Wednesday, March 18, 2009

A Taste of Summer

The ripe Chino grown strawberry tasted sweet and juicy as a pre-ride send off. The day was hot 85 to 91 F on the road. I was going to be happy just completing the ride with out falling over from heat. The afternoon sun made shade an enjoyable respite. My SDSR time was 33:45.

I use a Polar CS600 to measure heart rate, speed, time, distance, cadence and power. The question is how does the Polar CS600 Power meter calculate power? I have found two articles on the intricacies of how it does it. BikeRadar states it is computed by two key elements:

chain tension and chain speed. It then calculates the vibrational frequency of the chain – and thus chain tension – and derives power output from there.

The Bike Tech Article is a bit more colorful:

At the heart of this system is what is essentially an electric guitar pickup mounted to the chainstay. This sensor allows the frequency of vibration of the chain to be monitored by generating a voltage that is proportional to the up and down velocity of the chain.

Power is calculated

by measuring the chain tension and multiplying it by the chain speed. The small sensor that requires the replacement of one of the rear derailleur pulley screws measures the chain speed.

If you like equations the second article does a nice job leading one around the mathematics.

The CS600 measured my maximum speed at 31.4 mph.