More ZO Zen

A follow up to my post the other day about the setting of zero-offset / torque zero on power meters, and how we need to be sceptical about how auto-zero functions operate (if your power meter uses one).

I received feedback to suggest that auto-zero on SRM could not be as bad as I am suggesting, and that it might indeed be more accurate to leave the auto-zero on.

Well it hasn't been my personal experience that the auto-ZO is as reliable as theory might suggest, so I thought I'd do a test when I got the chance. More backyard science.

So today happened to be a lovely day, and I decided to go for a ride. Not a long one mind you because right now I'm about as fit as Harry the Hairy Nosed Wombat, but a long enough ride outdoors under fairly typical riding conditions for me.

For those that know Sydney - the ride went from Annandale, through Stanmore, past Sydney University, Redfern and onto Centennial Park where I did a 20-min "test" and rode back home again. I've done that ride about a bazillion times.

And before leaving I set my Powercontrol VI to use SRM's auto-zero function, and had my phone camera along to take a few snaps, to see what I noticed along the way. Here's the Powercontrol screen showing zero-offset before starting my ride:

The bottom line is the zero-offset value stored by the Powercontrol, and is the value used when calculating (and storing) power values. The middle, larger number, is the "live" zero-offset reading, akin to the offset number shown in the video in my previous post. i.e. if you apply some force to the cranks, that's the number that will fluctuate along with the force being applied. The "Auto" along the top just tells you that the auto-zero function is enabled.

The temperature inside and outside my home was not all that different, and my meter  had about 10-minutes outside before this initial check. This is a different SRM to the one in the video (different bike), although it's the same model of SRM.

So, my starting zero-offset was 409Hz.

After about 15-minutes I'm at some traffic lights near Redfern Oval, so I take the chance to pull over to the side and see what the zero-offset has done.

We can see that at some stage along the way in that initial 15-minutes, auto-zero has reset the zero-offset value to 417Hz, while the actual zero-offset is 407Hz, 2Hz less than when I left home 15-minutes earlier.

Just so that's clear, that incorrect zero-offset value is now being used to calculate all my power numbers. I have no idea when or how many times during that initial 15-minutes of riding the zero-offset was changed, nor what the size of those changes might have been, other than when I stopped to make this check.

OK, so I continue on to the Park and do a 20-minute test effort. Then after that I leave the Park to head back home, stopping on the bikeway alongside Moore Park to do another check. This is what I see:

Auto-zero has set zero-offset to 419Hz, when the actual zero-offset is 409Hz, same as when I left home about 50-minutes earlier.

Continue on home, and this is the final check I made after about 75 minutes of riding:

Auto-zero has set zero-offset to 403Hz, when the actual zero-offset is 407Hz.

So, to summarise in table format:

So, I have four actual zero-offset readings in a 1:15 ride that vary by only 2Hz (and this is pretty typical for my SRMs, i.e. not much drift in zero readings), yet the auto-zero function has reset the zero-offset value with a range spanning 16Hz. And that's just what I know it's done, let alone what I don't know it's done. As Rumsfeld would say, it's a known unknown.

Perhaps now you can see why I don't use auto-zero on my SRM Powercontrol.

The possible impact to my average power on this ride of a 16Hz differential in zero-offset is 3.5% and for my modest 20-minute test effort today, that's 8 watts. I reckon 8 watts is worth knowing about no matter how fast you are. If it were true, that's nearly one second per km in a time trial. But it ain't.

Now I have no idea whether the auto-zero performed better or worse than that on average because we just don't know. We can never know since no power meter keeps a log of zero-offset changes.

As I said in my previous post, such anomalous changes in zero-offset would make some analysis not worth doing (e.g. aero field testing when you are fine tuning equipment and position choices). I don't know about you, but I think a possible 3.5% variance is pretty significant. It's not something you can correct post-hoc either, since there is no record of what and when changes to zero-offset were made (power values are calculated based on the zero-offset value used at the time of recording).

At least with the Powercontrol, you can easily turn off the auto-zero function (just press the "Pro" button on the zero-offset screen), and checking the zero-offset is trivial press of the Mode & Set buttons at same time.

That's far better than having to navigate through various menus to perform one of the most important checks a power meter user needs to make every time they ride, let alone not being able to disable the auto-zero.

