Speed/distance calculation algorithm

Hi Alex,

It’s also something I’ve noticed with everyone’s Zwift rides but indoor ride speeds, and subsequently ride distance, often appear inflated versus a realistic ride outdoors for a given power and elevation.

Even a flat ride outdoors around a closed track with no stops would have wind resistance which would preclude the kind of numbers you see on smart trainers.

How does TrainerDay calculate these?

If I keep doing lots of indoor rides with our wet weather in Queensland at the moment, I’m going to beat all the A graders (Cat 1/2s) in our race team and top our leaderboard for average weekly speed, which will get me into trouble :joy:

I do use TrainerDay in combination with BigRingVR to ride virtual roads while following the training plan, and I find that TrainerDay is quite good with the distance. On flat/hilly roads, the numbers between both apps are really close. On mountainous rides, with loooong climbs and the arrival at the top, obviously TD can’t appreciate the real distance. But if the total up/down elevation is close to 0, TD is fine.


We had some complaints and recently I reviewed and compared our rides to Zwift and even to riders outdoors and found that we were consistently under estimating speed and distance. So I made an adjustment to our calculation about a week ago to be more optimistic. I dropped the elevation from 0.5% slope to 0.25% (except on virtual routes it uses actual slope) and gave people a very aerodynamic profile (closer to drafting most of the time). This now aligns closer to Zwift and to the outdoor flat rides we compared to some of the faster (non-racer) riders I was looking at.

You can see below the calculation values we are using (put in your own weight and power). This is all just estimates and there is no “right answer” for everyone. We could customize it and allow riders to enter their own slope, frontal area or drag coefficient :slight_smile: In a simplified manner like 3 values. Big rider, Speedy Guy, Ultimate Aero Racer… Right how we are probably between the speedy guy and ultimate aero racer…

Right now what you can do is add climbs in your strava profile and that will slow you down a lot :slight_smile:

You can pay with it yourself here

This will make you slow

Adding both checkboxes will make you more moderate… :slight_smile:

I also just change our drag coefficient from 5 to 5.5 to slow it down very slightly (less time drafting)

With 4.0 frontal area, you can see I did not go to the super extreme aerodynamic of the superman position :slight_smile:

Thanks Alex, that makes a lot of sense.

Especially when you’ve based it off a rider drafting and in a fairly aero position.

I was looking back at a couple of slower crits on a fairly flat, closed track and felt I was putting out more power to maintain similar speeds, but then crits are on/off the power way more than when I’m on the trainer.

Most my other rides have hills and stop signs or traffic lights so it’s hard to compare those.

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If I tick “climbs” in the Strava app settings does the ride then get some elevation too? :grin:

Not in the app :slight_smile: but we do have an automatic slope mode that does. Upload a ride, change our settings to slope in the app and uncheck ERG… It’s just like a ride outdoors with no visuals :slight_smile: And you are welcome.

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Ticking the “climbs” box gave me 900m elevation gain on my indoor ride this morning and the Speed was down to 11kmh riding in z2 :grin:

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Soon there will be a slight slowdown also but yes the climbs are brutal:)