Power Meter Confusion

I hesitated about posting this here because I recognize it is a fairly confusing topic, but since I’m running out of explanations, I thought it was worth a try in the event anyone has seen anything like this before. I also leaned toward posting this here since I think this might be the most popular forum with people who have true experience with power meters so your experience might offer me some help here.

There is a lot of background to this and I’ll try to be concise. I purchased a pair of Assioma Duo Power Meter pedals about 2.5 months ago. I use them between two bikes, a spin bike I use in the house and an M5 recumbent high racer that I use outdoors. The issue is that the readings I’m getting between the two bikes make no sense and they are the exact opposite of what one would expect.

Recumbents can be fast and faster than upright bikes due to their aerodynamics, but I’ve never seen a single claim that anyone could produce more power on a recumbent versus an upright bike. This has been fairly well documented throughout the years and one example can be found here: https://mccraw.co.uk/2013/04/20/recumbent-position-power-loss/. This example uses a recumbent bike very similar to my own, and again, the same person on an upright bike can produce more power than someone on a recumbent.

However, in my case the results are flipped! At the same heart rate and exertion I produce about 70% more power on a recumbent than I do on an upright spin bike?! Here are two examples I’ve provided Favero for them to investigate:

One-hour Spin bike ride indoors at 133 bpm HR = 116 watts:

One-hour Recumbent bike ride outdoors at 133 bpm HR = 204 watts

Favero has been investigating this for 6 weeks now and I’ve provided them with a considerable amount of data. Still, no answers. They agree that it does not make sense that I could produce this much more power on the recumbent versus the spin bike at the same heart rate, but at the same time they can’t see where the issue could exist. All the data looks perfectly accurate and I must admit, when I ride the data is very consistent. On either bike, even the slightest amount of resistance more results in a change in the power meter readings.

However, they both feel oddly different. For example, 150 watts on the spin bike feels labored. I can hold it for a while, but will probably have to fade down to 130 watts after 20 minutes or so. Conversely, 150 watts on the recumbent provides some resistance, but not nearly as much as I feel on the spin bike. I could hold it forever without a problem. Again, the striking thing here is that experience should be the opposite. On the spin bike 150 watts should feel easier than the recumbent based on other recorded data and peoples’ past experience.

Naturally, this all calls into question the accuracy of the power meter pedals. Favero continues to state that everything they see seems perfectly accurate, but they also state that those same accurate results don’t make sense for the bikes I’m using them on.

My apologies for the long-winded post, but I thought the details might matter here. If you’ve happened to seen anything like this before or have any thoughts, please let me know. Some items that have already been checked by Favero and myself are:

  • crank length
  • calibrations
  • static weight test
  • ant+ / bluetooth accuracy
  • tested against Garmin Edge 530, Rouvy App, Zwift, etc.

You probably already looked at this, but are you making sure that the pedals are not touching the crank arm? There are extra washers that can be installed if needed.

Can you borrow a trainer and test the outside bike inside?

What type of spin bike is it? Does it provide a similar road feel with a high enough inertia? Or does it force you to apply force throughout the entire pedal stroke unlike outside?

How does the max 10 sec power compare if doing a sprint when fresh, also that big of a difference?

Do you know if your outside power is close to accurate, using speed and some on-line calculator?

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See the answer above.

My first test would be to ride a conventional bike outside and see what figures you get. If those figures are roughly comparable to the recumbent (in terms of power/HR) that would point to the spin bike being the issue.

Three things I can think of. First, interference with the crank arm, requiring one or more washer, as already indicated. Second, the zero offset procedure vs a recumbent bike. The procedure requires the cranks to be vertical - this is top-bottom of the pedaling cycle for a regular bike, but not for a recumbent. I wonder if that screws up the power meter algorithm. Third, interference between the shoe and the power meter.

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Very interesting.

Do you mind sharing how you are recording the power from the pedals? Are you using the same head unit regardless of the bike?

Assuming the pedals are not contacting the crank arm on the recumbent bike, I think this seems most plausible to me. Try doing the zero offset with the pedals horizontal on the recumbent.

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I agree with trying to calibrate the pedals in the horizontal position. On a regular bicycle you put out max power at a different part of the pedal stroke (compared to the direction of gravity) than you do on a recombant bike. So the alternate orientation of the pedals during calibration might be important.

Also, let’s assume for a minute that the power reading is correct. There’s a few alternate explanations.

  1. First of all, you might be able to put out more power on the recombent bike than a standard bike if most of your riding is the recombent and your muscles are better adapted to that. The rule that says otherwise assumes that you ride both types of bikes and so have developed the other muscles. In that link you point to the guy regularly rides his upright bike.

