I’m running Eagle on my XC bike and using a GXP Stages L power meter.
I’ve attached some shots of a test I did this morning to compare the two power read outs.
Let me know if it’s just me? +30w on average is the norm for me.
There are two files where I used only the left leg to hold power and that seems to be accurate if you allow for drivetrain losses. WTF is going on with the rest?
The whole file that shows you where I was pedalling with only one leg vs two. Each left leg period is followed by a short stint with the right leg only.
Is your Kickr Core at the latest firmware update?
I also had a 20 watt difference between my Kickr Core and Powertap P1 pedals, especially with big chainring up front, while in the smaller ring up front, the difference was a lot smaller. I contacted Wahoo repeatedly with proof of the differences. But they kept saying it was within the margin of error (although it was clearly above 2% difference). Although the difference shouldn’t drift at higher speeds imo.
However they also noted that they were working on a firmware update which could possibly address the issues I was seeing. Now, a month later, the firmware update is there and the two align perfectly.
Add to that, that stages tends to give a higher reading as well… I would just use the stages power as the source and let powermatch handle the differences. But do update the firmware on your kickr core and do a spindown.
My apologies. I’m trying to manage several things at once.
The final image is the entire test.
The 1st image is me using 2 legs.
The second and third are using left leg only.
The fourth and fifth are using both legs.
I tried halving the reading from the left only segments and they give me numbers that kinda make sense. Everything else gives me approximately 30 watts difference.
When I first bought my Stages L it read 30w higher than the Kickr. The Kickr was tested and is supposedly accurate.
I’m trying to find out what’s going on with my power meter. Is it the problem? Or is it something strange with my legs that gives me a 30 watt difference most of the time?
It’s a really frustrating situation as I have the same distributor for both units. The Core was returned with the most perfectly beautiful data files from another Stages PM tested in house.
When I contacted Stages they told me to send the unit back directly which will void many of my consumer rights in Aus.
Powermatch is not really an option as I didn’t really like the way it worked compared to ERG mode and I don’t want to use my race bike on the trainer.
I actually don’t see much of a problem. The stages L measured only your left leg, and doubles the value. So if you go left only it shows 400 watts while you actually only do 200 watts on the kickr. (most of your screens look like that)
In your last screen - it looks like you had some left only (minute 6 => double watts) and directly a right only part (minute 8 => zero watts). Looks as expected to me (when I ignore your not matching comments - I can neither check that you plotted correctly, nor that you remember correctly how the workout looked like.)
In case power smoothing is enabled in the Kickr firmware try disabling it… This may bring the Core and the Stages closer together. Also both units with approx 2% accuracy does not mean both are inaccurate in the same direction. So Powersmoothing + Stages+2%High / Core 2% Low + some minor left right inbalance might explain your 30W average in my opinion.
I don’t think it’s anything to worry about. When you are pedalling normally, your Stages looks like it is reading slightly higher than the trainer, but this seems fairly predictable and easy to account for when you’re riding outdoors.
Your Stages will always double the power figure before sending it to your headunit.
Your Trainer will measure power at the cassette, which is from both legs. If this is 200W, then each of your legs will be doing 100W. If only pedalling with your left leg, and you want your trainer to read 200W, your left leg needs to do 200W. Your Stages measures 200W and doubles it to 400W before sending to your headunit.
The Stages is only measuring one leg, and doubling it. The Kickr is measuring both, so if your left leg is stronger than your right leg, the stages will read higher than the kickr. For example (numbers pulled from the air for sake of argument):
Kickr reads 200 W
Stages reads 230 W
Stages = 2 x Left leg power = 2 x 115 W
Kickr = Left leg + Right leg power = 115 + 85 W
L:R balance = 58:42% - high but not outside a normal range by any stretch of the imagination.
(who’d have thought simultaneous equations could be so useful )
The reason it matches (halved) when you ride with one leg only is that the right leg contribution is now not a factor.
Is there something in the stages app that allows you to correct for this maybe?
I’ve experienced the exact same thing as well as my wife. I haven’t actually thrown either of the bikes on the kickr but this is why I’m not a fan of Stages anymore, among others. If you have a slight variation in L/R balance it is never going to match or be on par with other power meters or smart trainers.
