Power Meter Question

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rstabler11
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Joined: Fri Jul 20, 2012 20:43 pm

Power Meter Question

Postby rstabler11 » Sun Dec 16, 2012 16:38 pm

Hi all,
I was reading about power meters and trying to understand how they work. As far as I can tell, the crankset / pedal meters all measure some degree of flex associated with pushing on them and convert that into power (I still don't really understand how the hub meters work). I know all the pro-riders use power meters, but surely it would be better to use a crankset that was so stiff there wasn't any flex? Or is this just not possible and flex is being measured at the nanometer level?
Basically, does anyone know if power meters are generally not as stiff as other top of the line cranksets (that just focus on stiffness and weight)?

Thanks,
Rob

(PS don't worry I'm not going to buy one or anything I'm just curious).

cougie
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Joined: Wed Apr 21, 2004 22:34 pm

Re: Power Meter Question

Postby cougie » Sun Dec 16, 2012 16:41 pm

They do use the best and strongest cranks. They're not losing any energy.

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schweiz
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Re: Power Meter Question

Postby schweiz » Sun Dec 16, 2012 17:01 pm

They all use strain gauges

http://en.wikipedia.org/wiki/Strain_gauge
http://en.wikipedia.org/wiki/Wheatstone_bridge

the very slight deflections in the crank, pedal axle or wheel axle are detected and are converted to a measurment of force.

Power = Force x Velocity (speed in a given direction) (velocity is of the moving part being measured for strain, not the bike)

danowat
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Re: Power Meter Question

Postby danowat » Sun Dec 16, 2012 17:56 pm

The strain gauges for crank based powermeters aren't in the crank arms.....

jezzpalmer
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Re: Power Meter Question

Postby jezzpalmer » Sun Dec 16, 2012 18:06 pm

danowat wrote:The strain gauges for crank based powermeters aren't in the crank arms.....

Some are, Stages Cycling definitely is, and from what I can gather so are the new Rotor power cranks.

The rotor looks really cool.
http://power.rotorbike.com/

danowat
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Joined: Mon Apr 19, 2010 13:58 pm

Re: Power Meter Question

Postby danowat » Sun Dec 16, 2012 18:36 pm

jezzpalmer wrote:
danowat wrote:The strain gauges for crank based powermeters aren't in the crank arms.....

Some are, Stages Cycling definitely is, and from what I can gather so are the new Rotor power cranks.

The rotor looks really cool.
http://power.rotorbike.com/


Sorry, should have said, strain gauges for decent powermeters aren't in the crank arms

jezzpalmer
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Re: Power Meter Question

Postby jezzpalmer » Sun Dec 16, 2012 18:59 pm

I can see the argument against the Stages offering being only 1 legged, though it probably still gives reasonable results for the general rider.

What is the issue with the Rotor (2 legged), and what is the issue with gauges in the arms?


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