it depends on exactly what patterns the system is looking for. for instance, when i'm at a steady effort and quickly increase my power to try to make a traffic light (not sprinting, just a firm increase in effort), my heart rate stays flat or even goes down a bpm or two before going way up. how does the system know that i'm increasing my effort and not actually slowing down for a sec? if it's going to be delayed as much as the heart rate is delayed in responding to the power increase, why bother with some estimated "power" figure at all? why not just train with heart rate as most people did before everyone had power meters?does the system also gather the speed and cadence numbers (and bicycle pitch?) to figure the stuff out? that would possibly work. but i doubt it as it looks like you can purchase the system with or without speed or cadence sensors.and what about when you become more efficient, increasing your power while lowering your heart rate? if this system is aimed at beginners, and beginners are going to make the biggest first increases in efficiency, then this is especially poor.

i asked some of these questions at least a year ago when some cycleops guys popped in the store. i got poor answers. i told them it was going to be bull-poo and they shouldnt do it. oh, well.

It's actually quite decent at matching up with my perceived effort. It's far more responsive to efforts than I thought it would be. And, it drops down to 0 (or pretty low anyway) surprisingly quick. I've found it to be pretty useful at pacing on longer efforts, climbs in particular.

All of that said, it's not a power meter. It's obviously inaccurate. And, I don't see how it will work well as fitness changes over time.

But, it's $99. And, it's unquestionably more useful than HR alone. Plus, it's easy to use it on all my bikes, including MTB.

I'll continue using it. I want to at least build up enough data to get better Stress Balance numbers out of Golden Cheetah.

If you have any more specific questions, let me know.

PS - for reference, I'm probably a strong 5, maybe a 4 on the road. But I don't race (much). About 132lbs, 5'9", age 40. I add this because I'm curious how well it might work for people of different ages, body makeup, and age. Or lookup swaits on Strava.

No. It has no way of talking or listening to that sensor that I know of. So, yes, because it's a power guestimator, sometimes my cadence is zero, and my 3s power average is bouncing around between 0W and maybe 50W.

I did wonder if the $130 bundle from PT which includes their speed/cadence sensor could do this. But, I couldn't find any info on that. Do you know?

And, in the end, it's a really very rough guesstimate. I'm under no illusion it's a power meter. But, it's still more useful tome than HR alone.

As a programmer with lots of interst in this sort of algorithmic approach to data estimation, I must say that they've done an incredible job with this. It's probably as good as you'll get with HR alone. It's far better than I would have guessed.

I am guessing this is how powercal calculate power. The powercal will receive data from Ant+ speed/cadence sensor, do the maths and throw the power & HR data to the computer. But since you mentioned with 0 cadence and it will jump between a range, I think it is possible that there is an accelerometer in it. It is to detect if you are pedalling/running.

Now the question is how does it measure power accurately? HR and power may correlate well but everyone power/HR ratio is different. The ratio may be determined by the speed. Maybe there is a GPS chip inside to calculate that if it doesn't use the Ant+ speed/cadence sensor.

The thinking here is that the PowerTap folks have access to massive amounts of fitness data over a broad spectrum of athletes. So, they built algorithmic models that attempt to estimate power from HR alone. The model is tuned until it best fits the data set. If you're unfamiliar with the area of machine learning, you'll just have to take my word for it.

As I said above... It's not an accurate power meter. It might be within 20%. But, it is more useful to me than HR alone. And it's $100, not $2000. Given that price difference, I personally find it to have value.

I'm just not getting this - if you want to measure HR, then surely the best approach is to train using HR zones etc? Otherwise this seems like a cheaper and worse version of an iBike, with similar calibration issues. And we all know the reception that got.

The key question for me is how you can use this gadget to track performance. With a real power meter you see bigger numbers. With HR estimated power I don't think there is any way of knowing. So you are back to square one - measure time round a given course, which is the same as HR.

So are the only people who should buy this the people who are too lazy to swap their power meter between bikes?

There are multiple current reviews of the Powercal around the interwebs. Here is one: http://goo.gl/JLs9R" onclick="window.open(this.href);return false;. It does factor in cadence as you can see the power data drop to zero if you stop pedaling.

I'm interested in it because:- it's cheap,- I have too many different bikes such that swapping conventional power meters between them would be a huge hassle,- I'm still waiting for a good pedal/cleat based power meter,- it's something new to play with...

That said: it seems there was an early model that has a "calibration stick" (e.g., on PBK or ebay) which was later removed from the package. Does anyone know whether there were other changes to the Powercal, e.g., do newer Powercals without that calibration stick have a different algorithm for calculating power?

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