Bottom tire area fairing/brush
 

I'm thinking about using a light broom or set of broom bristles to form triangular fairings around the front and rear tires of my car. What are some possible safeguards or loopholes in this approach, how would you recommend doing it, and what type of CD advantage am I likely to see?

In the early 1960's brush bristles were experimented with on some of the early small hovercraft (otherwise skirtless).

They discovered brush bristles didn't cut down on the dust being kicked up from the cushion pressure escaping out of the bottom of the hull, and did not contribute to forming an airtight seal either.

About the only thing they did was sweep a surface clean and perhaps soften coming off cushion slightly from impact with the ground. They were dead weight and were abandoned.

In your application I'm guessing the air would pass though unabated, perhaps turbulently and the bristles would not divert the air and the air pressure behind it would push through.

Think of the wind passing through a row of pine trees.

https://www.vwheritage.com/blog/2014...ed-hovercraft/
https://www.vwheritage.com/blog/wp-c...ure-2small.jpg

tires
 

So far,the best source of info is Goro Tamai's book,The Leading Edge,developed from his work with the MIT Solar Race Team.
Whatever you use,as mentioned,it needs to be non-porous.I'm using foam floor cushion padding,and I let the tires ablate it to 'size.'