# Garmin/Powertap users..do you 'zero/calibrate' before every ride?



## nyvram

I 'calibrated' mine the other day before I went for a training ride (Garmin's term..not mine) which makes it zero when the bike is standing still.

My power numbers seemed to be higher than before. Should I 'reset/zero/calibrate/whatever' with the bike standing still and PT awake before every training ride? How often do y'all do this? I only did it once back in Feb until earlier this week.


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## Wookiebiker

You do realize there is an "Auto Zero" function on the Powertap (at least the ANT+ versions)?

If you are not using that...then you need to do it before every ride for the most accurate power numbers.


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## nyvram

i *JUST* now saw that.  i have no idea if that is checked or not..will look when i get home. I kinda think it is.


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## Wookiebiker

nyvram said:


> i *JUST* now saw that.  i have no idea if that is checked or not..will look when i get home. I kinda think it is.


Default setting has it turned on...you need to go into the set up to turn it off...so chances are your Powertap has been on Auto Zero the whole time 

Generally speaking if the power numbers are reading zero when you're not pedaling, you are fine.


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## Alex_Simmons/RST

Wookiebiker said:


> Default setting has it turned on...you need to go into the set up to turn it off...so chances are your Powertap has been on Auto Zero the whole time
> 
> Generally speaking if the power numbers are reading zero when you're not pedaling, you are fine.


Except that Powertaps don't report when the torque zero is negative, only showing this as zero. So even if it says zero and you are coasting, the zero might still be wrong.

Best advice:
1. Allow the power meter a while to adjust to the temperature you are riding in. Bigger the temp change, longer it needs.

2. Always do a manual torque zero before your ride

3. Have the auto zero feature enabled (if it's a Powertap*), it will periodically check/adjust the zero point while coasting

Note that auto zero only work if the zero point is out by a little bit. If the zero point is out by too much, then auto zero doesn't work. Hence why you should always do a manual forced zero to start with.


* I don't however recommend this option for SRM wireless due to the way it performs the auto zero. Manual checks are better.


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## hrumpole

What is a manual torque zero?


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## Alex_Simmons/RST

hrumpole said:


> What is a manual torque zero?


When you manually check and set the torque zero on a Powertap.
Like you should do before every ride.


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## obiwan kenobi

Every ride


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## multirider

Alex_Simmons/RST said:


> * I don't however recommend this option for SRM wireless due to the way it performs the auto zero. Manual checks are better.


How come?


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## Alex_Simmons/RST

multirider said:


> How come?


Because zeroing an SRM while still clipped into the pedals is highly likely to result in a bad ZO. 

Certainly that is my experience where I have seen it incorrectly change ZO by several hundred Hz (when a typical drift might be just a handful of Hz).


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## nyvram

Alex_Simmons/RST said:


> Because zeroing an SRM while still clipped into the pedals is highly likely to result in a bad ZO.
> 
> Certainly that is my experience where I have seen it incorrectly change ZO by several hundred Hz (when a typical drift might be just a handful of Hz).


Alex is 100% correct. I just did a test yesterday. I was already riding my bike when i changed my garmin bike listing (3 bikes..2 dont have sensor or powertaps) and it started chirping "speed sensor found", "power meter found"..

then i went and did an hour of hill intervals.

it never zeroed. it would drop to 7 watts on every non-pedalling descent i did..but it never 'sorted itself out'. that seems to be a pretty good argument to take a few seconds and properly zero the power meter before getting on the bike.


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## ZoSoSwiM

I always wake up my wheel and let my Edge705 find all the sensors for a few minutes before I ride. I've never seen any weird power data come from doing it this way. Since it's been fairly cold out lately I try to coast a little after my first few minutes of riding to make sure it rezero's after the temp adjusts.


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## Undecided

I zero after the first few or several minutes of riding, on the theory that the hub is up to "operating temperature."


