Alternative to stompin'

Discussion in 'Power Training' started by daveryanwyoming, Feb 22, 2008.

  1. daveryanwyoming

    daveryanwyoming Well-Known Member

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    I promised I'd post a photo of the alternative method I use to torque test PT hubs. It basically replaces a weight (body weight or otherwise) with the crank tensioned in the upwards direction via a digital scale. The crank is rolled back into the upstroke position and tension is introduced by rotating the rear wheel backwards till you get the desired force. The cranks don't need to be horizontal, but the tensioned cord running to the scale should be at right angles to the crank arms for accuracy. I slide a wooden dowel between a spoke head and the stays to lock the wheel off when the cranks are close to perpendicular to the scale cord and the scale force is about where I want it. If the cords are stretchy it takes a few seconds for the system to settle out and then you read the scale and the PT computer in torque mode and follow the stomp testing math:

    Force(pounds)* crank length(mm)/25.4*cog_teeth/chainring_teeth = expected torque.

    If the cranks aren't perpendicular to the scale cord your expected torque will be higher than what you're actually putting on the system since it effectively shortens the crank length regardless of which direction you're in error. But the angle doesn't have to be dead on. A +/- 5 degree angle error relative to perpendicular only results in a measurement error of 0.4% and +/- 8 degrees of error is ~ 1% in measurement error.

    The biggest hassle is making sure the PT computer stays awake, the torque is zeroed and the scale is zeroed with just the weight of the connecting cords between tests. But it's a lot more portable than 50 pound barbell plates.

    Anyway, if you don't like the bodyweight test (didn't spend enough time balancing on concrete forms as a kid :) ) or don't have a lot of weight plates lying around then this is an alternative method.

    -Dave
     
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  2. kytyree

    kytyree New Member

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    thanks for posting the picture, I think I will have to try that
     
  3. swampy1970

    swampy1970 Well-Known Member

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    Nice wheels ol' chap!

    You know that says 40 and not 50lbs right? ;)

    I take it you calibrated the scale using scientifically tested calibration weights (where w >= 0.000001% deviation from Pi/0) during the median between a new and full moon and between high and low tides and when the planets were aligned such that Jupiter and Saturn were not on the same side of the Solar System and where the air pressure was 1013.25mbar exactly. :)
     
  4. daveryanwyoming

    daveryanwyoming Well-Known Member

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    Actually, it's 40.9 kg on that particular scale.....
     
  5. squidwranglr

    squidwranglr New Member

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    Have you considered putting a tight rubber band onto the rear brake lever to immobilize the rear wheel instead of using the wooden dowel? It just seems "wrong" to apply force onto the spoke head in that manner, but it's probably nothing...

    Berend
     
  6. daveryanwyoming

    daveryanwyoming Well-Known Member

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    That's how I previously locked the wheel, but I'd prefer a stiff toe strap to a rubber band since any wheel creep translates to torque changes and it can take a while for the numbers to stabilize. Yeah I sure wouldn't drop the dowel in and carelessly release all that force onto a spoke nipple and probably wouldn't use this method with carbon spokes. It's also why I used a wooden dowel instead of say a steel rod, figured it would be a bit kinder and gentler. But you don't have to lock the wheel at all if you can hold the wheel steady enough to take both readings. Personally the dowel method seems fine if you're careful, but don't do it if you're worried about stress to your spoke heads.

    -Dave
     
  7. daveryanwyoming

    daveryanwyoming Well-Known Member

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    You definitely got me thinking about this, but I took a look at the hub torque numbers measured during that testing. With 40.9 kg on the scale the hub torque was approximately 171 pound-inches. That's with my 172.5mm cranks and using a 50:14 gearing. The spoke nipples are just over 11 inches away from the axle. So there's ~15.5 pounds of lateral force on that nipple. I've gotta believe hitting an average sized pothole results in a lot more lateral force than that.

    -Dave
     
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