Watteam PowerBeat

If you already have one of a fairly large range of popular cranks, a Watteam PowerBeat kit turns them into an accurate, consistent double-sided power meter for a very sensible price, and the process of fitting them isn't nearly as scary as it sounds.

• Pros: Accuracy, value for money, surprisingly easy to assemble
• Cons: You have to glue it to a crank yourself, works with limited range of crank models, some vulnerability concerns

A double-sided power meter for under £400? What's the catch? You have to assemble it yourself, by glueing the strain gauges to the cranks yourself, that's what.

> Buy this online here

On the face of it, it's a crazy idea. Would you gamble almost half a grand on your mechanical skills and dexterity? We're all comfortable bolting lights and other gubbins to our bikes, but there's something scary about an assembly process that involves two-part epoxy resin adhesive.

If you've ever used Araldite then you're familiar with epoxy resin. You know that using the fast-setting version requires you to get a bit of a move on, and that once it's set, it's damn near impossible to shift without damaging the parts it joins.

I'll look at this aspect in more detail, but the executive summary is that Watteam has done an excellent job of de-skilling the process and if you follow the instructions carefully you'll be fine.

What's in the box?

The heart of the PowerBeat is a pair of sensors and transmitters (Watteam calls them 'comp units') that mount on your cranks, measure the strain on them, work out your power and transmit it to whatever device you're using to record and display power.

Along with them you get a connector to charge the transmitters via any USB charger and a micro-USB cable to add to the 3,762 you already have; two empty water bags that you fill to make 4.5kg weights for the calibration phase, with hooks to hang them on your pedals; protective gloves; plastic rulers to position the sensors; a pencil; sandpaper; a sachet of epoxy; glueing stick; alcohol pads for cleaning surfaces to be glued; sticky rubber strips to keep cables tidy; rubber bands to hold the sensors on while the glue cures; and washers that go between the comp unit mounts and the crank.

Watteam literally supplies everything you need aside from eye protection and a bowl of water to dampen the sandpaper.

Well, everything but a pair of cranks. PowerBeat is compatible with most recent hollow aluminium Shimano cranks from 105 up, Cannondale Hollowgram cranks and Campagnolo Potenza cranks. A sensor for carbon fibre cranks is in development.

UK Shimano importer Madison Cycles kindly donated a pair of Shimano Ultegra 6800 cranks to the project as I didn't have anything suitable to hand.

You can pick up a pair of Shimano 105 cranks for as little as £71, and a set of Ultegras will cost you about £130. Add on the £375 for the PowerBeat and you can have a double-sided crank power meter that's cheaper than anything with similar features. The closest rival, FSA's Powerbox Alloy chainset (RRP £599, but available for under £500), doesn't have Bluetooth capability or left/right data out of the box, and FSA says those features will be an in-app purchase.

Putting it together

As I was saying, I was nervous about assembling the PowerBeat to my cranks. Turns out it's really not terribly hard as long as you follow the instructions.

There's an app for iPhone and Android that you'll need to calibrate the meter, and through which you can watch the videos that show you how to fit the parts to your cranks. Use the app from the start; we'll get to why in due course.

The process goes like this. You use the supplied rulers and pencil to mark the spot on the cranks where the sensor mounts, then sand and clean the area to prepare the crank for glueing. Then you slide the mounting bands on to the cranks. You open and mix the sachet of epoxy glue, clean the crank again, and the sensor, then coat the sensor mounting surface with a thin layer of glue.

The sensors are small, but come in handles that make it much easier to grab and position them, and have grooves for the rubber bands that hold everything in place while the epoxy cures. You position the sensors on the prepared crank surface and put the bands in place to hold them. Watteam has very clear instructions for how the sensors must be positioned, and you've got a few minutes to tweak before the glue starts to set.

Once you're happy with the positioning you then wait for the glue to cure. This takes 24 hours at temperatures above 25°C, and 48 hours at 10-25°C. I put my cranks in the airing cupboard to speed things up.

I said before that you should follow the instructions, but I bent the rules in one way. Watteam says your cranks should be on your bike while you fit the sensors. I had a set of new cranks, so I figured it would be much easier to work at my dining table than with the bike in a workstand or turbo trainer.

I shot Watteam support an email asking if this was okay, and while emphasising that they recommend having the cranks on a bike, they said it was. That recommendation, I think, comes from the instructions and videos all assuming your cranks are on the bike. If you can get your head around aligning the cranks correctly, then you can work at a table with coffee and laptop for videos easily to hand. The email response came within an hour too; impressive.

Once the glue is cured, then you fit the comp units (transmitters) to your cranks, and plug in the sensors.