Of course YMMV

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Three, Two, One, Zero Offset

Some more backyard science. Well, training room science perhaps.

This one was prompted by occasional power training forum discussions relating to the setting of torque zero on a power meter, and the auto-torque zero feature on some crank based power meters.

It was also prompted by an addition to my training room set up, which now means I am able to view on a computer screen my SRM zero-offset numbers. That's kind of handy as anyone with an SRM Powercontrol knows, the zero-offset screen only stays on long enough to do a check and set the zero-offset, but then reverts back to the main display screen after a short delay, which is fine for its intended purpose. Since I'm doing something unintended, having the zero-offset on permanent display helps.

So for some fun I put my phone in front of the screen to video record my SRM's zero-offset numbers while testing a few things, namely, how the zero-offset numbers vary from unclipped to being clipped into the pedals. How stable was zero-offset when clipped in? When moving a little but still not attempting to put pressure on the pedals? And what happens when I back pedal?

And while this was on an SRM, the issues arising are applicable to all crank based power meters.

This was the result. It's a 3-minute long video.

OK, so my video ed skills ain't quite up to Francis Ford Coppola standards. The white noise you can hear is my fan that I had left running. Summary thoughts are shown in the video.

Just for the record - here are some more thoughts on this subject:


SRM have an auto-torque zero feature on their wireless units. If you are using an SRM Powercontrol, the auto-zero feature can be enabled or disabled. How the auto-zero function operates is not documented in SRM public literature or on their website, so when and how it invokes is a bit of a mystery. as follows:
1. Speed must be > 0.
2. Cadence must be 0 for at least 5s.
3. The Zero offset must not vary by more than +/- 4 Hz.
If all three of these conditions are met, the new zero offset is the average of the values over the 5s.
Thanks to the contributor that updated me on the SRM function from the German manual.

It's my personal experience that it can generate zero-offset values that are way off. I recommend disabling auto-zero and doing manual zero-offset checks (the same as you do with older wired models). Interesting that SRM says it requires speed > 0, as that implies it also requires a speed separate speed sensor for auto-zero to work.

However, if you use a Garmin device as your head unit with your SRM, then you will have no choice as you presently cannot disable the auto-zero function. In my view that's a significant functional flaw that Garmin and SRM should fix. How significant? For example, I would not rely on Garmin captured data from an SRM when performing aerodynamic field tests. Use an SRM Powercontrol.

Quarq does not have an auto-zero function, the user needs to choose to perform a torque zero (which is fine by me, it's far better than having an auto-zero you can't disable and have no control over or knowledge of when it happens). A torque zero can be done manually as normal or by back-pedalling the cranks a sufficient number of rotations (at least four).

Back pedalling to set a torque-zero is convenient for sure but introduces an error similar to that described in the video. The size of that error will vary and depends on how different your individual reading is compared to the fixed back pedal torque value assumed by Quarq. Best to check and set your torque-zero manually, and unclipped from the pedals, and preferably not when coasting either (on many bikes this latter item is no big deal but some have a bit of freehub drag that can apply positive torque to the cranks while coasting).

Power2Max enables you to do a manual torque zero check as normal and it also uses an auto-zero function which you cannot disable (at least not with a Garmin). P2M have publicly stated the auto-zero function will only trigger if the torque readings are stable for a period and presumably the crank is not rotating for a few seconds. The maximum torque variance that would trigger an auto-zero being no more than one "ppm", with "ppm" being the unit the P2M uses for torque measurement (each power meter reports using different units).

Using a filter of stable torque readings makes sense to prevent erroneous torque-zero values but I wonder how often that actually happens given it was not easy for me to keep a stable zero-offset even when on a trainer and able to focus on doing just that.

P2M's reported torque units are about one-quarter to one-fifth as sensitive as those displayed by an SRM. So the torque values a P2M would interpret as being a stable zero point and trigger an auto-zero, would be the equivalent of an SRM zero-offset value being within a range of ~5-10Hz. So while it seems likely that the P2M will trigger an auto zero when coasting with reasonable frequency, the consequence of this level of (in)sensitivity in the torque range used to trigger an auto-zero means it could well introduce a random error of up to +/- 5W in power readings.

Powertaps of course are not subject to the same issue of trying to deal with pedal forces when coasting since they are measuring torque at the freehub, and so an auto-zero can be invoked as the hub will know when it is coasting (and hence no torque is being applied). It's not perfect either, and there are situations when it might be fooled, but in general it works reasonably well.

