you choose is really up to you. Common thought is the shorter your legs the shorter the crank. Also, some cyclists feel if you prefer to spin shorter the better. If you want more torque applied or you have longer legs the longer the length.

So what does this all mean? There is no consenus. At 5-8 I've used 170mm on most road bikes, I currently use a 172.5 only because it came on my bike. I really notice no real difference. My MTB is 175mm, again that's what normally comes on a MTB supposedly for the extra power.

2.5mm is only 1/10 of and inch so there is only a very minor length differential. I'd simply go with convention as I stated above.  

Answer 2

Most of what M R said is correct, however its too flippant to say the difference between a 170 and 172.5mm crank is only 2.5.mm!!!!

On full stroke the difference is 5mm but as the stroke is a rotary action you need to remember your old school maths and incorporate πD this gives a circumference of 1067.9 for 170mm and 1083.6 for 172.5mm crank lengths. I could go on and start to incorporate gear ratios etc, but you get the picture.

Basically, with a 700c wheel a 170 to 175 should be ok ( I would go for a 170 )but if you have a large frame size or particularly long legs then look for a bigger one.

EDIT: Not me Glenn, I dont give any, - Lucky M R though, I'm usually a 'thumbs down troll' magnet!  lol.

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Answer 3

Hmmm....... I was under the impression that Dura ace was available in sizes from 165 - 180mm in 2.5mm increments. (But I've been wrong before.)

http://cycle.shimano-eu.com/catalog/cycl...

Assuming you are talking road bikes (Dura Ace cranks) Here is a rough guide.

Most road bikes sized up to 54 cm use 170 mm cranks

54 - 58 cm use 172.5 mm 58 cm or greater use 175 mm. This is usually close enough for most people.

Getting the BEST size for you is a bit trickier. The best way is to measure your output with a power meter. (power v heart rate)

If you aren't prepared to go to that trouble. Here is a formula that is somewhere in between.

Start with 10% of your height. (round down, this assumes stocky build, a touch long)

Then make adjustments for the style and type of riding that you are doing.

subtract 2.5mm for lean build. or if you prefer to spin

subtract 2.5mm to 5mm

if you are female (longer thigh to shin ratio)

for track racing (better acceleration)

if you have frequent knee problems (less knee bend)

add 2.5mm

for time trials

hilly rides

if you prefer to mash

add 2.5 to 5mm

pure hill climbs

(and MTB races)

(Out of the saddle you can get away with a much longer cranks since body movement reduces knee bend)

P.s. Looks like MR has the downthumb troll. Where's mine?

Answer 4

Most road bike crank arms are 170mm it should be marked on the back of the arm.  Make sure it will match up to your bottom bracket.