If you haven't been using the PowerBeat app with its how-to videos then the time to fire up the app is straight after you've glued the sensors in place. You need the app to calibrate the devices, and it won't let you do that until 24 hours have passed from you telling it you've glued the sensors in place, which is why you're best using it from the outset. Watteam can remotely change the clock on the app for you if you forget.

Next, you fit the cranks to your bike if they're not already mounted, and fill the bags with water so you have a pair of 4.5kg weights.

> Six reasons why you should use a power meter

Watteam suggests enlisting a friend to hold your bike steady for the calibration process, as it needs to be upside-down so the water bags clear the floor. I happened to be home alone so I clamped the rear tyre in a workstand to hold the bike in place.

The app guides you through the calibration process, which involves precisely positioning the cranks at 90-degree intervals. It took me three attempts to get it right; Watteam's support folks reckon I didn't have one of the comp units fully plugged into the sensor the first two times.

With the PowerBeat calibrated, I was done. I had a double-sided, ANT+ and Bluetooth-capable crank power meter that had started life as a pair of cranks, a kit of parts and a tube of glue, and I was feeling very pleased with myself.

Measurement and accuracy

Getting the PowerBeat talking to my phone or Garmin units was easy. You just spin the cranks to wake them up, indicated by flashing lights, then use the PowerBeat app to set the zero offset. Remember to have the left crank pointing at the floor when you do the zero offset.

I got data from the PowerBeat that tracked the programmed resistance of my CycleOps Hammer fairly closely, with no peculiar spikes or drop-outs. As you can see from the graphs, the power readings from the PowerBeat cranks also correspond fairly closely to those from PowerTap P1S pedals. I got a similar degree of correlation out on the open road too.

In short, the PowerBeat works well, and certainly well enough for my distinctly amateur training needs. The advantage of a power meter over the other main method of measuring effort, a heart rate monitor, is that it far more accurately reflects the work you're doing. For example, for almost every ride I did, the PowerBeat's measurement of the calories I'd burned was lower than the estimate from heart rate measurement. If one of your fitness objectives is to lose weight, that's very useful information.

Swapping between bikes

I find swapping a crank power meter between bikes is easier than swapping a pedal-based meter. Of course there's a catch: this is only true if both bikes have the same type of bottom bracket. Removing and fitting Shimano cranks requires just a 5mm Allen key (preferably on a torque wrench) and Shimano's nifty little plastic tool for the left crank preload.

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

It's close to idiot-proof. All the threads are right-handed, you don't have to worry about skinning your knuckles if a pedal spanner slips and because you're never fitting metal threads together, there's no chance of stripping a thread catastrophically.

Since all my bikes have Shimano bottom brackets, swapping the PowerBeat cranks was a simple matter of taking them off one bike and sliding the bottom bracket axle into another's bearings, then tightening everything up. A quick zero offset and I was good to go.

Durability

An obvious concern is that the sensors and transmitters are outside the crank, exposed to the elements and potentially to damage. I haven't had any problems and after a couple of months' use, my PowerBeats are still going strong, but we have had one report of the sensors coming unglued.

Watteam's use of rechargeable batteries is both a blessing and a curse. On the upside there's no need to ever replace them, but on the downside recharging involves unplugging the sensor from the comp unit and the plug and cable aren't exactly military spec. They're fine if you're gentle, and they don't take much force to plug or unplug, but it's still a vulnerability. A better solution might be a clip-on connection like Garmin uses for its fitness trackers or a magnetic connector as used by Tractive on GPS dog trackers.

Any proprietary connector has the problem that you're stuffed if you mislay it. It took me half an hour of rummaging to find the safe place I'd stashed the PowerBeat's connector last time it needed charging. Yes, I'm an idiot.

The comp units could in theory get damaged in a crash, but they're sheltered by the pedals and cranks, so it would have to be a freak impact. Likewise for damage from mishandling, though you'll want to remove the comp units if you pack the bike for a flight. You'll have to do that anyway as they're held on by the pedals.

Conclusion

The Watteam PowerBeat is accurate and consistent enough to be a useful training aid, and it's incredibly cheap for what you end up with. Start with Shimano 105 cranks and you have a double-sided power meter for well under £500. If you already have suitable cranks, then for £375 it's a no-brainer. While the number of compatible crank models is fairly small, they're the cranks you'll find on a hell of a lot of bikes from about £1,000 and up of the last couple of years. If I were directing efforts at Watteam I'd try to extend compatibility to Shimano's Tiagra cranks, the Praxis cranks Specialized is very fond of, and those bike manufacturer favourites, FSA's Gossamer cranks.

If £375 is still beyond your budget, Watteam offers a left-hand-only version, the PowerBeat Single, for £255. That's just over half the price of a PowerTap P1S pedal, a Stages crank or a Garmin Vector 3 Single. Bargain.

Verdict

Amazing value for money power meter; some assembly required