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Cranky

Power meter is doing weird stuff

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So today I have a 90 min spin, zone 1, @ 90RPM. 

I wasn't worried about how many Watts I'd be pushing, just wanted to try and keep 85-90rpm for the whole set.

I noticed at the start I was just over 50%ftp. No worries.  I'm now 58 mins in, haven't changed gears, cadence has only slowed by ~3rpm but I'm at 65% ftp. How is that possible? 

I've noticed similar before but as it wasn't one gear the whole way so I never really thought too much of it. I just thought I was finding it easier to increase my watts because I was warming up.

Does it have something to do with the tires warming up? Do they, when they are warm, expand and therefore create more resistance on the wind trainer? Or is my power metre shit!? I have just replaced the battery so it's fine.

I'll attach the graph when I'm done. 

Edited by Cranky

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It's stages isn't it?

what is the difference 10 watts to 50 watts , if it's only ten then it probables temp change

if it's 50 then it's your Pm

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What sort of power meter and what sort of trainer?

Trainer resistance can vary at same wheel speed due to temperature changes in the resistance mechanism (whatever form that takes). Depending on the trainer it may get harder or easier as it warms up.

Check your power meter's torque zero periodically during such a session to test whether or not it is also drifting. Torque zero drift will cause the meter's reported power to drift away from your actual power output.

If the meter is unilateral, i.e. only measures one side, then it's also possible your power balance varies during a session.

And it can be a combination of all of the above.

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Power meters, they're a load of crap. No good can come of them!

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24 minutes ago, Alex Simmons said:

What sort of power meter and what sort of trainer?

Trainer resistance can vary at same wheel speed due to temperature changes in the resistance mechanism (whatever form that takes). Depending on the trainer it may get harder or easier as it warms up.

Check your power meter's torque zero periodically during such a session to test whether or not it is also drifting. Torque zero drift will cause the meter's reported power to drift away from your actual power output.

If the meter is unilateral, i.e. only measures one side, then it's also possible your power balance varies during a session.

And it can be a combination of all of the above.

It's cranky you will need to either explain all of these or we will have threads asking how to do each thing...

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34 minutes ago, Alex Simmons said:

What sort of power meter and what sort of trainer?

Trainer resistance can vary at same wheel speed due to temperature changes in the resistance mechanism (whatever form that takes). Depending on the trainer it may get harder or easier as it warms up.

Check your power meter's torque zero periodically during such a session to test whether or not it is also drifting. Torque zero drift will cause the meter's reported power to drift away from your actual power output.

If the meter is unilateral, i.e. only measures one side, then it's also possible your power balance varies during a session.

And it can be a combination of all of the above.

Alex is on the money as usual 

my bet is it's a combo of all above 

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1 hour ago, Alex Simmons said:

What sort of power meter and what sort of trainer?

Trainer resistance can vary at same wheel speed due to temperature changes in the resistance mechanism (whatever form that takes). Depending on the trainer it may get harder or easier as it warms up.

Check your power meter's torque zero periodically during such a session to test whether or not it is also drifting. Torque zero drift will cause the meter's reported power to drift away from your actual power output.

If the meter is unilateral, i.e. only measures one side, then it's also possible your power balance varies during a session.

And it can be a combination of all of the above.

PM - Stages.

Trainer - Don't remember the brand. Nothing special. Hook it up, tighten the resistance, pedal.

Torque zero - What?!?!

I do calibrate it nearly every session.

 

1 hour ago, goughy said:

Power meters, they're a load of crap. No good can come of them!

What about a new thread? That's always a good thing!

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5 hours ago, ironpo said:

It's stages isn't it?

what is the difference 10 watts to 50 watts , if it's only ten then it probables temp change

if it's 50 then it's your Pm

Yeah, it changed about 30watts, give or take a bit

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3 hours ago, Cranky said:

PM - Stages.

Trainer - Don't remember the brand. Nothing special. Hook it up, tighten the resistance, pedal.

Torque zero - What?!?!

I do calibrate it nearly every session.

Stages - so the issue with variable power balance is a factor. That's an unknown and variable error that is a consequence of the type of meter and human physiology. Stages can't help you with that (except to sell you one of their newly announced bilateral side power meters).

Torque zero is what you call "calibrate". I recommend checking that "calibration" regularly during a trainer ride as an experiment to see if it changes. It may or may not vary but it is good that you are at least checking/setting before each session. If you find that it does vary during a session, then you'll need to include regular breaks during indoor sessions to re-calibrate.

Trainer - it's highly likely the resistance of the trainer is not consistent with the wheel's speed during a session. That's not an issue of the power meter but of the type of trainer and how it is set up. e.g. how tight the tyre is pressing against the roller, tyre pressure, type of tyre, how the temperature fluctuates during a trainer session (it can go up and down depending on what you are doing).

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1 hour ago, Alex Simmons said:

Stages - so the issue with variable power balance is a factor. That's an unknown and variable error that is a consequence of the type of meter and human physiology. Stages can't help you with that (except to sell you one of their newly announced bilateral side power meters).

Torque zero is what you call "calibrate". I recommend checking that "calibration" regularly during a trainer ride as an experiment to see if it changes. It may or may not vary but it is good that you are at least checking/setting before each session. If you find that it does vary during a session, then you'll need to include regular breaks during indoor sessions to re-calibrate.

