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20-02.-2008, 02:17 PM
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#1 |
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Registered User
Join Date: Apr 2005
Location: Kansas City, USA
Posts: 3,572
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Stomp Test is off -- what now?
I got suspicious when I noticed a large deviation between my PT Pro watts and my Kinetic Road Machine power curve, so I zeroed the torque and did the stomp test described here.
I repeated the test using 5 separate gear combinations, and got errors (from calculated torque) ranging from -6.1% to -7.7%. What's the typical tolerance for this kind of test, and is there any way to correct the unit? My hub is 2.5 yrs old and has ~11,500 miles on it. Here are the testing results (170lb rider, 172.5mm cranks): Gear Torque Measured / Calculated (in-lbs) 42x25 646 / 687 42x17 440 / 467 52x12 247 / 266 52x21 433 / 466 30x12 431 / 461 Last edited by frenchyge : 20-02.-2008 at 02:22 PM. |
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20-02.-2008, 02:45 PM
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#2 | |
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Registered User
Join Date: Jan 2007
Posts: 490
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Re: Stomp Test is off -- what now?
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20-02.-2008, 02:54 PM
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#3 | |
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Registered User
Join Date: Oct 2006
Posts: 1,383
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Re: Stomp Test is off -- what now?
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FWIW, all my PT hubs are within 3% when torque tested with a fixed weight, body weight tests seem to have a lot more variability depending on how well I balance on the pedal and how else I stabilize myself. 2 of my SL road hubs are within 1.5% and my Pro MTB hub is high by roughly 2.8%. I've started using a slightly different method for torque testing that's easy. I hang a digital scale from the seat rails in a position that allows me to attach the other end to a horizontal crank in the upstroke position. I use spectra climbing runners to connect the links to minimize stretch. I spin the wheel forward and then roll it backwards till the scale connection is tight and horizontal and the weight on the scale is in the 50 to 75 pound range. I then slide wooden dowel horizontally between a spoke and the seat stays to lock the wheel under load. Wait a few seconds for the system to settle and read weight and torque. It's easy to vary load and of course you can use various gears just like the other method. The results match a 50 pound weight plate very well and it's a lot more convenient. Personally if your measurements are solid I'd say you should send the hub in for service. -Dave |
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20-02.-2008, 03:20 PM
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#4 | |
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Registered User
Join Date: Apr 2005
Location: Kansas City, USA
Posts: 3,572
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Re: Stomp Test is off -- what now?
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I was being careful.... not Robert Chung careful.... but careful.  |
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20-02.-2008, 04:56 PM
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#5 | |
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Registered User
Join Date: Dec 2003
Posts: 238
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Re: Stomp Test is off -- what now?
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Not good enough. Borrow a couple of 25 lb plates and hang them from a pedal. I tried using body weight the first time I checked mine too...I was convinced it read 5% low. Then I used the plates and found it was spot on. The one nice thing about using separate weights is that you can VEERRRYYY slowly rotate the wheel by hand forward and back to find the peak reading. |
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20-02.-2008, 05:25 PM
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#6 |
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Registered User
Join Date: Mar 2006
Posts: 151
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Re: Stomp Test is off -- what now?
That's too bad when you're noticing differences between your KK and Powertap... and the PT is the one that's off! I was in your shoes awhile ago when my workouts started getting really tough. I did the stomp test with body weight and it seemed to be 7% off. It may not be as accurate as these other methods, but accurate enough for me to know it was time to send her in. Saris only warranties it for a year, so you'll have to pay for the re-calibration. I think they said it was $150, but don't quote me. I must say, though, that Saris' customer service is without equal. When I got mine back in the mail, I opened it to find a new chest strap, wiring harness, computer, and they overhauled the rear hub for me too. Excellent people. Now I just wish I could recover from those 2 X 20 VO2 sessions....
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20-02.-2008, 05:43 PM
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#7 | |
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Registered User
Join Date: Mar 2006
Location: Sydney, Australia
Posts: 1,574
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Re: Stomp Test is off -- what now?
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20-02.-2008, 11:31 PM
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#8 | ||
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Registered User
Join Date: Apr 2005
Location: Kansas City, USA
Posts: 3,572
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Re: Stomp Test is off -- what now?
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 I don't have access to much in the way of quality scales, weights, etc. to do a really thorough job or it. By using something as heavy as myself, I'd hoped that any errors in absolute weight would have smaller relative effect.Quote:
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20-02.-2008, 11:41 PM
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#9 | |
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Registered User
Join Date: Apr 2005
Location: Kansas City, USA
Posts: 3,572
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Re: Stomp Test is off -- what now?
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Heh, I didn't notice the difference while riding with both. I wasn't until I had to send my hub in for an overhaul that I decided to do a couple sessions using the KK speed chart to set my trainer target. The first thing I noticed was that the PT computer makes a *terrible* cycling computer when used with a separate wheel magnet. I don't know if it's because of the sampling frequency or what, but the speed resolution is in steps of about .7 mph (ie, speed is 20.2 mph or 20.9 or 21.7 mph, with nothing in between). Then, when I downloaded my workout file and applied the KK power formula I thought, hmmmm.... that didn't feel like a new FTP test.  |
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21-02.-2008, 01:04 AM
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#10 | |
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Registered User
Join Date: Jan 2007
Posts: 490
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Re: Stomp Test is off -- what now?
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Free weight plates might be labelled inaccurately, but you can weigh them at the post office. Every town in the US has a post office. |
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21-02.-2008, 01:08 AM
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#11 | |
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Registered User
Join Date: Jan 2007
Posts: 490
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Re: Stomp Test is off -- what now?
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21-02.-2008, 01:29 AM
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#12 | |
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Registered User
Join Date: Apr 2005
Location: Kansas City, USA
Posts: 3,572
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Re: Stomp Test is off -- what now?
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Yeah, that's what I was saying -- that I was thinking I could get closer than 5 degrees to horizontal and that my potential error would be less than .5% of the true peak torque as a result. I did check the crankarm with a bubble level, and raised it to the next ratchet pawl if it was below level at all to allow for the flex that came with me putting my weight on one pedal. Maybe I'll try some more measurements before sending it in for calibration. |
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21-02.-2008, 03:01 AM
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#13 |
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Registered User
Join Date: Jul 2003
Posts: 336
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Re: Stomp Test is off -- what now?
No real need for a level - eyeball it and then slowly roll the wheel back and forth. You'll see the torque peak and fall off. Record the peak value.
I got a 35lb weight and took it to the UPS store to verify - it happened to be 35.00 lbs exactly. Girl at the store thought I was a bit daft. |
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21-02.-2008, 03:08 AM
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#14 | |
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Registered User
Join Date: Oct 2006
Posts: 1,383
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Re: Stomp Test is off -- what now?
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21-02.-2008, 03:32 AM
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#15 | |
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Registered User
Join Date: Apr 2005
Location: Kansas City, USA
Posts: 3,572
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Re: Stomp Test is off -- what now?
Quote:
That approach does sound a lot more manageable than borrowing some weightlifting plates and taking them somewhere to be weighed. I'm visualizing that the dowel holds everything in place and steady for the reading, but this method doesn't give you the ability to rock back and forth over the peak does it? You're using eyeball or bubble level to find the right crank position then? |
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