I still think everyone is getting this bass-ackwards by focusing on flywheel… Its not the flywheel its the torque multiplier of being in lower gearing IMHO.
If you are in little chainring, and then the same amount of force is applied to pedals will deliver faster acceleration to trainer due to torque multiplier affect.
Wordy, but I suspect (without doing full analysis) that is the real explanation.
Here is one explanation of lower gearing https://www.superstreetbike.com/bigger-rear-sprocket-makes-more-horsepower-mythbusters#page-3
“The lower gearing therefore multiplies torque, increases the rate of acceleration, but does not measurably increase or decrease horsepower at the crankshaft.”
I sincerely believe the “flywheel speed” narrative should change to “lower gearing = torque multiplier for faster acceleration” narrative.
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That’s really interesting (to an anorak like me!), but the idea of low gear torque multiplier and flywheel speed/inertia are not mutually exclusive, it makes sense that both things apply simultaneously, does it not?
from the point of view of your pedals, all you know is that the trainer is sharply increasing resistance. How you respond to that depends on the gear ratio. The little chainring responds faster to that, due to torque multiplier affect.
you don’t even need to consider flywheel speed or Kickr gearing or Erg control algorithm.
I ride on a cycleops Magnus and experience the same damped effect where it feels like the trainer can not add load fast enough for the sprint interval. I end up hitting the correct peak power for the effort but will be 50-70 watts under where I should for the average of the interval. It seems like wheel speed contributes greatly to this effect. If I spin up the in anticipation of the interval the trainer is slow to respond and take 5 seconds or so to get to the target power. If I don’t spin up then the load bites very quickly but can drag the cadence down and then you pedal squares making the effort extremely difficult.
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I have a Tacx Flux and for short sprints I have to ride in the large ring or it won’t reach the power target. The reaction speed isn’t any difference but if I stay the small ring and middle or higher in the cassette the power doesn’t get high enough.
Not disagreeing, though in terms of the Kickr, or at least the model I have, the only measurement that is made is the (angular) velocity of the flywheel.
My understanding, albeit I haven’t really read into this too much, is that the Kickr knows the mass of the flywheel, thus inertia required to accelerate it and ultimately how much power has been applied when there is a change in flywheel speed.
My BIG assumption in all of this had been that due to a square law in rotational inertia a doubling in power at low flywheel speed makes a noticeable difference to flywheel speed thus was easier to respond to. Conversely, at higher velocities, doubling in power might not accelerate the flywheel all that much.
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My understanding from DCRainmaker review is that Kickr 2016 and later measure both resistance AND flywheel speed (optically) to estimate power. Due to reliability issues, they removed the earlier use of a strain gauge.
Every single time I compare big vs little chainring, my only conclusion is that less force is required on pedals to deal with power changes. During steady state (say 200 watt interval), the “trainer system” does a good job at making me output steady power regardless of chain speed. However I can clearly feel less pedal force being required to cover any acceleration changes required by a power bump.
Chris, without looking I’ll trust your statement about rotational inertia. But again, another analysis without a discussion how inertial changes due to Erg resistance algorithm are translated thru Kickr gearing and bike gearing. Since the Kickr is somewhat of a black box, all I know is that the little ring requires less pedal force (due to torque multiplier) to respond to the same increase in resistance. (inertial differences in flywheel speed might be minor/negligible by the time they arrive at your pedals, I don’t know)
Brian
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