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## multirider

nyvram said:


> Alex is 100% correct. I just did a test yesterday. I was already riding my bike when i changed my garmin bike listing (3 bikes..2 dont have sensor or powertaps) and it started chirping "speed sensor found", "power meter found"..
> 
> then i went and did an hour of hill intervals.
> 
> it never zeroed. it would drop to 7 watts on every non-pedalling descent i did..but it never 'sorted itself out'. that seems to be a pretty good argument to take a few seconds and properly zero the power meter before getting on the bike.


Do you have a PowerTap? or an SRM? I'm not clear from your post. Or both?

I have a PC5 on one bike and a PC7 on a different bike. Auto-calibrate is set to "on" for the PC7, I haven't seen any numbers that seemed wrong, but I haven't been diligent about checking. I'm going for a ride on that bike tonight, so I plan to pay attention and do some no-feet-on-the-pedals testing to see if it shows power when there is none. Except it might pick that up and zero automatically? I've always loved the auto-calibrate feature, but accuracy is more important. But if I can have both, that's what I want!

I forget to calibrate the PC5 sometimes. Recently, I've done several rides where I don't remember until the end, check it, and find it is not set correctly. For example, on at the end of a recent ride, the PC5 is at 512hz and should have been at 520hz. The discrepancy is usually less than 10hz, always less than 20hz. 

Is there some way to determine the impact to power readings? Can it be done on a percentage basis? That is, if it is 5hz off on a 500hz reading, would that result in a 1% error in watts? Or is it more complicated than that?


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## Alex_Simmons/RST

multirider said:


> I'm going for a ride on that bike tonight, so I plan to pay attention and do some no-feet-on-the-pedals testing to see if it shows power when there is none.


If the cranks are not turning, then power will report as zero anyway, no matter how far out a ZO might be.



multirider said:


> I forget to calibrate the PC5 sometimes. Recently, I've done several rides where I don't remember until the end, check it, and find it is not set correctly. For example, on at the end of a recent ride, the PC5 is at 512hz and should have been at 520hz. The discrepancy is usually less than 10hz, always less than 20hz.
> 
> Is there some way to determine the impact to power readings?


Just download/open file in SRMwin and you can go to files - properties and enter in a new ZO value to see the impact. Can do the same if you have the wrong slope in the powercontrol/file as well (I use one PCVI on two bikes and sometimes can forget to update slope).

5Hz might be a couple of watts.


The ZO readings on an SRM are way more sensitive than the torque readings displayed on a Powetap CPU, nearly 10 times so.


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## MontyCrisco

ZoSoSwiM said:


> I always wake up my wheel and let my Edge705 find all the sensors for a few minutes before I ride. I've never seen any weird power data come from doing it this way. Since it's been fairly cold out lately I try to coast a little after my first few minutes of riding to make sure it rezero's after the temp adjusts.


This is what I've been doing. But what I'm getting from this thread is that re-zeroing isn't the same as the initial calibration, which is a separate procedure. Since I keep my bike indoors I've been putting it outside while I get ready to ride, and then recalibrate before getting going. Can't tell whether this makes the least bit of difference though.

Me so confused.


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## nyvram

i'm confused too..seems my garmin only has 2 options:

- recalibrate
- a "checkbox" with 'auto zero' checked

i hit the 'recalibrate' button before riding and it seems to do something. you cant actually calibrate a powertap so i assume in this case 'recalibrate' = 'zeroing the powertap'


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## jspharmd

I know this is an old post, but I've had a similar situation and was wondering what everyone thought.

My power meter batteries died on a Saturday ride. I went home and replaced them. On Sunday, my watts seemed higher for the effort I was putting out.

When coasting my watts were zero. I realize that the zero could actually be negative, but it doesn't seem like that would contribute to my power readings being higher...maybe lower, but not higher. 

I did not "recalibrate" before the Sunday ride, so maybe that was the issue.

Thoughts?


