Why does a bicycle wheel spin when bike is on top of the car?

Answer 1

Well, are we talking while the car is in motion, or...?

If we're talking while the car is in motion, the answer is pretty simple (and slightly not):

The most likely cause is the friction caused by the air moving past the tire while the car is moving. As the air passes along the surface of the tire, it imparts energy into the tire in the direction the air is moving relative to the tire.

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Now, why is it that the tire spins in a particular direction and not the other? Since one would assume the tire is perfectly round and equal amounts of air would flow over one side and over the other, one would think the energies imparted would be equal thus no imbalance and no rotation... In a perfect system (with equal velocities and air paths on both sides of the tire). But the airflow around a car is NOT a perfect system, nor is it equal on obth sides of the tire due to the aerodynamics of the car in question.

You might want to take a look at some air flow diagrams of automobiles. It's kind of like the wing of a plane. As it moves through the air, the air tends to get less pressurized over the top of the wing, since the same amount of air has to travel in a warped path rather than a straight path (this is how we get lift for planes, by warping the wings slightly in one direction so we get buoyancy due to lower pressure above than below).

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Same thing likely happens with a car. So, I'd guess that the air above a car will tend to be slightly startified with the portion closer to the car moving at one velocity with the air at a slightly higher elevation will move at a different velocity or in a slightly different direction. The difference in speed and pressure probably accounts for the direction and speed of rotation in the tire.

IE if the air on the bottom moves faster than the air on the top, I'd expect the bottom to move backward and the top to move forward. or taken from the left hand or driver side, the tire would appear to move counter clockwise. Whereas of the lower air moved slower and the higher air moved faster, I'd expect a clockwise rotation. If the air was moving at about the same speed, i'd expect the tire to be relatively motion less (at low speeds).

Cheers!
~Michael

Answer 2

Well, yes it is the wind that spins the wheel, but the bike wheel is not a pinwheel. A pinwheel has twisted blades for torque generation.

What you are seeing with the bike wheel is a type of wind shear. As the air flows over the top of the car there is a boundary layer formed. The air speed (relative to the car roof) is low near the roof and faster farther above the roof. So the wind pushes harder on the top of the wheel than the bottom.

If I'm correct, you should be seeing the top of the wheel moving rearward and the bottom moving forward (relative to the car body).

Answer 3

There was actually a time there were some ships that were propelled by the fact they had spinning cylinders instead of sails. Wind blowing on a round object will cause it to spin, as air moves faster around a curved shape, lift is created on the other sides and the wheel spins.

Answer 4

Gerry - not particular. what's a push bike? My first bicycle - with a chain and pedals - it replaced right into a boys bike - it replaced into used and it replaced into 6 money. My father bought it from the neighbor at 6 O'clock in the evening throughout the time of a wild rainstorm. i replaced into in the sixth grade on the time. I rode that bike for a minimum of three hours. i replaced into ring and moist. i did not care about the elements. i replaced right into a baby with a sparkling toy and in no way some thing replaced into going to stop me - come rain or shine. I had 2 youthful brothers waiting on the porch for his or her turn. Is that what you advise - that type of motorbike? DeeJay.

Answer 5

Friction from air molecules because of the speed of the vehicle carrying the cycle.

Answer 6

Due to air drag

Answer 7

usually from the wind blowing it...

just like if you blew at a pinwheel...

Answer 8

wind

Answer 9

wind circulation

Answer 10

friction