  2. Cooling. If you don’t have really good cooling riding indoors you will quickly overheat and it will feel much harder to put out power. For every watt your body puts into the pedals it generates 3 times that much heat. Riding outside you get a lot of airflow when you ride, even more so on a recumbent. You should be in a tornado of airflow if you want to be able to put out as much power indoors as outdoors.

A combination of (1) and (2), especially (2), can make a huge difference in your actual power output.

I will add more reasons:
3) Bike fit. Poor seat height and fore/aft position as well as hip angle (heavily influenced by handlebar position and width as well as the seat position) can heavily affect power. It is very common to see differences in power output for a time trial position vs regular position. This is partly due to closing the hip angle and partly due to most of the training time (muscle adaptation) being in the regular position. If you primarily ride in one position (recumbent) you will naturally be strongest in that position. If being fast on the recumbent is your goal, ideally set up your recumbent on a trainer or try to match the hip angle, knee vs crank and leg extension between bikes.

  1. Like was said by others, on the Trainer you get zero breaks in pedaling (unless you consciously take them) and you have no inertia. Outdoors your legs are getting micro-breaks and you sometimes coast or pedal slower than your wheel is turning. On the Trainer it’s relentless resistance. That matters and you will feel it

Another question… what kind of spin bike do you have? Is it a fixed gear with a massive flywheel?

When I am training I notice that when I sit up to take a drink etc the power reading drops and when I go back down on the handlebars it increases. This is with a Stages single sided PowerMeter.

Wow. Such amazing responses. Thank you so much. I’ll try to answer the responses individually:

Yes, the spin bike requires a washer whereas the recumbent does not. Favero asked for pictures of both and said they looked fine:

Spin bike:


Honestly, I have acquaintances that I’ve ridden group rides with that have trainers, but I don’t know anyone well enough to ask to borrow one.

The pedal stroke of a spin bike does feel different than outside, but I’ve seen countless people on YouTube using power meter pedals with the same spin bike without problems. Favero also said spin bikes should not be an issue with getting accurate power meter readings. If you want to see how the Favero compares to a Peloton spin bike in regards to power accuracy, check this video out:

This is a great question and although I haven’t checked the data to see exactly, I can tell you what it feels like because I think it takes my heart rate and fitness out of the equation. The difference I feel can take place in the first 30 seconds of a ride. 200 watts is pretty darn tough on the spin bike. On the recumbent there is resistance, but nothing like what it feels like on the spin bike. I can hold 200 watts all day on the recumbent, but not on the spin bike. Common knowledge says the opposite should be closer to what I should be seeing.

Oops, I just realized I didn’t exactly answer the question. Max power on a sprint is different between the two. I think I’ve seen 800W spikes on the recumbent. At most I think I’ve gotten about 450W on the spin bike. Again, it just seems consistent with the crazy numbers I’m seeing. I don’t see how I could possibly put out more power on the recumbent over an upright bike.

Another great question. I know this isn’t perfectly accurate, but Strava’s weighted power averages for my outdoor rides (avg speed 21 - 22 mph) fluctuate between 200 - 230 watts. This is within the same range (power pedal meters) that I see on the recumbent, but not even close on the spin bike. I think my best average so far on the spin bike was ~137 watts and that left two huge pools of sweat on the floor and my HR was maxed out.

I wouldn’t feel comfortable asking to borrow someone’s upright bike right now given the pandemic. However, my sister-in-law does have an old upright road bike. It probably doesn’t fit me at all since I’m 6’4" and 235 pounds, but it would be worth a test just to see what numbers I might get. I’ll give this a try and report back.

Favero confirmed with me that the zero-offset was fine to do in the same way on an upright bike as a recumbent. There could be interference between the shoe and the power meter, but Favero doesn’t think so. The data I see and they see is so consistent and stable. Also, I’m using the cleats that came with the power meter so I would think this takes some of this out of the equation.

Exactly. I’ve used a Garmin Edge 530, Zwift, Rouvy, etc… They all show the same anomaly, spin bike showing much lower power numbers at the same heart rate versus the recumbent. Again, it should be the opposite.

I will do this as a test and report back. In case it helps, here are the Cycling Dynamics pedal stroke from the spin bike vs. the recumbent. They seem feasible to me, just not the power numbers.

Spin Bike (134 W):

Recumbent (213 W):

Thanks, but very unlikely. I ride the spin bike during the week and the recumbent on the weekends if the weather is ok. Recently, my recumbent rides have been rare.

Please keep in mind the difference I’m feeling between the two bikes and the amount of resistance that exists is immediate so cooling doesn’t enter into it. 200 watts is something I can do all day on the recumbent. 200 watts on the spin bike can be held for maybe 5 minutes.