Note that the Powertap auto-zero feature when using a Powertap Cervo head unit (Little Yellow Computer) will only work if the torque-zero is not too far out of range to begin with (up to 8 Powertap torque units I think but that's from the dark recesses of my memory), so it's important to perform a manual torque-zero check before starting any ride. I don't know if this function is the same when using Garmin head units. Auto-zero on a Powertap can be disabled on both the LYC and Garmin.

Finally, the torque units reported by a Powertap and used to invoke an auto-zero are about one seventh as sensitive as those on an SRM (it depends on the gearing used and the range is typically one-quarter to one-tenth as sensitive as an SRM for an equivalent crank torque), but at least it has the advantage of being isolated from pedal forces by the freehub.


As a general comment, power meter head units really should be recording torque-zero values and keep a log of when and to by how much those torque-zero values have changed. This is important data to enable forensic examination of a power meter's performance and accuracy. At present, the only power meter head unit to record a torque-zero value in its file is an SRM Powercontrol. Even then, it only records the most recently set zero-offset value.

So while we keep seeing a stack of features being implemented in each new generation of head units, the most basic, fundamental and important feature of a power meter, i.e. the quality of the power data, does not always get the attention it deserves.

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The bathroom scale analogy

Power meter accuracy and calibration 101

This is not a complex item, but I often see confusion* over the issue of power meter calibration, torque zero, zero-offset etc, so I thought I would use a simple analogy to help people understand the basic differences in what these terms mean.

There are many things that can affect the accuracy^ of power meters, but let's talk about one of the most important, i.e. the person using the power meter.

Most common on-bike power meters in use today (e.g. SRM, Powertap and Quarq, and more recent offerings from Power2Max and others) require a user to do three things for accurate data:

1. Pair the handlebar computer with the right power meter – this might be via a wireless protocol such as ANT+, or by simply plugging the two together via their wiring harness
2. Check the torque zero before and occasionally during a ride (torque zero or "zero-offset" as referred to by SRM are interchangeable terms in this context)
3. Check / validate the correct slope calibration of the power meter is being used

How you do #1 will vary depending on the type of handlebar computer and power meter used and as always, reading the manuals is a worthwhile investment of your time (ugh I hear you say). Typically it's not a difficult thing to do.

However I want to elaborate on #2 (torque zero / zero-offset) & #3 (slope calibration) via an analogy – the ubiquitous bathroom scales that many have a love/hate relationship with.

To demonstrate the difference between  "zero-offset" and "slope calibration" and their importance, I'm going to share with you a simple experiment - checking the accuracy of an old set of bathroom scales I have. They are the old fashion type with an “analogue” display that rotates around when you hop onto the scales.

Here’s a pic of the scale's reading before I place a known weight on the scales. The lower scale is kilograms (kg) and the upper scale is stone and pounds. I'll stick with kg for now.

We can see they are reading +4kg when there is nothing on the scales. Clearly the “zero-offset” is wrong. So if I placed a known mass on the scales, I should expect the scales will read 4kg too high.

So, let’s place an accurately known weight on the scales. I just happen to have an accurately known weight of 31.210kg. Rounding to 31.2kg will do for this example. This is what we see: 

That’s reading 34kg. But hang on, shouldn't we expect the scale to read 35.2kg  = 31.2kg (actual weight) + 4kg (the "zero-offset")? 

Well yes, we should, but it isn’t. Hang on to that snippet - we'll get back to it shortly.

The scales have a small “zero control” knob, which I can turn so the scales are reading zero when I am not standing on them. All we are doing is validating that, when no weight is on the scale, it displays a zero value. OK, so let's correctly set the “zero-offset” on the scales: 

and now put the weight back on the scales again: 

Now it says 30kg. Hmmm, so even though the “zero-offset” setting is correct, my scales are under reading the actual weight by 1.2kg or about 4%.

Let’s plot those readings.

There are four readings. The two for when the scale’s zero-offset was +4kg (the green triangles and line), and the two when the scale’s zero-offset was 0kg (the red squares and line).

The horizontal axis is the actual weight placed on the scales, which in this case is either 0kg or 31.2kg. The vertical axis is the reading provided by the scales.

