Magene PES P505 Power Meter

Ric Stern of CycleCoach takes a look at the Magene PES P505 Power Meter

It wasn’t so long ago that very few of us had power meters, but now they’re somewhat ubiquitous in the peloton. Whether you’re a pro cyclist, amateur racer, or someone who just wants to smash their friends! So I decided to test what’s available for the amateur rider, starting wit the Magene PES P505.

Power meters can be used to help guide your training, ascertain your energy expenditure, help with aerodynamics, or just amaze your friends with the numbers! (There are other uses as well!). You can get power meters that are crank, pedal, or hub based and potentially other types as well. It goes without saying that the cost varies from inexpensive to very expensive.

Power meters and me

I’ve personally used a wide range of power meters, and have used one almost continually since the mid 1990s. Potentially, I have the worlds largest collection of power data on a single person who is still racing. What’s more, having used them at University while undertaking my degree and with some very bright people I’ve been able to understand how to accurately calibrate a power meter and make sure the data is correct. This is the most important aspect of owning a power meter, as incorrect data is a waste of time. It’s important to note that this is different to the “calibration” that appears on your bicycle’s head unit (which is actually a zero tare).

I’ve been using the Magene PES power meter now for around two months. This has included both indoor and outdoor rides. It’s an extremely inexpensive power meter (perhaps the least expensive on the market), but is it any good? What’s the data like? (Or, should that be watts the data like? Anyway, pardon my awful dad joke!

Magene PES P505 looking good

First off, what does the unit look like? Obviously, beauty is in the eye of the beholder, but I think the Magene looks great. If I had some spare Shimano chain rings, I’d fit them for an even better look, but alas I don’t. It’s a great looking unit and talking about Shimano, it fits directly into their standard bottom bracket. This means that it’s likely to fit the widest array of bicycles on the market without having to change your bottom bracket. 

Once I’d installed the Magene, I zeroed the power meter on my head unit (you should do this on every ride for all power meters no matter what it may or may not say in your manual). I then went for a few rides. It’s important to let power meters settle and this can be done with one or two brief rides.

Calibration matters

At this point I wanted to check the accuracy of the power meter. The correct method to determine accuracy of a power meter is to hang a series of ‘known’ weights off each crank and see what the frequency response is of that weight. By multiplying out the actual weight by crank length and acceleration due to gravity (9.81m/s) you can ascertain the response that the power meter should give you. Note, you need a decent weight about 30 to 40 kg, and this needs to be measured accurately to about the nearest 50grams. Luckily, I have such weights. However, I was dismayed to learn that the Magene (this occurs with other power meters as well) does not display a frequency response. So it’s impossible (as far as I can ascertain) to accurately validate the power meter.

This left me in a quandary. I don’t personally want to use a power meter if I don’t know it’s accurate. Luckily, I had a reasonable test option left to use. It’s generally been regarded that the Tacx Neo indoor trainer is very accurate. Previously, I’d validated mine against a ‘calibrated’ SRM and Infocrank power meters I used to own. They agreed within 1 W against each other up to ~ 500 W.

Comparing power meters

So, with this in mind I rode my bike on the Tacx Neo 2 while recording the power from both units. I then used the website Compare the Watts to check the differences between the two data streams (Neo and Magene). This would allow me to see how the power data streams compared (albeit I would not know which one was correct, if indeed either of them were!).

These data streams show both power and cadence superimposed on top of each other. Average watts difference between the two files is 4 W. As you can see they track each other well. However, there were some larger differences when I sprinted at >800 W (about 40 W difference). 

I then did another ride a couple of days later and recorded these data (again, image is © Compare the Watts).

I then wanted to ascertain, what were the power outputs over specific durations

Magene PES P505

Total ride 99 W (avg) 173 W (normalized power)

• 5 secs 431 W
• 30 secs 397 W
• 60 secs 375 W
• 5 mins 199 W
• 10mins 175 W

Neo 2

Total ride 98 W (avg) 174 W (normalized power)

• 5 secs 414 W
• 30 secs 396 W
• 60 secs 378 W
• 5 mins 197 W
• 10mins 171 W

In other words…

In other words, I found that the differences between the two power meters was quite small. Around 4% at higher/brief power and ~2% for longer efforts. I’m unable to ascertain whether for brief efforts (e.g. 5 / 10 sec sprints) whether the greater magnitude of difference occurs due to non-linearity (i.e., the power meter becomes less accurate as the power increases) or whether it is the way that each specific power meter might sample data.

It might be that more samples are required to average things in a ‘better’ way. For example, in a 5 sec effort there will be 5 data samples. If one sample is out due to e.g., the way cadence is calculated, or something else going awry. Then that will make a bigger difference to a smaller sample size compared to if one sample is ‘awry’ in a 60 sec sample. Unfortunately, I can’t maintain my 5 sec power for 60-secs to check this issue….

Of course, I also rode the Magene outdoors on my road bike, but I wasn’t able to compare this to anything other than my own sensations. Having used a power meter on virtually every ride I’ve done since the mid 1990s the Magene felt like it was giving me the correct data. I didn’t look at my head unit and think “that’s wrong”. My power matched other rides I’d done outdoors previously. However, I was unable to actually check and make a comparison.

Conclusion

The Magene PES 505 power meter looks great, fits a standard Shimano bottom bracket and is sufficiently accurate in comparison to a Tacx Neo 2. I like the rechargeable battery (which supposedly lasts around 500 hrs between charges). I suspect it’s not as accurate as an SRM or Infocrank (which are likely the most accurate power meters) but, given that the price is ~£300 versus ~ £1500 for an SRM or Infocrank I think the Magene represents excellent value for money. I’ll be buying one for my spare bike.

Take a look at the Magene PES 505.

Ric Stern is the founder and head coach at CycleCoach, head over to their site for world class coaching. Ric is also testing the 4iiii PRECISION 3+ PRO power meter, read the preview here while he tests it.