Why cadence matters but not in the way that you might think

You’ll often hear us talk about cadence. It’s one of those terms we use a lot at Verve, but it’s not always the easiest thing to explain why it’s measured and how.

In the cycle racing world, cadence is a total rotation by both cranks. Cadence is measured by some power meters using magnets – a good solution since at least you know that the crank has passed a certain point in its rotation. However, most power meters use accelerometers to ascertain whether a crank has rotated, which leads to wild inaccuracies, particularly when large changes in crank speed occur or the bike is being ridden over surface irregularities.

In this slide from a recent webinar (available on our website) we described how a normal trainer views a 9 second sprint and entirely misses the highest power. It also gets the average cadence entirely wrong.

Now that’s a huge difference, which isn’t particularly helpful if you’re looking to make real improvements. Remember, cadence – along with torque –  is one of the key components of power, measured in watts. An error in either cadence or torque will inevitably lead to an error in the power number.

What is Cadence Image 1 Power Meter

So what does our latest innovation in power measurement, the InfoCrank IC2 do?

Firstly, we take the measurement of crank speed as seriously as we do in measuring torque. This matters because as noted above, the actual calculation for power is instantaneous torque X instantaneous crank speed – not cadence (which is just an averaging proxy).

To explain this, have a look at this series of pedal strokes – a standing start out of the seat into a sprint.

What is Cadence image 2 Power Meter

The first observation is that the stroke is not round but is more of a wave-form. We can see a definite top and bottom, which represents the highest and lowest torque sections of the pedal stroke.

What you don’t see but can easily imagine, is that the pedal stroke is not evenly pedalled. In fact, the crank is not rotating at a constant speed during its rotation. Therefore, there are such products as non-circular rings – where the idea is to rotate faster during the periods of low torque, often called blind spots, and rotate slower in the positions where the torque is higher.

Of course, only by measuring the crank speed during the entire revolution can an accurate rendering of power be always displayed. Much more information can be gleaned with this knowledge. For instance, you can work out whether you are best with round or non-circular rings.

The key learning here is that cadence is a hugely averaged value and in many circumstances, it is potentially a long way from accurate – such as the times when you would really like to know the speed of each pedal stroke and not an average.

Averages are not all that useful for improvement. However, crank speed measured many times per revolution on each crank is the measurement that is needed for the watts calculation. This gives the cyclist much more useful information and can definitely lead to pedalling improvement.

Which is exactly what you’d expect from the world’s most accurate power meter.

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