DIY Tire Spats

I’ve been de-aeroing my street car this year. What?

Last year I was experimenting with aero and I made an airdam and splitter on my 1993 street car. The airdam was a vertical 3″ extension under the stock lip, and the splitter was simply the forward part of the undertray. I built an adjustable spoiler to balance it out.

September 2019. Small airdam, splitter, adjustable spoiler (the black plastic part can be moved up or down into three different heights).

It went pretty well until I stuffed it into the weeds. The splitter dug in hard, bending the mounting brackets, the undertray got shoved underneath the car, and I sheared some bolts and plastic. It wasn’t pretty.

Oops. Shoved the splitter/undertray back and broke my side spats.

So like I said, I’m de-aeroing my car. My hope is, that if I go off track into the weeds again, the car just glides over them, rather than the splitter ruining my day.

The front end is now the popular R-package front lip. The brake ducts aren’t really functional, so I covered them up. Underneath, I still use a full undertray so I can duct the radiator and whatnot, but the front edge terminates right at the R-package lip.

One of the things I don’t like about the NA front end is that it doesn’t have enough tire coverage, and so I built some front spats to deflect air away from the tires.

R-package front lip with plywood undertray and covered brake duct. From this angle, it looks like the wheel spats cover the whole tire, but they don’t.

Last time I made spats out of extra HDPE plastic I had lying around from making the airdam. This time I decided to use metal, so I could pre-form the shape. It was pretty easy to mock up in cardboard, then bend it over and get the shape right.

I fastened the bottom of the spats to the undertray, sandwiching them between the undertray and the front lip. At the top I folder over the extra material, drilled a hole, and used the standard fender mounting bolt.

Angle aluminum wickers on each side.

Finally I added some 1/4″ angle aluminum to the edge of the spats. These required a bit of effort to form. I had to clamp it in a vise as I bent the arc slowly. If I hadn’t clamped it down, the whole piece of aluminum would have come out bent in more than one dimension.

The purpose of the angle aluminum is to act as a wicker (Gurney flap), and fool the air into thinking the spats are wider than they are. As air moves around the front end, it encounters this raised edge which deflects air around it, effectively making the spats wider.

Wicker helps deflect air around the tire.

I suppose the angle aluminum also makes the outer edge stiffer, but I’m not sure if that was an issue, these are pretty strong and very light. I don’t know if they can handle any serious grass cutting, but at least it’ll just be the spats I lose next time, and not the whole shebang.

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