I bought a PM used from ebay, installed on a set of wheels. Yesterday was my first ride outside with it and I wasn’t sure what to expect, and I truly hope this is normal. I was on a flat track so it was pretty easy to maintain constant speed and cadence. However, my power was all over the place. It was go from mid 100’s to upper 200’s second to second. I didn’t change gears, and my cadence was very steady. I was watching the power and cadence on the watch and the cadence wouldn’t change, but the power would jump back and forth.
I’m just hoping this is normal for one and that I didn’t buy a busted PM. And two, if this is normal, do you train with this? How on earth do you monitor your power when it moves all over the place.
Thanks for the quick reply. My only options are 3 seconds and 30 seconds. I did switch it to 3 seconds which reduced it a bit, but still fluctuated quite a bit. But glad that’s normal movement.
My guess is that you were showing instantaneous power. This varies due to the differing amount of force you apply to the pedals at different phases of your rotation. Switch to 3s or 5s power and you see more smooth numbers.
I was on a flat track so it was pretty easy to maintain constant speed and cadence.
The goal with a PM is to maintain constant power NOT constant speed. What you will eventually learn is that your speed will fluxuate wildly as wind and elevation change, but your power will remain consistent.
If you have access to a long hill use it to practice holding steady power. I found it took a lot of time on a trainer before I really understood how to maintain a steady output. The curse of the power meter is being able to see how often you soft pedal and how often you mash.
I was on a flat track so it was pretty easy to maintain constant speed and cadence.
The goal with a PM is to maintain constant power NOT constant speed. What you will eventually learn is that your speed will fluxuate wildly as wind and elevation change, but your power will remain consistent.
If you have access to a long hill use it to practice holding steady power. I found it took a lot of time on a trainer before I really understood how to maintain a steady output. The curse of the power meter is being able to see how often you soft pedal and how often you mash.
I shouldn’t have phrased it like that. I know the goal is to maintain power. I was saying that because I wasn’t totally sure the PM was working. But was just stating that i was on a flat course, not changing gears, cadence or speed. And I was assuming that based on that the power should be relatively flat.
If your not a super smooth peddler then it’s gonaa look choppy. Unless your working on Sprints then set it to either 3 or 30 and you will only view those averages. All the hills and valleys will be collected for analysis though once you get home.
I was on a flat track so it was pretty easy to maintain constant speed and cadence.
The goal with a PM is to maintain constant power NOT constant speed. What you will eventually learn is that your speed will fluxuate wildly as wind and elevation change, but your power will remain consistent.
If you have access to a long hill use it to practice holding steady power. I found it took a lot of time on a trainer before I really understood how to maintain a steady output. The curse of the power meter is being able to see how often you soft pedal and how often you mash.
maintain it within a narrow power band, not constant power. even climbing, power output could differ by at least +/- 5% or more
And I was assuming that based on that the power should be relatively flat.
Power meters do not really provide instantaneous real time data to your computer. Rather most pass on data only every second or so. If that is catching you at different spots in a choppy pedal stroke every few seconds, your numbers are going to jump around more than they might for someone else with a smoother stroke.
Thanks for the quick reply. My only options are 3 seconds and 30 seconds. I did switch it to 3 seconds which reduced it a bit, but still fluctuated quite a bit. But glad that’s normal movement.
I suggest you increase the number of fields displayed on that page of your Garmin so that you can display both 3 sec and 30 sec power side by side. With this configuration, you will still get fluctuations in the values for 3 second power but not near what you see with instantaneous power values. The 30-second smooths it out a bit and allows you to better meter your efforts during targeted/interval training.
And I was assuming that based on that the power should be relatively flat.
Power meters do not really provide instantaneous real time data to your computer. Rather most pass on data only every second or so. If that is catching you at different spots in a choppy pedal stroke every few seconds, your numbers are going to jump around more than they might for someone else with a smoother stroke.
The only common power meter that’s affected by intra crank variations in torque is a Powertap since it uses time based averaging of torque samples (every second).
It’s nothing to do with how “choppy” or otherwise your pedal stroke is but rather that the Powertap samples torque over a variable and fractional number of pedal revolutions each second. You could be a buttery smooth pedaller riding at a precise speed on rollers, and a Powertap can report greater second to second power variability than actually happens.
Crank/spider/pedal based power meters use event based averaging, IOW they average the torque samples over whole complete revolution(s) of the crank and are (mostly) immune to such aliasing.
As for the OP - power output is naturally variable, but a Powertap can also introduce some additional artificial variability in second to second power values reported due to this time based averaging of torque moving in and out of sync with your pedalling. It doesn’t affect power averages over longer durations, e.g. 5-10 seconds or longer. The level of artificial variability depends on cadence, e.g. it will be greater at ~80 or ~100 rpm (give or take a few rpm) than at 60, 90 or 120 rpm.
As others have said, 3 or 5 second rolling average for the display is usually helpful to smooth out such fluctuations, but get used to the fact that power does vary. It’s normal. Whether or not your level of variability is a problem, well that’s a different question, and I’m not sure you could answer that via a forum, I think that’s something that requires more personal observation of you riding a bike. At the very least an inspection of your power trace during a reasonable level of effort, e.g. a hard solo ride at tempo or threshold for 20+ minutes over consistent gradient terrain, and again over variable terrain.
The track you are riding on, I’m assuming it was Carrier Park, isn’t totally flat. IIRC coming out of the final turn towards the finish is a touch uphill and going into turn 1 is a little downhill. When I first came back from my crash and wasn’t 100% I rode a lot of my initial rides there and saw the same thing, although not as wide of a range.
One other thing to keep in mind. It was your first ride with power, don’t worry about the fluctuations. That happens to just about everyone. Over time, or at least it seems with the power meter user population I’ve dealt with, everyone learns to ride smoother.
Make sure you’ve set your head unit to Rolling 3 seconds as others have said.
Thanks for all the feedback, it was very helpful, and also good to know that the PM is behaving normally. Can’t help it but I’m always a little suspicious of expensive purchases on ebay. You just never know.
And yes it was carrier park. I didn’t think it was pancake flat, it seemed there was an ever so slight rise where you’re talking about. I did switch it to a 3 second average towards the end and it smoothed out a bit.
It’s sounds like your first ride with PM and mine were identical. You could have taught a trig class off my data chart. Cool thing is now you have identified one thing to work on and have a tool to help you address it.