I’m not expert cyclist so there is the possibility that my bike fit isn’t perfect, but I think it is close. After making a number of micro adjustments over a few months, I’ve noticed the power slightly tick up and down based on those changes. The fit now provides the most power, little to no hip movement, and absolutely no pain in my knees. Honestly, it just feels right. Also, please keep in mind that the difference in power numbers here is pretty darn big. 116 watts at 133 bpm on spin bike compared to 206 watts at 133 bpm on the recumbent. I don’t think tweaking a bike fit that I already feel is optimal could make up almost a 100 watt difference.

So true, but again, keep in mind I’ve used my HR to find sort of a baseline between the two and at the same heart rate the difference is pretty big, almost 100 watts. Also, the difference is felt immediately. 200 watts on the recumbent simply feels easier than 200 watts on the spin bike. This isn’t over time when conditioning could enter into the equation, but in the first minute of cycling.

Yes, it is a Joroto. It is nothing high-end, but has magnetic resistance and the flywheel is 35 pounds. I would say 35 pounds is about the average. Some have heavier flywheels, some are lighter. You can change the resistance, but not exactly a gear.

I notice the exact same thing. Do you notice that your heart rate increases when you sit up as well? As soon as I go back down to the handlebars my heart rate drops again.

In case anyone is curious. Here is a picture of my recumbent:

My prediction is that it’s the spin bike the pedals are having an issue with not the recumbent.

If the spin bike is direct drive fixed with a fairly big flywheel there is a chance you are putting a slight negative pressure on the pedals on the upstroke and the pedals won’t necessarily credit you for that exertion.

I’ll also echo what others have said though about heart rate not being the best metric to compare power meter setups.

As neither of the setups you are using are standard I really would recommend trying them on a “normal” bike if you can - just to see which one of your use cases is furthest away from that.

To be honest when I first read the post I was pretty sure it would have been just a case of the spin bike setup only recording data from the left pedal but this is clearly not the case based on the cycling dynamics data.

One more question… what are your torque effectiveness and smoothness scores like for the two setups?

This is really strange.

Not sure Strava accounts for lower drag of recumbent. So cant use Strava estimated power.

Reason I was asking about pedal stroke feeling inside is not that pedals wouldn’t work but that you will be able to output less power going to a low inertia inside bike. But that could account for maybe 10%-15%, you are experiencing much larger differences.

The outside Strava ride you posted, is that on a flat road with little wind? If I plug the numbers in http://www.kreuzotter.de/english/espeed.htm, then 200W should give you over 23mph speeds. And the speed you got is achievable with 115W. Now that is actually pretty consistent with the inside power you are getting.

You know…I wish I could tell you, but it wasn’t until you mentioned this that I started looking into it. I assumed Garmin collected this data, but I couldn’t find it anywhere. It turns out that my Garmin doesn’t enable this metric by default. Go figure. I didn’t buy a bike computer because I wanted less data! :slight_smile: So anyway, I just found the option buried in the menus and enabled it so I guess I’ll be able to start seeing this moving forward, but I have nothing historical. Thanks again.

True, but I do think it helps with ballpark estimates. Between Strava and my Garmin, they are pretty darn close.

For that specific ride, the speeds are inaccurate (low). This is because I used the Rouvy app on my iPhone to gather them and they use wheel circumference to gather speed. I did put in the accurate wheel circumference, but it still came out with what I know are low numbers. I then manually imported the FIT file into Strava.

Here is a more accurate outdoor ride on the recumbent leveraging my Garmin 530. This particular loop has some hills in it so that might account for the drop in speed, but still pretty close to your calculationes at an average 22.1 mph and an average 206 watts.

Again, thanks for everyone’s help here. I think I may have narrowed things down in some way today based on what many others suggested.

I decided to hook the power meter pedals up to my Son’s mountain bike. My bike fit is awful on this thing. It has huge tires and I don’t find it comfortable at all, but of course it’s for him, not me. I figured if I took it out on a somewhat flat road, I could at least see if 200 watts felt closer to my recumbent or closer to my spin bike. I only road for 30 minutes because that was long enough to get me the data I needed and as I said, it just wasn’t enjoyable due to the awkward fit. I tried to keep my heart rate around 133 in the same way my other rides had been.

Just so you know, putting the power meter pedals on this MTB required me to use the washer in the same way it does on my spin bike. I liked that since it was keeping things more similar.

