Structural role of the rear box sections?

[NoHachi]

@Firehawk..while you are working out there and removing bodywork, take a good look at the aerodynamics back there..this is you chance of drastically improving them.

The object is trying to make the air going under the car move faster then the air moving over the car. If you manage that, then you have just created yourself a mild form of groundeffect. Usually though you cant cleanup the airflow enough to do that, but any gain you make on the underside of the car improves downforce and reduces friction. Stuff on top of the car usually helps one but ruins the other..

The effect of good underbody dynamics can be VERY noticeably at high speeds and make a car far easier to drive at those speeds. Luckily, making improvements is fairly easy, so if you are building and N2 replica it is worth focussing on this part. The things you can do easily are:

Reduce the amount of air flowing under the car. : What doesn't get under the car, doesn't get to push it up..easy as pie. The way to do this is to get a lip spoiler as low as you can get it. Make sure its a sturdy item that will not bend under the pressure.. Once you do this, a high pressure area is created right in front of the lip. It would be a waste to not use this high.pres. air, so extend a thin tray forward of the lip to catch this air and generate a bit of free downforce (see picca). The effects are noticeable at the track at speeds from about 120kph.

Smooth undertray: Once you get the air under the car it makes sense to have as little restrictions as you can..Everyone knows the smooth trays racecars have..Actually, this part is not so interesting as it is very difficult to do correct. Racecars have a the trays shaped like a reverse wing, generating again more downforce (ground effect cars), but you'll be lucky to just clean up flow a bit. Make sure you don't forget to clad the area around the engine properly, but leave an exit for the hot enginebay air (use Naca ducts or similar ideally). Problems with full undertrays are dumping the heat of the diff and tranny...Do not use full trays if you do not run a transmission and diff cooler setup. Luckily there are easier areas to fix.

Wheel wells: Again the wheel wells catch a lot of air that cannot get out. You could think about running ducts from the rear of the car to the rear wheelwells, allowing air trapped there to escape.

Rear spoiler: Finally we get to the rear spoiler. In most old car this is a true air pocket and easy improvements can be made. The AE86 isn't that bad, because the fuel tank does a good job of directing the air under the rear spoiler..however, cars utilising a fuel cell usually do far far worse. The rear spoiler the acts just like a bag that's let loose in the wind, cathing lots of air and creating tons of drag. thankfully this is easily solved, by making a few ducts to release the pressure (read: holes). A better solution is replacing the fueltank by a piece of sheeting that gently widens toward the rear of the car..also known as a diffuser on cars that generate downforce.

All of these things combined can add up to a 10 to 15 extra km/h at the track. Best of all, they are dead cheap to do, you just have to use your head in stead of your sense of esthetics.