I’ve been noticing this with the wahoo kickr snap.
When you spin a big gear in erg mode, when there’s a lot of kinetic energy in the flywheel, the power readings of the trainer tends to remain high (constantly) but power from the powermeter reads almost 20 watts lower, it matches when cadences are a bit lower (approx 85-90rpm). Does this mean that erg mode power readings become inaccurate at high gearing / high flywheel speeds?
I noticed something not too dissimilar. Using the Kickr Core recently, in erg mode and the big ring, the resistance seemed to decline suddenly (target power stayed the same) forcing me to increase cadence dramatically to hit target power. Not sure if the TR software communication was dropped. Happened in two out three intervals I was doing. I’m new to the Kickr and erg mode so it could be operator error too.
There’s nothing wrong with your Snap, don’t worry.
On the issue of holding the target power, I notice the same thing but to be honest one single watt is a very acceptable amount of deviation in my opinion. It often sits (for the 240W target example) at 239, as you say, then will change to 240W for a while and sometimes go to 241 but let’s be honest, a 1 watt deviation is pretty good and neither here nor there. It is certainly not worth “chasing the watt”, just hold your cadence steady and it will follow the target within 1 watt above or below.
On the issue of power going to zero, the Snap has a very fast flywheel speed (especially when in a high gear and cadence), so when you come off the end of a big effort at high cadence (and especially in a high gear) the sudden drop in watts and the sheer speed of the flywheel means the flywheel sort of freewheels, or stays ahead of your pedaling, if you follow what I mean, until your pedaling “catches up” and your effort registers as a wattage again.
This latter effect can be minimised to a degree by riding in a lower gear (lower gear = lower flywheel speed). You can call it “inconsistency” but it’s not really a fault, just a consequence of the design of the trainer and the laws of physics. It doesn’t actually detract from the effectiveness of your workout so I wouldn’t worry about it.
Hope that helps. Happy training!
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I see similar behaviour with my TACX Vortex, I’m often -1 no matter what I try to keep up and I have both the odd power spike or drop right in the middle of an interval, not an over/undershoot on transition. I notice just a small change in my pedal stroke when it happens for a very short moment < 1s.
Yes, it does mean that, you’re dead right. The flywheel inertia effect been covered in various articles by TrainerRoad and also Wahoo themselves, as well as in several threads on this forum.
In a high gear, the flywheel inertia gives you “too much help” you might say, hence the general advice to use the small ring or at least lower gears.
In a ramp test in ERG mode, you can, if you want to, “cheat” your way to a false high FTP if you use, say, 50-13 or a similar high gear and wind up the cadence. It’s not surprising that the “reported power” in these scenarios is falsely high.
I don’t use the small ring except in the rest valleys, but neither do I use a really high gear. I use something like 50-16 or 50-17 so that
- I get the benefit of the flywheel to enough of an extent that it feels like I’m riding on the flat or up a slight incline/headwind and
- The chainline is still okay.
If you are aiming for hilly disciplines like MTB or CX or just lots of hill climbing, I’d incorporate some lower gear and lower cadence work into your plan but for me the idea of feeling like I’m climbing a hill in every workout (which the low flywheel inertia feels like) isn’t very inviting!
I think the Snap is a great trainer, super quiet and great road feel, as long as it’s used within its design limitations, which, as you’ve noted, includes avoiding very high gear/high cadence work.
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Welcome to life with data… data from sources of varying levels of accuracy and consistency 
Use the gear as directed and be as consistent as possible. It’s the best that we can do.
Simplicity has it’s advantages too.
Despite our current state of smart trainers, there is plenty of room to grow, IMHO. The emphasis has been on getting cost down, which is great, but we can’t sacrifice the actual purpose of the trainer along the way.
I feel there is a limit on “reliability” as a result of the downsizing on the pricing. We need to see improvement across the board, from low to mid to high level trainers. There are a surprising amount of issues with these devices.
Some do come from operator error, and can be remedied from people using them as intended. But there are also issues with inconsistency and reliability of these trainers. Trainer makers and app companies have some room to grow for making the experience more direct, error-free, and EASY… so that we can use them with less frustration and suspicion.
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There’s a difference between a ride feeling different in the big ring due to inertia, vs. a trainer systematically over-reporting power when in the big ring.
The former is acceptable and normal, the latter is not. Sounds like the OPs comment was about the latter - the trainer over-reporting power.
Anyone else have a comparison between power reported by the trainer vs a power meter, and if this varies big ring vs little?
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Why not just ride in a low gear? In Erg mode you lose nothing by doing that, but it will be quieter and more accurate.
I ride in my 39:25, it doesn’t look great on Strava when I cycle 30km in 90 minutes, but it means that the trainer works better, It tracked very closely (+4w) to my powermeter last time I checked.
