In this I will explain what power is all about. I should warn you in advance that it will get a bit technical at times. But by the end of this, you should be able to explain to your riding partners what your power meter is measuring and roughly how it does so. You should also be able to tell them how you’re using it to get fitter and faster.
The Basics of Power
Let’s start this discussion with the most common term used in power: the watt. On a power meter, we call the numbers displayed on the handlebar computer “watts.” Watts indicate how much energy you’re expending during the ride and, as you’ll see below, how fast you’re expending it.
The unit of power is named after James Watt (1736–1819), a Scottish inventor and mechanical engineer. He was a genius who is credited with discovering in 1765 how to make the steam engine more powerful and efficient (exactly what we are after for your bike performance), thus making the Industrial Revolution of the late 18th and early 19th centuries possible. He also developed the concept of horsepower and formulated the fundamental mathematics for power measurement.
Power in Physics
To help us get a broader understanding of power, let’s look at how physics describes it. After all, concepts about power arose in physics, and power—your individual power output, specifically—is the key to improving your cycling performance. Some of what follows may be a bit difficult to grasp on the first read, but hang in there with me as you think your way through this. In the end you’ll have a deeper appreciation for what’s going on when you ride your bike with a power meter. I’ll try to make my explanation as simple and painless as possible. Here we go.
The watt is a measure of power determined by calculating the rate (think “time”) at which work is done. By “work” I don’t mean your job but rather the physical act of moving something, such as standing up while doing a squat exercise with a heavy barbell on your shoulders in the weight room. That’s work. Work doesn’t care whether you stand up on the squat fast or slow; you moved the weight from here to there, so the amount of work remains the same no matter how long you take to do it.
Power itself is how much work you are doing and how fast you are doing it. The faster you stand up (shortening the time of the movement), the more power you are producing on the squat. Physicists express this relationship of power, work, and time as a formula:
P = W/t
This simply means power equals work divided by time. Let’s get a better understanding of work. Work is the result of an outside force (for example, your legs straightening while standing up during the squat) moving an object (the barbell) through a distance (from the low squat position to the fully standing position). So based on this we can say that work is force multiplied by distance. As a formula it looks like this:
W = F × d
Now, knowing what work is, if we go back to the first formula (P = W/t) and substitute force times distance (F × d) for work (W), since they mean the same thing, we get another way of expressing power:
P = F × d/t
This formula says that power results from force (you lifting the barbell) multiplied by distance (how far you moved the barbell) divided by time (how long it took you to stand up). For example, if you add more weight to the barbell and do another squat standing up (F) to the same height as before (d) and in the same amount of time as before (t), you’ve increased the power generated. That’s because force was increased. Or you could keep the weight on the barbell the same as before and stand up (F) to the same height (d), only faster (t). That would also increase the power because time decreased. Thus, power results from the interplay of force, distance, and time.
Still with me? If so, let’s see if we can simplify this even more. You know that distance divided by time is called “velocity,” which is what we more commonly call “speed.” In your car you talk about speed as miles per hour. That’s distance (miles) divided by (per) hours (time). So if velocity (v) is the same thing as distance divided by time (d/t), we can substitute v for d/t in the last formula, giving us an even simpler way of expressing power, especially when it comes to riding a bike:
P = F × v
This is the final formula: Power equals force times velocity. It’s where I wanted to take you with this somewhat roundabout way of understanding power from the perspective of physics. In bike riding, force and velocity are easier to understand than work divided by time, which is where we started. On a bike, force is what you put into the pedals and velocity is how fast you are turning the pedals. So now that we have the hard part out of the way, let’s move on to how power is produced through the interplay of force and velocity when you ride a bike.
Power in Cycling
When you are pedaling a bicycle, force and velocity are always present and determine how much power you are creating. As you push down on the pedal, you’re applying a force (F). The harder you push, the more force you are applying and therefore the greater is the power you produce. (In physics this turning force applied to the rotating pedals is called “torque.” That’s not too important for your understanding of power, but you may run across this word in your power meter software.)
When you are pedaling your bike, the rotating-pedal equivalent of velocity is called “revolutions per minute,” or “RPM.” That’s a term with which you’re already familiar. You may also call it “cadence.” As RPM, or cadence, increases—in other words, as you pedal faster—power is potentially increasing. I said potentially because the increase depends on whether you changed gears or not. Pedaling at a higher cadence in the same gear produces more watts because pedal velocity (v) has increased.
All of this means that in order to raise your power while riding, you can either increase the force (F) you apply to the pedals or you can increase the cadence (v). In the real world of riding a bike, the way to increase the force is to shift to a higher gear and keep the cadence the same. For example, you could shift from 53×17 to 53×16 while maintaining your cadence. Force will have to increase for this to happen (you’ll have to pedal harder). That will increase your power output, which in turn will increase your bike’s speed. Or you could keep the force the same by not shifting gears and instead increase your cadence by turning the pedals faster—for example, by going from 85 RPM to 90 RPM. This decreases the time it takes to do the work, thus increasing power.
Adapted from The Power Meter Handbook by Joe Friel, with permission of VeloPress.
The Power Meter Handbook