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ChazInMT · 08-12-2014, 11:18 AM

It is easy to think of a wheel spinning under a car as a fan, like when it is on the wheel balancing machine. How would the air be effected in this situation is pretty easy to see, it would act pretty much like a fan.

But this is NOT the environment a tire rolling under a car is in, the air is stationary and the car is moving through it. The contact surface of the tire is coming to a compete stop, and the top surface of the tire is moving through the air at twice the speed of the vehicle. The air at the hub is moving along at the same speed as the car. In light of this, you start to realize that the wheel and tire system rolling underneath the car is a Very complex aerodynamic environment.

When the wheel & tire are rolling along under the car and Through the air (which is stationary), to the air, the tire comes to a dead stop when it is on the ground and the tire surface is going twice as fast as the car when the tire is at the top of its rotation as the wheel spins on its axle. So it is a tremendously dynamic system that exists here.

This is known as a cycloid if you want to Google it. there is a lot of history behind it and a ton of math.

Looking at this, you can sort of see that the “blades” on a wheel are doing nothing when they are at the bottom, but, on top the blades are moving quite fast, so it would seem that they are moving most of the air when at the top half of the wheel and not so much in the bottom half.

Kind of a cool thought puzzle when you really consider everything going on. Way more to it than you would consider at first glance.

It is also another reminder that we need to be mindful of the fact that our cars are moving through the still air, the air is not “Blowing” around our cars. With a boat it is pretty intuitive that the water just sits there and the boat move through it, but for cars, we all seem to think that the car is just sitting there and the air is blowing over it, like in a wind tunnel. But it doesn't and a rolling tire/wheel system really illustrates the fact that it's much more complicated than you'd think.

08-12-2014, 11:18 AM	#9 (permalink)
ChazInMT Aero Deshi Join Date: Jan 2010 Location: Vero Beach, FL Posts: 1,065 MagMetalCivic - '04 Honda Civic Sedan EX Last 3: 34.25 mpg (US) Thanks: 430 Thanked 669 Times in 358 Posts	It is easy to think of a wheel spinning under a car as a fan, like when it is on the wheel balancing machine. How would the air be effected in this situation is pretty easy to see, it would act pretty much like a fan. But this is NOT the environment a tire rolling under a car is in, the air is stationary and the car is moving through it. The contact surface of the tire is coming to a compete stop, and the top surface of the tire is moving through the air at twice the speed of the vehicle. The air at the hub is moving along at the same speed as the car. In light of this, you start to realize that the wheel and tire system rolling underneath the car is a Very complex aerodynamic environment. When the wheel & tire are rolling along under the car and Through the air (which is stationary), to the air, the tire comes to a dead stop when it is on the ground and the tire surface is going twice as fast as the car when the tire is at the top of its rotation as the wheel spins on its axle. So it is a tremendously dynamic system that exists here. This is known as a cycloid if you want to Google it. there is a lot of history behind it and a ton of math. Looking at this, you can sort of see that the “blades” on a wheel are doing nothing when they are at the bottom, but, on top the blades are moving quite fast, so it would seem that they are moving most of the air when at the top half of the wheel and not so much in the bottom half. Kind of a cool thought puzzle when you really consider everything going on. Way more to it than you would consider at first glance. It is also another reminder that we need to be mindful of the fact that our cars are moving through the still air, the air is not “Blowing” around our cars. With a boat it is pretty intuitive that the water just sits there and the boat move through it, but for cars, we all seem to think that the car is just sitting there and the air is blowing over it, like in a wind tunnel. But it doesn't and a rolling tire/wheel system really illustrates the fact that it's much more complicated than you'd think.