IC Bike Power Readings: Actual or Estimated – Does it Matter?

After interviewing representatives from both Saris and Keiser, I have watched the debate about Indoor Cycling Bikes with power readings i.e. Keiser M3 vs. CycleOps Pro 300PT, with great interest. The CycleOps Pro 300PT IC Bike uses a version of their "Powertap" hub to sense torque. The Keiser M3's method of sensing torque isn't obvious so I decided to dig a little deeper and get some answers. I sent the following email to Darrin Pelkey, the VP Sales & Marketing at Keiser.Hi Darrin,
I didn't dig deep enough into this question during my interview with Suzette O'Byrne
How is the wattage computed on the M3? There is a lot of discussion / confusion on the forums as to how Keiser measures force without using a strain gauge in the pedal crank or wheel hub. Is it an estimate or actual power reading? Is there a calibration process?
See this thread - http://innercycling.com/f/showthread.php?t=8542I understand the basic physics and I'm an engine guy so I understand that most dyno's use eddy currents to load and measure power but I don't understand how you get your power readings on the M3.
I would love to inform my readers & listeners if possible.Here is Darrin's response:John,
First and foremost the M3 is not a ergometer nor is it priced like one. Power is an estimate. The price and our maximum usage (group exercise) dictate that it is not for testing and it cannot be calibrated. Though we do have some people that have used it with an IMET test when a bike is more preferential than a treadmill. One thing not mentioned below is magnet strength, they are consistent and do not weaken over the life of the bike.
To answer the question:Watts are calculated from the gear setting. A potentiometer is attached to the magnet holder (the round cone shaped disc at the end of the shifter cable). As the shifter is moved, the cable rotates the magnet holder. A potentiometer is rotated by the rotation of the magnet holder, thus feeding information to the computer on the position of the magnet holder. The rotation of the potentiometer is broken down into 24 gear settings. A dynamometer was used to test and develop a table of wattage at various gear settings and speeds. The speed of the crank is determined by a magnet attached to the large pulley on the right crank arm and a magnetic switch attached to the circuit board in the magnet holder assembly. Each time the magnet on the pulley passes by the magnetic switch, a signal is sent to the computer to compute the RPM's of the crank arm. Power equals force times velocity. The force is determined by the magnet position and the speed by the crank speed. The lookup table is programmed into the computer and the computer simply looks at the gear setting and speed and goes to the lookup table to find and display the Watts for those two settings.
Darrin Pelkey
VP Sales & Marketing at Keiser
The Power in Human PerformanceNow my two-cents as to Actual or Estimated — does it mater?
Short answer — No. My feeling is that any bike that displays consistent and repeatable power readings is a vast improvement over any non-power based IC bike.
There are many ways that you can pick both bikes apart; CycleOps has actual power readings yet the M3 has (in my opinion) a vastly superior system for creating load/resistance. The heaver flywheel on the CycleOps causes inaccuracies in power readings — in their words:
Why are there discrepancies between my PowerTap numbers and my Indoor Cycle numbers? Due to the external factors of riding a bike outside vs. riding a solid indoor cycle, you may experience different values on your PowerTap. Also, with the special design of the flywheel, inertia plays a much bigger role in your workout on the Indoor Cycle vs. riding your bike on the road. For instance, a larger effort is required to get your Indoor Cycle flywheel up to a certain speed, but due to inertia, the flywheel will spin much longer than a regular bicycle wheel, resulting in less work being required to maintain that predesignated speed.

For personal use, I'd say it's a wash... or more of a Ford vs. Chevy argument.If I was a club owner I would pick the M3. Less capital cost, it appears to need much less maintenance (calibrate 50 bikes each day? I don't think so) and how many of our members have a road bike with power?