The new Polar Power System was designed to make you a better cyclist. This system, along with the S710, S710i, 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 S710, S710i, 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.)

Measuring L- R- balance

The S710, S710i, S720i system allows a cyclist to track the efficiency of their pedal stroke in real- time. It measures the power distribution generated by the left and right foot respectively, and provides a constantly updating index number that appears on the receiver face. If a cyclist presses hard on the right, the index number will be skewed on the right side. This constant measurement will help the cyclist evenly distribute power between both pedals.

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 S710, S710i, 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 S710, S710i, 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

How does Power Output help cycling?

With regard to training it can give much more precision than heart rate and perceived exertion. Power meters also have a number of other users, including working on bike position and determining efficiency at different cadences among them.

FAQ

How Does this System Measure Power?
The Power Output Sensor measures power direct from the chain, unlike other models that use the crank or hub. This precision system combines measures from two key factors:
Chain Tension – using a sensor on the chain stay
Chain Speed – using a sensor on the rear pulley

How Quick Is the Set-Up?
It couldn’t be easier. All you have to do is enter – once, when setting up the unit – a set of key factors: chain length, chain weight and the length between the bottom bracket and rear wheel axles. And then away you go. Or, you may use the default settings to get started (Chain weight 304 g and length 1473 cm and Span length 420 cm). Precision pedal power – every time!

Does Chain Slap Affect the Results?
No. Because the chain is taut as you pedal, the Power measurement is unaffected by chain slap. The only time excessive chain slap occurs is when you’re not pedaling – and no power is generated.

What About Pedaling Efficiency?
Together the Polar S720i/S710i and Power Output Sensor let you track your pedal-stroke efficiency, in real time, by measuring two indexes, L/R Balance and Pedaling Index.

L/R Balance
L/R Balance is an index of the power output balance between the left and right strokes making up one rotation. The Polar S720i/S710i then compares the peak power produced during the left stroke against the peak produced during the right stroke. If you press hard on the right, the L/R balance will be skewed on the right side (i.e. L 37, 63 R). The Polar S720i/S710i display information helps you achieve balance between each push, avoid premature fatigue in either leg and contributes to improved pedaling efficiency with each rotation.

Pedaling Index
Pedallng Index (PI) measures the evenness of power output throughout each pedal rotation by comparing your power output at its minimum and maximum levels. A higher PI score shows that your power was produced more evenly throughout the pedal rotation –so your peak power for a rotation will be lower, even though your average power output remains the same.

How Does this System Measure Pedaling Efficiency?
A pedal stroke generates peak power as the foot pushes down on the pedal. In one rotation there are two power peaks: the first as you push down with the right foot and pull up with the left, and the second when you push down with the left foot and pull up with the right. In between, your power output is decreased. The Power Output Sensor measures the maximum and minimum power values and calculates your L/R Balance and Pedaling Index, using a cadence sensor on the right pedal to mark the right side.

Why Choose the Polar Power Output Sensor?
No hub, wheel or crank change: The system measures power direct from the chain – no need to change hubs, wheels or cranks unlike other models.

Right price for total system: The Polar Power Output Sensor option for the Polar S720i/S710i Heart Rate Monitors is an additional module, separate from the heart rate monitor itself. An athlete who already has the Polar S720i/S710i with its many heart rate monitoring features adds a whole new series including wattage, cadence and a pedaling index. The ability to incrementally buy into this power meter is one of its primary advantages.

Polar - the lightweight choice: The Polar S720i Heart Rate Monitor and the bike sensors weighs in at just 318g grams when installed - compared to over 550 grams for other models.