Posted by: Ian | February 21, 2010

Power and wheels

Yesterday I had my first ride with a power meter. Since reading Base Building for Cyclists I’ve been thinking about getting one but the expense and, even more, the array of choices have held me back. The obvious choice you have to make is what type of meter to get: one that’s built into the crank or one that’s integrated in the rear hub. Probably with insufficient research, I quickly settled on a preference for a CycleOps Powertap, which is a hub-based device. They’re a lot cheaper than the crank-based ones, and look easier to swap between bikes.

The next choice is what type of wheel to build it into. I could have just bought the hub itself and had my local bike shop build it into the Mavic Ksyrium Equipe that’s already on my Felt; but I liked the idea of getting lighter rims to offset the extra weight I was going to be picking up in the hub. My two primary candidates for better wheels (and there are obviously many more) were the Mavic OpenPro and the Zipp 404. The Zipp is a deep rimmed wheel that has a serious pelotonesque look about it. Although they do a clincher version, its aesthetic seems to demand tubulars. In response to a previous post, we recently had a comment arguing that tubulars are best. Well, they may be but I’ve had around a dozen punctures this winter and I fancy neither spending ~£50 on a new tyre nor unpicking and re-stitching the tyre to put in a replacement tube each time this occurs. The Mavic OpenPro is a traditional looking wheel that’s much cheaper, especially since if I get the Zipp’s, given the radically different design to my current Mavics, I’d really be committed to buying a pair. The clinching argument (forgive the accidental pun) in favour of the Mavic came from our colleague, Joerg, who pointed out the appeal of having separate powermeter-free race wheels – I could buy the Mavic for “training” and one day get some Zipps for any sportives at which I want to set a fast time.

The really clinching argument was that Evans had a Mavic OpenPro with Powertap in stock and on sale at a great price, so last week I bought it and took it home on Friday. I also ordered a chain whip and lock-ring removal tool from Wiggle and on Friday night took the cassette off my old wheel so that I could transfer it to the new one. Getting the cassette off was easy but I was surprised to find that it immediately dropped into nine or 10 separate parts. It was most satisfying to give them a thorough clean, making them almost silver again for the first time in months, and to reassemble them on the OpenPro.

At the same time, I cleaned down the front wheel – a Ksyrium Elite – that Emily had given me from her written-off Scott and swapped that onto my bike, keeping my Conti GP4000S tyres front and back. The weight that the Elite saved me over the Equipe on the front matched the net weight gain on the rear, which was itself mitigated by the lightness of the OpenPro relative to the Equipe. In effect, I’ve transferred torque-costly weight at the rims into inexpensive weight at the rear hub.

My unanticipated problem is that after doing all of this my rear gear adjustment has gone to crap. Given that correcting this involves the setting of only three screws and a barrel adjuster, I’ve been very tempted to monkey around and see if I can fix it. Getting the L and H screw positions right is presumably not too hard (they control the travel of the pulley at the extremes to the Low and High gears) and I could fiddle with the B screw and the cable until it worked. But since we’re covering this at bike maintenance soon I’ve decided to learn the proper procedure before risking any further incapacity.

So yesterday morning I struck out up Sticklepath hill and then along to Chaffcombe hill with a power reading on my Garmin 500 but without the use of my two largest sprockets. I configured the Garmin so that the power reading featured prominently as I rode. When I got home I uploaded the ride onto the Garmin Connect web software and, pleasingly, Power appeared on my Splits table (both max and average per mile) and as a charting option on the player.

From my Physics A Level (and probably even my O Level) I know that the integral of the power detected by the Powertap over the ride gives me the energy I expended on propelling the bike forward. Calculating this by multiplying the average power by the time (Exercise for the reader: Why does this overstate the true value?) and converting units gives me a figure of 210 cals. However, my Garmin reports just under 1,000 calories for the ride. I’m not sure whether this means that at least one of the devices is reading incorrectly or that (my) cycling is 80% inefficient or both. More significantly, I don’t recall from my book how to use the Powertap for effective training. I had a flick through Joe Friel’s book yesterday and read some of what he has to say on the subject – it begins with a requirement to find a quarter of a mile of flat road, and I haven’t yet been able to think of anywhere near to Hill Farm House where I might do this. But I love the new metrics that my Powertap is giving me and I’m sure that one day I’ll be able to extract benefit as well as interest.

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Responses

  1. 20% efficiency sounds about what I would expect?

    • I’ve no idea actually. It’s a bit disappointing to think that 80% of my effort is wasted on making me hot and sweaty. Obviously, energy dissipated post turning the wheel such as that lost heating the tyres etc. comes after the power meter measurement.


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