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I had my gravel bike, it's 1x, on the turbo and everything was hunky dory until I swapped the gravel bike for a road bike with a 50/34 crankset which creates a dilemma as to which chainring to use. Now in erg mode logically, at least to me, it shouldn't make a massive difference what chainring you're in as after all a watt is a watt, isn't it?
In erg mode I find it far easier to produce power in the larger chainring, for the same power my heart rate can be up to 10 bpm lower in the bigger ring when compared to the smaller ring which means intervals are easier and sustaining power is easier too. Also my self selected cadence seems to be around 10 rpm lower in the smaller chainring and I have a real hard time trying to match cadences which are easily achievable in the large chainring.
I suppose this is a long winded way of asking which chainring is best to use on the turbo?
I'm I over thinking it or is it best to do my high intensity work in the smaller chainring with a naturally lower cadence in an effort to replicate hill climbs and the longer efforts in the larger chainring taking advantage of the fact I can sustain the power for longer?
high intensity work in the smaller chainring with a naturally lower cadence is
?! 😟
I think the common recommendation is small ring and 3rd/4th cog down on the cassette. Something to do with producing a smoother ride (or less variation for the turbo to adjust too). Pick a cadence that suits, or the one the workout requires. The turbo should adjust resistance so that the power target is hit.
Higher rpm gives higher heart rate but each stroke is less fatiguing.
Total speculation alert!!:
If your turbo's not a very fancy one AND you're a bit of a wattmonster ... turbos find it easier to create resistance when "machine" speed is high, so high pedal revs at a lower resistance is easier for it to manage than slow revs
(I can't quite work out in my tiny mind whether this applies to both wheel-off and wheel-on machines)
I'm using an Elite direto wheel off turbo and I'm not a watt monster
The turbo will be measuring power using a combination of the speed at which the cassette is spinning and the measured torque. It doesn’t know what type of drivetrain is used to produce that power so there should be no difference.
Drivetrain efficiency might make a small difference but probably only a watt or three so probably negligible.
any difference is most likely to be from you being in an efficient riding style in on bike but not the other. Crank length, saddle position, bar position etc will all have an effect. The most likely one though is that you mention you’re pedalling at different RPM - one of those RPMs is likely a more efficient speed for you.
to test it out, I’d suggest keeping the same bike on the trainer but swapping gears to replicate the RPM change and seeing if you see the same HR vs power change.
The other thing that changes is the effective inertia of the flywheel (“reflected” inertia). In larger gears you effectively will feel like there’s more inertia so could lead to a change in both the feel you prefer and maybe a more or less efficient pedalling dynamic.
If your turbo’s not a very fancy one AND you’re a bit of a wattmonster … turbos find it easier to create resistance when “machine” speed is high, so high pedal revs at a lower resistance is easier for it to manage than slow revs
I think that's sort of it. I read that direct drive turbos find it easier to control resistance when the speed of the flywheel is lower, hence the recommendation to use the small front ring and a relatively big rear cog when in ERG mode.
GPlama discusses this here and reckons it does matter;
Older vid, tho, so maybe some newer perspectives out there. It sounds reasonable that you will have a natural inertia preference for your pedalling, just as you would naturally prefer a certain cadence. If you were training for short TTs, for example, I wouldn't use erg mode with big rear cogs and low inertia.
Definitely feels different in a low vs high gear on my trainer, I think it's basically that in the higher gear the flywheel is going much faster and therefore much smoother. Haven't compared effort levels but it feels much nicer.
I suspect it's down to flywheel effects. If you push a bigger gear then the flywheel is doing more work (e=0.5mv^2) every time you fluctuate the speed. Conversely it's not slowing down as quickly either.
My guess is that a high gear means you're pushing through the power phase of the stroke harder and recovering as the flywheel does the work over TDC. Which feels easy. Whereas a small gear the flywheel slows slightly over TDC and you have to add that energy back in each time.
It's the same power in both cases, but a big gear probably feels easier because it will feel more natural.
In the same way as however many times you tell yourself that climbing a hill in a low gear and ride on flat roads in a big gear should be at the same cadence. You still drop the cadence on climbs.
I'm sure there was something on DCR that said that the power is calculated by (most) smart turbos rather than measured directly, though TBH I can't see how a crankset power meter is going to be different in calculating it from torque x RPM.
In the small ring/big gear I find I have horrendous problems with tyre slip (Wahoo Kickr Snap)
I’m sure there was something on DCR that said that the power is calculated by (most) smart turbos rather than measured directly
They don't make it easy to figure out, I guess because the idea that a smart trainer is really just doing a better job of what Zpower used to do for dumb trainers isn't as marketable, but yes I think most are just measuring flywheel speed, and you can calculate based on how much it's accelerated/decelerated since the last revolution how much energy was added or taken out. And the physics behind the magnetic resistance part are pretty accurate and bulletproof with the spindown calibration giving it a zero point for a known resistance curve.
Elite have a torque sensor, it works by using a fairly simple light source and blades mounted on either end of an axle, the more you twist the axle the closer the blades get. But if you look at teardown/repair videos for most others there's only sensors measuring the flywheel's speed. It obviously works though because they all claim (and get reviewed with) basically the same sort of accuracy.