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1. Carlton Reid May 2nd, 2007 7:02 am

   Hey, David, great show, as usual. The London-and-women-cyclists story is shocking, if taken at face vale, but there’s a lot more to it than first appears. This blogger has the inside skinny:

   http://www.movingtargetzine.com/article/red-lights-and-media-spin

   Another example of a clued-up, enthusiastic blogger beating the mainstream press to the real story?

2. Ken May 4th, 2007 12:06 pm

   Hi David. Great review on iBike Pro, but you failed to mention its accuracy issues.

   Because it calculates your CdA (coefficient of aerodynamic drag times your frontal area) by the coast test, it will miscalculate your power output if you significantly change your position on the bike during the ride. For instance, if you calibrate it with your hands on the brake hoods and later go harder with your hands in the drops, your CdA will change significantly and your iBike Pro will calculate your power output lower than it really is.

   Also, although you said you think it’s a minor caveat, it can be a big issue that you cannot use it on the indoor trainer if you’re serious about training with power. Even if you have a trainer with power meter built in, how would you calibrate your iBike Pro and your trainer against each other? When training with power, consistency in power reading is one of the most important things. If you are set out to do, say, your threshold power workout, you should be putting out around your threshold power whether you’re working outdoors or indoors. +/- 15% difference in power output might put you in a different training zone.

   That’s why I use PowerTap SL; it seemed to be a good compromise on the cost and the consistency (accuracy) although I’m pretty much stuck with one training-and-race wheel. Well, guess I’m obviously biased 😉

   Cheers,

3. Ken May 8th, 2007 9:37 pm

   Oops, I made an error in the comment above.

   Your CdA changes as you change your riding position, and iBike Pro will calculate your output higher than it really is when you are riding more aerodynamically (it will mistake increase in velocity by decrease in CdA as increase in speed by increase in your power output), possibly misleading you to believe you can go harder than you really can.

4. Paul May 11th, 2007 10:04 am

   I think you should make a major distinction between power measuring devices, and power estimating devices.

   Currently only the PowerTap and SRM really measure power output – they have strain gauges that measure force, and are extremely accurate and repeatable.

   The iBike doesn’t measure power – it measures speed, slope of road, and wind velocity, and does a complex mathematical calculation to estimate power output. Anything that disrupts the measurements – like drafting other riders, a rough road surface, or as Ken points out any change in your riding position – will make the power estimate inaccurate. There are many people that have put an iBike on their PowerTap/SRM equipped bike, and seen errors of 10% or more. That is simply NOT accurate enough for serious training.

   Plus, for those of us in the northeast, who spend 4 months or more indoors, the iBike is useless. That may not factor in to southern California, but it is a major issue here.

   The Polar is similar, in that it only estimates power, based on how the chain is behaving. The Ergomo is an odd one, since it measures accurately the power output of your left leg – but then makes the huge assumption that your right leg is doing exactly the same thing.