wheel discs only on the front?

[freebeard]

Actually, I was thinking about forming the center hole. (Plywood buck = dimple die)

Detroit used to restamp their fenders to make low-production models.

The radius is achievable, but an aluminum cylinder or cup would need to be brazed on to complete it as shown.

[Vman455]

Originally Posted by kach22i

There is a typical pattern of the front wheels generating more drag than the rear wheels - on most cars. I'm sure that you will find an exception or two, but look for a majority pattern.

Most textbooks I've read put this around 60-65% of wheel drag associated with the front wheels, and 35-40% with the rear; reverse those numbers for buses and tractor-trailers.

Keep in mind that high pressure up front means low air velocity, and high air velocity means lower air pressure. This is not very intuitive, Mr Bernoulli got it right though.

Bernoulli's principle
https://en.wikipedia.org/wiki/Bernoulli%27s_principle

So now we want to ask if it's the pressure causing the drag or the air velocity.

This is what NASA has to say.

What is Drag?
https://www.grc.nasa.gov/www/k-12/airplane/drag1.html


So it's Velocity and not Pressure that causes drag.

Bernoulli's equation posits a relationship between pressure and velocity due to conservation of energy: the static pressure, gravitational potential energy, and kinetic energy ("dynamic pressure") of a fluid must remain constant if no external energy is added to the system. "Dynamic pressure" is proportional to the square of fluid velocity, and as that term increases static pressure must decrease (and vice versa), assuming no or minimal change in height.

The relative velocity of the air around a car body is zero due to the no-slip condition at the wall. Because of the viscosity of the fluid--a result of molecular attraction between fluid particles--the relative velocity increases with distance from the body until you reach the edge of the boundary layer, where relative velocity is now equal to free stream velocity. The drag from the "sheets" of air in the boundary layer sliding against each other is friction drag, and yes, it is caused by velocity differential.

But: the majority of overall drag force acting on a car (>80%) is due to pressure drag. I haven't read much about airplane aerodynamics (and that link isn't opening for some reason), but it may be that they don't operate in flow with a no-slip condition at the body; in that case, velocity differential probably contributes the majority of drag since they are streamlined shapes. But on bluff bodies at the speeds we're interested in on this site (read: low), I would not say that velocity and not pressure causes drag.

[freebeard]

Most textbooks I've read put this around 60-65% of wheel drag associated with the front wheels, and 35-40% with the rear; reverse those numbers for buses and tractor-trailers.
....
Bernoulli's equation posits [a lot of stuff].

It's simpler than that. The front wheels [typically] have to turn so the wheelwell is more voluminous than a rear wheel well. More volume leads to more turbulence. More turbulence leads to the dark side.

[kach22i]

Originally Posted by Vman455

Most textbooks I've read put this around 60-65% of wheel drag associated with the front wheels, and 35-40% with the rear; reverse those numbers for buses and tractor-trailers.

Bernoulli's equation....................

Fantastic post, I'm going to have to re-read it several times for it all to sink in.

Originally Posted by freebeard

........front wheels ..... wheelwell is more voluminous.......

That is part of it - I trust.

Would like to see a paper on it though.

[Vman455]

Originally Posted by freebeard

It's simpler than that. The front wheels [typically] have to turn so the wheelwell is more voluminous than a rear wheel well. More volume leads to more turbulence. More turbulence leads to the dark side.

That might be part of it, but also that the rear wheels ride at least partially in the wake of the front wheels.