Crankarm length

[Bullet]

I wonder if anyone can help me. How do you determine the crank arm length required and does it make a big difference? I want to buy a bike second hand and got the correct sizes required on everything except I do not know about the crank.

 

 

[Flemish Lion]

I went from 172.5 to 170 when I got my 'new' bike. I'm no giant though - ride a 52.

 

 

 

Acceleration was better, but I struggled with keeping a fast pace (forced on to me) in the beginning - but got used to it.

[Bullet]

I'm 1.94 m so I'm assuming 175 mm then.

[slowtwitch]

correct

[Jaco Steyn]

Personally I think it is mostly all in the mind, but I might be wrong because I am not speaking from personall experience. But, my reasoning is as follows. Calculate the circle that you need to complete to do one complete revolution. The formula for circumference is p x Diameter or 2 x p x Radius. The crankarm length will therefore be your radius of your circle. For a 17.25 cm crank your circumference is 108.38cm. For a 17 cm crank the circumference is 106.81cm. That brings you to a difference in circumference  of only 1.57cm.  I seriously doubt if you will really feel any difference. You might feel a small difference when you go from 17.0 cm (170mm) crank to a 17.5cm (175mm) crank, because your circumference now increases to 109.96cm, which equates to a difference of 3.15cm in circumference. Having said that, I might be completely wrong, so I am waiting feedback for the physics gurus. Johan Bornman, what is your take on this?

steynja2009-09-01 12:21:18

[Canaris]

the best explination I have heard for calculating crank length is based on some common sense. 

From basic mathematics regarding levers and also physological reality, you don't want to have to bend your knee more than 90 degrees, and you don't want to place pressure on your knee if you do.

So if you consider the diameter of the "circle" which the crank makes, the joints connecting levers in your leg, ie knee, hip and ankle, and the levers, ie foot lower leg and tibua (upper leg) the lever that is most mobile in the pedal stroke is the upper leg (Tibia).  THerefore, any change in this bones length should affect your loads on the joints during the pedal stroke.

 

So, if you had to link crank length with any measurement in the body, I would say it is the upperleg (tibia).  The rule of thumb is to measure the length of this bone, or from hip socket to knee, divide this length in two, and you should have a ball part figure for crank length.

[slowtwitch]

Tibia is lower leg frontal bone (shin bone)

[Johan Bornman]

Personally I think it is mostly all in the mind' date=' but I might be wrong because I am not speaking from personall experience. But, my reasoning is as follows. Calculate the circle that you need to complete to do one complete revolution. The formula for circumference is p x Diameter or 2 x p x Radius. The crankarm length will therefore be your radius of your circle. For a 17.25 cm crank your circumference is 108.38cm. For a 17 cm crank the circumference is 106.81cm. That brings you to a difference in circumference  of only 1.57cm.  I seriously doubt if you will really feel any difference. You might feel a small difference when you go from 17.0 cm (170mm) crank to a 17.5cm (175mm) crank, because your circumference now increases to 109.96cm, which equates to a difference of 3.15cm in circumference. Having said that, I might be completely wrong, so I am waiting feedback for the physics gurus. Johan Bornman, what is your take on this?

[/quote']

 

Sorry Jaco, I didn't even see this one.

 

I agree with you. Your argument is essentially about the distance your foot travels per revolution. You could have played it the other way and argued that the difference in force required to create the same torque would be whatever a 2.5mm shorter lever would have and that too is negligible.

 

I still think that for average people any average crank length is OK. My experience is that 170 through 175 makes not difference to me. I'm not sure which crank I'm using unless I get off and read the number. What seems to be underplayed in these arguments is that we simple switch to a gear that does what we want to do, in anyway.

 

And since I have the platform....crank arm literally means arm arm.   Crank is enough.

 

 

 

 

[Jaco Steyn]

Thanks for the reply Johan. Me and a colleague at work who also is a cyclist, had a very heated debate over this issue. He also insisted that since he changed from the 175mm Ultegra crank that came with his bike to either a 172.5mm Truvativ or a FSA crank his performance improved remarkably. I told him that it was all in the mind and that he only needed justification for the thousands he spent aver being duped by an excellent saleperson! He did not take too kindly to my comment! steynja2009-09-12 09:48:24

[Aquatic]

I remember seeing a general formula for this, your inseam multiply 0.21 in mm.Anyone knows about this?

 

 

 

 

[obree]

I am 1.91m and ride with 200mm cranks on the road and track

Check out www.zinncycles.com for equipment for taller guys.

Remember cycling is for the average sized person and not tall guys, who should be playing basketball, hence the manufacturers make equipment that they will sell the most of. If all cyclists were 1.9m and taller there would be plenty of manufacturers that made cranks 200mm and longer ...

[obree]

I remember seeing a general formula for this' date=' your inseam multiply 0.21 in mm.Anyone knows about this?

[/quote']

Yip, this will give you very close to 200mm if you are 1.94m tall

 

'Cream or foam' of course it matters, foam is the real deal, cream is a boere-ccino !!!

[Aquatic]

So obree would you round that of to 175?

 

 

 

 

[Rocket-Boy]

I agree with JB, if you were using a single speed it might make a difference. A longer crank arm would give you more leverage but take longer to complete a revolution. However you have gears so they should take all of the work out of that. If you need to bend your knees more or less then change the height of your seatpost!

[Sterad]

There should be more varience available in crank arm length, ie. if you are 1.6m tall a crank arm of 160mm +/-  and if you are 1.9m tall a crank of 190mm +-  but gears have basically eliminated this need.

[MaxUmis]

Don't know if anyone has run the numbers...

 

This is what I found for myself @ 84kg...

 

84kg on 170mm will generate 140 newton meter torque

 

84kg on 175mm will generate 144.2 newton meter torque

 

that's 3% more torque on the longer crank arm...