# Quarq vs. Powertap ?



## mmorales (Apr 28, 2012)

I have a general question for all those out there using or having experience with both power meters. I have the Quarq spider power meter on my Tarmac SL4 and a Powertap hub installed on my Giant TCR trainer bike. I have compiled some data from the use of the Quarq on the trainer and recently had to switch over to the Giant whilst my Tarmac fork is off the bike. My question to all those out there is, do you see a difference in the readings produced by the two different power meters? I have done some of the same interval training on the trainer with both power meters and have seen a slight difference in the data downloaded and reviewed with Power Agent software. ( -10 to -20 watts)

The drivelines are different from bike to bike. The Giant is Ultegra 6700 and the Tarmac is SRAM Red. I am trying to wrap my head around this difference in data from one bike to the other. I have noticed that the Powertap readings during the workout vary more than those recorded by the Quarq during the training. The Quarq stays pretty constant during the workout. 

Has anyone had issues with there drivetrain causing erratic readings or just differences between the two power meters? Please let me know what your experiences have been on this topic. Your input is greatly appreciated.


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## robdamanii (Feb 13, 2006)

I've seen a difference in the readings between the powertap and Quarq, but I've not noted any issues with the Powertap jumping a lot compared to the Quarq. 

Both have been pretty reliable in their consistency.


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## zone5 (Aug 21, 2012)

Simple...the Powertap is wheel based system. On the trainer you have different loads to the read wheel. You also have to consider how the resistance unit is affecting the gauges in the Powertap unit.

Quarq's input are all from your legs.


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## ukbloke (Sep 1, 2007)

zone5 said:


> Simple...the Powertap is wheel based system. On the trainer you have different loads to the read wheel. You also have to consider how the resistance unit is affecting the gauges in the Powertap unit.
> 
> Quarq's input are all from your legs.


This makes no sense to me. The delta between an idealized wheel PM and an idealized crank PM is just the power loss in the drive train. Neither system is impacted by whether it is on a trainer. I have heard that drive-train efficiency losses are of the order of a few percent, so at 300W one might expect the PowerTap to read up to 10W lower. I think a hub-based system has a potential inherent advantage over a crank system in that hubs spin faster than cranks and a hub design can take multiple readings per revolution to estimate hub rotation speed more accurately than a crank can measure cadence. Also wheel rotation is much smoother than cadence. I don't know whether PowerTap internals really get any benefit from this though.

Now, neither approaches are idealized PMs. And any PM can be poorly calibrated. I would not worry about a 10W discrepancy, especially if the two PMs differ by that much consistently over time. At 20W one might start to worry. I'd be more concerned over the erratic readings. If you are putting out a constant effort on the trainer (e.g. no variation in speed and a smooth pedal stroke) and the PowerTap wattage is wandering up and down significantly, then I'd be inclined to think that your PowerTap has an issue and contact Saris for technical support. On the road it is very difficult to spot this as power is up and down all the time within this 20W range.

OP - You have everything you need to figure this out - put both PMs on the bike at the same time and hit the trainer and do some testing.


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## mmorales (Apr 28, 2012)

Tech support has responded with the following answer:

To me it sounds like your hub is functioning normally. Being that the PowerTap has a sample rate of 1 second, you are getting averaged power readings every second. If you are seeing erratic numbers, this is because of pedaling efficiency, not calibration error. Your calibration numbers, being between 500 and 524, are within spec. If you are looking to see more consistent readings, I would change to power smoothing on your Garmin to something like 5 or 10 seconds. This will average your power over that amount of time. I would also recommend not zeroing the torque (called "calibrate" on your device) while on the trainer. It is possible that the roller on the trainer can put small amounts of torque on the hub and thus you would be zeroing the torque with that torque on the hub, causing inaccurate calibration values.



That said, zeroing the torque should not be necessary before every ride as long as you have the Auto-Zero function enabled. This is a default setting on Garmins. Let me know if you have any more questions.



Best,



Michael Fioretti 
Customer Service Representative
Saris Cycling Group 
5253 Verona Road
Madison, WI 53711
[email protected]
Phone: (608) 274-6550
Fax: (608) 274-1702


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