I think you are missing several factors, as Erg mode has a control loop that applies braking force if power exceeds target.
That braking force gets transmitted to crank thru gears, and counteracting the braking force requires less torque when in small chaining at same cadence.
Torque is proportional to force on pedals, so less torque requires less change in force applied to pedals.
So at the same rpm, and same power output, the small chainring requires less pedal force to adjust to changes in force that Kickr is constantly applying thru cassette.
Without running some numbers on the impact of flywheel, we donāt know how much flywheel speed impacts the Erg control algorithm with different gearing.
thanks dave, I think you answered my question up there, actually I have more facility in out-power in climbs than in the flat
I understand that watts are watts, but for me it was a psychology thing in my mind, Actually, for what you wrote, I probably have better muscle groups to climb.
Exactly this! Great post. When I first started training the big ring high cadence stuff was way more taxing on me than lower cadence small ring stuff (plus I like to ride the hills). Back then my 8 or 20 min tests were done on big ring and probably gave me a moderate or lower number than I could actually sustain pushing up a grade.
Now many years into training Iām more developed all around so there isnāt a huge difference between the two. Plus this year Iām using Assioma pedals so Iām not relying on KICKR power any more. I feel the pedals report power more accurately with no concerns of the KICKR over reporting.
@DaveWh if this were true, then you would expect to see smaller power variations in big ring vs small ring. But it is just the opposite in my testing.
The narrative here is that the primary consideration is flywheel speed and inertia, as if the Kickr didnāt have Erg mode control algorithm applying resistance.
The reality is that Kickr is behaving like a black box. If you didnāt know it had a flywheel, and I showed you smooth power in small chainring then according to your outdoor analogy that would mean small chainring on trainer is just like riding on flats in big chainring. (p.s. I have plenty of ride data that shows at same cadence there are similar power fluctuations on climbs at 5% grade as on flat rides at 18-22mph)
Iām pretty sure there are higher variations in angular velocity of the chain ring riding uphill vs flats. This is just an outcome of the physics of riding at slow speed uphill vs high speed on the flats.
But this by itself doesnāt mean there are higher power fluctuations, as power = torque * rotational speed, and the other variable in the mix here is torque, which varies a lot more over the pedal stroke than angular velocity does. And itās the multiple of the two that counts.
By the way, Iām referring to variation within the 360 degrees on a single pedal stroke here - not variation between different pedal strokes - which is a different topic.
Iām not sure where you ride on the flats, but around here there is usually some wind. And I do a lot of solo training rides, not protected inside a paceline/echelon. When smoothly spinning at 80rpm on a nice 5% grade there isnāt a lot of change in torque, just like on the flats in a tailwind. When the pitch is changing, it is a lot like riding into a crosswind, and changes in torque are required to maintain cadence.
This is getting a little off the topic of my long post above - which was an attempt to explain why riding uphill/low inertia feels different than riding on the flats/high inertia.
I agree with your point about how torque and power applied varies under different riding conditions.
So what you are saying is on a dumb trainer/rollersā¦where the drum size is constantā¦big ring or small ring does not matter. 350w is 350w no matter how you get there in that context?
The point of your long post was to provide a narrative for why the Kickr behaves in certain ways in little vs big chainring. However when I apply that reasoning, it tells me I should see smoother power on the trainer while in big chainring because of the flywheel inertia. But Iām seeing the opposite.
Assuming a constant cadence of 90 rpm, a dumb trainer/rollers will only have one āspeedā that will achieve that 350w. You will need whatever is the appropriate gear selection on the bike to spin the device that fast, to get that power.
This is different from a smart trainer that can achieve the same power at a wider range of gearing selection on the bike.
It is a āmanyā gearing option on smart trainers compared to a āsingleā gearing option on dumb/rollers.
Then why confuse the issue by bringing the trainer into your post?
I routinely ride 40-60 minutes into at 10-20mph headwind, using big chainring. It is great training for long climbs even though those are often 34 chainring and 32 cassette (or one/two steps above it on 11-32 cassette). The big difference? On climbs the sweat rolls down my face, into my glasses, and onto my bike. That doesnāt happen riding into a head wind. Not joking, Iām being completely serious.
thanks for the discussion Dave, it gave me some ideas for additional testing as time permits. Interesting topic, and so far the answers to my questions appear quite elusive. Iāll not bother posting again until doing some more homework over the next month or two. Have a great Thanksgiving!
Pretty interesting stuff that makes sense to me.
Does it apply to the Tacx Neo? I have seen this mentioned multiple times, but I donāt experience this on my Neo.
Has anyone using the Tacx Neo attempted to use other power meters to see if the power reported by the Neo is accurate?
Thanks in advance
The Tacx Neo doesnāt have a flywheel, but the Neo attempts to mimic road riding, so Iād expect the issue to be similar on the Neo i.e. slow speed riding in the small ring will feel different than high speed riding in the big ring (which just to be clear, doesnāt necessarily mean youāll find big ring easier than small, as it likely depends on the rider). But I donāt know for sure as Iāve never ridden a Neo.
Its hiding, just press and hold your finger on the empty space at bottom right, to the right of āFirmwareā and under āToolsā in this screenshot:
