PowerTap SL+ short term accuracy

Discussion in 'Power Training' started by 886014, Feb 5, 2010.

  1. 886014

    886014 New Member

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    G'day, I'm migrating from a Polar based power set-up to a PowerTap SL+ and Garmin 705. Unfortunately I've always felt the Polar got a really bad rap (typically by those who had never used it) and after some years I can honestly say I was pretty happy with the whole system. The reason I changed to PT/Garmin was because I now swap bikes quite a bit, I travel a lot and liked the GPS function, and I'm interested in sprint/short-term power measurement.

    The latter is the reason for the post/question. Today was the first time I used the new system and sadly it was pouring with rain ... and I mean POURING! A really crappy ride so I just went out for an hour before it all became a bit silly out on Sydney's roads. On the way home however I decided to open my legs up a bit and was surprised to record a max power of around 1100W which settled down to around 8-900W for the effort. I say I'm surprised as it wasn't really what I'd call any sort of "sprint" just "revved the engine" a wee bit. The Polar samples only every 5 seconds and is accurate to only 1000W anyway so data like that I would just manually delete.

    Pretty soon I start incorporating more intensity in my training and wonder how accurate shorter-term efforts are on the PT? While I sure as heck aren't looking at a display when sprint training, it would be nice to track the data and know whether it's accurate or not over a period of time (ie increasing or ... not :eek:)?

    Pete
     
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  2. Alex Simmons

    Alex Simmons Member

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    Boy, hasn't the weather here been shocking this week! Just as well we now have the Turbo Studio in Sydney ;)

    In answer - the PT, provided it's properly zeroed and in spec, will be accurate for sprint wattages.

    I would take 1-sec peak power with a grain of salt but 5+ second power averages will be good. That's due to the time based sampling of torque data at the hub, which can contain a different number of pedal down strokes from one sample to the next. The longer the duration, the more these average out and don't affect accuracy.

    Having said that, I did find from many years of track and road sprint efforts using a PT that peak power numbers were pretty consistent.
     
  3. 886014

    886014 New Member

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    Ok thanks Alex, I'll have to re-investigate the zeroing, though the rest of that ride was pretty much what I was expecting power wise, so I think it wasn't too far out over the longer term. I've felt the Polar used to short change me in some instances and that seems to be confirmed from what I've seen so far; the Polar was very accurate over the long-term with calculated figures but got more and more stochastic as the duration came down. Shame in a way as the whole package was lighter than the PT wheel I now have.

    Yes the weather is absolutely dreadful, checked the radar, got kitted up ... then proceeded to get drowned. I don't mind if I'm out on the open road, but not around Australian drivers! Fortunately I've been away riding so have been getting plenty of DRY kms in :D

    Pete
     
  4. lanierb

    lanierb New Member

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    Trust the PT. It's accurate (except as Alex said on 1s or so durations because in 1s you can catch 2 downstrokes and only one deadspot by chance and then you get too high an avg power, though it's not technically wrong it's just misleading).

    Also, don't be surprised if you also find some systematic differences between the PT and the Polar. I found the Polar was often significantly off in certain gears, for example.
     
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