How Do Power Meters Work? - Sea Otter Classic 2017

 REAL WORLD EXAMPLE!

 Don't you hate it when you push buttons on a dummy gadget and nothing happens?

 @denomerdano: It's not a dummy unit, it just doesn't have a touchscreen (as not everyone likes those since they're a pain with gloves)

 We should do a little contest on it.

 @AJBarlas: whats the ROTOЯ hat he;s wearing at the end? I like it

 I still didn't learn anything. Why is this important tool of measurement, how does one use a power meter to get better, and do I really need it if I am riding for smiles and not KOMs?

 On a regular group ride with friends it's a very good measurement of how douchey you are.

 @abzillah: If you're riding for fun and not racing/performance oriented then yes it is useless. However for road cycling or XC it allows you to measure your effort and analyse your data when training and helps you to pace a lot better when racing :

for example, if you come up to a climb and go full gass at the start, you might not make it to the end , however, by having the power data in real time, you can pace yourself to a certain power output that you know is sustainable for the duration of the climb.

Powermeters give you an accurate measurement of the effort you are exerting regardless of the conditions (wind, speed, gradient, etc).

 @vicrider222: rotorbike.com

 The strain gauges in the meters only measure deflection in one axis.

 The deflection of only the crank is measured, though the crank assembly will deflect differently, the force that the crank sees will still be the same. If the force is same, then the power is the same because power is torque times angular velocity. There would be compensation required if the crank length changed. Spindle length does not change the measurement.

 You can create power, measured in watts in this case, by either pedaling very fast with less force, and slower with more force. This is just like the engine in a car, you either need a big engine turning slower or a small engine spinning really fast to create the same amount of HP. So, the accelerometer tells the system what your rpm is, and the strain gauge tells you what your torque is. RPM x Torque = Power. This is doubled with only one side of the crank measuring power because when your left leg is on the up stroke your right is applying the power. More expensive units have meters in both sides, which can aid in looking for imbalances in your pedaling, but is overkill for what most customers want, and are willing to pay for.

 the powermeter is tuned for different cranks of different stiffnesses

 great question, do you have to calibrate each crank arm in a vice by loading it with a set weight? Or maybe stages have catalogued every crank and have profiles ready to go?
presumably even identical crank arms will have different 'flex' though?

 They calibrate EVERY SINGLE powermeter-equipped crank in the factory. Even 2 cranks of the same model can have varying stiffness. The calibration process deal with that problem.

 @K4m1k4z3: Thanks for that!

 @K4m1k4z3: does the non-pickup side crank arm get tested as well? I would think by having only one pickup on one crank you're not accounting for most riders that will have a dominant leg, so doubling of the figure produced is inherently inaccurate

 @dirt-unit: AFAIK they only sell the left crank(s).

 @K4m1k4z3: But AFAIK Stages don't calibrate the crank arm for use with ANY pedals. Seeing as pedals will vary in axle stiffness too this power meter cannot really be as accurate as we are led to believe. but it will be consistent for the same rider using the same pedal so still a handy training measurement.
well... for your left leg anyway.

 @G-A-R-Y:
No influence of the pedals. Whatever pedal you use, you will put a certain amount of force (more or less your weight) on the axle of the pedal which is 175mm (usually) from the bb axle. For a given crank arm stiffness, this force applied at a certain distance will give a completely predictable deflection, which is measured.
So deflection allows to calculate torque, and torque X speed = power (Nm x rev/sec = watts).

Which is then doubled because the left side doesn't work much when the right side is pushing...
I am curious to know how much difference there can be, thus if this "left side power x 2" principle is accurate...

 @Uuno: If stiffness in pedal axles makes no difference to power transfer. Why is it we lead to believe we NEED stiff carbon soled shoes for better power transfer?

#PROFIT

We might as well just use £3 resin pedals and slippers from Aldi, eh?

 @G-A-R-Y: It's calibrated with a steel pedal spindle installed, as seen in some previous videos. The weight is applied around the same spot it would be on the pedal. Power meters can only be accurate at the point that they're measuring, so no device couple possibly take into account the power loss at all spots (i.e. a hub based system might read very slightly lower than a pedal based system), but the differences are pretty minor in the readings (compared a stages to a left pedal system once and it was different by an average of 1.5w).

 there are things called manufacturer websites. they are set up to inform people. try it for yourself, maybe you'll learn something new there...

 I wonder if i mounted on my arm it would mesure poo time strain?!

 @ilovedust: Is that what the bars are for in the bathroom stalls, to brace for those massive marathon poops?