Trainer Difficulty vs Gear Ratios

Is there any way to calculate the effect of trainer difficulty on gear ratio? For example, if I have 12-28 and set trainer difficulty to 100%, climbing in Zwift will be exactly the same as climbing outdoors. If I have the trainer difficulty set to 50% or 75%, is there anyway to work out what the equivalent cassette would be?

Hi @Keith_Leng I split your question into its own thread.
Are you asking what cassette to put on your smart trainer for a given course in Zwift in the way that a professional racer might choose a cassette for a mountain stage? I’m not sure if that’s what you’re after.

First let’s define what the Trainer Difficulty setting in-game does for smart trainer users. By default, this is set to 50%, and you can slide it from 0% all the way to 100%

If the climb gradient in-game is 10%, the default setting will reduce the resistance of your controllable trainer to feel half as steep. So the feel at your pedals is more like a 5% gradient climb. But it also alters the feel of the descent on the other side by 50%. So you won’t reach the same speed if you coasted all the way down the hill.

Here’s Rahsaan Bahati explaining Trainer Difficulty in our video:

Regarding gearing in Zwift - the game doesn’t know what gearing you have on the bike, and doesn’t care. What matters to the game is the power you’re putting out, and more specifically, the watts / kg you can sustain. Whether you achieve X watts by mashing a big gear at 60 rpm, or spinning a smaller gear at 120 rpm is of no consequence to the game. It’s the same X watts either way. That is - unless you’re in a training plan, where the idea is to train your body to work efficiently across a wider range of the cadence:effort spectrum.

If you’re saying that the hills are easily climbed at 100% Difficulty Setting and you have a 28 tooth low cog - you’re probably a strong rider. Is that 28t cog low enough for the steepest climbs in-game? If it is, it might be time for another FTP test.

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Hello Shuji. Thanks for the reply. My question is this: if I have 50/34, 11/25 gears on my bike, is there any formula I can I apply to determine what difficulty setting I need to simulate, say, 50/34, 11/28 or 50/34, 11/32, etc.

(edit to add that I’m talking about climbing. I appreciate that Zwift essentially has three sets of virtual gears: flats, ascents, and descents).

@Keith_Leng I think you just need to find the ratio between the gear ratios.
To simulate a 34 (cogs on chainring)/28 (cogs on cassette) gear with a 34/25 gear it’s (34/28)/(34/25) which is about 89%.

@Keith_Leng
I’m really not sure how to answer your question, because we’re thinking very differently about how the game works in general, and the Trainer Difficulty feature specifically.

I want to dissuade you from the idea that Zwift has “virtual gears” because it doesn’t. Again, the game has no idea what gears you have installed on your bike, has no way to deduce what particular gear combination you’re in at any given time, nor does the game care.

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Keep your trainers limitations in mind too, some max out at 8 or 14% max

Changes in resistance due to gravity are proportional to changes in gradient. If a gradient is twice as steep the power required to overcome the gradient will be twice as much.

Changes in the power required to overcome a gradient are also proportional to changes in gearing at a fixed cadence. If you double the gear ratio you also double the power required to overcome the gradient at the same cadence. If you exclude air resistance and frictional forces the power required to move up a hill in a 34/14 gear at 70 rpm is double the power required to move up the hill in a 34/28 gear at 70 rpm.

As the power required to overcome a gradient changes in direct proportion to both changes in gradient and changes in gearing the trainer difficulty setting can be used to simulate changes in gearing.

Thanks Michael. That’s very helpful.

Hi Shuji. Thanks for your help, anyway. I agree that we seem to be looking at it very differently. To me, gears modify the cadence required to produce a given power output, and that’s exactly what the difficulty slider does.

Thanks, Ward. I have a Kickr Core, so 14%

Thanks again, Michael. Assuming that the difficulty setting is linear (and I have no reason to believe it isn’t), it sounds as though I could create a simple spreadsheet to do the conversion.

Going back to your original question, if I am understanding Michael’s response correctly, it sounds like your 12-28 in Zwift at 50% would be equivalent to a 24-56 IRL.

Yes, that is correct.

To take this idea further imagine pairing that 12-28 cassette with a compact 50-34 chainset. If you climb a gradient in a 34/28 gear (1.21 ratio) at a given power and cadence then in a 50/41.18 gear (1.21 ratio) you should be able to climb at the same power and cadence.

No one will have a 41.18 tooth cog on their cassette but at 50% trainer difficulty a 20.59 tooth cog is equivalent to a 41.18 tooth cog. A 50/20.59 gear (2.43 ratio) at 50% difficulty should climb at the same power and cadence as a 34/28 gear (1.21 ratio) at 100% trainer difficulty.

I don’t have a 20.59 tooth cog on my cassette but I do have a 20 and 22 so I did a quick test. I climbed a hill in 34/28 at 100% trainer difficulty, and again in 50/20 and 50/22 at 50% trainer difficulty, all at the same cadence to see if the power matched. My power in 50/20 was a little higher than expected but in 50/22 it was a good match. Maybe the slider wasn’t quite on 50%, my cadence wasn’t quite the same, or there’s something else going on, but overall the result was close to what was expected.

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I’m with Michael here. So much so that I hope that my laboratory and my project students will have a “conversion” table ready by the end of the year (sorry, even without the coronavirus I would not have much luck asking my students to come to the lab in the summer)

I wondered this as well because other programs like Fulgaz (share sub with my sister in law) have recommended settings for what cog you use to set the “virtual” wheel size to help the game do the conversion.

I know all about trainer difficulty and have it set at 100% which feels very close to how I feel outside on certain gradients.

But why does 100% seem like a necessity to replicate IRL? I have a bike with 11-25 in the back. If I wanted to go and do Alpe d’ Huez in France I would certainly be changing my cassette to a more climb friendly range. In Zwift I simply turn down the setting and do Alpe du Zwift.

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This is a dangerous path to go down I know, but to back up Shuji’s point. Trainer difficulty does not replicate an easier gear set up, it reduces the gradient reality / feel.

If you want to do Alpe d’Huez IRL you could change your cassette to a climb friendly range. You could also do that before riding in Zwift. The second option you have in Zwift, which you don’t have in real life, is to make Alpe d’Huez less steep so that you can keep your 11-25 cassette on.

The caveat is always that reducing the gradient feel does not make it faster to ascend - watts are watts to calculate speed.

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