The left / right distribution of power meters in practice

In connection with the Stages powermeters now available in Europe, which measure power only at the left crank, we have received an increasing number of inquiries regarding the importance of this left/right distribution. In our first two practical tests we have already compared the Stages Powermeter with the Garmin Vector, the Quarq Powermeter and with a calibrated Cyclus 2 ergometer. In this test all tested power meters had only a small deviation among each other.

In the following test we have compared the left-right distribution of the two systems Quarq SRAM Red and Garmin Vector. For this purpose, we mounted both systems on a test bike and ran a step test over 100, 150, 200 and 250 watts on a Cyclus 2 ergometer. In the table we have summarized the average values from this test.

First of all, it can be stated that both power meters once again deliver very good wattage values in relation to the Cyclus 2. The Quarq SRAM Red Powermeter even has no deviation at all at 100 and 250 watts and only a max. deviation at the other two levels. This is also in line with our previous tests, in which the Quarq SRAM Red Powermeter always achieved very good readings.

The Garmin Vector system also delivers very good measured values and has a deviation of only 3-5 watts to the reference value of the Cyclus 2 ergometer at all levels.

The left-right distribution shows a different picture. Here both power meters deliver clearly different values. It is particularly noticeable that the Garmin Vector pedal in this test shows a tendency towards the left leg. The Quarq SRAM Red Powermeter, on the other hand, shows – from 150 watts – a tendency to the right leg. So there is not only a difference in the measured percent values, but even a different tendency (more left or more right) is output. In direct comparison, therefore, both values must first be seen as not really meaningful.

In the following we have shown both evaluations (power + left/right distribution).

In order to be able to go into more detail on the left/right distribution, one must first know how this is determined in the systems. Spider based power meters (SRM, Quarq,, P2M) measure the power in the crank spider with strain gauges. The spider is mounted between the right crank and the chainrings. A “real separate” measurement of the left side therefore does not take place. This can only be done by power meters that really measure left/right separately (e.g. Garmin Vector and Rotor Power). However, spider-based power meters already measure all the power that is fed into the chain via the crank. This is different from the Stages Powermeter, which actually only measures power on the left side and then extrapolates to total power.

But how does a spider-based powermeter now divide power between left and right? Here is Quarq’s official description of the PowerBalance.

Power Balance™
Power Balance uses crankset torque to display the ratio of power generated in the right drive stroke (first half, 0-180°) versus the left drive stroke (second half, 181-360°) for each crank revolution.

This means that the entire force application over 360 degrees is divided into two phases and then assigned to the left or right leg via the spider position. However, this has some potential for error. This is because the system cannot distinguish between tension and compression phases. Therefore, crank power meters will always show power on both sides, even if you have one foot unclicked and are only pedaling on one side. The move phase on the clicked side is then assigned to the wrong side.

With the left/right distribution, it is possible to detect whether force is being applied unevenly. A real statement whether this happens on the left or on the right side can not be made from our point of view.

The Garmin Vector Pedal, on the other hand, is strictly speaking two power meters. One in the left pedal and one in the right pedal. This system is therefore technically much better suited to measure the left/right distribution.

However, there is a problem here as well. Since, strictly speaking, we are dealing with two power meters, these two power meters naturally also have a scattering of accuracy between them. This can be seen e.g. with all crank-based systems. If you test several power meters of one type, they will of course differ slightly from each other. Since the left / right distribution is usually only about small differences, a difference between the two sensors here can easily have an influence on the displayed distribution.

Observations in the test

Very noticeable especially in the test with the Garmin Vector is that the variation between left and right becomes significantly smaller with increasing power. While at very low powers of 100 watts there is still a larger deviation of 53.7% – 46.3%, this decreases significantly at 200 watts and is only approx. 1% at 250 watts.

This tendency can also be clearly observed in the graph and the measured values with the Quarq SRAM Red Powermeter. Here the deviations to a 50/50 distribution from 200 watts are about 0.5%. In addition, you can clearly see in the graph that the max. deflections in both directions becomes significantly smaller with increasing power.

This is also consistent with research by Andrew R. Coggan (Training and Racing with a Powermeter), which shows that as power increases, the influence of the left/right distribution appears to diminish. Again, this is a single case analysis from Adrew’s own training data. R. Coggan.

Conclusion

Andrew Coggan says he has a DLF-45 of about 280 watts. From about 175 watts it has very little variation between left and right. Above 200 watts in the direction of the continuous power threshold (DLF 60 will probably then be at 265-270 watts) the deviation goes clearly towards 0.

Our test rider currently has a DLF-60 of about 310 watts. Basic endurance training (GA1) normally takes place in a range around 200 watts for this test rider. Threshold training (EB) in the range around 300 watts. According to our test results, the left/right distribution has only a very small influence on this driver from 200 watts and also goes towards the continuous power threshold towards 0.

If one transfers the present results to the Stages Powermeter, which measures the power only on the left side due to its construction, then one must say that this will probably have no negative influence on the measuring accuracy in all relevant training ranges. This is also in accordance with our previous tests with the Stages Powermeter, which showed that there are slight deviations from the Cyclus 2 and the Quarq Powermeter at very low braking values (e.g. 100 watts), but with increasing power the deviation of the STAGES system from the reference systems becomes smaller and smaller.

We suspect that Team Sky found something similar during the test phase, because deviations of about 5 watts to a calibrated reference value for power in relevant training ranges is absolutely within the range of proven top systems like SRM or QUARQ. In practice, fluctuations due to zero offset and temperature may even have a greater influence than the left/right distribution.

Whether current power meters can reliably display the left/right distribution in these higher power ranges remains unclear at present. Spider-based systems (SRM, Quarq, P2M) can at best show trends here.

We will schedule another comparison test between a Garmin Vector and the Rotor Power System on this topic shortly. These two systems have a “real” left/right measurement.