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Total vs. Left-Only Power Meters

With an ever-growing array of power meters to choose from, there is a lot to consider when trying to determine which power meter is best for you. However, perhaps the biggest difference in power meters, and the subject of this article, is in regards to how they measure power. Some power meters measure your total power. Others, called left-only power meters, do things a bit differently. Determining which one is right for you is going to be one of the biggest decisions you’ll need to make when shopping for a new power meter. So let’s dive in and see what all the fuss is about.

Left-Only Power Meters

 

Total vs. Left-Only Power Meters

Total Power Measurement

Originally, all power meters measured a rider’s total power – that is they naturally considered the force or torque applied by both the left and right leg. For example, if at a specific point in time, your left leg was generating 150 watts while your right leg 140 watts (exact 50/50 power distribution is rare – more on this to come), the power meter would measure all 290 watts. Simple right.

As an example, take an SRM crank-based power meter, the very first power meter ever produced. SRM measures power at the crank spider. Forces generated by both legs pass through the crank, so a crank-based power meter such as the SRM measures total power.

Let’s consider PowerTap’s popular hub-based power meter, the G3, as another example. Like the crank, all of a rider’s power passes through the hub. Left leg, right leg, it doesn’t matter – the hub measures your total power.

 

Left-Only Power Meters

In the last several years, new power meters have been introduced that do not measure power from both legs. Rather, they measure left leg power only. These left-only power meters take your left leg power and double it, in order to derive your total power. The two most common types of left-only power meters are crank arm-based and pedal-based power meters. With a left-only crank arm-based power meter, the user simply swaps their current left side crank arm for a new, power equipped on. With left-only pedals, you buy a new set of pedals with only the left pedal having a power sensor.

To be clear, not all crank arm and pedal-based power meters are left-only. You can absolutely buy these power meter types as complete power meters that measure left and right leg power.  Verve Cycling for example, places a power meter on both the left and right crank arms and can therefore measure both legs. Likewise, PowerTap, bePRO and Garmin all make pedal-based power meters that can measure total power.

The Good

In general, left-only power meters are cheaper than their total power counterpart. The reason is easy enough to understand. Fewer sensors and strain gauges means cheaper to buy. In addition, they are often quick and easy to install. For example, a left-only crank arm-based power meter like the 4iiii PRECISION or Stages can be installed in about 10 minutes.

The Bad

By doubling your left leg power, these power meters are making the obvious assumption 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 rare. Most of us have an imbalance – some larger than others. In fact, imbalances as large as low 40s one leg/high 50s the other leg, aren’t uncommon.

I will use myself as an example. On average, I produce approximately 45% of my total power from my left leg and 55% from my right. So when riding at 200 watts, this equates to 90 watts from my left leg and 110 watts from my right leg (again, this is on average and depends on things such as intensity, fatigue, etc.). The power meter will take my left leg power, 90 watts, and double it to arrive at a total power of 180 watts. Note that this is 20 watts less than the actual 200 watts I’m really producing. If my left leg produced greater than 50%, then the opposite would be true – the power meter would read high.

So, because left-only power meters are forced to make the assumption that both legs are generating the same power, if you are not 50/50 balanced, then we can conclude that your power will either be under or over reported.

 

Does it Matter?

It depends who you ask. There are two schools of thought on this. I will provide both sides…and you can decide for yourself.

‘No it doesn’t matter’ school of thought

So what if your power meter reads 180 watts when you are doing 200 watts (example above). You can still train to your power meter just the same. You can establish training zones and monitor your progress. It can even be said that your true wattage is a bit meaningless.

Here is an analogy. Say you are trying to lose weight. You step on the bathroom scale and it says 175 lbs. Let’s assume your scale is under reporting your weight and you really weigh 180 lbs. Does that matter? You can still use the scale to track your weight loss progress. The fact that it is under reporting by 5 lbs. isn’t super important. Do you care how much you actually weigh…or how much you weigh vs. the last time you measured (hence your progress). This school of thought argues the latter and therefore believes a left-only power meter is just as good.

‘Yes it matters’ school of thought

Yes, but there are some situations where knowing your true power is important.

  1. You move from a left-only power meter to a power meter that measures total power. All of a sudden, your watts jump by 20 (example from above). This will make comparisons with past training data difficult.
  2. You use race simulation calculators such as those found at Cycling Power Lab and Best Bike Split. These sites rely on accurate total power estimations.
  3. It would be okay if your left-only power meter always read too high or too low by the same amount. However, there is mounting evidence that an individual’s left/right power balance can fluctuate during a ride or from day-to-day based on things like intensity, fatigue and cadence (the magnitude of fluctuation is dependent on the individual). This makes training to left-only power a bit more complicated.

 

So Which One is Best?

We can’t tell you…in fact no one can. And it’s not about better or worse…they are just different. Each type has some pros and cons and it’s really about deciding what is best for you. If it’s important that you know your true total power and you don’t mind spending a little more, go for a power meter that measures both legs. On the other hand, if you are okay with the limitations of a left-only power meter and want to save some money, then a left-only power meter might be better. Lastly, we would note that if you know you have a meaningful left/right power imbalance, perhaps due to a previous injury, we would recommend a power meter that measures total power. In this extreme situation, left-only power meters might not be a great choice.

 

Examples

Power Meter TypeTotal Power MeasurementLeft-Only Power Measurement
Crank - SpiderSRM Crank-Based Power Meters
Crank - ChainringPowerTap C1 Chainring
HubPowerTap G3 Hub
Crank ArmVerve InfoCrankStages Cycling
4iiii PRECISION PRO4iiii PRECISION
WATTEAM POWERBEAT
PedalGarmin Vector 2Garmin Vector 2S
xxxxxxxxxxxxxxxxxxxbePRObePRO S
PowerTap P1PowerTap P1S
Bottom BracketROTOR 2INpowerROTOR INpower
Opposing ForceVelocomp PowerPod
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