- POWER METER BASICS
- OPPOSING AND APPLIED FORCES
- HOW THE NEWTON CALCULATES POWER
- HOW OTHER PRODUCTS CALCULATE POWER
- COMPETITIVE COMPARISONS
- PRODUCT REVIEWS
POWER METER BASICS
Power, measured in watts, is the work you do each instant of your bike ride.
Physics measures power by this formula:
Power = Force x Speed
So, power meters measure both force AND speed during every moment of a bike ride.
The primary technical challenge power meters face is force measurement.
OPPOSING AND APPLIED FORCES
Cyclists pedal against the opposing forces of Nature: wind, hills, acceleration, and friction.
FORCES OPPOSING FORWARD MOTION
According to Newton’s Third Law, “Opposing Forces equal Applied Forces“.
So, the opposing force caused by wind, hill slope, acceleration and friction is EXACTLY THE SAME as the applied pedal force.
HOW THE NEWTON CALCULATES POWER
Accurate, Proven, Solid-State Sensors
Digital accelerometer and dynamic pressure sensors, the kind used in aerospace applications, are mounted inside the Newton. These sensors measure forward acceleration and opposing air pressure. Because the sensors experience very little stress they require no maintenance or periodic recalibration.
A wireless sensor mounted on the chain stay measures bike speed.
Aerodynamic and Frictional Drag Coefficients
As part of initial setup the user enters total bike/rider weight, tire size and road surface, rider height, and ride position. From these inputs the rider’s CdA (coefficient of drag), and bike Crr (coefficient of rolling resistance) are determined.
iBike “Physics Engine” Converts Sensor data into Power
On the road, the iBike Newton’s “Physics Engine” converts air pressure, accelerometer and speed measurements into opposing wind, hill slope, acceleration, frictional forces.
The total opposing force, multiplied by bike speed, equals cyclist power.
Because it accurately measures opposing forces and speed, the iBike Newton accurately measures power.
HOW OTHER PRODUCTS CALCULATE POWER
When cyclists apply power, bike components (hub/chain/bottom bracket/crank/pedals) flex in response.
Traditional, “direct force power meters” use strain gauges, mounted in the pedals/crank/hub, to measure bike component flexing.
Electronics located inside the hub/crank/pedals convert strain gauge measurements into applied torque (rotational force) measurements, averaged over the entire period of rotation. Measuring applied forces over the complete turn of the crank is a very solid technical approach that has been proven over the years.
A slightly different approach is measuring strain/torque in one leg only, then multiplying the result by 2.0, using the assumption that both legs apply power identically. Since this approach measures cyclist torque only 50% of the crank stroke, actual power accuracy over the complete pedal stroke is unknowable.
Torque measurements are multiplied by cadence (rotational speed) to calculate power.
Because power meter hubs/cranks/pedals experience the full, concentrated torque of the cyclist, for extended periods of time, they periodically require factory recalibration/refreshing.
We’ve been comparing iBike data with other power meters for 10 years. So have many of our customers.
Below is second-by-second ride file data, complied by cyclists around the world. Judge for yourself!