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To be fair, I think you’re quoting me slightly out of context there, since you missed out my preceding comment where I said that yes, it is the case that the trainer is over-reading in such circumstances.
But the flywheel inertia is intrinsically tied to the same issue. These trainers have their limitations but so long as you work within them then all is well. In the case of the Kickr Snap, if you avoid very high gear/high cadence work then you can expect acceptably accurate power reporting, if you consider such a thing vital - see below for further thoughts on this specific point.
You don’t need a power meter to demonstrate that the real, actual power that you have to put through the pedals, in ERG mode, to hold say, a “reported” 300W at 95rpm in 52-14 is less than needed to hold the same “reported” power and cadence in 39-14. Ergo, at some point, as gearing and cadence (and therefore flywheel inertia) increase, actual power becomes less than reported power.
Does this matter?
The question is, does the phenomenon detrimentally affect the quality of a training workout to such a degree that it’s inhibiting the physiological improvements we’re all seeking? I think the answer is no, it doesn’t. For me, therefore, it’s an irrelevance.
The other question is, given that this is a limitation of this trainer, can it be easily avoided? The answer is yes, by avoiding the highest gears when working at high wattages and cadences.
I’m seeing far more than 1W variation. I’d be happy with +/-5W at this point!
2018 Kickr, Trainer Road, Stages power meter. All seems to work fine in Erg mode at first (after borrowing an iOS device since the Android app won’t pair devices). Part way into my ride at steady cadence and effort the power readings on my Garmin 520 are suddenly +15 to 20W over what TrainerRoad reports. After pausing the workout and resuming the difference was then approximately -10W offset. Everything is the latest firmware. All spindowns and calibrations are within a day or two. This is getting rather frustrating…tempted to send it back since I’m inside the 30 day window.
Without the Garmin reading the Stages crank I’ll have nothing to compare the Kickr power output to. When I rode it prior to pairing with the power meter it was consistently reading off by 25W. If I can’t count on the Kickr being within +/- 10-15% then what’s the point of a smart trainer?
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Not sure if this is the same issue, but I noticed much better responsiveness connecting the Kickr to my computer via ANT+ instead of Bluetooth.
Of course you’d need to be using TR on a PC (with a dongle, which is not expensive).
Hope this helps.
S
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I am connected through an ANT+ dongle on my PC already. Thanks for the suggestion though.
@Darkgerbil, I think @DaveWh is correct. Gearing doesn’t effect power output in erg mode, but the inertia or kinetic energy from riding in a big gear will generally make pedalling seem easier because of the muscles being engaged vs what muscles are engaged and when they are engaged when pedalling in a small gear. But the power output itself is the same. This link and accompanying video explain it well.
KICKR Bike ERG Mode: Why Gear Selection Matters | Wahoo Fitness Blog
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@GlenJ I am with you on this. I have just bought a kickr core and it is strangly reading lower than my Vector 3 pedals by around 20-25W. i have done factory spindowns and standard calibrations.
Like you I expect an £800 trainer to give me consistent and reasonably accurate readings. This reads out so 106W is showing 86 and 233W is showing 203W. That is a big difference when it comes to a workout.
I note @Chad’s comments about the trainer market and problems across the board. I have just swapped from a direto which was just so slow to respond to changes it was unreal. The kickr feels much more realistic in ride and power changes in erg mode.
Some people suggest using your powermeter to powermatch and override the trainer. For goodness sake! I don’t think it is reasonable to spend £800 on a smart trainer with a power meter, and then have to use another £800 pair of pedals or cranks to make sure it is reading consistently.
(Like you I am using Ant+ with a PC. )
Phil,
I’m done with the Kickr. It’s currently for sale. I’ve gone back to my e-Motion rollers and Stages power meter, manually matching the TR workout profiles with gearing etc.
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It’s a shame Glen I have just done Disaster on my Kickr core and it was brilliant, even allowing me to hit decent levels on the 5 seconds 200% job.
I am pushing Wahoo to fix this. If you swap, I think the tough one is choosing something different.
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It’s because of the hundreds of threads like this one that I won’t even consider a smart trainer for Trainerroad. With my 4iiii power meter and simple Tacx magnetic trainer I only have to concentrate on the essentials, like food, sleep, motivation and what to watch on Netflix - and of course, pedal hard and meet the power targets! With the trainer in high resistance and small ring in front, it’s quiet as well.
Just don’t stop using Trainerroad because the smart trainer technology is not mature enough yet. Try to go simple before you give up. Even virtual power with the cheapest and simplest possible trainer and speed sensor can be good and consistent enough for trouble free training and progression.
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I love my Snap. Flywheel inertia is a great future for the real road ride feel. There is no power meter that ever matches another one, not sure why it is such an issue for many people… It is a great kit and works well, just need to get used to its drawbacks.
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