Gearing & watts on the trainer

Can gear ratio be used to estimate watts on the trainer at a fixed cadence and resistance?

I.e. at a fixed resistance level , if I spin at 85 RPM using a 33/33 = 1.00 gearing to produce 100w. If resistance and cadence are kept constant, and I shift into the 33/21 combo, are my projected watts = 1.57 x 100w = 157w?

That depends entirely on the resistance unit of the trainer, and it’s response (power curve) as speed increases. It’s impossible to know without more info about the specific trainer to begin with.

This site has a range of trainers and their posted power curves. Many are non-linear and will not map like your proposal above.

What is your real goal in this question?

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Wouldn’t holding resistance constant eliminate this as a variable? Maybe positioning my question slightly differently by conducting the experiment outdoors on a flat, consistent surface with no other variables at play like wind; can the gear ratio be used to predict wattage loosely (e.g. ignoring cda)?

My question is twofold:

  1. I’m trying to ascertain how best to set resistance on the TR app, when I am targeting specific cadence ranges for my workout intervals.

  2. I’m trying to troubleshoot issues I’m having with my new Wahoo KICKR bike, which is not responding properly in resistance mode in the TR app.

  • Maybe? That assumes a direct linear response of of the set resistance level, and who knows if that is really the case?
  • Outdoors is a mess, even if you ignore “wind”. It’s impossible to ignore wind resistance (riding forward in still air) because the response to input and change in speed is very non-linear. And with where it seems you want to go, there is likely no relation between this example and your Kickr Bike.
  • OK, the Kickr Bike aspect brings a totally new set of issues. Being an electronically controlled resistance unit makes this all more complex.

  • Considering that you are using “virtual gearing”, it is all down to the programming in the device. Who knows how “real” the entered chainring and cassette settings relate to “real bikes”?

About the best I can envision is to set the bike in a specific gear, and a set resistance level in the TR app. Then proceed pedaling from low to high cadence range (60-120 rpm?) with no other changes. You’d want to hold each cadence (maybe 5 rpm steps?) for at least a full minute, maybe 2-3, so you can get stable power data at that cadence. Then progress to the next step in the range.

Repeat this test process in a number of gear ratios and then compare the resulting cadence to power data results. And repeat again with the entire process, and Resistance changed. You should see how the progression in one gear compares to another gear.

That is a LOT of potential testing, but depends on the precise number of gearing, cadence and resistance settings you want to cover. You might start with something like

  • High, Medium, and Low Gearing
  • Cadence checks at 80, 90 and 100 rpm
  • Resistance at 10%, 20%, 30%

That leads to a reasonable matrix of total tests for a starting point, and my give enough picture to see where to spend more time testing at finer levels.