I assume you read the rest of the sentence - wheel speed, power, and (in some cases) cadence. What else do you think a trainer reports?
Smart trainer = trainer using ANT+ FE-C and/or BLE FTMS protocols.
Yes, I did and it says
I am not native speaker, English is my 4th language but if the other options are inside (), is it for me another part of the sentence. Sorry if I misunderstood it.
But the fact is, that direct drive trainers (Neo, Kickr, Saris H3) do not report wheel speed.
I can guarantee you they do. Try connecting one with a head unit as controlled trainer, you will see your speed displayed as you start pedaling.
Zwift ignores this data and calculates speed from power. But just check how many threads are started every year on this forum with the topic “Zwift is completely wrong, disagrees with my Garmin/Wahoo”…
Most of those are people using a speed and cadence sensor with a classic trainer.
Tacx Flux S connected ANT+ to Garmin Edge 520.
Try imagining how you could do a spin-down calibration from Zwift if spindle speed was not reported by the trainer.
Robert, I am speaking about trainers with powermeter all the time.
About your speed in Zwift:
How does your head unit calculate your speed outdoors? GPS, not wheel speed.
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All smart trainers have a form of built-in power meter. You can’t do erg mode without measuring power. You can’t run Zwift without sending it a power measure (unless you use a dumb trainer and let Zwift calculate power based on… wheel speed).
Head units that are compatible with BLE FTMS or ANT+ FE-C can control smart trainers and display information from the trainer - as shown in the example above. Basic FTMS and FE-C protocol include transmission of speed, power, resistance level, and control in resistance and power modes.
And again: how could apps like Zwift allow you to do a spin-down calibration of a trainer without receiving an input of rear wheel rotation speed from the trainer?
I use a Garmin 1030 to monitor my averages that Zwift doesnt provide so its purely secondary to the Zwift data. Power and cadence are always replicated perfectly as they are pulled via ANT+ from the trainer and cadence sensor. But speed is rarely the same as Zwift as Garmin doesnt know what Zwift knows about me (height and weight) and all the other variables that Zwift knows (gradient, draft, wheels etc) - thats why WATTS are what determines speed in Zwift.
This is not guessing, I use a Garmin 1030 to dual record all my activities and speed is never accurate.
Spin-downs are a red-herring - manufacturers just want you to spin up to a certain flywheel revolution and they measure the time it takes to get down to a target revolution - its not about ground “speed”.
@Milan_Rost FYI, I have to use Garmin speed sensors (they use wheel revolution) on all my outdoor bikes as GPS is only accurate speed wise on 0 degrees gradient. As soon as you hit hills etc, it becomes inaccurate as GPOS is not 3D. More applicable for mountain bikers but throwing it out there as an FYI. I used to use GPS when speed skiing though but for competition had to use two GPS units, which is why race boats always have two speedos.
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Hi Dean,
GPS (and all other systems, such as Galileo, …) are 3D, but the precision is the problem. And how the route is calculated from GPS data is another question - I get different length and altitude numbers from the same source data in Polar, Strava and TrainingPeaks.
But I doubt the calculation with wheel diameter and revolution is much better. Do you exactly measure the wheel diameter for each ride?
Garmin produce the wheel sensor for this purpose - that their GPS is not accurate. They actually suggest if you want the most accurate distance and speed data, buy the sensor. I do track building and we have to measure stuff accurately - only people that use wheel sensors get close to approved measurements.
The problem with so called 3d is that coverages is not guaranteed hence you dont know when you are receiving 1 signal for your speed/distance vs 20 - polling is the other known issue with any GPS - as its not realtime or even close to it for consumer grade kit it simply cant be accurate by definition and you/we rely on smoothening.