wheel question
01-20-2002, 12:11 PM
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wheel question
let say you have two wheels one is a 15" wheel and one is a 16" wheel they both weight exactly the same weight lets say about 20 lbs. Would the car be quicker with the 15" wheel or would it not matter since they are both the same weight.
01-20-2002, 07:07 PM
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Ok. that's funny. Now the physics answer...
Assume the wheels weigh the same. And assume the tires that go on them weigh the same (perhaps not a good assumption, but it's the question you asked).
It's actually not the weight that matters directly. It's the polar moment of intertia of the wheel/tire combination.
Skipping all the math: The more mass you put out near the *perimeter* of the wheel/tire the larger the moment will become.
You can feel this yourself which a simple experiment. Pick up a hammer. Flip over 180 while holding the end of the handle. Then flip it over holding the head of the hammer. Same weight in each case. One is harder on the wrist than the other. That's polar moment of intertia.
So, all other things being equal the 16" wheel/tires will slow the car down more than the 15" under acceleration. Because the 16" ones have more metal (heavy) out near the perimeter of the wheel/tire.
Now in real life, the larger sidewall tires that go on the 15" wheels weight more than the ones on the 16" wheels (unless you also went with wider tires in the upgrade). And in any case, the effect will be very small and hard to notice.
But the physics say: more spinning mass out near the perimeter of the disk = higher polar moment of intertia = harder to accelerate.
It's actually not the weight that matters directly. It's the polar moment of intertia of the wheel/tire combination.
Skipping all the math: The more mass you put out near the *perimeter* of the wheel/tire the larger the moment will become.
You can feel this yourself which a simple experiment. Pick up a hammer. Flip over 180 while holding the end of the handle. Then flip it over holding the head of the hammer. Same weight in each case. One is harder on the wrist than the other. That's polar moment of intertia.
So, all other things being equal the 16" wheel/tires will slow the car down more than the 15" under acceleration. Because the 16" ones have more metal (heavy) out near the perimeter of the wheel/tire.
Now in real life, the larger sidewall tires that go on the 15" wheels weight more than the ones on the 16" wheels (unless you also went with wider tires in the upgrade). And in any case, the effect will be very small and hard to notice.
But the physics say: more spinning mass out near the perimeter of the disk = higher polar moment of intertia = harder to accelerate.
01-21-2002, 04:36 AM
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Good explanation, zain. There is a fairly simple way to measure..
polar moment of wheel/tire combinations.
Make a cradle from three thin cables (1/16 dia works) to hold the tire/wheel horizontally, then suspend it 8 feet or so on a single strand of that cable from the ceiling joists. Rotate the tire to wind up the cable a few turns. Accurately measure the period for a few turns of wind up and wind down. Direct comparisons are easy, and with some math you can calculate the actual polar moment.
Make a cradle from three thin cables (1/16 dia works) to hold the tire/wheel horizontally, then suspend it 8 feet or so on a single strand of that cable from the ceiling joists. Rotate the tire to wind up the cable a few turns. Accurately measure the period for a few turns of wind up and wind down. Direct comparisons are easy, and with some math you can calculate the actual polar moment.
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01-21-2002, 09:50 AM
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I'd take the empirical approach with a test pc. of known polar moment.
A 24 inch dia. circle of 5/16 thick steel plate, about 40 lbs. and easily calc'd I.
That's not easy to get so how about eight 24 in. dia. circles cut from a 4' x 8' sheet of 3/4 in. MDF available from your home center and glued together. That should be about 36-37 lbs, and make a good calibrator.
I'll find the math if anyone wants it.
That's not easy to get so how about eight 24 in. dia. circles cut from a 4' x 8' sheet of 3/4 in. MDF available from your home center and glued together. That should be about 36-37 lbs, and make a good calibrator.
I'll find the math if anyone wants it.
01-21-2002, 10:20 AM
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Oh yeah, I got a "known polar moment reference" in my garage someplace ;)
It think it's next to the NIST traceable mass standards and under the calibrated radiometers.
Sorry, couldn't resist.
Sorry, couldn't resist.