4iiii Precision Power Meter

The 4iiii Precision is a crank-based power meter that delivers sound, usable data, as long as you're happy with the limitations of a single-sided system.

The Precision system consists of a tiny pod that's bonded to a non-driveside crank arm. In that way it's similar to a Stages unit. I used the Shimano Dura-Ace version (they're all Shimano) and it weighed just 9g more than the crank I took off, and that includes the battery. A Shimano 105 version is £349, and a Shimano Ultegra version is £429.

> Find your nearest dealer here

You can also provide the crank yourself and have 4iiii install the pod.

The pod sticks out 8mm from the centre of the crank so the Precision is compatible with the vast majority of frames out there.

You just turn the battery cover with your fingers to remove it. That might not instil confidence that it'll stay put but it hasn't gone walkabout during testing or even looked like it might. The battery is good for over 100 hours of use and as it's a CR2032 3V you can get replacements all over the place.

The pod itself is waterproof – nothing got inside when I took a hose to it – and it's unlikely to take a knock thanks to its low profile and its positioning. I mean, it could, but the inner face of a crank is hardly the most exposed area of your bike.

Setting up

Setting up the 4iiii Precision is simple: you take off your existing crank, you put this one on, and that's yer lot. It transmits via both Bluetooth Smart and ANT+ so it'll talk to the vast majority of devices you're likely to want to use, including Garmin Edge computers and your mobile phone. I used it most often with a Garmin Edge 500 and Edge 510. Pairing is simple and you only need to do it once.

Unlike some power meters, the 4iiii Precision does require zeroing at the start of every ride. You need to point the crank down and hit the relevant button on your computer. 4iiii says that you should zero the system after any large temperature fluctuations too. I guess that could be an issue in a mountainous area, but for most UK rides that's unlikely to be a problem.

Okay, so getting started with the Precision 2.0 could hardly be easier; what about actually using it?

Well, it is a single-leg system. It measures your left leg only and then doubles it to give your total power. Is that a problem? Not if your power is equally distributed 50:50 between your legs, but most of us have some discrepancy, at least some of the time. When I use dual-sided systems they tend to say my power averages 50:50 between right and left most of the time, but sometimes it's 51:49, and occasionally it's 52:48.

Obviously, if your left leg is only putting out 48% of the power and the system is assuming it's putting out 50%, you're going to have a level of inaccuracy in the data.

Say you're putting out 300w. If that's split 52:48 between right and left, that would mean that 156 watts would be coming from your right leg and 144 watts from your left leg. If the 4iiii system measured your left leg at 144 watts and doubled it, it would give your total wattage at 288 watts rather than 300 watts.

I've been using the 4iiii Precision alongside Garmin Vector pedals and a PowerTap hub over the past few months. We've chosen the PowerTap G3 hub system as our benchmark because we can use it alongside most other power meters to get data we can compare. PowerTap is a proven, well-trusted system, and we need to have something as our standard. Rightly or wrongly, the PowerTap is it.

The average power figures I've got from the 4iiii system have usually been within 3% of the PowerTap's over a full ride, just occasionally larger than that, although that doesn't really tell you much. If a power meter over-measured peaks and under-measured troughs, for example, it might still give an accurate average.

In graphic detail…

It's more revealing to look at graphs of power measurement over the course of rides in more detail, and when you do that the 4iiii tracks the PowerTap much more closely than you might expect.

Take this hour-long ride, for example. I tried a bit of everything I could think of to mix it up and challenge the power readings: a 5min climb at the start, some pootling along, some little humps and bumps, some 10 second bursts... In terms of intensity, the ride was all over the place.

The purple line shows data from the PowerTap hub, the blue line is the 4iiii Precision power meter.

The average power figures for the whole ride are within 3% of one another, with the 4iiii figures tracking those of the PowerTap pretty closely in all situations. Smooth the chart (below) and it's easier to see that the PowerTap measures a little higher in most situations.

Let's look at a short section of the ride (below).

I did five 10-second intervals off a minute. For four of the five efforts, it's actually the 4iiii peak figures that are slightly higher than those of the PowerTap – we're talking about 440 watts versus 437 watts (a difference of 0.7%), and 504 watts versus 477 watts (a difference of about 5%), as a couple of examples.

The 4iiii's troughs tend to be a little lower in this section too.

However, elsewhere the PowerTap peaks are a little taller. It could be that minor left/right discrepancies in my power output are getting doubled by the 4iiii system or it could be something entirely different. It could be that the 4iiii system is measuring accurately and the PowerTap system is at fault, or perhaps a bit of both.

What we can say, though, is that the two different systems follow one another closely most of the time and that they match one another in terms of response to increased intensity. It's not that one lags behind the other.

Check out this 10min section (below) where the power figures range all over the place from under 100 watts to nearly 500 watts. The 4iiii troughs tend to be a little lower than those of the PowerTap but the peaks and the rate of increase/decrease in power (the slopes on the graph) are very similar indeed. They don't track one another perfectly, but they're close considering that one set of data comes from a hub and the other comes from a left-side-only crank.

It's quite easy to make the 4iiii data diverge from that of dual-sided (or hub-based) systems – you just pedal really hard with one leg and really easy with the other. Inevitably, the accuracy of a one-sided system that assumes parity is going to suffer in that situation. In the real world, an injury to one leg that means power output on that side is significantly diminished could affect the system's accuracy. That's pretty obvious.

However, my experience is that the 4iiii Precision offers sound data across a variety of situations with no obvious flaws – other than the single-sidedness discussed above. I've heard of other people having trouble with the ANT+ signal dropping out occasionally but I've not experienced that.

> How to choose a cycling power meter

Given the choice, we'd all rather have a dual-sided power meter – and 4iiii now offers the Precision Pro which is exactly that. Prices have yet to be finalised but they'll obviously be considerably higher. If you have the extra money, great, go for it. But if you don't want to spend that much, the left-sided Precision provides sound data from which to build your fitness/training.

Verdict

Solid power meter that delivers highly usable data, as long as you're satisfied with a left-leg-only system