# How Does Zwift Determine My Speed?

How Does Zwift Determine My Speed?

For Wheel-on Classic Trainers:
Zwift takes the wheel speed broadcast by your speed sensor and the power curve (see example below) of the trainer you select on the Pairing Screen and converts that to virtual watts, also known as “zPower”.

Your in-game speed is determined by:
• zPower watts
• In-game bike frame and wheel set
• In-game drafting (Tri and TT bikes get no drafting boost)
• In-game road surface (Mountain bikes are fastest on dirt roads, but slowest on pavement)
• In-game virtual elevation changes.

Activating a Power Up such as the Truck (draft boost), Feather (weight reduction), or Helmet (aero boost) can further alter your in-game speed for a short duration.

This is not a real trainer curve, only to demonstrate the concept.

Example: Your wheel turns at 40 km/h then Zwift will use the curve and determine your Power as 200w. Using a set of complex equations taking your weight, height, road incline, rolling resistance and other factors in to account Zwift will calculate your virtual Speed.
So that 200w can be 45km/h on a flat road or 12km/h on a hill.

For Smart Trainers and Power Meters:
Your in-game speed is determined by:
• Your weight and height entered
• In-game bike frame and wheel set
• In-game drafting (Tri and TT bikes get no drafting boost)
• In-game road surface (Mountain bikes are fastest on dirt roads, but slowest on pavement)
• In-game virtual elevation changes

Activating a Power Up such as the Truck (draft boost), Feather (weight reduction), or Helmet (aero boost) can further alter your in-game speed for a short duration.

*****Also note that there are no stop signs, cars, or wind, and you don’t need to slow down for curves within Zwift.

Thanks to @Gerrie_Delport and @Mike_Rowe_PBR for all the help in putting this together.

Ride On!

2 posts were split to a new topic: Custom Zwift speed

Is there a calculator somewhere where I can put in the data and see what the watts should be? My spouse is on a kickrSnap and we can’t ride together because he is sooo slow. He’s not like that on the road.

He calibrates every time he rides.

But 5kph? I can walk up hills faster than that - it’s just totally demotivating and random. And the randomness means I can’t even compete with my yesterday time. Maybe Zwift isn’t for me

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What incline are you on when you’re doing 5kph and what wattage are you doing at what bodyweight? 5kph could be perfectly reasonable.

A rough estimate of climbing speed on steep gradients comes from the VAM formula.

Without any wind or rolling resistance 1 W/kg is 367 meters/hour. So if I add in rolling resistance 1 W/kg is around 350 meters/hour. So if you’re doing 3 W/kg, triple that (1050 meters/hour). If you’re doing 2 W/kg it’s around 700 meters/hour. Etc.

Suppose you’re on a 10% gradient. That means you go approximately 10 meters forward for each meter gained. So 350 meters altitude gained per hour is 3.5 km/hr forward at 1 W/kg. If it was a 5% gradient that’s 1:20 so it would be 7 km/hr. It it were a 20% gradient it would be 1.75 km/hr.

Climbing the steepest gradients will be slower on a bike versus running because bikes are heavy, assuming you’re not carrying your bike with you as you run. It’s a good question where the cross-over is, but 20% is a reasonable guess, depending on how heavy the bike is.