Polar Power Kit for the S710, S720i, S725
Power along with heart rate is the ultimate information for a cyclist to have.  Combine that with almost unlimited memory and software to download and keep track of the info then you really have something.  The Polar Power kit for the S710, S720i, S725 allows a cyclist to accurately and conveniently measure power thru a light weight device that measures chain tension and speed and then transmits that data to the S710, S720i, S725 watch.

How It Works

Power = chain tension or force X chain speed

All the power is transferred to the wheels via the chain.  The polar power kit measures the chain speed from a sensor at the derailleur pulley and the the chain tension is measured by an infrared device that notices chain movement and vibration.  This vibration can be converted to chain tension.  This technology is similar to the way the fitness test calculates your VO2 Max and the Tanita body fat scales measure you body fat.

Polar S710, S720i, S725 Power Kit

You then enter chain length, chain weight and the distance between the bottom bracket axle to the rear wheel axle.  The manual will provide info to help you.  The S710, S720i, S725 provides a real time readout of power that is accurate to 5 percent.   The S710, S720i, S725 also records Power, Cadence, Speed, Heart Rate and  Time all you have to do is pedal.


  • Light weight system only 225 grams

  • Measures pedaling efficiency of both legs

  • No Hub or crank change making an easier set up

Your pedal stroke provides max power at 3 o'clock and minimum power at 6 o'clock.  The S710, S720i, S725 uses this to calculate the power produce by each leg and then display that on the watch in percent. (i.e. 47 percent right, 53 percent left)  The cadence sensor provides a marker to distinguish each leg.

Sark Products Price  $329

We have developed a special program to predict times on real cycling courses using the S710, S720i, S725
click here for free download