So I finally got around to hooking up the powertap and did all that zero’ing stuff. I put the bike on the computrainer and did the typical 10 minute warmup to calibrate the CT. I was then expecting the power numbers to be within 5 watts of each other. What I find is that the powertap is registering ~30% higher watts at all speeds. WTF’s with that? Which is the accurate one?
the PT is supposed to be more accurate.
The problem is…
I can average 270 Watts on the CT for an hour at max effort. If the PT is accurate and say 25% higher, that would mean I can really average about 335 Watts for the hour - which I can’t.
on the CT I’d use those #'s. Do some field testing to figure out what your PT #'s should be and when riding outside use those.
That sounds a bit much of a difference…but as DD says…take the PT on the road. It SHOULD register SLIGHTLY higher watts than the CT, being mechanically closer to the source, and therefore incurring less mechanical loss before the measurement is taken…but no way should it be 65 watts different…
My powertap training wheel reads 5-10watts higher. My race wheel with a tufo S3 lite reads 25-30watts higher…ahem…
Anyways, its hard to know for sure which is giving the ‘off’ reading. I have noticed that if I calibrate the powertap with a high number (3+) the difference widens. If I keep the calibration number around 2.3ish the difference is consistently 5-10watts. Also, I find the CT and PT to be more equal in a higher gear (all the more reason to just use ergometer mode)…say a 53x15 or so.
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You can do a stomp test to verify the accuracy of the PowerTap. Do this and then you will have some confidence in how accurate the PowerTap is. You have no way to do this with the CT except by using a known PT or a calibrated SRM or possibly Ergomo on it.
you guys have too many toys
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600+ hrs, man you need to move to a better climate.
Yeah, well… right now YOU need to move to a better climate 
heheh…DD has his trainer set up in the kitchen just in front of the fridge that has to remain open while he rides…he then manages to get a ride at only 115F
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What are you guys talking about? Better climate? pfffft. It’s 97 and 8:30 in the morning. Should be 116 today. I need to go finish running some more dripline. Even my native desert plants are wilting
Do not compare power measuring apparatus. It does not work with the level of accuracy that these devices have in meauring power.
No currently available power meter is very accurate in meauring power.
All of them do, however, demonstrate a degree of precision: i.e., they can produce repeatable results that will facilitate comparison of data with the exact same apparatus.
The problem comes when you try to compare power workouts from one apparatus to the other.
Based on my (admittedly limited) exposure to power measuring apparatus I have found out the following basics: The most *accurate *power meter may be SRM. The other appear (at least) to be significantly inaccurate, but adequately precise. All power measuring devices currently available are mechnically unreliable and problematic. They are complex for installation and susceptible to problems from water intrusion and road film. This makes them a little problematic to own. I anticipate rapid improvements with the eventual introduction of a value-priced, precise (but not accurate) power meter that has improved weather/real-world survivability. The numbers are only as good as your interpretation and implementation of them in your training.
Stomp test? What is this and how do I do it?
There is a way to measure how accurate the PowerTap is measuring torque, known colloquially as the stomp test. There is also a way to measure this for the SRM, but the SRM allows one to input the measurement so that one can calibrate a SRM. The PowerTap does not let one adjust the offset but one can correct for this, assuming in both cases that the torque measurement is off linearly when measuring between both high and low torque.
Also, I can’t remember if the computrainer is linear - if it is off by the same amount at high and low loads. There’s a really big discussion group on topica or yahoo.
http://www.midweekclub.com/powerFAQ.htm
Q: How do I calibrate my power meter?
A: (Andrew Coggan) Neither the Polar S-710 nor the PowerTap require calibration after initial set-up. Calibration of the SRM via slope adjustment can be performed by the user, as described in the Owner’s Manual at http://www.srm.de/index.php?la=3&lb=3&lang=ger (click on “Troubleshooting,” then “Calibration check”), and a more complete calibration procedure is now available un the U. S. as well.
Technically, the PowerTap cannot be user-calibrated, but its accuracy can be checked using a simple test that is similar to the SRM calibration check. First, check that the transmission icon is on, and if not, give the rear wheel a spin. Then, enter the torque mode by holding the “Select” button down for 2 seconds or longer (the “WATTS” designation will disappear from the top line.) Apply the rear brake sufficiently to lock up the rear wheel. Now, measure torque as follows: with the cranks exactly horizontal (right crank at 3 o’clock), hang a known weight of at least 50 lbs from the right crank, or simply stand on it – hence the name ‘stomp test’! Measured torque = (weight in lbs) × (crank length in mm) × (1 in/25.4 mm) × (cog teeth/chainring teeth). For a 159 lb rider standing on a 175 mm crank, with the chain on the 39 tooth ring and the 23 tooth cog, 159 lbs × 175 mm × 1 in/25.4 mm × 23/39 = 646 in-lbs. Compare this to the displayed value by calculating % error as (measured torque - displayed torque)/measured torque.
in three winters I have 600+ hrs logged on my CT. Is that a toy?
I think kids do better than that playing video games. Are they toys?
OTOH–I have a CT. I wish I had a PT, too.
I took the bike out for a ride on the road today. I held a steady 270 watts along a flat stretch for about 10 minutes in the aero position and the speed was 38 km/h. From the QCW website, I figure the PT must be pretty accurate.
Funny thing on that site, you get +.4 kph if you use tubulars instead of clinchers, just leaving the “base” numbers (whatever comes up when the site first loads – 300w, 80kg rider, etc.). Somebody better introduce whomever wrote that to Slowtwitch… He needs to add a field for tire width, tire pressure, quality of gluing job, etc. And of course, a special SUPER slow setting for Tufos…
That’s why it says “admittedly limited”.