Speed sensor calibration

(Joe W HPC/GKV VRT) #1

I have an unsupported classic static trainer with a speed / cad sensor.  It only has one wheel so the speed and cadence revolutions per minute are the same.  Zwift thinks I’m doing 12MPH at 200RPM.  Clearly this is not the case.  On my Garmin Edge 800 I had to set a very large wheel size for the speed to be realistic - but there seems to be no way to do this in Zwift.  Am I screwed?  I tried disabling the speed sensor in Zwift, at which point the virtual rider never moved.



(Eric Min) #2

Hi Joe,

What kind of trainer do you have and which profile did you select in Zwift? Did you set your weight in Zwift? 

(Joe W HPC/GKV VRT) #3


It’s an old Pro Form Fitness 790 static bike.  I bodged a BTwin spd/cad sensor into it so I could see metrics on my Edge 800, and manually tweaked the “wheel size” setting in my Garmin until it matched the speed given on the bike’s own speed sensor (not ideal, but close enough for training).  The setting in the Garmin is 5369mm as a wheel size (probably similar to a Penny Farthing, lol)

Because of the layout of the bike’s internals, the speed magnet rotates at the same RPM as the cadence magnet (there’s only one flywheel).

I used the “Speed Sensor+classic trainer” profile, linked the cadence sensor, speed sensor and HRM to Zwift, and set my weight correctly (170lb).

Is there anything I can do to make this work?  For various reasons I can’t currently afford a turbo and don’t have a power meter.  I’m working on the assumption that this setup would not work and have therefore borrowed a friend’s rollers, but having never ridden rollers before it’s going to be a steep learning curve…

(Scott) #4

Hi Joe - I’m afraid that setup is going to be problematic. Even if you could set wheel size, it still wouldn’t give you accurate results as there is no way for Zwift to know your actual effort. We map the power curves of classic trainers to translate any given speed into watts which is what powers you in game. 

More modern stationary bikes that broadcast watts via ANT+ work great or retrofitting an older bike with power meter pedals is another solution.

Your experience on rollers with just a speed sensor will also be limiting for similar reasons. Zwift has no idea what resistance you’re doing at a given speed without a power meter on rollers so the profile is quite limiting.

I hate to be the bearer of bad news, but you really do need a power meter with classic rollers. 

(Joe W HPC/GKV VRT) #5

Thanks for the info - I’ve bought a supported trainer.  I was thinking that some metric involving HR was involved, but I suspect HR is only recorded for completeness - thanks very much for putting me on the right track.