# Should I "zero torque"?



## ridenfish39 (Jun 20, 2008)

Power Tap question????

My friend says to disable the auto zero torque function and manually zero torque every ride. Is this really necessary? I have owned the power tap for about a year now and have zeroed torque once. It has not given me any out of the ordinary readings so far.


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## chase196126 (Jan 4, 2008)

ridenfish39 said:


> Power Tap question????
> 
> My friend says to disable the auto zero torque function and manually zero torque every ride. Is this really necessary? I have owned the power tap for about a year now and have zeroed torque once. It has not given me any out of the ordinary readings so far.



Who ever gave you the advice about turning off Auto zero does not know very much about powertap systems. 

If you want reliable and accurate data you should have auto zero turned on as well as zeroing the torque on the hub before EVERY ride you do. 

Auto Zero on the PT is awesome. As long as you have zeroed the torque at the beggining of the ride the hub will keep itself zeroed out even if there are changes in temperature. The older SRMs do not do and auto zero, and even with Quarq you have to remember to do the back pedal auto zero while riding if the temp. changes. 

In order for the auto zero function to work properly, however, the hub must be manually zeroed before every ride.


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## AdamW (Oct 26, 2010)

Of course, your mileage may vary ... I have always had problems with the auto zero function. Do a hill interval, coast back down, the next interval wattage would be way off. So I also turned the auto zero off and now manually zero before each ride and mid-ride if the weather is changing. Haven't had a problem since I switched it off. I believe that with a constant offset throughout the ride, it is also much easier to post-correct any "affected" values if you put in some coasting time mid-ride. Then again, I'm a bit of a control freak and don't trust automated anything.


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## chase196126 (Jan 4, 2008)

AdamW said:


> Of course, your mileage may vary ... I have always had problems with the auto zero function. Do a hill interval, coast back down, the next interval wattage would be way off. So I also turned the auto zero off and now manually zero before each ride and mid-ride if the weather is changing. Haven't had a problem since I switched it off. I believe that with a constant offset throughout the ride, it is also much easier to post-correct any "affected" values if you put in some coasting time mid-ride. Then again, I'm a bit of a control freak and don't trust automated anything.


How sure are you that the numbers after coasting were off? Did you manually check the zero offset to see if things had changed?


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## AdamW (Oct 26, 2010)

Pretty darn sure. PE did not match power output and non-zero power numbers were recorded when cadence was zero. I know there are others out there who experienced the same thing on timetrial turn-arounds. Just saying this isn't the first time I've heard somebody recommending turning the auto-zero off.


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## chase196126 (Jan 4, 2008)

Hold old was the powertap unit? Was it one of the newer ANT+ setups, the original wireless signal, or a wired set up?


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## AdamW (Oct 26, 2010)

2009 wired PT Comp; not ANT+


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## zender (Jun 20, 2009)

I run an Ant+ Powertap. I have never had any reading other than 0 when coasting. Should I be worried? Might I be able to qualify for the TdF but I just don't know it because my numbers are so off?


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## chase196126 (Jan 4, 2008)

If you are getting numbers that are really off after coasting (assuming you zeroed at the beginning of the ride after allowing the temperature of your PT to stabilize) you could either have very low hub batteries, or the unit needs to go back to Cycleops to get checked.


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## dwgranda (Sep 8, 2009)

I've never had to manually zero my wireless powertap and my numbers match up almost perfectly with my wired powertap that I have to zero all the time. I have auto zero on the wired CPU turned off because one of my bikes is a fixed gear conversion. Sometimes torque goes up to 2 or 3 on the wired if I forget to auto zero. If you have another bike that shares the same wired CPU then you really want to make sure you manually zero.



zender said:


> I run an Ant+ Powertap. I have never had any reading other than 0 when coasting. Should I be worried? Might I be able to qualify for the TdF but I just don't know it because my numbers are so off?


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## chase196126 (Jan 4, 2008)

dwgranda said:


> I've never had to manually zero my wireless powertap .



I dont mean to bug, but why would you not zero your powertap? Even if the head unit is showing zero watts while coasting the system could be reading a negative torque value, and thus giving low power readings. 

If you have invested in an expensive power measuring device, why not use it the way it was designed to be used


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## dwgranda (Sep 8, 2009)

Because the auto-zero works on the wireless one? Not sure if this is true (that auto-zero is better on the wireless), but I use the garmin 705 or joule 2.0 and I'm not sure/haven't bothered to check torque on them. I have occasionally checked the torque with the default cpu and it has always registered 0.




chase196126 said:


> I dont mean to bug, but why would you not zero your powertap? Even if the head unit is showing zero watts while coasting the system could be reading a negative torque value, and thus giving low power readings.
> 
> If you have invested in an expensive power measuring device, why not use it the way it was designed to be used


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## zender (Jun 20, 2009)

This was my suspicion too... that autozero is working on my Powertap Ant+ If it records a negative value with zero cadence, it uses that new number as zero torque. But, this seems easy enough to program in the firmware. How big an error are you all seeing? More than 10W? I suspect below that, you're within the "noise" of the system anyway.


