Tire Wind Deflector Question

[daqcivic]

Tire deflectors need to be designed in consideration of the whole area around the tires—I'm sure that goes without saying. What's been said so far about angling the deflector shape to face the outward direction of the airflow as it approaches the tires is helpful, but we also need to consider how this airflow interacts with the wheel wells and the outer sides of the car body and wheel.

It's my opinion that without any aero modification the wheel wells, and the spinning inner surface area of the tires and wheels, contributes a significant amount of the drag attributed to the front wheels/tires. The wheel wells are a negative space with low pressure that will draw air coming under the body and disrupt the airflow behind it, so that even with a smooth underbody aft of the wells the airflow will be somewhat turbulent.

Also because the airflow underneath the front underbody is bowed outward, if the deflectors are designed with only the leading edge of the tires in mind there will be quite a lot of air directed toward the inner side of the tire and wheel, which will create a lot of drag since the area is recessed and the surfaces are spinning.

You could just create an airfoil deflector that would be wide enough to divert air around the tires and the wheel well at the appropriate angle, but that would be so wide that it would greatly increase the frontal area, and it would be hard to make the deflector long enough to create an effective airfoil shape. I think a better solution would be to 1) create an undertray that will cover as much of the wheel well as possible, leaving room of course for the wheels to turn and the suspension arms to pivot through their full ranges, and 2) design airfoil deflectors angled to face the bow angle of the airflow, with a front area wide enough to mask the tires' profile at that angle. I would also curve the deflector so that the rear end is close to 0 degrees, as you don't want it to push air further out from the side of the car than it already is.

Here's a very rough 2D sketch:

[Otto]

I agree. Nothing is going to be perfect, but your basic design is probably about as good as practical, esp. if tweaked for cruising speed airflow.

[skyl4rk]

Remember that 95% of the time your wheels will be straight and not turned, especially at higher speeds. Better to optimize the design for the 95% period rather than try to optimize under all angles of turn.

[botsapper]

Ditto. KISS solutions also recommended.

[Otto]

Originally Posted by skyl4rk

Remember that 95% of the time your wheels will be straight and not turned, especially at higher speeds. Better to optimize the design for the 95% period rather than try to optimize under all angles of turn.

Yep. Any time you're going slow enough to crank the steering over, you're too slow to make much aero drag. Better to set it up for max efficiency at cruise speed.

[doviatt]

so just ignore the dotted line in the drawing. The shape still applies to the angled oncoming air, does it not?
And for my clarification this is angled because of the lower front of the car pushing the direct line of travel air to the sides correct?

[winkosmosis]

Originally Posted by doviatt

so just ignore the dotted line in the drawing. The shape still applies to the angled oncoming air, does it not?
And for my clarification this is angled because of the lower front of the car pushing the direct line of travel air to the sides correct?

The flow that hits the tires is under the car though, not to the sides of the body. It should be going straight back unless there's something else underneath to push it to the side.

[daqcivic]

The design represented by my sketch does not consider the aero effects of angled wheels. The dotted-line wheel demonstrates the area that the deflector must clear to allow the wheel to pivot throughout its steering range—otherwise the wheels would just rip up the deflector or tear it off the car. The lengthwise angle and curve of the deflector will not push air out past the width of the car anymore than the high pressure zone in front of and beneath the car already does. As many others have noted, with citations to proper scientific studies, the airflow that reaches the front tires does so somewhat diagonally rather than head on, so the deflector is designed in a NACA airfoil shape to directly face the airflow in order to minimize the drag it's frontal area creates, and also gently 'bend' it to parallel with the length of the car in order to aid smooth flow underneath the full length of the car and between the underside and sides of the car body.

Some seem to misunderstand what we are saying about the angle of the airflow underneath the front of the car, so to put it another way, if I'm the airflow, the frontal area of the tires that I 'see' is not from directly ahead of the car but from something more like a 30-45 degree view.

[daqcivic]

I forgot to mention as evidence of the direction of airflow under the front of the car the position of factory deflectors: they are all offset from the front profile of the tires, because that way they deflect the airflow around the tires from the angle the air comes at them.

I was actually in the dark for a long time about this until a few posts set me straight.

[skyl4rk]

Thats new to me. Why does the air flow in an angle, if the car is going straight? Pressure from the front end of the car moving to the outside?