Powertap torque test: revisited

Prior to sending my Powertap back to Saris for calibration, I decided to do one more torque test (previous posts: my wheel, comparison with other wheels), this time bypassing the Powertap internal zeroing, and go directly to the raw torque numbers, which are available in torque mode on the Cervo head unit.

My procedure:
  1. weigh test load (a bucket of fresh kitty litter) with my Ultimate hanging scale = 14.63 kg
  2. go into test mode
  3. for each gear tested:
    1. shift into gear of interest: 36/12-13-14-15-17-19-21-23-26 on my wheel, 36/13-14-15-17-19-21-23-25-28 on C's wheel.
    2. record torque
    3. load pedal with mass
    4. find orientation which maximizes torque reading
    5. record torque
    6. remove mass
    7. record torque
    8. subtract avg of unloaded torque readings from loaded torque reading

I plot theoretical torque delivered to the hub on the x-axis, with the y-axis showing the measured value, for each hub.

Powertap torque test
It looks good for C's wheel, but as before, my wheel is off. Curiously, each hub reported a slightly larger number than trend when in the largest cog. I'm not sure if there's any significance there....

Anyway, a similar test was reported on the Wattage list by Doug. I plotted his data. His procedure differed, and his test mass was weighed with a Tomita body weight scale, which is much less accurate than the Ultimate hanging scale. Still, the comparison between wheels is telling, and the weight load is the same in each case.

Doug's torque test data
Black seems fine within the accuracy of his scale. Blue on the other hand seems substantially lower. Of course, I can't vouch for his procedure. It may be his results are a measurement artifact. But certainly these results should have you questioning whether your Powertap hub is reporting accurately.

Okay -- time to ship that wheel back to Saris...

Comments

gregclimbs said…
Dan,

You're killing me....

Weights in KG, torque in lb-in, and extra scales (w@18minOLH??)

That said, I think it would be interesting to plot the error bounds of torque that would still keep the ptap in spec (+/-1.5%) and see if you hub is outside those bounds...

g
March 22, 2010 at 7:38 AM
djconnel said…
Yeah -- I can't help the lb-in. That's what Saris uses, so since their reported number is to such low precision, changing units from what they report would be unwise.

"W@18min OLH" is watts at the speed it takes to climb Old La Honda Road in 18 minutes. Old La Honda time is my canonical metric of cycling power.
March 22, 2010 at 9:23 AM
djconnel said…
Oh, yeah -- WRT the weight in kg.... Newton would be better, I admit, but it's the metric unit reported by the Ultimate.
March 22, 2010 at 9:25 AM
Anonymous said…
This comment has been removed by a blog administrator.
March 23, 2010 at 2:10 AM
Post a Comment

Popular posts from this blog

Marin Avenue (Berkeley)

Image
On Saturday, I got an email from Tim telling letting me know that Murphy's Spring Classics was going to start up Marin Ave. I'd missed that. Murphy announces his courses the day before, and indeed it was already well into the night before when the course announcement went out. I'd skipped over the details, jumping straight to middle and end games. Marin? I laughed at the irony, because not long before when he'd told me he was choosing to repeat the Nifty Twn Fifty which we'd done together last year versus selecting Murphy Mack's Stage Mullett two-day, I responded I didn't think I had it in me to ride Marin Ave two years in succession. Well, so much for that. I was committed, so I went downstairs to swap my 11-26 cassette for my Recon 12-27. The name is easily overlooked: "Marin Ave" fails to strike the same impression as, for example, "Redwood Gulch" or "China Grade". But among those in the know, Marin Ave is infamous i
Read more

hummingbird feeder physics

Image
I couldn't figure out how the hummingbird feeder worked. Why didn't it overflow? Forces need to balance, of course. Neglecting surface tension, there liquid level is higher in the inner reservoir than in the feeding chamber, so there must be a corresponding pressure difference. Suppose the pressure in the inner chamber were zero. Then the column height difference would need to be atmospheric pressure / (density of liquid × gravity). But this is over 9 meters! Obviously the height difference is only approximately 1% of this. So the pressure difference inside versus outside is only approximately 1%. The inside is only slightly below atmosphere. So air is getting in. How? Does it diffuse through the liquid? If this were the dominant mechanism, it wouldn't take long for the pressure inside to go from 99% to 99.3%, for example, which should be plenty to push the column of liquid down in the inside chamber and thus push liquid out through the holes. It would over
Read more

Gallium Pro geometry

Image
On Caltrain yesterday, I was surprised to see among the usual set of commuting bikes a SRAM Red-equipped Argon Gallium Pro. The bike immediately stole your attention. The design was one which has been trending: clear-coated carbon with plenty of text detail so the viewer is sure to realize he's gazing upon the result of advanced, proprietary engineering and not at yet another paint job on the same old OEM frames being pumped out of the same old Taiwanese factories. But it worked: the bottom bracket area was huge, the downtube a large-diameter "inverted Kamm tail" design seemingly designed to maximize wind resistance, the top tube a broad, eccentric shape which screamed "vertically compliant yet torsionally stiff". All it lacked were the pencil-think seat stays Cervelo popularized, but this was a machine designed for stiffness over anything else. It seemed dramatic overkill, since the bike was small (the Argon "XS" I suspect). There wasn't muc
Read more