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## dwgranda (Sep 8, 2009)

I suspect that the error is much less than 10W for these things for the sustained powers. Claimed accuracy is within 1.5%. For most of us that's going to be within 5 watts and between two different powertaps 10 watts would be the extreme.


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## Borti (Aug 1, 2008)

What number should I be getting after zeroing the torque on the PT with my Garmin? Sometimes huge numbers show up and I am simply unclear as to what it should be reading after zeroing the torque value.

Thanks,


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## IAmSpecialized (Jul 16, 2008)

Also, have you checked torque calibration with a stomp test? Sometimes the strain gauges can have problems and calibration can be off. You might want to do a stomp test if you haven't done one before. If the torque is more than about 1.5% difference in multiple gear combinations, cycleops will repair the problem. Start with 39/17 and also do a couple of gears above and below that. Make sure to use atleast a 50lb weight.


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## Alex_Simmons/RST (Jan 12, 2008)

chase196126 said:


> If you want reliable and accurate data you should have auto zero turned on as well as zeroing the torque on the hub before EVERY ride you do.
> 
> Auto Zero on the PT is awesome. As long as you have zeroed the torque at the beggining of the ride the hub will keep itself zeroed out even if there are changes in temperature.


This is the advice I give to any client using a Powertap.

The reason is that while auto zero feature is good, if the torque zero value is outside of a given narrow range, the auto zero feature no longer functions. So best to start it off at a proper zero point.

This may be an issue if you have large changes in temperature during a ride, such as doing hill repeats with longer fast descents. You are constantly pedaling on the way up, so the autozero never gets a chance to kick in and it may have drifted somewhat in that time.

I have noticed for example that having the bike on a car roof rack on a cool/cold day can make the torque zero go out quite a bit. Enough so that auto zero would not function.

I personally use SRM now days. My advice for SRM users is different - I suggest disabling the auto zero function as it is IMO and IME unreliable due to the way it functions. Doing a ZO check however is trivial.


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## Borti (Aug 1, 2008)

Just checked the power tap after I 'zero'd' it. The number consistently read 13805. Is this correct?

Thanks,


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## chase196126 (Jan 4, 2008)

That seems like a really high number.. when I calibrated my powertap using the Joule I usually got a zero offset of around 412-415 depending on the temp. 

Are you sure you didnt somehow get the signal number of the hub somehow?


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## Borti (Aug 1, 2008)

Hey Chase,

Thank you for your response. What is the signal number you are referring to?


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## chase196126 (Jan 4, 2008)

The signal number is the ID code the powertap transmits to the head unit. The unit remembers the specific ID of the hub so it doesnt accidentally read another hub that is not on your bike. 

Could you describe the method you are using to Zero the torque?


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## zender (Jun 20, 2009)

I did a "test" last weekend without zeroing torque.

It was 68 degrees in my house and 54 degrees outside when I started riding. It warmed up to 77 degrees by the end of my ride. The PT (Pro with Ant+) read 0W on rolling downhills from my driveway (downhill) to the last downhill at the end of the ride. 

Not very scientific, I know, but either the autozero function is working or temperature doesn't affect the strain guages over this range of temps.


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## Alex_Simmons/RST (Jan 12, 2008)

zender said:


> I did a "test" last weekend without zeroing torque.
> 
> It was 68 degrees in my house and 54 degrees outside when I started riding. It warmed up to 77 degrees by the end of my ride. The PT (Pro with Ant+) read 0W on rolling downhills from my driveway (downhill) to the last downhill at the end of the ride.
> 
> Not very scientific, I know, but either the autozero function is working or temperature doesn't affect the strain guages over this range of temps.


It's likely the auto zero did it's thing and the starting ZO was close enough to actual zero point that auto zero functioned properly from the get go.

However, keep in mind that the meter does not display negative numbers, only a zero. So if the error is on the negative side, you won't know. 

Hence why it makes to sense to do a manual zero before you start and all but eliminate the risk that the auto zero function doesn't do its job from then on.


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## Roadiedvm (Apr 29, 2007)

*Zeroing PowerTap*

For what it's worth, I've learned from experience that a PowerTap computer has to be zeroed to give accurate readings. I have an SL+ with the old yellow computer. This replaced an older wired unit which clearly had to be zeroed or it would show positive wattage when coasting. The SL+ shows zero wattage coasting so I assumed it wasn't necessary to zero it. I also use a Garmin 500 most of the time outdoors and readings on the Garmin are always higher than on the PowerTap, by 15-20 watts for a similar workout or course, which I assumed meant the Garmin was wrong. After reading the manual and learning I should zero the PowerTap, I did a trainer workout using both the PowerTap computer and Garmin; power readings were almost identical. I hope this means I had been generating more power than I thought and not that both units are artificially high.


