The new Polar Power System was designed to make you a better cyclist. This system, along with the S720i, will redefine the way you train by combining heart rate and power (wattage) data side by side. Now you can analyze power output, pedaling efficiency and heart rate simultaneously. The weight savings versus traditional methods make it a tool you can use even when you're racing.

The system derives wattage from the chain, not from the crank or hub as other models do. The system works by measuring two key factors:

Chain Tension - measured using a sensor on the chain stay
Chain Speed - measured using a sensor on the rear pulley

POWER = Chain Tension X Chain Speed.

All of the drive force of the bike is transmitted through the chain, allowing the S720i to calculate watts very accurately. The sensor positioned on the rear pulley works magnetically and measures the speed of the chain. The chain stay sensor calculates the tension of the chain by measuring the vibration as it passes over the sensor (The user will enter one time during set up: chain length, chain weight and the measurement between the bottom bracket axle to the rear wheel axle. A chart in the manual will help provide some standard parameters.)

How the Unit Measures Pedaling Efficiency?

There is a cadence sensor also built into the system. A cadence magnet is located on the bottom of the drive side crank, when the crank is in the 9 O'clock position, near the pedal. This provides real time cadence to the S720i.

A normal pedal stroke will generate max power when the foot pushes downward on the pedal. Example: I push my right foot down and pull up on the left and I get a power spike. When I push down on the left, and pull up on the right, I get another power stroke. In between is a spilt second "dead spot" where power disappears. The S720i measures the spikes and calculates the difference between pedal strokes. This is converted into a percentage measurement. It uses the cadence sensor on the right foot to mark the right side from there, it provides an index for the right and left.

Example:

Right side pedaling generates	300 watts
Left side pedaling generates	300 watts
Pedaling efficiency	50% right 50% left