Any tips for syncing workouts with different power meters?

Equipment
1

I’ll try to keep this simple upfront, then add more context at the end for anyone who wants it.

I have power meters on 3 different bikes (indoor, analog MTB, and e-bike) that I use for training. My issue is that the three are seemingly not giving the same power outputs, so figuring out the proper effort to give during a workout can be tricky.

I assume this happens because:

  • Power imbalance between my legs (outdoor meter on at least one bike is left-side only)
  • Error tolerances & differences between power meters in general

If I was only running one of the power meters it wouldn’t matter, but I do workouts on all three bikes.

Are there any tips for figuring out the difference between power meters so that I can adjust my workouts accordingly? Or do I simply have to guess?

For the longer version with more context, continue reading:

I have a Wahoo Kickr bike that I use for indoor training, a SRAM left-side power meter on my MTB, and a Specialized Turbo Levo that also records power (no idea how accurate that one is). I train using all three, depending on where I am riding and what the workout is.

I previously had a dual-sided power meter on my XC bike and a Wahoo Kickr that I put my bike on. When using that setup, I was able to run both power meters at the same time while doing indoor rides. When doing this, the XC bike’s power meter consistently read 8-10% higher than the Wahoo, even if I would recalibrate both power meters.

At the time my FTP was ~300W, so the power difference was in the 25W range when doing sweet spot workouts.

Without knowing about the difference I could do an outdoor workout at a much lower power output than I should be doing, but once I knew the difference I could adjust my outdoor rides and shoot for slightly higher power numbers.

With this new setup I cannot easily compare the two, so I’m not sure how to figure out how they compare. Especially since I will be using a left-side power meter, which means there could be even more variation if my left leg puts out less power than my right.

Does anyone have any tips for coming up with a ballpark estimate of how the two power meters vary? I don’t expect it to be perfect, but having a ballpark idea of how they compare will make it a lot easier to keep my workouts close to the correct power ranges.

2

The only way to even have a shot at it is to add a 4th power meter (or smart trainer) that can be used simultaneous on each bike, to validate the delta to the “common device”. Once you have that, you may have a shot of at least knowing what delta’s likely exist. But doing this needs to be done carefully and along a range of powers, as the delta between any two devices is likely to be non-linear.

3

Thanks.

I don’t know of any power meters I can attach to the Wahoo Kickr Bike (the one with a bike built in), so it sounds like my only option is to try to do some FTP tests over time and compare for a ballpark idea of the differences. Even that might not be very telling, but it will hopefully point out any major differences.

4

Pedal power meters are the only option for a smart bike in a comparison like this.

To be honest, FTP testing is fraught with enough variability in the best of circumstances, I’d never try to use that as a way to compare power data devices.

1 Like
5

I agree, but was mostly just trying to catch wild variations.

Power pedals would work, but are a bit expensive for what I’m doing. I could potentially get some used and then sell once done.

What are others doing who ride an outdoor bike and use an indoor trainer? Just ignoring any potential differences?

6

If they lack a way to cross compare, likely they are guessing at best or ignoring it. Without proper tools, there is not much more to do as a “human” power meter is a flawed device :stuck_out_tongue:

7

Guess I need to get Mark Rober to build me a robot that pedals at exact power outputs so I can put that on all of my bikes :joy:

1 Like
8

I use the same bike on trainer and outdoors with power meter pedals to keep the power consistent. If I had different bikes outdoor/indoor, I’m not sure what I’d do - possibly keep the pedals on outdoors and use the smart trainer power for indoor and try to create some kind of crosswalk for the two using perceived effort.

9

Try learning riding to feel, it’s the cheapest option by far and it’s a super useful measurement for riding.

1 Like
10

FWIW: I have 2 crank power meters, PM pedals, and my smart trainer. I’ve compared them by putting a bike with a crank PM on the trainer, putting my PM pedals on that bike, and recording all three to do a three way comparison. In my case they were close. It varied minute to minute, but averaged over many minutes they were within a couple of Watts of each other, which IMO was not enough difference to significantly affect training. I haven’t thought through what I’d do if any was widely different, but that could range from getting one fixed if way off, doing a static calibration with a weight, adjusting the output in the app, or just recognizing the difference and mentally accounting for it.