A Hub-Axle Treatment Evaluation Protocol

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Stan Pope
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Re: A Hub-Axle Treatment Evaluation Protocol

Post by Stan Pope » Sat Apr 03, 2010 9:30 am

FatSebastian wrote:Has it been discussed anywhere why the outer diameter should be maintained at ~2"?
That was based on the radius of the ring on which the 4' of line would be wound. That is the leverage by which the falling weight would accelerate the ring. All were approximations I made when working out the analogy. From http://www.stanpope.net/pwafm.htm:
Attach a weight of about 1/4 ounces onto a 4 foot(+) thread. Loop the other end. (The weight is chosen to induce the approximate the angular velocity of an actual wheel on a car at the bottom of the slope.)


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Jeff Piazza
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Re: A Hub-Axle Treatment Evaluation Protocol

Post by Jeff Piazza » Sat Apr 03, 2010 10:40 am

FatSebastian wrote:How well does it workl? Also, does it work continuously like an analog tachometer? I would like to be able to measure, say, the decay rate of spin down. (For example, how much time does it take to go from X RPM to X/2 RPM?)
There are definite limitations with this device; one might have to pay more than $13 for a more versatile instrument. It updates the display about twice a second, and afterwards can report highest, lowest, and last reading. I don't think one could measure decay times to sub-second accuracy.

Still early days with this, so still tinkering. I ran a few trials the other night, but that's all so far. Trials with the single (1.5 oz.) weight gave max-RPM readings of 1700, 1664, 1655, and 1668, and there's some chance of operator error on the first reading. (Using two hands to hold the tach and the spin ring, respectively, I may have lost aim briefly. I may try to construct a jig that holds the tachometer and pin vise in the correct relative positions.) The two-weight (3 oz.) readings were 1868, 1820, and 1875 rpm.

I'm not sure if this would be reliable enough to detect friction differences at the level we're interested in. Re-reading the original description, a 1/4 ounce weight is suggested; perhaps the lower forces would help bring smaller differences more readily to light. I've got to read up more on angular momentum, something I'm certain was not covered in my high school physics class.

/JEP



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FatSebastian
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Re: A Hub-Axle Treatment Evaluation Protocol

Post by FatSebastian » Sat Apr 03, 2010 12:57 pm

Jeff Piazza wrote:There are definite limitations with this device; one might have to pay more than $13 for a more versatile instrument.
Does it require you to attach a reflector?



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Re: A Hub-Axle Treatment Evaluation Protocol

Post by Jeff Piazza » Sun Apr 04, 2010 3:27 pm

FatSebastian wrote:Does it require you to attach a reflector?
Although it comes with reflective tape, it seems not to require it. As you can sort of see in at least one of the photos, I just darkened the ring edge with marker, except for a small window of bare aluminum.



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