Gradient to resistance change math; in Zwift or Indoor trainer?

General Discussion
1

Hi,

can someone help me to understand how the gradient during riding in Zwift is brought to the indoor trainer and a change in resistance?
Does Zwift calculate the more/less Watt as a function of the gradient in game and sends it to indoor trainer bike or does the trainer bike has to calculate the change of resistance by itself as a function of transmitted gradient in %? If last assumption is correct, how can I know how accurate the manufacturer has implemented the function? Could be totally off, or?

Thanks,
Lars

2

I believe it is off, by a lot for most people. When I looked at the %gradient message that is sent to the trainer, weight is not in there.

Now, I don’t think this is a big deal - all that really means is that you’ll be in a different gear climbing the same gradient on Zwift than you would IRL. Which is really the same as Trainer Difficulty. For some (very lightweight) people, trainer difficulty of 100% will actually end up with higher resistance than they would feel climbing IRL and something less than 100% would be more realistic.

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3

In SIM mode (as opposed to ERG mode for workouts), Zwift transmits to the trainer the rider’s weight and also the gradient at each point. The trainer then determines the resistance according to the weight and gradient and sets that accordingly.

As for how you can trust the trainer manufacturer to do it properly, well, there’s no easy answer there. However, remember that we’re only talking about resistance here, and what gets you up a gradient is power, no matter what the resistance is.

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4

Thanks, Steve and Craig!

I know that it’s not super important but a lot of manufacturers advertise the realistic ride feel and gradient up to 20% and 2200 W max power… Would be nice if the new purchase brings whats advertised. I think the big players invest a lot of effort that its like this but I think lower price segment it will differ more if they have to invent the transformation from gradient+weight+gearing to change of power. And who will disprove that its not true…

5

Everybody who has a power meter can double record and prove. I am sure not the only one (hobby rider!) with two different power meters (SRAM and Power2max), using a Tacx Neo for Zwift.
Tested all combinations, BLE and ANT+ in the last 4 years - never more than ±2.5% difference.

6

Power is power. Gradient and weight only dictate the resistance the trainer is producing. The amount of power is still totally down to how hard you push those pedals.

7

Yes you can check easy if power from trainer is equal to power from pedals like Assioma.

But my question was how and how accurate a specific gradient in real life is transfered to my/your setup at home.
Let’s say you climb in real life 10% with 80kg, 48/36 and 75 RPM (and need power XXX for this). Now you climb with the same settings in Zwift on a Kickr bike shift. How much resistance has the bike to deliver that you also have to generate XXX Watt with the same cadence. This math (you have to build a model of the real physics) has to be done in the equipment I learned here. But who knows exactly how much power is needed in real life, it alters every second on a climb. Do you know if the 10% from your Tacx Neo are only 8% in real life?
Also an everesting with 100% makes no sense when Tim has 200 Watt at 10% and Tom 250 Watt at 10% because of different manufacturer model calculation.

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Power meter (and good smart trainers) are measuring the power, not calculating!