Effectiveness of side skirts?
 

I'm planning to build an undertray in the near future and I'm looking at possibly doing side skirts. The Aerocivic uses skirts lined up with both the inside and outside of the tires. Side skirts would help when used in conjunction with an air dam when there's a rough underbody, but I'm planning to smooth mine out. Alternatively, I could do a boat tail behind the front wheel and then a contoured deflector in front of the rear wheel. Any idea if one will work better than the other?

i think that the side skirts give you some resistance to cross-winds corrupting the smooth air under the car. In racing, they give you some downforce because the low pressure air underneath is trapped better... take that as you will.

With a smooth underbody, do side winds going under the car really matter? Side skirts would direct more of the air around the car, but they would also present a larger area for the wind to blow against.

I think side skirts might be easier for me to make, but I could build deflectors and boattails that wouldn't catch on speed bumps. Anything more than about 2" on a side skirt would have a good chance of scraping.

I debated both approaches when I did my car, but figured the side skirts would help to keep the airflow travelling straight and parallel under the car, unaffected by the presence of any crosswinds, that could slant and redirect the airflows under the car, possibly increasing the drag. I'm containing the turbulence created by the wheels into a "pod" running down each side of the car with a straight, parallel airflow underneath the car in between the "pods". I recall discussing it with Aerohead on another site, who recommended the approach I took. Adding the side skirts made a noticable reduction in the dust trail kicked up by my car when driving on a dirt road.

My side skirts are made of coroplast and are designed to flex out of the way when scraping on bumps in the road. I deliberately made them overlong, sprayed them with black paint, and then after several days of driving, noted how far up them the paint had been scraped off, and trimmed them appropriately.

After trimming, about how much ground clearance were you left with?

3 inches. I left it slightly long so it still occasionally touches bumps in the road, figuring that it will eventually get abraded down to the optimum length.

I have seen some side skirts that run from the inside of the front tire to the outside of the rear tire, which moves some of the underbody flow to the sides. I can see where they might be good for performance but could they be developed for fe? Somewhat similar to the Insight's front wells that dump to the side?

^^^ I want to know that one to, it kind of makes sense...

I would think it is good for FE, I've seen the same setup on a few tractor trailers.

This setup would move the turbulent air coming off the back side of the front wheels to the side of the car just outboard of the rear wheels, while at the same time moving some air from under the car and feeding it into the front of the rear wheels. This might help FE on a car or semi with a non-aerodynamic underside by diverting some of the air out from under the vehicle and relocating it from contact with the rough underside to the smoother sides of the vehicle.

It would hurt FE on a car with a smooth underside and aero shielded (wheel well skirts, front air diverters, wheel boattails) wheels, since it would disrupt the linear flow of air under the car by redirecting it into the rear wheels. Any time you change the direction of an airflow, you create drag so you want to keep the airflow as linear as possible while still getting it past the car.