Ok so what is the purpose of the little air dams mounted under the car forward of the rear wheels?

Any guesses?

 

The wheels on a car are a major source of aerodynamic drag. The small dams in front of the rear wheels divert air either into the central void, or outboard of the car and away from the wheel.

When the air is diverted into the central area its pressure reduces and helps create downforce (especially if the flow is the ejected from the underside using a diffuser). However, the main reason is for drag reduction and hence fuel economy.

wombat

 

Yes sometimes called Areotabs. As posted above it's for air flow management. Trying to reduce the air turbulence the spinning tires cause. Usually they are mounted in front of all 4 wheels. Look under the front of your XJ and you will see them mounted in front of the tires.
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club,

Aerotabs, or Airtabs, are actually reverse NACA ducts. They are designed to excite the airflow close to the boundary layer (Vortex Generators) and excite it away from the body in question. They are often used on trucks and trailers to reduce 'Boat tail drag'.

The air dams fitted to most cars build a body of air that is forced, usually through induced flow, into the central void. A very small aero device can have a dramatic effect on distance separated airflow. One beneficial effect, that you quite rightly mention, is that the turbulent air created by wheel rotation is spilled overboard.

Aerotabs are not air dams....Trust me, I'm an aeronautical engineer, and test pilot.

wombat

 

Thanks for the explanation, my F-Type has the same little dams/tabs (I think all F-Types have them) and I have never seen an explanation of what they are for on any F-Type sub-forum. I thought maybe they were some type of mini mud flap or road debris blocker but then I thought nah, they are way too small for that.
A question - I had to saw mine in half which left around a 1 mm slot in the middle so I could remove the lower panels which they are rivetted to so I could fit illuminated tread plates, would those slots make any difference to if/how they work?

 

OzXFR,

I think I follow your explanation, but without seeing a photo it's hard to be sure. I think you should be OK, even very small devices can have large aero impacts and parts of an aero device can be missing and the device, as a whole, will still work! Think I'm crazy? Look at most Russian designed Fighter jets and you will notice that the vertical fin is clipped at the top. They worked out that the sweep angle, combined with flow characteristics made the cut-out section redundant. Western companies took a different approach for different reasons. Another example is the tiny saw tooth 'windshield' fitted to some F1 cars. Designers worked out that they could save weight buy removing 50% of the panel as the remaining area would have sufficient effect on the airflow.

wombat