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## Alex_Simmons/RST

You can't set the calibration of a Powertap, you can only check its calibration (you can set the calibration of an SRM and a Quarq Cinqo). 

Calibration is validation that the power meter is accurate and involves knowing that the torque values it is reading match actual torque being applied - at zero and at other torque values (this is what we call the "slope"). 

Setting the zero point is one thing, but no good if the meter's slope is wrong - e.g. it reads zero when no torque is applied however it registers 100N-m when only 90N-m are actually being applied.

You can manually check and set the torque zero, and you can also set it to perform an auto zero check and reset when the hub recognises it is coasting for a while. Auto zero will only work if the torque zero is out by a little bit, if it is out by a lot, the auto zero doesn't work.

Always perform a manual torque zero first before each ride, then allow auto zero to look after things from there.


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## Creakyknees

Alex_Simmons/RST said:


> You can't set the calibration of a Powertap, you can only check its calibration (you can set the calibration of an SRM and a Quarq Cinqo).
> 
> Calibration is validation that the power meter is accurate and involves knowing that the torque values it is reading match actual torque being applied - at zero and at other torque values (this is what we call the "slope").
> 
> Setting the zero point is one thing, but no good if the meter's slope is wrong - e.g. it reads zero when no torque is applied however it registers 100N-m when only 90N-m are actually being applied.
> 
> You can manually check and set the torque zero, and you can also set it to perform an auto zero check and reset when the hub recognises it is coasting for a while. Auto zero will only work if the torque zero is out by a little bit, if it is out by a lot, the auto zero doesn't work.
> 
> Always perform a manual torque zero first before each ride, then allow auto zero to look after things from there.



These look like English words to me but I have no idea what you are talking about.


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## Alex_Simmons/RST

Creakyknees said:


> These look like English words to me but I have no idea what you are talking about.


Well if you don't use a power meter, then I suppose I can understand that.

But an analogy:

Consider Bathroom scales.

When you are not standing on the scales - do they read zero lbs? 
That's the same as checking the torque of a power meter reads zero when you are not putting any pressure on the cranks.

When you know the scales read zero OK, but if you place a 100lb weight on them but they only read 90lbs, then that's the same as the power meter calibration (slope) being wrong.

IOW for bathroom scales (or a power meter) to be accurate, it needs both the zero and the slope calibration to be correct.


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## jspharmd

Alex_Simmons/RST said:


> You can't set the calibration of a Powertap, you can only check its calibration (you can set the calibration of an SRM and a Quarq Cinqo).
> 
> Calibration is validation that the power meter is accurate and involves knowing that the torque values it is reading match actual torque being applied - at zero and at other torque values (this is what we call the "slope").
> 
> Setting the zero point is one thing, but no good if the meter's slope is wrong - e.g. it reads zero when no torque is applied however it registers 100N-m when only 90N-m are actually being applied.
> 
> You can manually check and set the torque zero, and you can also set it to perform an auto zero check and reset when the hub recognises it is coasting for a while. Auto zero will only work if the torque zero is out by a little bit, if it is out by a lot, the auto zero doesn't work.
> 
> Always perform a manual torque zero first before each ride, then allow auto zero to look after things from there.


Please note the quotations around my recalibrate. I read this post and understand you don't calibrate the power meter. I was using terms others used. You suggest that the power meter calibration may be off, and if it is significant, setting the zero point won't help.

My actual question (probably not clear from my post) was: Does it seem reasonable that just changing the batteries would cause the power meter calibration to be off significantly? If I zero it and it still seems off, how do I tell if the calibration is off (preferably without sending it back to Cycle Ops)?


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## nightfend

Huh, I never re-calibrate my Powertap before riding.


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## chase196126

jspharmd said:


> If I zero it and it still seems off, how do I tell if the calibration is off (preferably without sending it back to Cycle Ops)?


