History of the Power Meter
The first record of power output being measured on a cycle ergometer dates back to 1896 however it wasn’t until almost a 100 years later in the 1980’s and the invention of the SRM and LOOK Max One that cyclists and coaches gained the ability to easily measure power output on the road.
Since then the growth has been exponential with each subsequent decade seeing an increase in manufacturers, reduction in price and incredible rise in popularity in all levels and disciplines of cycling.
To unlock the secrets of the power meter a cyclist must first understand the basics of its recording and outputs, its relevance to performance and some of the tools for analysing the myriad of data it creates.
WHAT IS POWER?
As the name suggests a power meter measures “power” but what does this actually mean and how can it benefit you’re training and racing. From a scientific standpoint power is the product of speed and force
- Power (Watts) = Speed (m/s) x Force (Newtons)
WHAT’S A WATT?
A Watt is a measure of energy. You may have heard the term used in relation to light globes, a TV or boiling the kettle – this is the same as a watt that cyclist’s generate whilst pedalling on a bike. Scientifically it is the measure of the amount of energy required to move one newton one meter in one second
- 1 watt = 1 Newton x meters/ second
So when we measure power output for cycling what we are actually measuring is the combination of the force that is applied to each pedal stroke and the speed at which this force is applied. When cycling this is manipulated by cadence and gearing choice and it is the interplay of these two variables which allows a cyclist to attack or relax. Your power meter records this interplay and displays it as a wattage.
HOW IS THIS POWER MEASURED?
There are two types of power meter on the market: direct force and indirect force. This terminology relates to how the power output is calculated.
As the name suggests indirect force power meters (IFPM) do not directly measure the force being produced instead they utilize complex mathematical modelling and input from a variety of sensors including a digital accelerometer, speed and dynamic air pressure sensors.
The most well-known indirect force power meter is the IBike Newton. The initial setup of these power meters requires the rider to enter bike/ rider weight, tire size, road surface, rider height and ride position then these inputs are used to calculate CdA(coefficient of drag) and Crr(coefficient of rolling resistance). As you ride the computer automatically calculates and displays power output as you ride.
Numerous tests have shown indirect power meters to be highly accurate however due to the large number of input metrics and the variability of these a rider needs to make sure that they are accurately set up for correct calculation of power to occur.
All of the early power meters and most of the common brands used around the world are what’s known as direct force power meters (DFPM). These utilise a piece of technology known as a strain gauge which is commonly located in the hub, crank or pedals.
Direct force power meters rely on the measurement of two main components torque (rotational force) from the strain gauge and angular velocity.
These are then combined in the equations previously mentioned to directly measure power output. Accuracy of DFPM relies on 3 major components consistency of the strain gauge, translation of the strain gauge response to torque (zero offset) and good measurement of angular velocity. Inaccuracies or errors in any of these 3 areas can result in an inaccurate power calculation with the most likely area being an inaccurate translation of the strain gauge response. This is why it is important to perform a zero offset or calibration of your power meter at a regular interval.
As direct force power meters are the most commonly utilised we will be referring to these for the rest of the article unless otherwise specified.
To make matters a little more complicated we also have dual and left side only power meters this refers to whether data from both legs is being measured. Traditionally most units have measured both left and right side data however as companies have looked for a way to reduce costs left side only power meters have entered the market. To calculate total power output left side only power meters simply double the output recorded. The major downside of this is that it does not allow for pedalling dynamics to be recorded and may not necessarily be a true representation of a riders’ total power output.
Data transmission needs to occur to allow the user to access the information. Again this occurs in two main formats: ANT+ and Bluetooth, with the vast majority utilising ANT+ due to the popularity of Garmin head units. SRM and Pioneer units use a variation in ANT+ technology which requires the user to use a specific SRM or Pioneer head unit. Some companies including Stages utilise a combination of both Bluetooth and ANT+ to allow the user to decide on the head unit and transmission method.
The claimed accuracy of the most common brands of power meters is ±2% with some brands claiming accuracy within 1%.
Power meters come in a wide range of brands and the installation and placement on the bike also varies dramatically. In the next article of this series we will do an in depth review of the various power meters available and work out which one is most suited to your needs. For now, the table below looks at which brands are located where.
