Crank-arm vs. Trainer power measurement

Potentially a stupid question :).

I have a stages left arm power meter and a Hammer trainer. I’ve only ever used power-match, but i realized that when i look at the readings between the two, if I’m getting 175watts at the crank, i’m getting something like 140 from the trainer. That makes sense to me as the drivetrain will eat up wattage on the way to the trainer. Power-match makes this moot, because it reduces the resistance on the trainer until the crank-arm measurement hits the target.

My question is, how does it work without power match? Does the trainer estimate how much power would be lost to the average drivetrain and account for that? Otherwise, everything else being equal, people not using power match would be training at a higher wattage than they’re seeing. I’m certain some of you guys know the answer :). Thanks for indulging me.

Without actually answering your question, training is best when done with the same “tape measure”. In this case, we use “watts” as the units, and a smart trainer or power meter as the “tape measure”.

When you have two power measuring devices (power meter and smart trainer like your example), there is absolutely no guarantee they will “match”.

  1. Each device will have a stated tolerance range on power. That means the reported power data may be above or below the “real” power data value in the true sense of the data.

  2. Because of #1, this means that two devices may well differ because one could read high in the tolerance, while the other could read low on the tolerance. Giving a gap, but one that is within spec.

  3. Add to that the fact that you happen to be comparing a smart trainer effectively measuring data from both legs while your power meter is measuring one leg and doubling the value. The fact that people may have a variable power split from left to right means the data may be skewed from the actual rider, above and beyond the power data tolerances.

  4. Then you get your question, which really boils down to where each power data point is measured, and the potential impact from drivetrain efficiency comes into play.

All this ends in power comparison between different devices being far messier than any of us would like. Apparent “gaps” may be real and “right”, while apparent “matches” could be actual errors.

“A man with a watch knows what time it is. A man with two watches is never sure.”

  • Replace “watch” with “power meter” and “time” with “wattage”…

“A man with a power meter knows what wattage it is. A man with two power meters is never sure.”

  • Then we have a present day problem for cyclists with too much money :stuck_out_tongue:

Well said, and I get that you want an apples to apples comparison when you’re tracking your own power data, so so long as you’re consistent with what you’re measuring with, you’re golden.

But that being said, if say i’m 4 w/kg ftp at 75kg (both would be nice) and i’m basing that off the crankarm measurement. That’s 300watts, to achieve that.

Now my evil twin, also 75kg, is also putting out 300watts, but it’s to the trainer, that isn’t measuring the drivetrain friction, and let’s pretend that resistance is 5%. His power measurement is going to come back at 285watts. And my sad evil twin is at 3.8 w/kg (no wonder he’s evil).

I feel like if that was the case, when the alphas at the races are all touting their ftps, they’d have to qualify how they’re measuring it, to get proper “bro props”.


Which is why you need three power meters


Pedals, crank arms, crank spindle, chainring spider, wheel hub (or wheel-off smart trainer) for the whole shebang :stuck_out_tongue: Max it out!!!


I always use the power meter on my bike because that’s going out on the road with me while the trainer stays home.

  • And there in lies the reason people like “real” bike races with rider on rider action that take place in the physical world. Be that mass start like a road race, or individual like TT, you are measured against each and every rider that is there competing. At that point, power is effectively irrelevant and totally unnecessary, because the reality forms from the efforts provided by and against each rider in those moments, moving the riders in the race environment.

  • Your scenario becomes much more relevant when you start looking at online competition (in a virtual world) that is actually based on the power data provided to move the rider in that fake space (like Zwift). That is a messy proposition and one that is hotly debated with questions about how and where power is measured, not to mention potential for “bad data” (be that intentional or accidental).

Outside of those two situations, you are looking at the everlasting and totally futile experience of EGO (and other stuff…) measuring via various methods and units. Same old, same old… with different tech, and it usually is meaningless at the end of the day.

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that’s always been my method, the crank arm measures everything.

It’s funny you bring up the online stuff. I’ve been tinkering with Zwift during the pandemic, and having connectivity issues with the different devices and that’s what made me think about it. When I switched to the trainer as the power meter instead of the crank arm, things were noticeably harder at the same “wattage”. You confirmed what I thought, and that there isn’t some sophisticated shenanigans going on under the hook, you just need to pick one and stick to it.

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There can be real issues where some devices (brands and or models) will typically report data higher or lower than “real” values, as judged by using multiple “known and validated” power sources. This is effectively what Shane and Ray do in their power device testing when they install one or two additional power devices when evaluating another one (ex power pedals and crank to evaluate a smart trainer).

With mixed testing, they can better judge how various devices measure and how accurate certain ones are (at least based on the examples they have in hand). We know that some devices typically measure high or low as a rule when compared to other “good/accurate” devices.

That leads to some presumptions that Smart Trainer A is more accurate than Power Meter B or reversed. It’s all part of the mess involved with virtual racing and something that Zwift and others will have to tackle depending on how “legitimate” they plan to claim or expect that racing to be in the future.

This is effectively what Shane and Ray do in their power device testing when they install one or two additional power devices when evaluating another one (ex power pedals and crank to evaluate a smart trainer).

That was a big source of my confusion, when they compare power meters they always “look” like it’s apples to apples, but it’s apples to squirrels, and they just account for the squirrel offset.

Thanks Chad, I’m been training with power for almost 2 years, and I’m just now learning what the heck is going on :).

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It can be done though!
Just need to pick one unit and have official calibrations.
kind of what they are doing with indoor rowing championships, where everyone is using the same Concept2 trainer, calibrated at the event.

I’m in the same situation here. I recently got a Kickr Core after years of training with powermeter + dumb wheel on trainer. I did a few workouts in ERG mode using the trainer power reading. After I completed a couple workouts things started feeling much harder…so I checked the power reading from the stages gen 1 on my Garmin and found out that the trainer power is 20-30W lower. Then I started using powermatch but I’m still getting used to it…my OCD self really likes the smooth lines from ERG mode and Kickr power reading…
I believe the Stages number is correct because it’s in line with other power meters I have (Vector, stages gen 2 and powertap P1). And because it gives me a higher FTP!

I use the gen1 hammer and assioma uno pedals, I run them separately between TR and zwift (tr owns the hammer), I’ve chosen to accept the hammer as the source of truth because I find it consistent and pretty darn accurate. The Uno readings are really close, but I think it can be influenced by my pedaling technique. I realize I’m talking pedals and not crankarm here, but imagine it’s similar enough.