There is a process known as the "stomp test" that uses a known weight to determine if the powertap is reading accurately based off of how the zero offset number changes in relation to that weight. It is close to the process of calibrating an SRM, but you cannot change the internal slope of the PT. It may also require the PT yellow computer, but I am not sure. Check on google for powertap stomp test to find out how to do the calibration check. I also recommend subscribing to the Google Wattage Group. They have a ton of really good info on power training and how to check and maintain different meters. 

Also: *Always calibrate your powermeter before EVERY ride!* No matter what powermeter you have, you should get in the habit of checking before every ride and even checking before starting important intervals


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## Alex_Simmons/RST

There are two things to test for.

Put the Cervo computer into test mosde and see what the torque zero point reading is. It should be about 510 +/-10. It may still work OK if it's outside of that range, but it is a first sign something may be up.

Secondly you can validate the calibration of your Powertap by the following:

You will need to set up bike so you can hang an accurately known mass (to nearest 50g) of at least 20kg from the pedal spindle of a horizontal crank arm.

Gym plate weights and a cord for hanging are good, all items weighed on calibrated postal scales.

Placing the bike in a turbo trainer is good for performing calibration tests. It keeps the bike stable so you can manage the weights. You can raise the front wheel of the bike up onto a chair to give more clearance for hanging masses.

You will need a means to prevent the rear wheel from turning. Either have someone hold the brake, or use something else to prevent the wheel from turning when needed.

Record both the mass used and crank length.

For Powertap:
1. Select the gearing for the test (chainring and rear cog).
Record gear used (number of teeth of front chainring and rear cog).

2. Wake up the Powertap, ensure it is receiving a torque signal from the hub
Put Powertap in test mode (refer manual) - which will show actual torque reading as well as the torque zero setting

3. Brace the rear wheel so that it can’t rotate
Rotate the cranks backwards until left crank is horizontal and towards front of bike
Record the zero torque setting value.
It will typically be ~ 512 +/- 20 inch-pounds (if it's not, then the unit is mostly likely out of spec without even proceeding with the check)

4. Hang the known mass from the pedal spindle
Record the torque value.
Then take away the zero point value to give you the measured torque (in inch-pounds) for that mass, crank length and gear combination.
Optionally you can very slowly rotate rear wheel until you attain the maximum torque value reading while the wheel is not moving
Record that value

5. Remove the mass from the pedal spindle

6. Repeat steps 3 to 5 using the right crank arm

7. Repeat steps 3 through 6 one or two more times to validate readings

8. Repeat steps 3 through 7 using a different chainring/cog combination


Compare the reported torque values with the known torque value and express the difference as a percentage.


Known torque value = test mass (kg) x 2.2046 x crank length (mm) / 25.4 x rear cog (teeth) / Chainring (teeth)

The 2.2046 & 25.4 are factors to convert the metric units to the inch-pounds reported by the Powertap CPU

e.g. 30kg on a 170mm crank in the 53 x 16 chainring/cog combination should give a reading of:

30 x 2.2046 x 170 / 25.4 x 16 / 53 = 133.6 inch-pounds


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## nightfend

Seems like a lot of effort.


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## Alex_Simmons/RST

Takes about 10-minutes. Which to me is tiny compare to the amount of effort required to train - and validating your equipment is working properly will assist that.


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## nightfend

Alex, I am sure you could do it in 10 minutes. I would most likely make the situation worse by trying to re-calibrate my Powertap with the weight system.

I will take your advice and do the auto calibration on my Garmin before each ride. But, I don't really need an exact power number anyway, as long as it's fairly close and repeatable, I'm good. My computrainer never registers exactly the same as my powertap anyway. They are usually 5 to 10 watts apart.


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## chase196126

nightfend: It really is easier than it sounds. One good reason to do the check is to allow yourself to be able to compare data from year to year. If the slope drifts a few points in 6 months you will not have a baseline to determine if you are actually improving/decreasing in ability or if your meter has simple shifted. It helps make your numbers accurately repeatable


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