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## dwgranda (Sep 8, 2009)

Roadiedvm said:


> For what it's worth, I've learned from experience that a PowerTap computer has to be zeroed to give accurate readings. I have an SL+ with the old yellow computer. This replaced an older wired unit which clearly had to be zeroed or it would show positive wattage when coasting. The SL+ shows zero wattage coasting so I assumed it wasn't necessary to zero it. I also use a Garmin 500 most of the time outdoors and readings on the Garmin are always higher than on the PowerTap, by 15-20 watts for a similar workout or course, which I assumed meant the Garmin was wrong. After reading the manual and learning I should zero the PowerTap, I did a trainer workout using both the PowerTap computer and Garmin; power readings were almost identical. I hope this means I had been generating more power than I thought and not that both units are artificially high.



I don't know about the 500, but the default on my garmin 705 was to not include zeros in average power. For the powertap cpu it was the opposite. Not including zeros will make your average power look higher. When you did the workout on the trainer it probably didn't make a difference because you did little to no coasting (where you would get the zeros).


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## Roadiedvm (Apr 29, 2007)

dwgranda said:


> I don't know about the 500, but the default on my garmin 705 was to not include zeros in average power. For the powertap cpu it was the opposite. Not including zeros will make your average power look higher. When you did the workout on the trainer it probably didn't make a difference because you did little to no coasting (where you would get the zeros).


That's a good point but what I found after I zeroed my PowerTap was higher wattage readings at any given speed than before I zeroed it. The only change was zeroing the PowerTap. I also had records of some trainer workouts, Spinervals, Sufferfest, etc., using either the Garmin or PowerTap in which the Garmin always read higher until I zeroed PowerTap. This was the first time I used both units during the same workout. 

As an aside, I also discovered that the PowerTap HM belt works for both PowerTap and Garmin but the Garmin belt works only for Garmin. They look identical apart from the logo.


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## Alex_Simmons/RST (Jan 12, 2008)

No matter what power meter you use:

1. Always zero torque before a ride. Occasionally check during rides. Just because it says zero watts when coasting doesn't mean the meter is correct (since they don't show negative values).

2. Include zeros in average power calculations. It is meaningless otherwise.


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## dwgranda (Sep 8, 2009)

Alex_Simmons/RST said:


> Just because it says zero watts when coasting doesn't mean the meter is correct (since they don't show negative values).


Another reason coasting is useless as a zeroing check is that one's cadence is 0 when coasting so of course it will likely report 0 watts.


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## Roadiedvm (Apr 29, 2007)

Alex_Simmons/RST said:


> No matter what power meter you use
> 
> 2. Include zeros in average power calculations. It is meaningless otherwise.


On the trainer, there are no zeros, so an hour with an average of 240 watts might have a range of 120 to 400 watts. But on the road, unless you're climbing the whole time, you've either got some zero or at least very low power readings for some of the workout, even during intervals. So to average 240 watts, in this example, your power range might be 0-400 watts but you're going to have to have a higher average wattage while pedaling than on the trainer. So you can't really compare a trainer workout to an outdoor workout.


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## Alex_Simmons/RST (Jan 12, 2008)

dwgranda said:


> Another reason coasting is useless as a zeroing check is that one's cadence is 0 when coasting so of course it will likely report 0 watts.


While that's true for an SRM or Quarq, the Powertap doesn't use cadence to calculate power - it uses the rotational velocity of the hub, which of course is not zero when coasting.


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## Alex_Simmons/RST (Jan 12, 2008)

Roadiedvm said:


> On the trainer, there are no zeros, so an hour with an average of 240 watts might have a range of 120 to 400 watts. But on the road, unless you're climbing the whole time, you've either got some zero or at least very low power readings for some of the workout, even during intervals. So to average 240 watts, in this example, your power range might be 0-400 watts but you're going to have to have a higher average wattage while pedaling than on the trainer. So you can't really compare a trainer workout to an outdoor workout.


? 

It has nothing to do with indoor / outdoor rides.

Average power is just that. kJ of mechanical work done divided by the number of seconds over which the work was performed.

Taking an average which excludes periods of time where you were not pedaling is meaningless, since the time spent not pedaling (i.e. resting) affects what you are capable of when you are pedaling.


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## Roadiedvm (Apr 29, 2007)

I know what an average is. My point was that it's easier to be consistent in your intervals, whatever the length, on the trainer since you're not dealing with variations in terrain. If you're tracking progress, you need to compare similar workouts, i.e. trainer workout to trainer workout, climbing workout to climbing workout, etc.


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## Alex_Simmons/RST (Jan 12, 2008)

Roadiedvm said:


> I know what an average is. My point was that it's easier to be consistent in your intervals, whatever the length, on the trainer since you're not dealing with variations in terrain. If you're tracking progress, you need to compare similar workouts, i.e. trainer workout to trainer workout, climbing workout to climbing workout, etc.


With that I agree, but this has nothing to do with whether or not you should exclude zeros from the calculation of average power.

If you want to understand the different demands of performing similar intervals on a trainer versus outdoors on the road, then Quadrant Analysis is your friend:
http://alex-cycle.blogspot.com/2009/01/quadrant-analysis.html

Then there is of course the issue of whether or not one experiences a variance in power between training indoors and outdoors:
http://alex-cycle.blogspot.com/2009/01/turbocharged-training.html


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