How does the Powerpod work? Unknown and changing variables

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Joined: Fri Nov 04, 2022 12:48 pm

How does the Powerpod work? Unknown and changing variables

Post by aerogunni »

I guess that this is a frequently asked question so I am posting it in this subforum:

"There are lots of things that affect CdA: wheels, bike helmet, ride position, and seat height are just a few."

As far as I understand the power measurement of all Powerpods (PP) / Aeropods depends on a CdA value that is "entered" during the set-up. I never had a PP so correct me if I'm wrong. I assume that during the initial setup I have to enter how I sit on the bike (endurance vs race position). And based on that the software guesses my CdA. But isn't that much to rough if an estimate? What about the use an aero helmet, deep section wheels, my body shape etc? Also the software would have to assume a value for rolling resistance which can easily vary by 5-10 watts per tire depending on the tire or is the software exact enough and I have to enter the specific tire I ride? Then there is drive train friction which can easily be 5 watts between a waxed chain and a dry one. How is that accounted for?

It seems that the end result is a rather accurate power measurement (I gathered this from reviews) but I'm confused how this is achieved given all the variables (see above) that can vary and have to be taken into account.
Velocomp CEO
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Re: How does the Powerpod work? Unknown and changing variables

Post by Velocomp »

These are questions which we have been answering for 18 years now. :-)

1. During PowerPod setup you'll enter height/weight/ride position/road type and tire type/pressure. From these inputs PowerPod estimates your CdA and Crr (aero drag and road resistance). How good are those estimates? We have 18 years of data to make them extremely good--good enough such that PowerPod is routinely within +/- 2% of direct force power meter measurements.

2. Chain losses are set at 2% (or about 4W at 200W). This is the same factor used by PowerTap...

3. User settings are fixed for each profile. But yes, if during your ride you significantly change ride positions (e.g. hoods to TT), or if your road surface type changes radically (e.g. smooth asphalt to soft sand), watts can vary. But as a practical matter, for most riders, ride position and road surface type don't vary enough over the course of an entire ride to cause significant measurement differences. We have 18 years of side-by-side power meter data to support that statement. :lol:

4. If you're worried about accuracy...what about the assumption of single-side power meters that left and right leg strengths are IDENTICAL? What about the temperature sensitivity of strain gauges, which can cause watts to vary on hill climbs? And what about price/value? You can use PP on four different bikes, of any type, and switch PP between bikes in less than 60 seconds, without switching out any of your bike components.

5. In fact, the most important thing about ANY power meter is consistency--the factor required to train and race confidently. Most modern power meters, including PowerPod, deliver excellent consistency.

6. If you're trying to account for the aero differences between helmet types, ride position, or clothing, then AeroPod (in conjunction with a good direct force power meter) very accurately measures CdA differences.

PowerPod is a very practical, very good device. For tens of thousands of cyclists, it meets their needs just fine.
John Hamann
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