Crank Arm Length

[blacksmith]

Should one not rather look at the amount of power a person can transfer. To the wheel? Ie if you have stronger legs then a shorter crank will do you good as you can apply the same amount of torque to the wheel, as opposed to someone who is a bit weaker who might benefit from longer cranks. Of course taking clearance etc into consideration

[andydude]

Should one not rather look at the amount of power a person can transfer. To the wheel? Ie if you have stronger legs then a shorter crank will do you good as you can apply the same amount of torque to the wheel, as opposed to someone who is a bit weaker who might benefit from longer cranks. Of course taking clearance etc into consideration

Isn't it the otherway around? Shorter crank easier to spin and less power while for longer crank you need more power?

 

According to my length I need a longer crank, but I like to spin it to win it which would require shorter crank, therefore taking both into account I'm happy with my 175.

[GoLefty!!]

Inseam 81.5cm

 

road cranks = 172.5

Mtb Cranks 175

 

always has been

[blacksmith]

 

Isn't it the otherway around? Shorter crank easier to spin and less power while for longer crank you need more power?

 

 

No. Torque = force x length. Thus, the longer the length the less force you need to deliver the same torque.

[Christie]

 

Isn't it the otherway around? Shorter crank easier to spin and less power while for longer crank you need more power?

 

 

No. Power = (torque) x (rotational speed). This means that for more power, you need to turn the longer crank at the same rpm as the shorter one. However, data from testing indicates that power per person remains konstant, regardless of the length of crank. I saw a study where they used an adjustable crank that could go from 140mm to 200mm, where power per rider stayed the same - conclusion seems to be that your lungs and legs can do a certain amount of watts, if you fit a shorter crank, your body will adapt to spin it a bit faster, till you reach the same limit as before. Same story for a longer crank. There were some exception, if I remember correctly, shorter cranks were beter for sprinting.

 

Bottom line is that getting bit longer or shorter crank wont gain any more power.

[Witkop]

I did a bit of reading on this when I worried about the smallest technical detail.

THe first premise is that the knee loses power if it bends more than 90%. THerefore, the knee should be bent at about 80% at the top of the downward power stroke. (which is about at 2Oclock on the cycle.) Any more bend in the knee will put a lot of pressure on the knee and may result in over use injuries if you consistently push heavy gears.

 

Secondly the longer your crank the lower your saddle will have to be in order to have a 0-2% bend in the knee at the bottom of the stroke. Which in turn will result in a greater knee angle at the top of the stroke. Thus each 1mm change in crank will have a 2mm effect on the top of the stroke.

 

Thus leg length is more important than height. (Some people have short legs and long torso's). Of the leg the upper leg is the most active during pedaling, where the lower leg effectively just joins the knee to the ankle. Foot size may also play an influence if you have a dynamic foot (flexible ankle) ehen pedalling).

 

THUS, the best indcator to crank length would be the length upper leg (Femur?!). A guide would be about half the length of this bone, give or take 2mm to take foot size and ankle dynamics into consideration.

Edited June 26, 2014 by Witkop

[GoLefty!!]

I did a bit of reading on this when I worried about the smallest technical detail.

THe first premise is that the knee loses power if it bends more than 90%. THerefore, the knee should be bent at about 80% at the top of the downward power stroke. (which is about at 2Oclock on the cycle.) Any more bend in the knee will put a lot of pressure on the knee and may result in over use injuries if you consistently push heavy gears.

 

Secondly the longer your crank the lower your saddle will have to be in order to have a 0-2% bend in the knee at the bottom of the stroke. Which in turn will result in a greater knee angle at the top of the stroke. Thus each 1mm change in crank will have a 2mm effect on the top of the stroke.

 

Thus leg length is more important than height. (Some people have short legs and long torso's). Of the leg the upper leg is the most active during pedaling, where the lower leg effectively just joins the knee to the ankle. Foot size may also play an influence if you have a dynamic foot (flexible ankle) ehen pedalling).

 

THUS, the best indcator to crank length would be the length upper leg (Femur?!). A guide would be about half the length of this bone, give or take 2mm to take foot size and ankle dynamics into consideration.

 

 

correct, hence through bike fit wit dr. Swart we came to the conclusion for my set up 172.5 to175 would work. I use the longer crank on the mtb for more leverage

[Witkop]

correct, hence through bike fit wit dr. Swart we came to the conclusion for my set up 172.5 to175 would work. I use the longer crank on the mtb for more leverage

 

I suppose on a mtb there is the option of standing to get stop leverage where needed........

[andydude]

No. Torque = force x length. Thus, the longer the length the less force you need to deliver the same torque.

No. Power = (torque) x (rotational speed). This means that for more power, you need to turn the longer crank at the same rpm as the shorter one. However, data from testing indicates that power per person remains konstant, regardless of the length of crank. I saw a study where they used an adjustable crank that could go from 140mm to 200mm, where power per rider stayed the same - conclusion seems to be that your lungs and legs can do a certain amount of watts, if you fit a shorter crank, your body will adapt to spin it a bit faster, till you reach the same limit as before. Same story for a longer crank. There were some exception, if I remember correctly, shorter cranks were beter for sprinting.

 

Bottom line is that getting bit longer or shorter crank wont gain any more power.

 

Thanks guys!

 

Should it be read in conjunction with the chain ring then? For example, isn't it much more difficult to turn a 60t than a 30t with the same crank? I assume you can draw two lines for the different chain rings and see where they actually intersect depending on different length cranks?