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How to choose a power meter

Powertap hub
Peter Stuart
12 Nov 2015

With a host of power meters on the market, Cyclist examines how one power meter can offer very different things to another.

Using a set of strain gauges to measure the power being pumped into a drivetrain was once the stuff of Tomorrow’s World. Even when the technology came into production it was a luxury confined to only the top professionals. But now, not only are power meters affordable to most riders, there’s a huge variety to choose from. Making that choice can be tricky though, because one power meter can be ideal for one rider but less than perfect for another, and there are several reasons for this.

First, power can be measured from different parts of a bike. One obvious place for measuring it is at the crank. ‘The crank is the place where power is being put into the bicycle, where torque is generated,’ says Dr Auriel Forrester, the UK distributor of the original power meter brand, SRM, and a cycling coach ( ‘It’s protected from damage, and we have space there for strain gauges and an internal battery.’ No surprise, then, that the crankset is where most power meters are located, albeit in slightly different places: the spider, the crank arms and the axle, for example. But power also travels through several other parts of the bike, so it can be measured at the pedals, the chainring and the rear hub, each having advantages and disadvantages.

SRM Crank

The crankset in itself is a pricey component, and with a power meter installed it can become prohibitively expensive for many. The SRM Campagnolo Super Record crankset costs a penny short of £3,350 with the latest head unit. There are also practical problems with several crank-based systems, which often require a specific groupset. Rotor, for example, produces a power meter that works only with its own components. Crank-based systems can also require a lot of work to switch between bikes, and typically carry a weight penalty.

PowerTap’s hub-based power meter was the first popular alternative to crank-based systems. ‘You don’t even need a tool to switch the PowerTap hub from one bike to another, plus it has easily replaceable batteries and communicates in ANT+ [neither of which the SRM offers],’ says Justin Heinkel, PowerTap’s product manager. But there are downsides. Not every brand produces PowerTap compatible wheels, and many people use different wheels for training and racing. 

Pedals are another viable location for power meters, yet while these are transferable between bikes and easy to install, they restrict the very personal choice of pedal and cleat system. However, price is the main factor that informs buying decisions, and thanks to the emergence of power meters that cost less than a mid-level wheel upgrade, their popularity has boomed.

Power to the people

In 2012, Stages launched a unit that weighed only 20g, and cost as little as £600, that used an algorithm to estimate the power in both legs based on the output of only the left. Other manufacturers, including Garmin, Rotor and Pioneer, soon followed suit. Cycling coach and former national champion Dave Lloyd sees this level of information as ample for most amateurs. ‘For me, affordability is the main thing,’ he says. ‘A lot of my athletes have power meters because they’re cheaper now. Not many of my athletes have £3,000 to spend on a training tool.’ Lloyd, like many coaches, sees power output as a useful metric once an athlete has got to terms with heart rate training. 

There is a but. Troy Hoskin, product manager at Quarq (Sram’s spider-based power meter), says, ‘Single-sided power meters are a good introduction to power, but there are inherent problems. All of the science, and the testing we’ve done ourselves, shows that cyclists commonly have 8-9% asymmetry, and that the asymmetry varies depending on your level of effort or fatigue.’ 

Stages crank

This is important because if an imbalance between legs changes, it could either mask or exaggerate a rider’s fatigue. ‘A single-sided option works well to give you better training feedback than you get from a heart rate monitor, or to reflect overall training load, but in terms of the precision you need for doing interval or threshold sessions, or even pacing a long race or time-trial, you should definitely be measuring both legs,’ says Hoskin.

PowerTap doesn’t offer products that only measure output on one side. ‘I think they give a false sense of accuracy,’ says Henkel. ‘I’ve seen plenty of power files, and a simple imbalance of 3% becomes 6% when you’re measuring from one side and doubling it. Now, 6% of 300 watts is nearly 20 watts, and if my power meter was off by 20 watts I’d be pretty upset.’ 

Unsurprisingly, Stages sees things differently. ‘Our data doesn’t support the argument that riders become significantly more imbalanced with fatigue,’ says marketing manager Matt Pacocha. ‘In riders we’ve found with slight imbalances, as they increase effort the balance comes together in a consistent manner.’

Measuring the total power output is very different to measuring the input from each leg individually, though. While most power meters measure overall power, only a handful can measure each leg independently and compare the two.

Left and right

The emergence of pedal-based power meters, which measure power output on each side using separate units, has addressed this for the first time. ‘It could be that your left leg is pulling up more and allowing the right to produce a higher peak,’ says Pacocha. ‘In that case, your weaker leg could be reported as the stronger leg. That means pedalling drills to strengthen the left leg could actually make the problem worse. These combined systems [that measure only the total power through the crank] just aren’t a viable balance measurement tool.’

Garmin Vector

The opportunity for right and left isolation may seem like a minor detail, but the balance between legs reveals a lot about pedalling form, and so elevates the power meter from being a mere recorder of output to offering a level of technical coaching.

There are those who dispute the significance of the balance between legs. ‘I’ve never met anybody who has a hugely problematic discrepancy,’ says Lloyd. ‘And there’s not much you can do to correct it. You can try leg exercises and one-legged drills but I think that just confuses matters.’ 

That’s not the end of the argument, though. ‘I think the jury is still out on that,’ counters Henkel. ‘The power absorbed when you’re dragging on the back stroke is definitely something you can improve.’ He goes so far as to suggest that power meters will play a big part in the development of bike fitting. ‘I think the fit business is going to benefit the most from the advanced metrics, as you can gauge pretty closely how somebody’s pedalling changes as their position changes on the bike.’

As with most component choices, there is no simple answer. But as Henkel says, ‘I don’t think there’s a perfect power meter out there – it’s just whatever suits your situation best.’

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