Doubting power meter readings after service & re-calibration

MishMash95

Hey,
So I had been having a fair bit of mechanical trouble with my bike, virtually unable to get out of saddle/put out higher wattage due to the chain dropping. Had a service, but next few rides ended up yielding mediocre power. Tried out a local chaingang, and on a rolling course, broke away early with 2 other riders and averaged 38.5km/h over the course, though my avg power only came out at 205w despite pulling pretty much 1/3rd on the front and a high perceived effort. Never quite sure if this was normal, as I hadn't done too much riding in that format since getting a PM, so don't quite have a perception of how much you save.

Bike was still dropping the chain under load so took it back to the shop. Got some new chainrings fitted and decided to recalibrate the powermeter after, just to be sure. Then went out on a ride and suddenly felt incredible! Despite carrying a bit of fatigue into the ride, the power readings were coming out rather high. Not out of this world, and technically within my ability, but it felt a fair bit easier than the previous days.

I'll also throw in that the seatpost was raised about 1.5cm (as before it was a bit low, but had seized) and am wondering if that can make a difference to power output and rate of fatigue.
https://www.strava.com/activities/1696028987/overview

This was my ride, do you reckon is 34km/h reasonable for 263w over mostly flat terrain with a few small humps and bumps? (multiple riding positions, mixing between aero and upright), or is the PM likely over-reading?

Obviously I know equipment can make a difference and there are a million other external factors, but wanted to know if you guys had any tips for telling if readings are consistent? I'm ideally hoping that i've just gotten fitter, but am always doubtful whenever I have a crazy ride which massively exceeds my expectations.

My hope is that those "bad" rides I had before re-calibrating were just the result of the PM being mis-calibrated after the bottom bracket was stripped, but no idea if this is reasonable.

cougie

Almost impossible to say. What's your FTP ?

MishMash95

Yeah fair :P! Haven't tested in a while, should be atleast 270w, that's what I used to feel i could sustain for an hour (though never did in practise as I don't really do TTs often, and tended to focus on intervals). For a 20 min effort, i'd probably try and target 295-300w and see where that goes. Not sure if I could sustain it, but feel I could get close. But that could be based on wrong numbers. So being optimistic, perhaps even 280w.

Probably time to get a HR monitor actually.

cougie

You've got a power meter but no HRM ? Weird.

They're your figures. None of us can tell from one ride how accurate your figures are.

Alex_Simmons/RST

Perform a static slope calibration of your power meter.

paul2718

I looked at some segments on your ride, against other riders on the same day with power, nothing obviously anomalous. You could do the same, particularly against your previous previous efforts.

Doesn't mean it's right, just not obviously way out. I have a Stages that got into a mess recently and was obviously over-reading at lower outputs, under-reading at high. It was really good fun on Road Grand Tours being able to hold 350W up hill... The PM wouldn't calibrate from Garmin and I ended up removing the battery to completely reset it. Since then it has behaved itself and my athletic ability on the turbo is back to normal.

craigr

MishMash95 wrote:

I'll also throw in that the seatpost was raised about 1.5cm (as before it was a bit low, but had seized) and am wondering if that can make a difference to power output and rate of fatigue.
https://www.strava.com/activities/1696028987/overview

Yes if your seatpost is too low it can affect your power. I moved mine whilst cleaning bike and put it at wrong height, like you a bit too low. Just felt like i couldn't get any power in the climbs whilst seated and fatigued a lot quicker too.

Sometimes some power meters take a bit to settle after new installations if i remember right, what one are you running?

As for calibration, i calibrate mine before every ride as only takes a few seconds to do.

sloppyschleckonds

cougie wrote:

You've got a power meter but no HRM ? Weird.

Here we go again

Matthewfalle

What tyres are you using?

Alex_Simmons/RST

craigr wrote:

As for calibration, i calibrate mine before every ride as only takes a few seconds to do.

That only checks/sets the torque zero (i.e. confirms it reads zero torque when no torque is applied). It doesn't however calibrate the power meter. To do that also requires validating that measured torque = known applied torque.

It can be like having a tape measure made out of a stretchable material. You align the zero mark OK but anything above that can still be wrong depending on how stretched the tape is.

Which is why I suggest performing a slope calibration check.

Not all meters however permit one to do so.

MishMash95

Cheers for the replies! Yeah the main difficulty is that I recently moved to a new area at exactly the same time as the mechanical issues started, so I don't have much frame of reference against normal segments. I did the same loop yesterday at a lower power and speeds do seem to still correlate reasonably with what you would expect (~220w yielding a 32km/h avg speed vs the 270w @ 35.6km/h).

I guess the main doubt came from the fact that I just didn't feel that fresh going in but seemed able just to really hit out :P I have done efforts close to that before but am normally need a few min recovery after doing 300w for any extended period of time (8 min in this case), but after my first 8 min interval, was able to carry on riding "hard" for another 8 min after a 20s rest at a junction. Though on the flipside, have been training decently hard recently.

Matthewfalle wrote:

What tyres are you using?

Continental GP4000s, though front one does need replacing soon. Running at 100 psi, 25mm.

craigr wrote:

Sometimes some power meters take a bit to settle after new installations if i remember right, what one are you running?

Running a single-sided 4iiii precision power meter, on shimano 105 crank.

Alex_Simmons/RST wrote:

Perform a static slope calibration of your power meter.

Alex_Simmons/RST wrote:

Which is why I suggest performing a slope calibration check.

I had a search online but could not find information on how to do this? Do you know of any resources where it is explained? The 4iiii App only offers scale factor and zero offset options, which I do understand to only as you said, adjust the offset, but not necessarily account for accuracy across the full range.

I guess the numbers do still fall into a mostly reasonable range, so for now i'll hope it's actually the result of a better position and training. It's definitely close enough to past efforts that I know its possible, and while it was a hard session, it did feel easier than it should have done. Perhaps not significantly, but it was about 8-10w higher than a similar session of riding threshold/VO2max intervals that I did 5 weeks ago, with a slightly lower perceived exertion.

Perhaps i'll just leave it and use it as an opportunity to brag about increased numbers that may not actually bear any practical performance increase :P

Alex_Simmons/RST

If you can get a head unit to display torque values, that's all you need (along with an accurately known mass of say ~20kg).

That said being a unilateral meter means there is an unknown and variable error due to the assumption made by such meters.