While I love the simplicity, accuracy, and reliability of my PowerTap G3 power meter hub, I would very much like the flexibility of swapping wheels and tires at will, especially as I change bikes often depending on the conditions, season, and route. Namely, I really wanted a power meter that I could swap between my road bike and my cross/winter bike, but I didn't want to shell out over a grand for Vector or P1 pedals. After consideration, the Stages crank arm power meter seemed like the best option. This is possible because I've made sure both my bikes have the same length crank arms (170mm) and spindle type (GXP).
However before I start relying on the data from the Stages, and setting my FTP and sprint targets based on data from the new left-side only power meter, I thought I'd conduct a little experiment to compare the accuracy of the power meter readings. I don't pretend to be anywhere near as thorough as the DC Rainmaker Stages Power Meter In-Depth Review
but it was still quite illuminating. Read on to find out what I discovered.
Stages vs PowerTap G3 Test Setup
Indoor test methodology:
- Riding Sufferfest video “The Rookie” using Perceived Exertion (RPE), good mix of steady-state and sprints
- Bike mounted on Kurt Kinetic Road Machine 2
stationary/turbo fluid trainer
- Recording PowerTap G3 power meter
with Garmin Edge 800 over ANT+
- Recording Stages Rival power meter
with Wahoo Fitness app on iPhone 4s over Bluetooth Smart/BLE
- Both power meters calibrated at room temp, Wahoo & Garmin auto pause turned off
- Analysis on Golden Cheetah 3.3
A few things are immediately apparent from the GoldenCheetah power data graphs. First, there seemed to be some time drift between the Wahoo Fitness app on my iPhone 4s and the Garmin Edge 800. Not sure who is the culprit there, but the activities drifted about 2 seconds apart over the hour. The Stages data at the crank arm is shown in magenta
on the graph, while the PowerTap G3 data from the rear hub is shown in cyan
Stages vs G3 Entire Activity
So, let's line up some segments of the ride to minimize the drift.
It's easy to see that the Stages crank arm power meter is reading consistently higher watts versus the PowerTap G3 hub.
It's also apparent that the G3 data is more fine-grained, which makes perfect sense when you think about it; since the Stages is left-leg only, it therefore has to double it's readings to approximate the output for both legs, and therefore halve the resolution or frequency.
Stages vs G3 Rookie Part 1
Stages vs G3 Rookie Part 2
Drum roll please... here are the results from the Golden Cheetah summaries for each of the entire efforts, with percentage difference:
PowerTap G3 Power Meter Results
Total Work (kJ): 528
Avg Power (watts): 162
Max Power (watts): 515
Relative Intensity: 0.861
Stages Power Meter Results
Total Work (kJ): 561 +6.25%
Avg Power (watts): 172 +6.17%
Max Power (watts): 591 +14.76%
Relative Intensity: 0.907 +5.34%
Quite a large difference!
Part of the difference probably friction loss in the chain or rings. The bike is moderately clean, but the chain could have used a lube.
But, I think the bigger reason is that my pedal stroke is not ideal. Keep in mind that the Stages is left side (non-drive side) only and doubles that figure, so if the trailing leg (right or drive side) is not completely unweighted
on the up stroke, the left leg will have to apply more force to compensate - wasted power that is seen at the crank, but is never transferred to the rear wheel where the G3 hub is measuring.
When I made a very conscious effort to unweight my legs on the up stroke, the numbers lined up much more closely. So, that's one area that I definitely have to work on, and be mindful that it will effect the results. On the plus side, it means I should be able to go quite a lot faster for the same effort if I just optimize my pedalling style.
Which measuring method is "better" in this regard? Well, the Stages is more accurately measuring the effort that I'm making, so it should be more accurate in terms of calories and workout intensity. But if the power isn't making it to the back wheel, it's not propelling you forward; consequently the G3 would better for comparing absolute speed between riders, i.e. the Coggan power charts to see where your W/kg stack up against other Cat 1,2,3,4 riders.
At any rate, I'm excited to finally have power on my CX bike, and to have power numbers AND be able to use my nice rear Dura-Ace wheels at the same time. I've become a big fan of the PMC chart this winter, and now I don't have to make guesses about any more stress scores. If I really work on my pedal stroke over the 2016 season, I can always do the test again and see if the numbers are any closer together. I suspect they will be, as the DC Rainmaker tests showed a very minor difference between the Stages and G3 - and I suspect Ray's pedal stroke is a lot smoother than mine!