Arofly power meter

The Arofly power meter is lightweight and affordable but we've found that it simply doesn't provide measurements that are consistent enough that it can be considered a useful training tool.

Arofly is unique in that it attaches to the valve stem of your rear inner tube. It's larger than a dust cap but is still very small with a 20mm diameter and a weight of 11g, including the CR 1632 battery that lives in there and the Presta adaptor (if you need one, see below).

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If you use a Schrader valve the Arofly just screws directly on to it. The valve pin needs to make a good contact with an 'Air Post' in the centre of the Arofly, beyond the thread, in order for you to get readings.

If you use a Presta valve you need to loosen the valve screw, then tighten an Arofly adaptor in place – just a small threaded piece – and finally screw the Arofly on. Again, the valve pin needs to contact the Arofly's Air Post. I've used the Arofly with Presta valves and found that it works easily on some but not on others. You need to find an inner tube with a tall enough valve pin. Once up and running it has stayed up and running without dropping out.

Then you need to download the Arofly app, available free for both Android (OS 4.3 or later) and iOS (iPhone 4S, 5, 5S, 6, 6 Plus or later), on to your smartphone and pair the device with the app via Bluetooth. You input a few details: the units you want to use, the type of bike you're using – road, mountain or small wheel – and so on, and you're good to go.

The Arofly will not talk to other head units so you can't have the power figure displayed on your Garmin, say, you have to mount your smartphone to your handlebar/stem. A basic smartphone mount comes as part of the pack, although you'd get better security with something like a Quad Lock Bike Kit.

The app must be open and running while you ride. It gives you this information as you go:

* Current speed

* Time of day

* Arofly's battery status

* Cadence

* Power

* Ride time

* Distance

* Heart rate (if you've paired a heart rate monitor with the app)

* Cumulative ascent

There's just one display page and it isn't customisable. You can't, for example, have your power as a 3-second or 10-second average, and there's no auto-pause function – you need to do that manually if you stop.

Assuming you do that, the app gives you this info when you're done:

* A map of your ride

* Ride time

* Ride distance

* Average speed

* Average cadence

* Average power

* Calories burned

It also gives you graphs that show:

* Speed

* Power

* Cadence

* Heart rate (it you've paired a heart rate monitor)

* Ascent

Those graphs are small on a smartphone and you can't zoom in on them so you're better off pressing the 'Upload to Strava' button and viewing the information over there. If you like, you can then export a TCX file of your ride from Strava and transfer it somewhere else.

How does it work?

Okay, let's go back a bit: a power meter that attaches to your inner tube valve – how does that work? 

"Originating from aerospace technology, Arofly is the result of years of research and cooperation between aerospace scientists and sports biomechanics professors, developing a patented air pressure differential technology based on the pitot tube design," according to Arofly.

'Through core and patented algorithm and advanced calibration technology the precise pedalling power is registered and with that the cycling performance.'

Hmmm, okay. That doesn't reveal a lot. Like me, your first reaction is probably scepticism. How could it possibly work? Fitting strain gauges in a chainset spider, crank arm, pedal or hub: that makes sense. Fitting a power meter to an inner tube valve? It's a mental hurdle, to say the least. The big question is, does the Arofly provide useful power data?

We always review power meters by comparing the results from one system to those of others. It's difficult to say categorically that one is right and one is wrong, but we can look for discrepancies.

I've used the Arofly alongside a CycleOps PowerTap hub and a Stages crank-based system. The results I've got from the Arofly are very different from those I've obtained from either of the other two.

Take this ride as an example, where I've ridden a bike with the Arofly paired up to an iPhone and a PowerTap hub paired to a Stages Dash head unit. I've transferred the data from both devices to GoldenCheetah in order to compare the results. The power readings from the PowerTap are in green, the power readings from the Arofly are in violet.

What I did on this ride was warm up, then do four hill reps of about 5:50mins each, riding downhill and on the flat for around 3mins between each, then rode home. In other words, it was a structured workout with definite start/finish points for every section.

The first rep starts at 11mins, the second one at about 20mins, the third one at 29mins and the fourth one at 38mins.

The PowerTap data shows what I'd expect: four periods of power with very little effort as I went back to the start between each of them.

The Arofly data for the first rep is hardly distinguishable from the warm-up in the main, although there's one peak at the end of the interval. It does indicate that there was less power in the rest period between intervals, but with several peaks in there that I can't explain.

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The second interval is clearer, although I was aiming to ride it in a similar way to the first (and that was reflected in the PowerTap data).

The Arofly picks up the rest period after the second interval and then the work during the third interval, although the peaks and troughs don't track those from the PowerTap particularly closely. Then there's quite a big spike in the Arofly's data halfway between the third and fourth intervals. I really couldn't tell you what that's about because I wasn't pedalling at the time... And so on.

I rode the first two intervals seated, and stood up on the pedals at times during intervals three and four to see if that made a difference to the data. That seems to have resulted in higher peaks from the Arofly.

Even trying to be generous, it's difficult to reconcile the Arofly measurements with those of the PowerTap to any great degree. The power graph goes up during the work intervals (well, it does for intervals two, three and four, at least) and drops during the rest intervals, but this was a very defined session and you'd be hard pressed to discern that from the Arofly's data.

Conclusion

I'd love to be able to tell you that, against all odds, Arofly supplies power measurement that's good enough to train by, but my experience is that it doesn't. I've not managed to use it as the basis of training sessions because the figures are too inconsistent. Sorry.

The team behind Arofly says that development has been continuing over the past few months and that it jumped the gun in sending us a product back in March.

> Buyer's Guide: How to choose a cycling power meter

It says that any shortfalls in performance are down to the app, which is being improved, and will be addressed by the Arofly+ which is a rechargeable head unit that's designed to work with the valve-mounted device. The Arofly+ will be unveiled at the Eurobike cycle show in Germany in a couple of weeks.

The team says it has ploughed a huge amount of money into the development of the system and it really believes that it has a 'game changer' of a product here.

There will have to be big changes in the results before I could think of recommending this product.

Verdict

An affordable power meter, but we didn't find it to deliver useable data