REAR WHEEL POWERTAP HUBS
- Crank spider – Quarq/ SRAM, Power2Max, SRM, PowerTap C1
- Crank arms – Rotor, Stages, Pioneer, 4iiii, WatTeam
- Pedals/Cleats – Garmin, PowerTap P1, Polar/Look combo, bePRO Look-only option, Xpedo, Brim Brothers
- Bottom Bracket/Axle – shton Instruments, Dyno Velo, ROTOR INpower
PUTTING WATTAGE IN PERSPECTIVE
The power electrical appliances use can also be measured in watts, so how does this compare to the power production of cyclists.
- Light Globe – 60w
- TV – 240w
- C Grade FTP (60 min) – 250w
- A Grade FTP (60 min) – 350w
- Chris Froome 2015 TDF Stage 10 (Peak 40 min) – 414w
- Toaster – 1100w
- Road Sprinter – 1200-1400w
- Microwave oven – 1500w
- Sir Chris Hoy (reported max power) >2300w
Check track sprinter Robert Fortemann’s effort to power a toaster on youtube.
UNDERSTANDING WATTAGE FOR CYCLING
The amount of power a cyclist can produce depends on a wide range of variables including, body size, training history, muscle fibre type and the time frame over which they are completing an effort.
There are two main ways that a cyclist can produce power
- High force produced slowly (low cadence)
- Low force produced quickly (high cadence)
We can see these different methods of achieving power in the pro peloton, Chris Froome and his high cadence spinning accelerations or Alberto Contador dancing on the pedals. To achieve really great power outputs cyclists need to be able to utilise both high force and high cadence
ranges simultaneously the best proponents of this are typically track sprinters. These guys regularly reach cadences of > 150rpm and can top out over 180rpm. In comparison a road sprinter may be 130rpm and a climber maybe anywhere from 80-110rpm for a prolonged period of time.
As the time frame over which an effort increases the amount of power that can be produced decreases the rate at which this decrease occurs is known as a rider’s power profile curve. This can be utilised to help identify the relative strength weaknesses of a rider. Look out for more details on the power profile curve and its implications will be more fully explained in the articles to come.
WATTAGE IMPLICATIONS FOR PERFORMANCE
As we have already discussed wattage is the way we measure cyclists input onto the bike. How this translates into speed is dependent on a number of other variables including aerodynamics, weight and rolling resistance. The example below looks at how 2 riders with different capabilities of producing power can complete a climb in the same time.
Picture this a cyclist has his local climb 5km with an average grade of 5%. He is currently 80kg and can tackle it in 16:30 minutes.At this stage he needs to produce 280watts to complete this climb. He has a goal to lose 5kg and gets down to 75kg.It subsequently only requires 265watts for him to complete the climb in the same time.
However, he has managed to lose weight and maintain his fitness as such for 280watts his climbing time is down to 15:48 minutes and he has whacked a 42 seconds off his time.
Recording and tracking your power can also be used to predict your performance on climbs prior to racing this can be incredibly powerful in calculating your pacing strategy on longer rides.
WHY TRAIN WITH POWER?
So far in this article we have examined how a power meter works but the question of why you would want to train with power may still remainunanswered. Training with power is the best purchase that you can make to improve your cycling. It will open your eyes to the way you ride and the effort level required to compete with the best.
I can distinctly think of six main ways in which a power meter can benefit you as a cyclist.
- Identifying strengths and weaknesses
- Pacing efforts
- Identifying the demands of racing
- Evaluating the effects of equipment and positional changes
- Tracking training trends
- Analysing racing and training
The data collected allows the identification and quantification of a riders’ strengths and weaknesses, it allows and Ironman triathlete to pace their ride and smash the run! We can use it to calculate the demands of a race or evaluate improvements gained from positional changes.
In my opinion however, the greatest benefits of a power meter are in prescribing, tracking and analysing a riders training and from a coaching perspective the power meter has completely changed the way that we look at training and racing.
In the next issue we will be exploring your power meter purchase. What are the pros and cons of different brands and how these suit riders in different situations.
(this article originally appeared in Bicycling Australia Magazine)