Trainer - it's highly likely the resistance of the trainer is not consistent with the wheel's speed during a session. That's not an issue of the power meter but of the type of trainer and how it is set up. e.g. how tight the tyre is pressing against the roller, tyre pressure, type of tyre, how the temperature fluctuates during a trainer session (it can go up and down depending on what you are doing).

Thanks. Will give the re-calibration a go during a couple of workouts and see what happens.

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As Alex said - the type of trainer is important - Specifically fluid trainers have a very different power curve when cold compared with when they are warm. Check what trainer you have and get back to us.

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On ‎5‎/‎09‎/‎2017 at 1:48 PM, Cape_Horn said:

As Alex said - the type of trainer is important - Specifically fluid trainers have a very different power curve when cold compared with when they are warm. Check what trainer you have and get back to us.

CycleOps Fluid 2 Trainer

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Emailed Stages Support. Here's what happened:

ME -

Today I hada 90 min spin, zone 1, @ 90RPM. 

I wasn't worried about how many Watts I'd be pushing, just wanted to try and keep 85-90rpm for the whole set.

I noticed at the start I was just over 50%ftp. No worries. But about an hour in, hadn't changed gears, cadence hadonly slowed by ~3rpm but I was at ~65% ftp. How is that possible? 

I've noticed similar before but as it wasn't one gear the whole way so I never really thought too much of it. I just thought I was finding it easier to increase my watts because I was warming up.

I have just replaced the battery so it's fine. And I calibrate it every couple of uses.

 Here's the link to my session. I only changed gears in the first 10 or so minutes. I then stayed in the same gear for the rest of the session.  https://connect.garmin.com/modern/activity/1956846812

I don't have the serial number. I can get it when I get home, if that helps.

 

THEM -

Hi Jo,


Thanks for email and I am happy to help. Unfortunately I was unable to download your ride for analysis but what I have seen on indoor trainers is that over the course of a ride, as the trainer warms up, resistance will change and increase. So, sitting in the same gear at the same cadence over time will be harder. If you have a heart rate monitor I would like to see if you see if your heart rate drifted up as well. That would also be indicative of the resistance in the trainer skewing the values higher. 

 

Thanks and let me know what you find, 

 

ME -

My apologies. The session was locked so that’s probably why you couldn’t access it.

 

Here’s the link again https://connect.garmin.com/modern/activity/1958832369 It has HR data too.

 

Also, here is todays https://connect.garmin.com/modern/activity/1962557377

 

You mentioned that as the trainer warms up it might get harder? It actually gets easier. I am able to produce the same power in the same gear with a lower cadence.

 

I also stopped once during both sessions to recalibrate, to double check.

 

Session 1 –

4:10am: 841 (before session)

4:46am: 845 (during session)

5:28am: 841 (end of session)

 

Session 2 –

4:53am: 841 (before session)

5:08am: 842 (during session) – after one of five efforts. Watts were staying the same but cadence was slowing (ie – easier to make the required power)

 

 

THEM -

Hi Jo,


For whatever reason Garmin Connect is not allowing me to get a look at your files. With a calibration values in that range, the meter will be ready consistently accross the ride. If the ADC were shifting in the 50-100 point range we would start to see extreme variations caused by the meter. That being said I am pretty limited to troubleshooting remotely and if you would like to coninue troubleshooting the distributor in your market prefers to work with Stages dealers in the field. For the fastest support we recommend reaching out to your local Stages dealer, as we are limited to remote troubleshooting.

 

A list of dealers in your area can be found via the following link: https://www.fesports.com.au/storelocator.php?id=253

 

Thank you,

 

 

 

Any ideas why they couldn't open it? Can anyone here open it?

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I could see them, you Monster you! Granted it was only like a summary without any charts or anything, but this was on my tablet and was a mobile view.

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I am on a pc and only get the summary stuff also... I have always found that as the trainer warms up (if fluid) that it becomes easier not harder which is at odds with what he is suggesting...

 

 

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Since torque zero is not changing much, then that rules that out. So the only other factors are the trainer's resistance changing during a session and power balance changing during a session.

I can open the Garmin link but it only shows summary stats. not a plot of power and speed data.

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"You mentioned that as the trainer warms up it might get harder? It actually gets easier. I am able to produce the same power in the same gear with a lower cadence."

Cranky, if you are producing the same watts at a lower cadence doesn't that mean that it is harder? Ie if you kept cadence the same you would need more power.

You seem to think that pedalling slower is easier, no less power is easier.

 

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On 08/09/2017 at 0:10 PM, goughy said:

I could see them, you Monster you! Granted it was only like a summary without any charts or anything, but this was on my tablet and was a mobile view.

That's weird. I thought it would take you to what I would see.

19 hours ago, ashley_s said:

"You mentioned that as the trainer warms up it might get harder? It actually gets easier. I am able to produce the same power in the same gear with a lower cadence."

Cranky, if you are producing the same watts at a lower cadence doesn't that mean that it is harder? Ie if you kept cadence the same you would need more power.

You seem to think that pedalling slower is easier, no less power is easier.

 

But I'm pedaling slower, in the same gear, so speed is slower and I'm not having to work as hard to keep the same power.

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