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Today in physics i learned that solid disc's spin faster then hollow discs.

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Today in physics i learned that solid disc's spin faster then hollow discs.

Old 01-26-11, 07:43 PM
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Today in physics i learned that solid disc's spin faster then hollow discs.

Disc rim VS traditional laced rim?

I learned that a solid disc will always spin faster due to the center of gravity.

You think this would apply verywell with a disc rim?

I've done multiple tests in class, And it's so far been proven true. I would do the test with a disc and traditional lace. But that means i need a mirror surface large enough for the rim, And i would need to spin the rims on its lip not tires ):
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Old 01-26-11, 07:53 PM
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Spokes have more air resistance than the disc; cyclists learned this long ago. They also learned that in crosswinds, the fool with disc wheels is in trouble. Back in the '80's, they ran time trials in the Red Zinger races with disc wheels on the front. In Colorado. Where the wind never blows. yeah, right. They no longer do such foolish things. But you do see roadies with 40-60mm deep rims.
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Old 01-26-11, 07:56 PM
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Originally Posted by evilcryalotmore
Disc rim VS traditional laced rim?

I learned that a solid disc will always spin faster due to the center of gravity.

You think this would apply verywell with a disc rim?

I've done multiple tests in class, And it's so far been proven true. I would do the test with a disc and traditional lace. But that means i need a mirror surface large enough for the rim, And i would need to spin the rims on its lip not tires ):
Where do you study physics?
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Old 01-26-11, 08:00 PM
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Oh boy, this could get ugly.
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Old 01-26-11, 08:01 PM
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Originally Posted by evilcryalotmore
Disc rim VS traditional laced rim?

I learned that a solid disc will always spin faster due to the center of gravity.

You think this would apply verywell with a disc rim?

I've done multiple tests in class, And it's so far been proven true. I would do the test with a disc and traditional lace. But that means i need a mirror surface large enough for the rim, And i would need to spin the rims on its lip not tires ):
For wheels of equal mass and uniform mass distribution this is true. So you can't just slap on a cover and magically reduce the wheel's moment of inertia.
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Old 01-26-11, 08:09 PM
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Originally Posted by KyleOndy
For wheels of equal mass and uniform mass distribution this is true. So you can't just slap on a cover and magically reduce the wheel's moment of inertia.
Exactly. Plus, you can't really model a spoked wheel as a "hollow disc" because it isn't. The rim alone might be, but it has all the spokes and hub attached to it.

I think Post #2 sums it up pretty well. Although I'd still like to try a rear disc wheel on my roadie and see how it works. I've been thinking about making my own wheel covers, just out of curiosity.
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Old 01-26-11, 08:13 PM
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Post the numbers of your tests please.
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Old 01-26-11, 08:14 PM
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Huh, Just wondering because i had done this inscale with a uiler's disc one hollow ring and one disc, And the disc almost spinned for over 3-4 minutes.

How about those fancy rims i see that are semi disc. They have 4 small small holes maybe 2-3 inches for air to go through.
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Old 01-26-11, 08:15 PM
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What axis were you spinning them on?
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Old 01-26-11, 08:18 PM
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Originally Posted by evilcryalotmore
inscale

a uiler's disc

the disc almost spinned
I'm so confused! Maybe you should pay more attention in English?
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Old 01-26-11, 08:20 PM
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Originally Posted by FastJake
Although I'd still like to try a rear disc wheel on my roadie and see how it works. I've been thinking about making my own wheel covers, just out of curiosity.
Rear discs are used routinely on outdoor tracks, where it can be windy, so I don't imagine it would be a problem on the road. However, most of the aerodynamic benefit comes from the front wheel, so IDK if you will notice a very significant improvement. What does make a big difference IME is using bladed / oval spokes instead of round spokes and to minimize the spoke count in the front and to a lesser extent in the rear. Finally, use the narrowest possible front rim and tire.
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Old 01-26-11, 08:20 PM
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It doesn't matter if you can spin the disc faster, it's different when it is on a bike with wind resistance, road resistance, drivetrain of the bike, physical condition of the cyclist etc etc.

