Garmin Vivoactive 3, B'Twin speed sensor, Tacx Vortex, ANT pairing

Hi all. Newbie here trying to fumble my way round. I’ve got a Garmin Vivoactive 3 which I’ve paired to my B’Twin speed & cadence sensors. I’ve recently got a Tacx Vortex and my laptop/Zwift detects the cadence but not the speed sensor. My Garmin sees both fine. I’ve got a USB ANT sensor in the laptop. Why can’t I see the speed sensor in Zwift? Also, can I choose/force the laptop/Zwift to use ANT instead of Bluetooth? Thanks.

you won’t need to pair the speed sensor.

if you select the vortex as the controllable trainer and power source then zwift will calculate your speed based on the power you are doing at the time.

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As Chris said the speed sensor isn’t needed. And in fact if you select a controllable trainer in Zwift it removes the speed sensor selection. The only time you will see a speed sensor box is if you are using a non-smart trainer that can’t control resistance.

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Did my first ride today. With the speed sensor issue, Zwift reported my speed as 31kph via the trainer, whereas the Garmin reported 25kph via the hub mounted sensor, so there’s a significant discrepancy there! Personally I think the hub mounted sensor is right, What can I do to adjust the Zwift reading? The tyre/wheel size is correct. Thanks v much.

Zwift and your Garmin head unit will never be the same. Garmin doesn’t know the virtual terrain you are riding it only knows how fast your rear wheel is spinning. Whereas Zwift uses your weight, height, in game bike/wheels, grade of the road, pavement type (dirt, gravel, tarmac, etc), watts being produced and rolling resistance to calculate your speed.

Search the forums this as it has been covered a million times.

the sensor on the hub assumes you are riding on a flat road - zwift takes uses your power, the gradient you are currently riding in game and if you are drafting others (among other things) to work out your in game speed.

imagine driving your car and not moving your foot on the accelerator if you are driving on a perfectly flat road your speed would be different to when you are going up hill or downhill or following a car and drafting them etc. it’s a similar thing.

Thanks for the replies. It’s helping my understanding. I still reckon Zwift was being somewhat generous… My scenario last night was a flat course (Richmond), road bike, no drafting of anyone, I rode solo pretty much the whole 40 mins. My Zwift speed was approx 31kmh average, the Garmin said 27kmh average.

Please read this: How Does Zwift Determine My Speed? - #4

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What rolling resistance, height weight and route profile did you enter in the Garmin?

The Richmond Fan Flats is 5.1km with 19m of climing so you did 3 laps. 15.3km with 60m elevation. the Garmin does no know anything about any of that.

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Right, so shouldn’t the Garmin show higher average speed, not lower?

Ignore the speed the Garmin is reporting. Remember you are stationary, but moving through a virtual world (Zwift).

Setup an indoor riding profile on your Garmin and just have Watts, Cadence, HR and Time.

Depending on what cda and rolling resistance you input in garmin.

As Paul said ignore the Garmin it is wrong if you are not moving.

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Thanks for all replies. Basically I should ignore the Garmin, which is what I’ve been using for the last few decades, so it’s a mind shift for me. Also from riding outdoors I know I wouldn’t achieve the speeds Zwift says I’m doing. But no prob, I’ll give it another try tonight.