If you have a power meter that measures both legs on any of your bikes or your trainer than you have the chance of shooting yourself in the foot by using stages as it will never be comparable to the others. Some people can get away with it because they have a 49/51 or 50/50 split most of the time. But there are other people that end up pushing to larger splits. If you only have a stages then no big deal. It is comparable ride to ride. But if you have multiple power meters now your see how it’s not comparable.
A 5% difference at 285 watts is adds up to a 14.25 watt difference in power when the single leg measurement is doubled. If you’re trying to hold FTP for intervals you’re going to be off by 5% which can cause you to fail a workout.
I use Race Face Cinch power on my mountain bike, Quarq on CX, Vector 3 on road, and kickr for trainer. They line up pretty well but the race face is single sided so there is more variation there. I just go by feel if I need to drop or raise the wattage if I’m doing intervals on that bike.
An even bigger issue IMO is if your LR balance varies with power. Mine is about 50:50 at threshold, but away from the can be between 40:60 and 60:40 depending on the zone. With a single sided meter that’s a whole world of potential issues
Exactly. Common to see larger swings through the ranges from what I understand. Here are multiple examples of intervals on the Vector 3 which shows true R/L balance. First one is SST, second is VO2, and third pic is 90sec of VO2 into 3m at LT. Once again if you’re only riding stages I don’t really think this is a problem, but I have multiple power meters.
I get that bit. I’ve been back and forth trying to figure out why the difference always seems to be a certain number of watts, compared to a percentage based variation.
There is something fishy with the numbers I’m getting back over multiple tests. I really just need help making sure I’m not missing something simple.
No doubt, I will have some imbalance issues. My left leg is pretty beat up and there are severed nerves all over the place. My back is pretty bad on that side as well.
I’ve got pages of calcs and multiple workouts to allow for different losses and imbalances. My problem is that this is as close as I can get it.
If I have an imbalance issue it shouldn’t manifest in single leg drills right? I have another data set that shows a similar test done in two different gears. The result was a +19.5w average in one gear and a 8.8w average in another. Both were performed at the same target wattage in erg mode.
It’s a strange set of data I have on file that leads me to believe something isn’t right. I’ll try and do some more testing when time allows to post up some of the variations I’m seeing.
Thanks so much for sharing that info. Thanks to the other replies as well. Much appreciated.
I haven’t quite figured out the TR forum features, so excuse the multiple posts.
I’ll get some more data and report back with variations in power using my left leg only. That should eliminate the margin of error for leg imbalances.
On that subject, is the claim of 1.5% accuracy for a Stages L G3 in related to it’s left only measurement, or is it while being used with 2 legs at 50/50 balance?
I have a Stages on my TT bike. It consistently under-reports power by 30W compared to my P1S and Neo (which are identical). It really doesn’t make any difference as it’s completely consistent so I make allowances when I’m on the TT bike. The number might be different, but the effort is the same. I wouldn’t worry too much about it.
I get that it’s not really a problem if it’s constantly out by 30w. More of an annoyance than anything else, but it sure isn’t what I paid for.
At this stage it’s just an assumption that the Stages is the odd one out. It’s totally possible that the Kickr is all over the shop. The substantial differences reported when using different gears is very odd.
I may be way off the mark here, but this implies to me that the Kickr is more likely to be the issue… I can’t see why the Stages should be impacted by gearing, whereas for the Kickr it results in a change in flywheel speed, which could.
Anyone else you know have another smart trainer or power meter you could validate against?
I agree totally. My gut feel is that both units are out of spec.
I promise to post more data once I get a chance. My drive train was swapped recently so I’m not comfortable posting prior data.
I’m thinking a ramp test with one leg is the best way to go. Do it in 1st, 6th and 12th.
My poor left leg which tends to be weaker than my right. I guess it’ll toughen it up a little at least.
In the interest of getting faster, I’m in the week after an XCM event on Saturday. The break from structured training is well deserved.
I know that any deviations from claimed accuracy in either unit will not make me faster. Hard work is hard work and there are no shortcuts. Although, it would be nice if measuring devices were easily proved accurate.
PS. Anyone in Syd Aus that would like to give some comparison data with my Kickr unit?
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