A question to any Powertap users

akbond1973

Hi guys,
Have recently bought a powertap Pro and am finding it very hard to keep within zones.The torque figures are very jumpy on the road and will jump a few hundred watts without much change in my pedalling force. Perhaps this is due to my unsmooth pedalling technique but I am wondering if any other users find this. On the turbo it is a lot easier to stay within zones.

Many thanks for any advice

Andy

Herbsman

Put your display on 10 or 30 second average and keep it within a certain range of your 'ideal' power output.

Instant power is no good for road riding.

danowat

Depending on the length of the interval I am measuring, I either use 10s avg, 30s avg or lap avg

SBezza

As above use 10 or 30 sec power, also don't get too hung up if you go outside the training zone you are aiming for, try and make sure AP, or NP for the ride is within the training zone, just try to avoid blasting up hills and then coasting down the other side, try and keep the effort steady.

okgo

Herbsman wrote:

Put your display on 10 or 30 second average and keep it within a certain range of your 'ideal' power output.

Instant power is no good for road riding.

Why do you say that? It is possible to keep it steady with practice.

Herbsman

okgo wrote:

Herbsman wrote:

Put your display on 10 or 30 second average and keep it within a certain range of your 'ideal' power output.

Instant power is no good for road riding.

Why do you say that? It is possible to keep it steady with practice.

Yes, it's possible to keep it steady, but (and this really depends on your definition of 'steady') is it beneficial to do so?

There are so many things that will change your power output when you're out on the road... if you're riding smoothly at a steady pace, minor undulations, changes in wind speed / direction, road surface, air pressure etc will cause your power output to fluctuate if your cadence stays the same. Is it really beneficial to react to these things every time they occur, which can be quite frequently, by adjusting your cadence or changing gears constantly? You lose some momentum every time you change gear. Unless you're riding on a really flat, smooth road with no wind I don't see any reason why you would try to keep the instant power value close to one number all the time.

amaferanga

I find instantaneous power (non-smoothed) useful in addition to 30s rolling average.

Fredster42

As above I had the same problem when first using the powertap on the road. I would suggest tutting the power to 30 second average and then try to keep this in roughly the right range. Also I now try to keep a steady effort but the keep the lap power average in the right range. The power tap is really just an indicator of a cerain power level so once you get the feel of what certain zones feel like its a lot easier.

Alex_Simmons/RST

Power, especially outdoors, is naturally quite variable - stochastic. It's normal because that's what happens.

Layered on that natural variance is an artificial variance in the displayed/recorded power numbers which is a result of the Powertap's fixed time based sampling of torque data - otherwise known as aliasing. This artificial variability does not occur with crank based power meters, which sample torque with event based sampling (i.e. full crank revolutions).

Powertap aliasing impacts the variability in reported power differently at different cadences. e.g. if you pedalled at 60rpm, 90 rpm or 120 rpm*, at which cadence there are an equal number of pedal down strokes per second, then the artificial variance is minimised, but if you pedal at cadences away from those, then the artificial variance is increased as each second of torque data contains a different number of pedal down strokes, and so affects the calculated torque average for that second.

So even though you may be perfectly smooth in power output, if you are pedalling at say 80-85 or 95-100 rpm, then the powertap will still display variable power from second to second. Of course over time these variations average out and the data is good, but it is one reason why you should also take very short duration power numbers (5 seconds or less) from a Powertap with a large grain of salt.

The normal way to address this is to apply a moving average to the displayed data. On Powertap head units, 5 seconds was a good choice for power display. I think 30 seconds is a bit long.

Also keep in mind not to fall into the trap of "chasing your power tail", where you are constantly reacting to what the display says and over compensating.


* the cadences at which this occurs is different with older model powertaps that averaged torque every 1.26 seconds, rather than every second with current models.

maryka

akbond1973 wrote:

Have recently bought a powertap Pro and am finding it very hard to keep within zones.The torque figures are very jumpy on the road and will jump a few hundred watts without much change in my pedalling force.

A few hundred watts? :shock: I find it hard to make mine jump that much without a serious change in effort on my part.

3s rolling avg seems to work best for me, 10s and 30s are way too long, 1s is just too annoying on the eyes.

If you really are jumping a few hundred watts for very little discernible effort, I would recommend getting the powertap checked out to make sure there's nothing wrong with the torque tubes.

neeb

Because of the interaction between cadence and the 1 sec averaging that Alex has described above, you can get quite big jumps from second to second even if your output is more or less constant, maybe not 100s of watts but possibly 100 or so... You get used to it, I use both 10s and 30s averages (as well as overall average) and have all displayed simultaneously. After a while your brain sort of integrates them and/or tells you what figure is important at any given moment. The 10s average is probably most useful, you should be able to keep that more or less constant with practice. However, on anything but a perfectly flat road or a constant climb you will still vary your output quite a lot even if you are not intending to - that's why the 30s average is also very useful. On the turbo it is much easier to keep the power more constant because the resistance is exactly the same much of the time. Incidentally, if you are comparing the powertap to the (rather inaccurate) power readings you get from VR trainers such as the Tacx ones, I'm pretty sure that the "instantaneous" power readings you are getting from the latter are artificially smoothed and/or averaged over a few seconds too. With the powertap you are seeing the raw data, i.e. the averages for each successive second, which is why it appears to jump around a lot if you are turning the cranks once in one second and two times in the next second. <edit>: or more accurately, if you are pushing a pedal down twice in one second and three times in the next second.

fish156

maryka wrote:

3s rolling avg seems to work best for me, 10s and 30s are way too long, 1s is just too annoying on the eyes.

+1
I have 3s and lap/interval power displayed.

moonshine

fish156 wrote:

maryka wrote:

3s rolling avg seems to work best for me, 10s and 30s are way too long, 1s is just too annoying on the eyes.

+1
I have 3s and lap/interval power displayed.

+2