## Power Meter Question

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rstabler11
Posts: 62
Joined: Fri Jul 20, 2012 20:43 pm

### Power Meter Question

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
Posts: 19008
Joined: Wed Apr 21, 2004 22:34 pm

### Re: Power Meter Question

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

schweiz
Posts: 1618
Joined: Sun Aug 31, 2008 18:51 pm

### Re: Power Meter Question

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
Posts: 2830
Joined: Mon Apr 19, 2010 13:58 pm

### Re: Power Meter Question

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

jezzpalmer
Posts: 384
Joined: Tue Jul 20, 2004 10:46 am
Contact:

### Re: Power Meter Question

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
Posts: 2830
Joined: Mon Apr 19, 2010 13:58 pm

### Re: Power Meter Question

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
Posts: 384
Joined: Tue Jul 20, 2004 10:46 am
Contact:

### Re: Power Meter Question

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?