Friday, May 25, 2007

Power meters.. the ibike again

OK, an iBike update, firstly. The 3V battery dropped below the recommended 2.75V so I swapped in a new one. First battery had lasted for about 26 average 1-hour rides, or about 40 days. I used the "coast" function about 7 times. Maybe 6 rides were about 90 minutes. The new battery lasted 2 whole rides (and 3 "coast" tests) before dropping below 2.75V! Aaaargh. I rode yesterday and today with the battery below the minimum and the results are fine, so I guess there's a margin for error here (if you start below 2.75V and ride for 6 hours I'd guess you may lose some data, or get screwy data). Now I know why iBike sell batteries in bags of 20!

I also filled the memory once and have taken to dumping the ride list more often. The unit is still reliable but when analysing the data I suspect (and I cannot prove this) that it:
  • undervalues flat-land efforts by 20-50W (ie shows 150-180W when my manual calculations suggest 200W is closer)
  • overvalues sprints by a considerable amount - as much as 50% higher (ie shows 1500W when manual calculations point to maybe 1000W) but only for a second or 2
  • is most accurate at sustained high or medium-effort climbs, where the output is often within 2-5W of manual calculations based on speed, time and inclination
  • lags the actual effort by 10-30 seconds
  • loses it's pretty little head in corners and over bad bumps.
Now that reads pretty bad, but it's not so bad, really because you can:
  • smooth your data and remove 'outliers' such as spurious high-Watt readings
  • normalise your data in a spreadsheet or online tool
  • fine tune your friction and aero values on the provided USB-link software - this is better than re-doing the "coast" setup, I reckon, but it just may be that I've never done the 'coast' correctly (hmmmm...)
  • ride on smooth roads and never go around corners.
OK, I'm kidding with the last bit. Overall the data is consistent and relative to the values entered and it remains a useful training tool at a great price.

Some other quirks are:
  • It alters altitude overnight - presumably as the barometer rises and falls - so adjusting it is a good idea fi you want your data to be consistent
  • It adds 100kg (or maybe just defaults to a really high weight) when you swap batteries - make sure you check your setup after changing batteries!
If I hadn't bought the iBike (and I don't regret it, BTW, if only because it's still the easiest and least-cost way to get into full-function power meters) I would have considered Polar's new CS600 with power reading. It looks like a real hassle to fit but at least is wheel-independent and would work on an indoor trainer (which alas iBike can't - as yet - do). Pez has a good, detailed review of the CS600 here.

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