WATTS IT ALL ABOUT?
The real-world why, what and how to get maximum gains from using a power meter
The whys and wherefores of training with power meters, plus a round up of six of the best crank-based meters.
Pro riders have them fitted to their bikes as standard, coaches love them and forum braggers are obsessed by them. But what are power meters and why are the watts they measure so important? More importantly, what’s the best way to use them to get fitter and faster?
Watt are we talking about?
According to Wikipedia “The watt (symbol: W) is a unit of power. In the International System of Units (SI) it’s defined as a derived unit of one joule per second and is used to quantify the rate of energy transfer.” To most cyclists, a watt is the best way to measure how much power you can produce to propel your bike.
While a watt is a time-specific measurement, it can also be used over different time spans for different purposes. Peak/maximum wattage is useful for sprinters, and some training apps can sample wattage across 10-, 30- and 60-second or two- and five-minute periods to produce a power graph of your workout. Most GPS-based cycling apps ‘smooth’ out your data by averaging your wattage over a 20-30 second range so your numbers aren’t jumping all over the place, either on your bike’s display or the graphs produced post-ride.
Of all the numbers a power meter produces it’s FTP (Functional Threshold Power) that most riders are keen to improve. That’s because it’s a good gauge of power/fitness. It’s also the figure that most training programs and online group-ride ‘games’, such as Zwift, are based upon.
Why Watts?
The best thing about wattage is that it’s a pure measurement that isn’t influenced by anything else. Whatever bike you are riding, whatever the weather is doing, if you’re warmed up or worn down, a power meter just doesn’t care. Provided your chosen device is correctly calibrated, the number that appears on your readout is how hard you’re pushing the cranks at that particular moment. It’s indisputable data. That makes it brilliant for measuring every minute of your ride, tracking your training progress and setting targets in a way that other metrics simply can’t match.
It takes a while for your heart rate or RPE (Rate of Perceived Exertion) to catch up and then stabilise around exertion levels, which means they’re not always reliable at the start of your workout. Complicating matters further is that certain factors can cause your heart rate to stay high or even increase while your power output drops out of the useful training zone. As Craig Stevenson, head coach with vitfor.com, explains: “If you’re not using power for intervals it’s really easy to go harder than you ought to for the first three or four reps until your heart rate or RPE catches up. [Then] because you’ve gone too hard too soon, your next few reps probably won’t be hard enough and the final few will just be useless from a training point of view. In other words, you will have totally missed the training zones that you need to target for physiological adaption and ended up killing yourself for very limited gain.”
Other metrics, such as speed or time over Strava segments, may serve as a rough guide although, obviously, they’re entirely at the mercy of everything from weather and road conditions to your choice of clothing.
Why not Watts?
Perhaps confusingly, the best thing about watts (that they’re a pure measurement that isn’t influenced by anything else) is also the worst thing about them. If you dive down the wattage rabbit hole without considering other factors such as your heart rate, diet and sleep patterns, you can make yourself really ill, really quickly. Training purely to increase your FTP is only useful if you ride events such as time trials or track pursuit at a fixed wattage. That’s why, as with any training plan, you need to make sure one built upon power data is geared towards the improvements you want to make and fits in with the lifestyle you lead.
“Cross-referencing power against heart rate and RPE is better, as only having three metrics to compare can give you a full understanding of what is going on,” says Stevenson. “For example, Charlie Tanfield – one of the riders I coach who went to the 2018 Track World Championships to race the team and individual pursuit – has only been doing about 13 hours of training a week because any more than that would mean he wouldn’t hit the power numbers required to stimulate adaptation.
Maintaining quality of efforts and hitting the right numbers is hard to do without power and is one of the most important factors for improving. Charlie’s now the second fastest British individual pursuit rider (only Chris Boardman went faster in the now banned ‘Superman’ position).”
As effective as it is, power is also the most expensive data to gather in terms of the technology needed to measure it, which comes in a number of different forms.
Meet the meters
As you can tell from their prices, power meters are complex, problematic devices to design and make. The strain gauges at the heart of them have become a lot smaller, more accurate and more energy efficient but ensuring they can handle the rigours of real-world riding while still delivering consistent readings can be a struggle. That’s why it’s important to pick a good, reliable model. You can find useful information to help you do that in the mini reviews here and the more extensive ones online at bikeradar.com. Before you pick the power meter that’s best, you need to know the pros and cons of each different method of measuring power.
Hubs
Building the power meter into the rear wheel’s hub makes it easily transferable between bikes, presuming they share the same axle standard, and generally works out to be good value, at around £500. Fewer variables for the strain gauge to cope with mean hub-mounted power meters tend to be very accurate and reliable too, although your output will change depending on which bike it’s being used in and the condition of the chain, cassette and chainrings. The disadvantages of hubbased meters are a lack of left-/right-leg power splits and the need to decide whether to fit the hub to your training or race wheels or pay to double up.
Stationary trainers
With basic, speed-calibrated wattage readings available on Tacx’s Smart range of turbo trainers, which start at £280, and ‘proper’ strain-gauge readings appearing on other manufacturers’ models from £350, a stationary trainer is the cheapest way to add a power
meter to your training arsenal. Accuracy on cheaper trainers is generally calibrated less precisely and they rarely give you the option of left-/right-leg power splits. Plus, you only get power data from training inside, with no way to collect or compare data from ‘proper’ rides outdoors.
Pedals
Power-measuring pedals seem like the ultimate easy-switching option if you want consistent power monitoring across your different bikes and, unsurprisingly, they provide the most accurate left-/right-leg balance by far. The latest designs are a lot more user friendly and consistent in terms of readings than older-generation pedals. They still have some issues though and at £850 for a pair of Garmin Vector 3 and £1000 for PowerTap’s P1 pedals they are an expensive option.
Cranks
Cranks are the most common form of power meter for several reasons. They’re generally reliable and accurate, and they position the extra mass of the meter low on the bike. While cranks can’t be swapped between bikes as easily as pedals or a wheel, you can move them between bikes that share the same bottom bracket standard. Pricing varies between around £500 for basic, single-crank setups to £2000 for THM SRM carbon setups, but there are lots of options to suit different budgets. We’ve tested six of the most cost-effective for this grouptest.
What to do with your Watts
Choosing a device to measure your power is only half the battle. What should you do with that information once you start collecting it?
As wattage-based training is all calibrated on your personal power levels, the first thing you need to do is to find out what they are. Obviously some tests will be less applicable to your riding goals than others but we’d still suggest doing a full battery of them to get a more complete picture of your overall baseline before you start following a training plan. Before doing any high-exertion efforts check your general health with your GP and while self-testing is easiest, getting tested by a qualified coach will give you a greater insight
into the numbers you are generating and what they mean. Do each test on a different day to avoid fatigue affecting your results and be sure to repeat the same test protocol each time so your data is consistent and can be used to gauge your progress.
Once you’re all set up and you’ve got the all-clear from the doc, there are three main areas to find your numbers for:
1. Sprint (or maximal) power
Simply the highest maximum wattage you can generate in a 6-10-second sprint from a standing start.
2. VO max
A true VO max reading can only really be done in a lab with proper respiratory test equipment, but the average wattage for a flat-out three-minute effort will be close enough for most purposes.
3. Functional threshold power
Also known as FTP, this number is the maximum power output you can hold for an hour of riding. FTP tests are generally done as a 20-minute max effort for convenience, or a 30-minute block where only the last 20 minutes are measured (on the basis that your heart rate will have stabilised after the first 10 minutes). This figure is then divided by 0.95 to predict your hour output.