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How advanced does your power meter need to be?

Peter Stuart
27 Feb 2017

With so many power meters on the market, we investigate just how advanced they need to be to give us the numbers we actually want

The invention of the bike computer gave us everything we needed to train efficiently. We could measure speed, cadence, heart rate and even altitude gain all at once. It was ample data to track our progress on the bike.

Then it all changed. In 1986 the SRM arrived – it measured mechanical power being applied to the chainset with the use of multiple strain gauges, answering many of the great unanswerables in coaching and training.

Since those early days, the sort of serious data that a power meter provides has gone from being the preserve of professional coaches with expensive software to something that simply pops up on Strava or Garmin Connect, begging to be over-analysed.

Today there are dozens of products on the market where once there were only a few options. Choosing one, though, has never been harder.

One-sided perspective

If accuracy and time-tested credibility are the main considerations, perhaps the search starts and ends with the initiator of the power game.

‘The likes of Garmin and Stages are now on their second iteration,’ says Dr Auriel Forrester, SRM’s UK distributor and cycling coach (scientific-coaching.com).

‘For us, the power meter has just gone through its seventh build, while the head unit is now on its eighth.’

But the competition has one thing that SRM has historically lacked: affordability. At well over £2,000, the SRM system has always been limited to professional athletes or very serious (and rich) amateurs.

The new arrivals have done an exceptional job of cutting costs, but there are those who debate whether a cheaper power meter compromises the data on offer.

‘I don’t think there’s any one perfect power meter,’ says Hunter Allen, co-author of Training And Racing With A Power Meter.

‘If you have a limited budget, you’re probably looking at options that take power from one leg rather than both – something like Stages, Rotor’s LT Power or Garmin’s Vector S.’ 

That’s because the most obvious way to cut costs is by conducting only half the measurement, using either a single crank or pedal. These power meters come in at close to £500 – something unthinkable just a few years ago.

But this approach brings with it the most obvious of compromises, as the power from the other leg isn’t measured at all. 

‘One-leg systems give a false sense of accuracy,’ says Justin Henkel, product manager at PowerTap.

‘A simple imbalance of 3% becomes 6% when you’re measuring from one side and doubling it. And 6% of 300 watts is nearly 20 watts. I’d be pretty upset if my power meter was off by 6%.’ 

It’s an issue that divides the market. ‘In the most basic sense of training with power, the most important aspect is consistency,’ argues Matt Pacocha, marketing manager at Stages Cycling.

He suggests most riders have close to a 50/50 split in power, and that small inaccuracies are unimportant as long as overall improvements or losses in power are consistent.

‘In the riders we’ve found with slight imbalances, we’ve recorded an imbalance at lower effort and generally as they increase effort the balance comes together in a consistent manner,’ he says.

Not everyone sees it the same way. Allen’s experience of training riders has led him to the conclusion that single-sided power meters give false results: ‘It’s correct half the time, because it’s only your left leg, but there’s a lot of stuff that happens with the other leg, believe me.’ 

In the course of his coaching and power analysis he has found that one leg can fluctuate in power in a very different way to the other.

‘Most people have a “lazy leg”,’ he says. ‘When riding along at recovery or endurance pace there’s one leg that doesn’t do as much work as the other. It’s subconscious – it just happens.

'As you approach your FTP [functional threshold power] that lazy leg starts to add more and more to the gross power and the balance moves towards 50/50 because that lazy leg is coming into it, but at maximum effort it can swing back to 47/53 or worse as the dominant leg takes over again.’

For many amateur riders, this imbalance will matter little, and an inexpensive single-sided system such as Stages will adequately suit their needs.

For those in serious training, however, it’s important to assess not just effort but technique too, and to do that you need a system that can measure the power from each leg independently.

A balanced argument

‘I’m a huge fan of power meters that measure left and right independently,’ says Allen.

‘I’ve been doing a lot of research, looking at right and left data, and there’s a lot to be learned.’ 

Despite the claims of many power meter companies, there are actually very few that can measure the two legs separately.

To isolate the right and left leg truly, you effectively need two power meters in a single system, one to measure the output of each leg.

When the strain gauges of a power meter are placed at the crank spider, hub or chainring it is difficult to isolate the forces each leg is applying individually.

These systems largely generate a balance figure by separating the power in the first 180° of the crank rotation from the power in the second 180° of the rotation, and calculating a consequent balance between the two.

That’s a fairly accurate measure, but does not fully consider that a rider may be exerting force on the upstroke. 

