# How does Zwift work out speed?

(C orinne Black [Consett]) #1

I was on Zwift for the first time tonight, whilst also running Wahoo app on my phone. I use the Wahoo speed/cadence sensor. Wahoo had me doing around 15mph, at the same time Zwift had me doing nearly 30mph! Then, Zwift lowered my speed on the ‘hills’.

I know in the Wahoo app, I set my wheel size to get an accurate speed. But, how does Zwift calculate the speed?!

Over my first 30 minute ride, Zwift had my average speed at 21.8mph, Wahoo app at 15.3mph.

(Stuart Davis) #2

Zwift doesn’t use this metric, instead it takes your power (either from actual power meter data or some flaky guestimate based the known speed/power curve of your trainer).

It then calculates your in-game virtual world speed given it knows your power, weight and road gradient, so 200w can see you doing 10mph on a gradient and 30mph downhill.

(nhat tran) #3

I guess that’s a lot of excuses for Zwift inaccuracy.   The trainer machine should simply simulate uphill or downhill with increase or decrease resistance.   That would make it harder or easier to pedal and therefore; how slow or fast the bike wheel would turn.   What’s left is simply measuring the wheel revolution for speed.

15mph vs. 30mph?   What’s the point of measuring speed if there is such a discrepancy?    That difference is an ocean wide.   In other words, that’s simply wrong (period).

This is my first winter season using ZWift.   I thought I was getting faster … a lot faster!   That got me excited.   Unfortunately, I sense that Zwift is fooling me with its inaccuracy measuring that favors showing faster speed.   Maybe that’s Zwift’s strategy, get people excited so they will do more Zwifting.

(. Bath Salts (84Kg) (EV) C) #4

FYI  nhat tran , they talking about Zpower we all know that it can be up to 100% off , limit it to just 200watts max. not power meter / or smart trainer.

(Paul Allen) #5

For zPower Zwift calculates in-game speed using the rear wheel speed and the power curve of the trainer selected and converts that to watts. With those watts, the weight you entered, in-game bike used, in-game drafting, and elevation changes Zwift calculates your in-game speed.

Your rear wheel speed shown on the Wahoo app will very rarely match the speed within Zwift.

(75 years Lynn 75 Years M70) #6

I was trying to keep up with another rider  on Zwift recently and it took 3.8w/kg to match his speed when he was delivering less than 2 w/kg. What is the explanation?

(Steve Ellis) #7

Lynn: on the flats, watts matter more than watts per kilogram. A heavier rider putting out 300 watts will go faster than a lighter rider putting out 250 watts, even if the lighter rider is doing more w/kg.

The greater the incline, the more w/kg matters. At 14%, for instance, the lighter rider at 3.8 w/kg will pass the heavier rider at 2 w/kg, even if the heaver rider is putting out more watts.

In professional races, it’s generally the larger riders who win time trials on flat courses, and the riders who generally win on the massive climbs are thiiin.

(Andre Hufschmid) #8

You can’t be serious. You think you can read the speed on a device. Everybody knows that Zwift depends on a lot of factors. Bike, Weight, Wheel Size, Power, Road Surface and calculates speed. Its like in real World. It depends on a lot of factors. Zwift is absolute accurate, you’re the one that obvious has no clue how things work.