How does an indoor trainer measure cadence without cadence sensor?

[alcatraz]

Hey WWs...

I recently started using an indoor trainer (tacx t2180) and I noticed it reports my cadence. Quite accurately it seems. I can change gears and even if my speed increases but cadence drops, the measurement remains accurate.

I was just wondering how an indoor trainer can measure cadence when my bike has no cadence sensor?

Thanks
/a

[mentok]

It measures torque which spikes at 9 o'clock and 3 o'clock in your pedal stroke. It detects the peaks in torque, measures their period and converts to an RPM measurement.

[TonyM]

Like for the Stages powermeter etc....

[mentok]

stages and other crank based powermeters that don't use a reed switch and magnet combo use an accelerometer

[alcatraz]

Thanks for explaining it. Makes a lot of sense....

[mattr]

And if you are very jerky or very smooth at pedalling, it can't always distinguish the individual pedal strokes.
Needs to have a measurable and repeatable difference between on power and off power.

[Leviathan]

Also its perhaps worth noting that at the cheaper end the cadence output is worthless when accelerating or decelerating (ie all the time), Pretty sure Kinetic "estimates" my cadence using a random number generator.