How the Newton Works

And Other Secrets about Power Meters

  • Advanced technology means
    Simplicity and Portability

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.

 

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OPPOSING AND APPLIED FORCES

Cyclists pedal against the opposing forces of Nature: wind, hills, acceleration, and friction.

FORCES OPPOSING FORWARD MOTION

opossing_forces

 

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.

 

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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.

Newton “Physics Engine” Converts Sensor data into Power

On the road, the 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 Newton accurately measures power.

SensorImage

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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 full turn of the crank (“full-crank DFPM”) is a very solid technical approach that has been proven over the years.

A lower cost, but accuracy-compromised approach measures strain/torque in one leg only (“half-crank DFPM”), then multiplies the result by 2.0, using the assumption that both legs apply power identically.  Since this approach measures cyclist torque only on one leg, the actual power accuracy over the full 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.

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COMPETITIVE COMPARISONS

We’ve been comparing Newton 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!

Newton vs SRM

Newton vs Stages

Newton vs PowerTap

Sprints, Newton vs PowerTap

Newton vs PowerTap, 23 rides

Newton vs Quarq

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PRODUCT REVIEWS

If you’re skeptical that the Newton has accuracy comparable to other products, you are not alone.  Here is a July 2014 review published in road.cc  The reviewer freely admits he was very skeptical, but after 3 months of testing concluded the Newton was within 2.5% of his direct force power meter.  His overall verdict? “ [An] Innovative power meter that offers results comparable to others, along with features to improve your position and technique.”

Another review (from a PhD cycling coach!) says the Newton seems magical.  Well, we’re not magicians; we are physicists, engineers and cyclists, focused on one thing: making the Newton better and better.

If you’d like to read what real customers have to say about the Newton, check out the testimonials sprinkled throughout the website.

For more independent reviews of the Newton, from publications around the world, click here or here or here or here or here.

And here is another recent review of the Newton, published June 2014.

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