It took probably only 5 minutes to realize the data I was getting back was much more akin to my recumbent than the spin bike. 200 watts just felt much easier than it did on my spin bike and very close to what it feels like on my recumbent. Here is the ride on Strava in case you are curious:

So in summary, at a constant heart rate of 133 BPM using the Favero Assioma Duo power meter pedals and leveraging the same Rouvy app on my iPhone to gather metrics (Garmin Edge 530 was almost identical):

Recumbent: Avg 204 watts
Mountain Bike: Avg 209 watts
Spin Bike: Avg 116 watts

So something is happening with these power meter pedals and the spin bike and there is nothing staring me in the face as to what it could be. Tens of thousands of people use spin bikes to train with power meter pedals. There are examples all over YouTube so I’ve got to believe it has nothing to do with the flywheel or the belt. There could be some kind of interference with the Ant+ or Bluetooth signal from the power meter pedals to the bike computer / computer / iPhone, but as I’ve just stated that has been tested on multiple platforms in many different ways and all the results are consistent. Favero has confirmed the data is consistent as well.

I don’t see that it could be the crank arms because the power meter pedals appear to bind solidly against the crank arm. However, I obviously can’t see inside the little recessed area to be sure. I just know it doesn’t move, but that also doesn’t mean that the seating is flush so I guess this is where I’m going to start. I’m not sure where to find replacement crank arms for a spin bike, but I’ll start hunting. I think this is the first logical step. I’ll keep you this post informed and thanks again for all the help.

Ok, so it looks to be the inside numbers that are too low.

If you go into the assioma settings, there is an option to make it a uno. It will then take the data from just one pedal and double it. Have you tried this Uno setting and do the same differences exist?

Do the crank arm lengths differ between outside/inside bike? And if so, are you adjusting the pedals for this? If crank arm length is adjustable on the bike computer in the pedal settings, this will override the crank length setting for the pedals in the Assioma App.

I did try this once and noticed no difference, but just did it again for good measure. My power came out a little higher than what I’ve seen previously at the same HR. I think that is probably because of a few things:

  1. I came off a rest day and felt stronger than normal.
  2. I forgot that the app I was using before was Rouvy Mobile. It doesn’t differ much from the results in Zwift, but they are there. Rouvy Mobile is usually about 2 watts less. Not so sure what to attribute this to, but I’m fine with it.

The crank arm lengths do differ between outside and inside. 172.5 outside and 170 inside. I always make the change and do a zero offset usually before riding, but always when switching bikes. I’ve forgotten a couple times to set the crank arm length on the bike computer when using it, but since its default is 172.5 it hasn’t seemed to make a big deal. Even if I intentionally adjust the crank arm length between 172.5 and 170 I don’t notice much difference, maybe a few watts here and there. It certainly doesn’t make up for the 80 - 100 watt swings I’m seeing from the spin bike to the recumbent / MTB.

Here is the Strava record from the Zwift ride riding the DUOs converted to UNO in the app on an iPhone and at a HR of 133.

I think there could be slight differences, but not much really when you are solely comparing HR and watts. Now as soon as you enter speed into the equation then it becomes something else entirely. Keep in mind that watts is similar to effort and effort is similar to what your HR should be. HR can slightly move based on a number of different things like sleep, diet, fitness, etc., but as you can see by the numbers above they are pretty close.

Where things get fun is where you use your example and look at one person’s speed at a specific HR and Power and then see how speed changes as you move from an upright position to the drops. You should see higher speeds with the drops at similar HR and Power. This is just how I understand it based on things I’ve read many times over as well as my own experience, but I’m not an expert. I am the person asking the original question here so if someone has a different understanding, please feel free to correct me.

For crank size settings, you are changing it in the Assioma app for the Zwift/Rouvy rides and it is in the correct setting on the garmin for the ride outside?

I dont know what will happen if the garmin is connected inside while using the zwift app to record. Pedals might default to garmin setting also for Zwift?

You had also mentioned that the static weight test is producing accurate readings inside?

Final thing to test is if the pedals are reporting the right cadence. Pedal to the beat of music with known bpm or count pedal strokes yourself with a timer to check.

What happens if you let some one else ride the mountain bike vs inside?

Two things:
The power numbers are calculated based on whatever crank length setting is in the device they are connected to. I would try recording power using the same device (Garmin) inside and outside, to rule out a bug somewhere using a really off crank length. Obviously 172.5 vs 175 doesn’t change power much, but maybe Rouvy or whatever else you are using is using a really bad number.

Again, I think you need to look at your cooling situation. You keep using how the effort feels vs power to “prove” something is wrong with inside power. If you have inadequate cooling this is exactly what is expected. Your body’s self defenses make it feel really hard when you are overheating. If I ride the trainer with the fans off and a shirt on, it takes about a minute to feel too hot. In five minutes you absolutely will be too hot if riding anywhere near threshold without lots and lots of cooling and little clothing. When you ride outside you are getting 15+ mph wind over your entire body. Inside even with fans it’s only partial airflow over your body.

Edit: If you don’t have enough airflow to feel COLD when you start your ride, heat will be limiting your performance, not your fitness.