What causes car drag?

[JulianEdgar]

This video looks at pressures and areas, and takes into account what is happening at the front of the car as well as the back.

We generally think of the rear of the car as being more important than the front when it comes to drag, but I wonder how much of that is a carryover from older-shape cars that had a lot of rear separation (eg from the trailing edge of the roof on sedans / notchbacks).

This video deliberately tries to keep things simple so doesn't look at trailing vortex drag, undercar drag or friction drag. Note that the diagram below is not showing pressures but actually the drag/ thrust caused by each area of the car.

https://www.youtube.com/watch?v=2qXwLaiRtYg

The video was in part prompted by Vman455's measurements on his Prius, and also a comment I got on my channel that questioned how much drag was actually being caused on this Hyundai by the low pressure wake:

https://www.youtube.com/watch?v=3zX4cU59gBk

[oldtamiyaphile]

Hi Jullian,

Is there a place where we can get your books within Australia?

[JulianEdgar]

Originally Posted by oldtamiyaphile

Hi Jullian,

Is there a place where we can get your books within Australia?

Yes - https://www.booktopia.com.au/search....Julian%20Edgar

They don't have all of them, but most.



Oh and I should have said, Car Aerodynamic Testing for Road and Track is available as Kindle download here.

[JulianEdgar]

And, talking about wake negative pressures not being that great in the scheme of things, look at the old Porsche 924.

[JulianEdgar]

..and my Honda Insight...

[kach22i]

I enjoyed these two videos even more than the others.

I might be getting use to your voice.

[aerohead]

At either the 2008, or 2010, Los Angeles Auto Show, Ford's Chief of Global Design, told journalists that the rear was responsible for drag.
I don't have that particular citation with me. I'll try for Friday.

[Vman455]

That Jaguar image is especially interesting; I wonder how they calculated contribution to overall drag--gauge pressure and surface inclination?

Scibor-Rylski, again (I find myself quoting him a lot lately--he has more of a way with words than a lot of other technical authors):

The aerodynamic force is the net result of all the variably distributed pressures which the airstream exerts on the car surface.

(Scibor-Rylski, A.J. and D.M. Sykes, ed. Road Vehicle Aerodynamics, 2nd ed. [London: Pentech Press, 1984], 1).

[JulianEdgar]

Originally Posted by Vman455

That Jaguar image is especially interesting; I wonder how they calculated contribution to overall drag--gauge pressure and surface inclination?

Yes, I imagine that's how the CFD software estimates it.

[freebeard]

Originally Posted by kach22i

I might be getting use to your voice.

I think the delivery is fine, but the room acoustics are 'boomy'. Could you hang some drapery behind the camera to absorb some reverb?