Powertap hubs are time sampled, SRM cranks are event sampled with one data point recorded per complete pedal stroke but then recorded into the file based on preset recording interval which implies appropriate interpolation or decimation (averaging or sub sampling) of the data points depending on storage rate and pedaling rate. IIRC, the Quarg is also event sampled.

The differences in sampling techniques can lead to data artifacts such as aliasing in time based sampling methods (e.g. PT Hubs) because you're not recording complete pedal cycles so you can get more erratic data than normal but even small averaging as in two to three seconds of data smoothing tends to cancel that out. But those data artifacts are just that, artifacts and not on a sample by sample basis accurate in terms of the power that was being generated. So depending on your power meter and your cadence you might have some sample by sample errors but for time intervals of interest including efforts as short as several seconds your data is accurate. But it is one reason to take instantaneous peak power from PT hubs with a grain of salt even though 5 second peak power and longer periods should be fine.

That's the sampling stuff, in terms of the inertial influences. From a physics and measurement standpoint a watt is a watt and low inertia vs. high inertia watts (or joules per second of energy delivered) are exactly the same. The question is whether it's the same from a physiology or ability to produce that power perspective. Lot's of discussions on just that subject and usually focused on why indoor training seems so much harder for the same or lower sustained power than outdoor training. Inertial effects are definitely one of the prime suspects there as are cooling issues and motivational and focus issues. And the question always comes up as to whether a workout that felt just as hard or harder but generated lower power is as effective from a fitness building standpoint. Lot's of debate on that one, but in the end you can only do what you can do and if you're stuck indoors and the power is lower then that's what you have to work with. It's certainly better than not working out at all.

Search these forums, the web or various cycling and power training forums for discussions on 'indoor vs. outdoor trainer power' or 'lower power indoors' or other variations on that theme and you'll find many discussions on the topic.

-Dave

Thank you. This is interesting and helps me sort some of these questions out.

My trainer has a function that allows me to change resistance (increase resistance and it effectively decreases inertia). I did two intervals at the same wattage but at different trainer resistance settings (changed gearing to accomodate the increased resistance and keep cadence similar), and I could swear that the lower resistance setting (higher inertia) was "easier" despite the same wattage. Mental? Small change in cadence? Who knows, but it just got me thinking...

Edit:
Some good info here:
http://alex-cycle.blogspot.com/...harged-training.html

Also interesting:
http://www.ncbi.nlm.nih.gov/pubmed/8945654

watts are watts period, but how many watts your body can produce can change on the trainer, or uphil, or downhill due to differences in inertia, heat, etc.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

No, it's likely not mental many if not most folks can sustain more power when they have more momentum carrying them through the dead spots in the pedal stroke. It's unlikely that your trainer's inertia actually changes, but at lower resistance setting you maintain higher rear wheel speed and that rear wheel is basically a flywheel, spin it faster and you carry more angular momentum which helps carry you through the pedaling cycle. Trouble is, even a fairly heavy wheel is still not much momentum compared to just rolling your body weight down the road and the linear momentum that provides. Some trainers like the Velotron or Lemond Revolution have much bigger flywheels that carry more momentum.

There's no doubt momentum is part of the equation and many folks find extra momentum whether via bigger flywheels or maintaining higher rear wheel speeds helps them sustain more power but interestingly some riders don't seem to struggle with this part and it's not clear why it impacts some a lot more than others. My main point above was that once the sampling stuff is removed the inertial question is one of physiology not one of physics in the sense that inertia does not change the accuracy of the power meter or the wattage displayed but inertia can definitely be a contributing factor in terms of what it feels like to sustain those watts in the first place.

So for instance on a Computrainer ridden in ergo mode it pays to ride bigger gears and keep that rear wheel speed up. In ergo mode the CT will adjust the resistance to try to keep the power constant but that extra rear wheel speed translates to extra angular momentum and that's generally a good thing in terms of riders sustaining the required power. But for some reason there are folks that seem a lot less sensitive to this and do just fine grinding away on a CT at low wheel speeds so it doesn't seem to be universal.

-Dave

He's right about the sampling. I think the Zone is supposed to be different and measure instead of sample power. I don't understand it fully but it's supposed to generate a bit different numbers than those with strain gauges.

Hopefully someone who knows more about this can chime in.

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CEO at TrainerRoad
Co-host of the Ask a Cycling Coach Podcast