Interesting Articles on Power

[bruce]

http://www.slowtwitch.com/Training/Intro_to_Power_151.html

[gianni]

 

I'm not an Engineers etc so please forgive the question but he says; "The power output is determined entirely by your muscles: how fast and how hard they contract.  (This explains why you can?t improve your power output by changing to cranks that are a little longer or shorter.) "

 

but if I turn a 170mm crank at 90rpm and a 180mm crank at 90rpm I am surely using more power to trun the longer crank.

 

Is the difference in lenght is too small to make a difference or is the only relevant value the speed at which we spin the cranks ?

gianni2008-01-07 13:30:47

[Minion]

 

 

but if I turn a 170mm crank at 90rpm and a 180mm crank at 90rpm I am surely using more power to trun the longer crank.

 

 

 

In a rotating system (with some simplifications):

 

 

 

Power = torque x rotation speed and,

 

 

 

torque = force applied x distance from axis of rotation.

 

 

 

So, if your power at 90rpm is constant, then a 170mm crank means you need to apply a higher force on the pedal than if you had a 180mm crank since you are applying the force closer to the axis with the 170mm (it's like a lever).

 

The other side of this is how the tangential speed (basically the speed at which your foot stamps downwards on the pedal) is related to the rotational speed:

 

Foot speed = angular speed x distance from axis

 

The result is that, while a 180mm crank allows you to use less force to get a given power, your muscles will have to contract quicker than if you were using a 170mm crank.

 

Bruce will have to confirm or refute this, but it might start making a difference to your overall power if your body is particularly adapted to a certain force and contraction rate i.e. if your legs 'like' harder, slower contractions you would have a better peak power with a 170mm crank. Likewise for the 180mm crank if they 'like' faster, lighter contractions.

 

[bruce]

Edman as you say, longer cranks should not make a difference to your overall power, the reduction in force that must be applied is balanced by the increase in tangential speed because the longer crank has to travel a greater distance in one revolution.

 

Biomechanically though, a longer crank will suit riders with longer legs, and hence longer leg muscles.  A muscle has a range of contraction, so riding with short cranks and long leg muscles means that your leg muscles are not operating in their full range of contraction.

 

See http://www.zinncycles.com/cranks.aspx for the generally accepted metirc for calculating optimal crank length.

[Pantani]

 

I used to think I was clever...

 

[bruce]

I used to think I was clever...

 

Whatever gave you that idea?? 

[Big Boy]

Bruce where were you this am? Going long tomorrow?

[bruce]

Bruce where were you this am? Going long tomorrow?

 

Was out at the Cradle - doing my Tuesday morning TT

 

Will go Loooong tomorrow.

[lowracer]

 

See http://www.zinncycles.com/cranks.aspx for the generally accepted metirc for calculating optimal crank length.

 

Mine works out slap bang at 174.96

 

but personal choice (based on reasoning) I prefer a shorter crank (if I have a choice) as the knee has a smaller range of motion, and it is easier to spin.

 

Instead of gear inches (which most of us know) Sheldon Brown prefers to talk about Gain ratio, which takes crank length into consideration

http://www.sheldonbrown.com/gain.html

 

as an aside: limitations on junior gearing does not take the crank length into consideration. is the limit to "protect" the juniors, or just as a way to standardise and equalise the field.  If it is the first, shouldnt they look at crank length as well? (I am not sure if they do)

 

 

 

 

[bruce]

The limit is there to try to protect the juniors.  But if you think about it, low cadence/high torque situations that can apparently damage knees occur during climbing, not on the flats or downhills, so there is nothing stopping a junior rider from pushing too big a gear on the climbs.

As the articles point out - gears are just linkages, they don't add anything to the overall production of power, so restricting gear ratios doesn't level the playing field, it provides an advantage to riders that have a greater percentage of Type I muscle fibres.

[Minion]

 

See http://www.zinncycles.com/cranks.aspx for the generally accepted metirc for calculating optimal crank length.

 

That implies that my optimum crank length is roughly 194-200mm. Is it even possible to get such a thing?

 

[Norris]

I thinks so!

[bruce]

Edman you'd have to have custom made cranks - I know someone that has bought from Zinn.

 

The correct length for me is around 190mm, so I use 180mm Dura Ace.