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Why are my power meters reading differently?

We frequently get questions from customers who want to know why one power meter is reading a few watts higher or lower than another. This is always a tricky question to answer as there are several factors that can be associated with the difference in wattages. So, let’s dive in! 

 

Manufacturer Difference

The first is the manufacturer difference. Each company uses a proprietary formula to get to final wattage numbers. All are a combination of torque x cadence x a constant value, which is sometimes referred to as “power meter slope”.

 

Power Meter Type

The next possibility for differences is the power meter type, or where wattage is being measured. For example, when using pedal-based power, you will see head units ask for crankarm length. The length of the lever arm is critical because of where the torque is being applied. Spider and crank-based power meters usually do not require this parameter. 

 

Drivetrain Loss

The more complicated examples tend to be when we compare stationary trainers. Since the wattage is taken at the base of the cassette freehub body, the flex and drag of all the components leading to this matter a great deal. This is referred to as drivetrain loss. This hysteresis of chainring flex, drag of each roller and chain plate, drag of chain on cassette teeth and flex of the freehub body add up markedly.

 

Left vs. Dual-Sided

In the standard cost/benefit analysis, power meters run true to form.  Many lower cost meters will derive power from the left leg only and yield one power number on your head unit. They do this by taking your left leg power and multiplying by 2 in order to estimate your total power (both legs). However, by doubling your left leg power, these power meters assume that you are producing the same power from both legs (50/50 power balance). However, studies have shown that perfect 50/50 power distribution is somewhat rare. Most of us have an imbalance – some larger than others.

Therefore, if you left leg produces less than 50% of your power, your left-sided power meter will take that number, double it, and will therefore be underpotting your true power. The opposite is the case if your left leg produces more than 50% of your power. So as you can see, power comparisons with a left only power meter with one that natually records power from both legs…can be problematic.

Although potentially less accurate, these lower cost left side power meters should not be misconstrued as lower quality as many manufacturers offer both left only and two-sided options. However, the two-sided power meters will give you more detailed and accurate power numbers. 

 

Head Unit

Your head unit or App may also yield slightly different readings from others.  This usually comes down to how “zeros” are recorded.  Depending on sampling time, a head unit may pick up coasting at the exact second or not.  This is usually not an enormous consideration and can be smoothed by using an average power in your data fields versus an instantaneous snapshot.

Dave Harris

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