rear wheel aerodynamics (undertray/diffuser/boattails)
 

i've recently completed stage 1 on my rear undertrat/diffuser that basically covers the area behind the rear axle up to the rear bumper.

this leaves the area behind the rear wheels uncovered. part of the problem covering this is that the rear wheel wells are open at the back.

i'm wondering if there are aerodynamic reasons for this design, as air can vent into the space behind the rear bumper (wich is relatively high) and excape further to the middle of the car.

i have a few options

-leave things as they are and place fences at the side of the existing undertray

-continue the undertrat and make it flush witht the bumper and extend the wheel wells so no dirt gets on the tray

-continue the undertray but curve front of the parts behind the wheel up so they meet with the curent wheel well, creating something ala reverse break cooling duct on race car undertrays

-build full rear wheel boattails (not likely to happen)

Boat tails, like on Aerohead's trailer, using rigid, sculpted or hotwired foam. Chord line canted inward, for better confluence of flows around the outside of the wheel and from under the car, like Prius, but bigger/better.

Some smart person on this website suggested using plastic or rubberized waste baskets cut in half for wheel fairings. Oval ones would work well for leading edges on front sides of tires, maybe also rear sides of tires if the oval were cut at the top of the arch, pinched, and rejoined. Foam probably best for this app, though. Not really structural, this part could be also made of Coroplast if you can find a way to permanently glue it.

YouTube for "hotwire foam" or similar.

I think that the first step would be to simply extent the undertray behind the rear wheels then build a vertical air dam above the undertray behind the rear wheels to keep most of the dirt out, build and mount boat tails behind the wheels on the underside of the undertray. I suspect that it will take two or three versions befor you get what you want,and what has the best aerodynamic affect. There are as many ways to do this as there are people doing it, have some fun and see what works for you.:thumbup: