use generic ANT+ speed sensor in place of kinetic inride, allow to enter wheel size or roller size for accurate speed measurement

(Kuba Tyszko) #1

Hey Zwift, 

I’ve just completed my 14day/50km trial and going through my 2 months Strava premium now. I’m very likely to continue and become your paying patron as I really like the software. It solves one important problem - boredom on a trainer ;).

I know the Kinetic InRide is supposed to be supported in beta stage, with iPhone as BT smart receiver - and my InRide pod arrives in a few days so I can’t wait to try it out.

BUT - what I do not like is having to use my phone purely as the receiver.

So here’s an idea, first a few assumption and observations


  1. I’m assuming that InRide pod is simply an ordinary wahoo speed sensor - and the InRide app knowing very precise circumference of the roller uses that information to calculate accurate speed, and then power using power curve.

  2. One issue with ZPower (using speed /cadence sensor) is that it assumes 700/23C tire (that’s fine) but given tire pressure, deflection, roller pressure and other factors this will never be accurate - unless you allow user to enter tire circumference - just the way cyclocomputers do.

  3. If the 1 is true, then it would be safe to assume that your iPhone app does exactly the same - and then transmits the power to Zwift.


— NOW - the idea - if one could use an ordinary ANT+ based speed sensor, garmin or whatever, snap it somehow on a trainer (any trainer, doesn’t have to be Kurt), have a magnet.

Zwift would have allow to enter the roller type and circumference -> and use that *very accurate* speed data to achieve better power reading.


All it takes is another box in the “pair” screen, for a “ANT+ based speed sensor mounted on the flywheel” - and have the user enter circumference (and provide a few known ones too).


I know that’s rather poor man’s speed sensing and power estimation - but far more accurate than what cyclocomputers and ZPower do.

The presumption is that the roller’s movement represents the actual distance covered… 


Cheers, keep up the good work.

(Nate Kemp MNSTRS🚴🏽545) #2

The InRide value should be a superior power estimate because of the calibration step (assuming it’s been performed correctly) – calibration takes an indirect measurement of the trainer’s resistance (by seeing how long it takes for the wheel to slow down after pedaling stops) and is intended to remove the errors associated with tire pressure, trainer resistance adjustment, etc. 

It’s true that it’s only a software algorithm and that Zwift could technically do it directly as part of a ZPower calibration setup step, but the accuracy will not be as good as InRide.  This is because the sampling rate at which the speed is measured – it depends on the number of times the magnet passes the sensor in a given amount of time.  A once-per-revolution wheel sensor is much more coarse that a once-per-revolution measurement on the trainer’s aluminum hub where the Inride pod measures it.

If there were multiple magnets installed on the wheel’s spokes, one could improve the sampling rate of the wheel speed during calibration (would require the user to input the number of magnets installed – not that this is difficult).

What is really needed here is an improved workflow when it comes to calibration of the Inride sensor directly from the Zwift GUI at 10min into the workout. 

(Kuba Tyszko) #3

I agree but my point was to basically:


take ANY ANT+ speed sensor

snap it into the trainer

mount a magnet somehow (like it mounts in a kinetic trainer)


Measure roller circumference, calibrate and use speed read this way to match curve.


The assumption is simply that InRide is exactly what it is - a speed sensor with a magnet on the roller.

The benefit is that one doesn’t have to depend on iPhone for BT connectivity and use ANT+ directly…

(Nate Kemp MNSTRS🚴🏽545) #4

Right- I understand… one additional complication that I failed to mention in my earlier post:  ANT+ vs. BLE are different in terms of the number of times it can transmit a measured speed to the head unit.  With the much faster rotational frequency of the trainer’s cylinder, I’m not sure ANT+ could “keep up”.