So now we can visualise two things:

• the “zero-offset”, which shows us how much the scales read when there is no weight applied, and 
• the “slope”, of the scale – in other words, how much weight the scales report increasing by for every kg of actual weight placed on the scale.

This slope can be calculated as follows:

[Reported weight - Zero-offset weight] / Actual weight

In this case for both sets of readings, the slope is 0.96.

Hence, if I stood on these scales, and the zero-offset had been set correctly to 0kg, and the scales read 83kg, I would actually weigh 83 / 0.96 = 86.5kg.

So even though the “zero-offset” has been correctly set to zero, this does not mean the scales have been calibrated, nor that they are accurate. All we know after performing a "zero-offset" is they will read correctly when there is no weight on the scale - but that does not ensure accuracy when we step on the scales. 

In order for the scales to be accurate, we need to know not only the zero-offset is correct but also their slope is correct. In this case the slope of the scales is wrong, and hence the weight reading will be wrong unless I apply the correct slope to the "raw" data.

The exact same principle applies to bicycle power meters. Instead of weight on a scale, most power meters measure the torque (twisting force) applied to a bicycle component (using special gauges). The most common meters measure the forces at the crank spider or at the rear hub but forces can also be measured at the pedal or cleat, the crank arms, or the rear cog (or even the chain). Besides measuring the torque applied to the component, all that is required to determine power is the the rotational velocity of the component (revolutions per unit time).

So to complete the analogy:

• The zero-offset (or torque zero) of a power meter is the torque reading when there is no force being applied to the crank (or hub) and is analogous to the bathroom scale's reading with no weight on them. Various power meters report in different units.
• The slope of a power meter is a value indicating the increase in the reported torque readings per unit of actual torque applied to the crank (or hub) and is analogous to knowing how much the bathroom scale's reading changes for each kg of actual weight we put on them.

Checking and/or re-setting the torque zero (zero-offset) of your power meter before and occasionally during a ride is a necessary and sound practice, 

however

unless you also know the correct slope of your power meter is being used, then the data may still be inaccurate.

Torque zero / zero-offset is something that will naturally vary, in particular with ambient temperature, but other things can affect it too, which is why it is good practice to always check it and do so regularly. The better meters have predictable and minimal zero-offset "drift", and some have firmware designed to automatically adjust the torque zero while riding, which may or may not be user enabled (depends on the meter).

This auto-zero / correction feature may or may not be a good thing depending on how it has been implemented. In my opinion, I consider knowing how and when such changes occur to be useful and valuable information when evaluating the possible errors in reported power data.

There are also some things that can affect the slope of your meter between when it left the factory to when it is finally installed on your bike, so I encourage you to have the slope validated while the meter is actually on your bike. Slope checks are best done at a 6-12 monthly intervals, or whenever you make changes to the crank's set up (such as changing cranks arms or chainrings).

Some power meters have far more stable slopes than others. It’s not a difficult thing to check yourself, but I’ll look at providing an example of that process in another post.

In the meantime, the good folk at Quarq have provided a video to demonstrate the slope checking process for their power meter. It's a similar process for other meters but the means to obtain the torque numbers and calculate the slope will vary.

As a final comment - it's possible to post-hoc correct power data that has had an incorrect slope applied but an incorrect zero-offset/torque zero can be a lot more difficult (if not impossible) to correct, and especially so if that zero-offset has been drifting. For SRM users, applying a slope or zero-offset correction is pretty trivial to perform using SRMwin software.

*  The confusion hasn't been helped when one of the major manufacturers of bicycle computer recording devices (i.e. Garmin) use the terminology "calibration" for their device, when the specific function they refer to as "calibration" it is not a true calibration. If you use a Garmin computer, and "calibrate", I suggest in your own mind to replace the Garmin word "calibration" with the words "torque zero".

^  When using a power meter, we want to ensure that the data is as accurate and precise as possible. We do this for many reasons, in particular so we can make valid comparisons of performance changes over time (keeping in mind that the gains at high levels of relative fitness are only a handful of percent and people may not use the same power meter their entire lives). There are also many performance analyses that require accurate and precise data to make valid but important choices about performance matters, e.g. the testing of aerodynamics, or tyre rolling resistance. Anyway, I’m not going to labour why accuracy and/or precision is important, that’s for another discussion.

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