Bunk test is bunk.
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Old 01-26-11, 08:26 PM
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If you are talking about an Euler Disk, https://www.youtube.com/watch?v=ug2bKCG4gZY, Thats a whole different ball game of physics.
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Old 01-26-11, 08:26 PM
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Originally Posted by TejanoTrackie
Rear discs are used routinely on outdoor tracks, where it can be windy, so I don't imagine it would be a problem on the road. However, most of the aerodynamic benefit comes from the front wheel, so IDK if you will notice a very significant improvement. What does make a big difference IME is using bladed / oval spokes instead of round spokes and to minimize the spoke count in the front and to a lesser extent in the rear. Finally, use the narrowest possible front rim and tire.
Does my Specialized tri-spoke count? According to this: https://www.sheldonbrown.com/rinard/wheel/grignon.htm it's pretty aero.

Still, it can really throw me around on windy days. I can't imagine using a front disc outdoors.
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Old 01-26-11, 08:38 PM
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tomorrow i learned in calculus that skinny jeans have a lower drag coefficient than baseball caps leading to higher fuel efficiency
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Old 01-26-11, 08:41 PM
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Originally Posted by FastJake
Does my Specialized tri-spoke count?
Does it wooosh when you bomb a hill?
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Old 01-26-11, 08:46 PM
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Originally Posted by FastJake
Does my Specialized tri-spoke count? According to this: https://www.sheldonbrown.com/rinard/wheel/grignon.htm it's pretty aero.

Still, it can really throw me around on windy days. I can't imagine using a front disc outdoors.
Sure, its very aero, which is why triathletes use them, but in the front any solid spoke wheel, just like any deep section wheel is going to be hard to control in the wind. I used to own a Mavic IO front 5 spoke solid carbon wheel and it was murder to control on windy days. But on calm days there was nothing faster.
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Old 01-26-11, 10:00 PM
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Originally Posted by xkillemallx16
tomorrow i learned in calculus that skinny jeans have a lower drag coefficient than baseball caps...
duh.

that's why i wear a fidel hat.

Last edited by dookie; 01-26-11 at 10:03 PM.
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Old 01-26-11, 10:13 PM
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Originally Posted by bbattle
Back in the '80's, they ran time trials in the Red Zinger races with disc wheels on the front. In Colorado. Where the wind never blows. yeah, right.


My regular ride includes the Morgul Bismarck from the old Red Zinger races.
Not only is it windy as holy hell in the winter, there are always a few days every winter when I must give up and turn around because it it literally impossible to keep from getting blown off the road on the Northbound leg.
You couldn't pay me enough to run a disc.
I won't even use Deep-V's.
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Old 01-26-11, 10:19 PM
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Old 01-26-11, 10:21 PM
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I'm glad that you're trying to apply what you learn in class to real life, but you should do some more reading one why a solid disk will spin longer than a hoop. Your experiment probably had nothing to do with aerodynamics, and everything to do with the moment of inertia of the objects.
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Old 01-27-11, 10:32 PM
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Originally Posted by TejanoTrackie
. Finally, use the narrowest possible front rim and tire.
Is this true? I know some of the newer Zipp rims are wide, and you use a narrow tire on them to create a leading edge or something to that effect.
For example:
https://www.zipp.com/technologies/aer.../firecrest.php

Last edited by clink83; 01-27-11 at 10:43 PM.
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Old 01-27-11, 10:49 PM
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Originally Posted by clink83
Is this true? I know some of the newer Zipp rims are wide, and you use a narrow tire on them to create a leading edge or something to that effect.
Yeah, there's some newfangled aero stuff out there now where the rim kinda bulges out, but all the older wheels like the Mavic IO are very narrow, and designed to accept narrow tubular tires. I'm not really into the latest aerotech, and just use a Zipp 404 tubular front road wheel for all my TT stuff on the track and a narrowish 20mm tire.
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Old 01-27-11, 10:54 PM
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Originally Posted by yummygooey
I'm glad that you're trying to apply what you learn in class to real life, but you should do some more reading one why a solid disk will spin longer than a hoop. Your experiment probably had nothing to do with aerodynamics, and everything to do with the moment of inertia of the objects.
hehe yep yep...

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