Wheel Spats and wheel well liners
 

I'm trying to do some drag reduction work on my NB Miata while avoiding attention on the street, and tire spats/wickers are high up on my list. I've searched around a bit and while I can find lots of good discussion on front wheel spats, I've found nothing for the rear of the car.

The NB Miata has this large sill overhang in the front edge of the rear wheel wells that angles towards the inside of the car. It's almost 3" deep. I feel like this would grab a lot of air that's spinning with the tire and fling it back into the underbody area, adding to the turbulence and high pressure in this area.

https://cimg0.ibsrv.net/gimg/www.mia...3f8bc4697d.jpg

Pretty much any time you look in the wheel well of a newer car, like my girlfriends 2017 Chevy Volt, and you'll find a smooth wheel well liner with no ledge, and a spat hanging below the rear sill to deflect oncoming air away from the tire. The thing is I have no idea how much this is for drag, or if the primary focus is NVH. I certainly can hear rocks flinging into the sill every time I go over gravel or sand.

https://cimg3.ibsrv.net/gimg/www.mia...00db4844c6.jpg

It seems like good engineering judgement (TM) to replicate these features. Something like this Cardboard Aided Design mock up, but you know, better executed.

https://cimg3.ibsrv.net/gimg/www.mia...ca781992e0.jpg


I'm curious if anyone else has gone down this road before for aero reasons, read a post/article/book about it, or done experiments in the real world? Do these low hanging spats I see only on production cars only reduce drag, or do they have a predictable change in lift? I'm sure this is far from the best place to spend my time and effort for drag reduction, but my hobby is overanalyzing this kind of stuff.

I got some generally positive feedback on the Professional Awesome Facebook page, so I went ahead and made something up.

https://cimg3.ibsrv.net/gimg/www.mia...a04c3c003a.jpg
This old beer box was such a good fit with almost no trimming. Too bad cardboard isn't waterpoof.

https://cimg2.ibsrv.net/gimg/www.mia...9f8d4449ef.jpg
I took it with me to Walmart and found the cheapest plastic piece that covered the template, a garbage bin missing a lid or something to put it in the discount section.

It's polypropelene, so not the toughest thing out there, but for $4 it's worth a shot.

https://cimg1.ibsrv.net/gimg/www.mia...6f674e99eb.jpg

Trimmed to fit. I used some VHB tape to bond some tabs to the wheel well, then drilled holes in them to use standard 8mm fender liner fasteners.

The next step is to create the spat that hangs below the sill. Ideally I would have made it all out of one piece, but the trash can wasn't big enough.

https://cimg2.ibsrv.net/gimg/www.mia...0448ec2266.jpg

Keeping the theme going up front, I added RGR Engineering wheel spats and a knockoff RS Factory lip. I thought about making the spats on my own, but given the quality of finish on these and the time I would have spent, I felt they were well worth the $40.

I've always wondered if the little anti-drag flaps on modern cars can be retrofitted to an NB, but I am nowhere near smart enough to work out if its actually effective in a measurable amount on an NB so haven't put much thought into it. I will be following this with interest.