‘A combined left/right measurement – such as through SRM, Quarq, P2Max and others – can’t tell you why a peak in power associated with a left or right leg is occurring,’ says Pacocha.

‘It’s possible that they tell you your right leg power is 2% higher than your left, but you won’t know why. It could be that your left leg is pulling up more and causing the right to produce a higher peak.’

Allen says, ‘The true left and right meters now, as far as I know, are the Garmin pedals, the Powertap pedals, the Infocrank and also the Pioneer crank.’

To that list you could add Rotor’s new dual-sided system 2InPower, which has a strain gauge on the bottom bracket and the crank, which the brand claims can isolate both sides of the drive.

The advantage of these systems is the data they offer for analysis purposes. They can define exactly how much power is being lost by exerting power in the wrong direction – pressing down as the pedal moves back up.

‘I’ve seen people resisting the positive forces of the downstroke with negative forces on the upstroke by as much as 45 watts. That’s massive!’ Allen says. ‘People who are really effective in their pedalling stroke absorb 8-10 watts of power on each upstroke. These folks are among the best. The norm is between 10 and 15 watts.’

If we’re absorbing all this power, it’s surprising that even two-sided systems don’t display pedal drag as standard (save for Pioneer’s). Instead, they usually require third-party software such as Allen’s WKO4 to present this metric.

Knowing this information could give riders an insight into how efficient their pedalling stroke is. What it can’t do, some argue, is teach a rider how to pedal better.

‘All of the science we’ve seen so far seems to say that for pedalling efficiency your preferred pedalling style is going to be the best way for you to pedal a bicycle,’ says Troy Hoskin of Quarq.

That doesn’t mean the efficiency data from dual-sided power meters is worthless, however. It can still play an important role in bike fitting.

‘You can gauge pretty closely how somebody’s pedalling changes as their position on the bike changes, so the fit market is actually going to benefit a lot from the new metrics,’ says PowerTap’s Henkel.

Does accuracy matter?

‘That’s the age-old question,’ says Allen. Accuracy, the argument goes, isn’t really a factor if you always use the same power meter.

‘Consistency is the key. It doesn’t really matter if it’s off by 30 watts, so long as it’s consistently off – you’ll still train to make gains on that figure, and see when training is and isn’t working,’ Allen argues.

But he admits there’s an issue with always reading the wrong number: ‘Mentally it’s very challenging. Let’s say you’re a category 1 racer and you are being told your FTP is 250 watts, and your power meter is off by 50 watts – you’re going to think, “Man, I suck, and I can’t stay with these guys.” Even if it’s just that your buddies ride at 270 and you think you only ride at 250, you’re going to feel deflated.’

If accuracy is your thing, SRM claims it still has the upper hand. ‘When people talk about a power meter being plus or minus 1% or 2%, the standard they’re comparing it to is an SRM,’ says Forrester. 

Verve has actually claimed greater accuracy with the InfoCrank, but it still has to stand the test of time and gain the WorldTour seal of approval.

Almost all brands claim a margin of error of below 2%, with inconsistencies coming down to quality control from one unit to the next.

Here, it’s the pedal and dual systems that suffer the most, as having two separate power meters increases the risk of error.

However, these inaccuracies pale in comparison to the losses from not using a power meter properly – specifically the art of manual zeroing.

This is the process of resetting the torque sensors on each ride to adjust for temperature and pressure – a necessity unless you have a system that does it automatically.

‘This is extremely important as temperature and pressure differences can easily throw the readings off by 30 watts,’ Allen says.

So no matter how accurate the power meter, it’s only as good as its last manual zero, meaning that making sure a system has a straightforward manual zero process could be worth more than a 100% accuracy claim.

Perhaps that’s the issue at the heart of the search for a power meter. The systems are so advanced that both the software and consumers themselves need to catch up to make the data useful to the rider.

‘Our hope and our goal is to make people understand and appreciate the value of that data,’ says Andrew Silver, product manager at Garmin. While Garmin Connect and a host of other software is extremely advanced, it speaks in a language lost on most cyclists.

‘We’re working with many third parties to look at how better to represent that data to the consumer.’ 

Allen, similarly, thinks other data about a rider will need to catch up with what power meters can offer. ‘We’re increasingly going to see other sensors being integrated – sensors for breathing rate, ventilation rate, metabolic rate, and what’s actually happening with the foot in the pedalling stroke. 

'A sensor in the shoes, sensors in the shorts that show you what your knee is doing. We’re going to see more digging deeper into the body side. That’s the future.’

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