Structural role of the rear box sections?

[firehawk]

Just wondering - what role do, if any, the rear corner box sections which are usually hidden below the rear bumper (see the photo below) play in the structural integrity of the car's body? Would it be possible to just remove them and then weld in some sheet metal to fill the open bottom? A small triangular support could be added in its place to support the rear bumper's sides. As far as I can see, the body of the car wouldn't lose any of its rigidity, despite the rear corners being absent.

Apparently, at some point these boxed sections have rusted through on my car and have been repaired with some sheet metal. They have then been covered with body filler and it seems that the moisture trapped underneath it has already started to rust the sheet metal again.

Removing the boxes would allow water/dirt to be thrown from the rear tyres to the inside of the rear bumper but I don't see that as much of a problem on a race car that's driven exclusively on asphalt-covered circuits. I haven't found any high-res photos of the SS Work's N2 AE86 which has a tubular subframe at the back of the car but from what I can see, these boxes have been removed and just a small support added for the bumper.

[NoHachi]

Anything aft of the rear wheels is just there for crash safety..no problem in removing them.

[kipsy]

I chopped them off mine but replaced them with sheet steel! I modded them not to hold lumps of dirt etc as my car is used on road rallies in the forest etc! Not structural as far as i can see!

[firehawk]

All right, thanks! Looks like if I cut them off from the floor level, there'll be left just enough of them to support the top of the bumper and all I need to add is a small support for the forward edge (which will be left under the N2 flare anyway).

[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.

[firehawk]

Wow, thanks for the excellent information! I hadn't really put as much thought into underbody aerodynamics as I probably should have. So far I've only been thinking about a rear diffusor-like thing which would guide air away cleanly, instead of making turbulence on the fuel cell, swirl pot, diff oil cooler/pump/filter, whatever stuff I have there.

Quote: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. ...
Smooth undertray: Once you get the air under the car it makes sense to have as little restrictions as you can..

The TRD N2 kit has actually a very nice front lip which extends quite low and includes small intake holes for brake cooling ducts. On several photos I've seen of N2 cars, an aluminium sheet has been attached to the underside of the lip - I don't know how far it extends back from the center. Most probably not further than the front crossmember. It also has the additional bonus of shielding the lip from getting damaged when taking the apex a bit too far over a curb.

As for cooling the engine bay, maybe I could use two small NACA ducts on the undertray just behind the radiator diverting air in and two larger ducts on the rear edge of the bonnet mounted shallow end rearward to exhaust air both coming through the radiator grille and the two small ducts. The air passing over the bonnet should create a low pressure point on the duct's ramp and help to pull out hot air from the top of the engine bay. Some cars also seem to use those large "drop vents" on the center of the bonnet.

Quote: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.

This I hadn't thought of at all - excellent point! Cutting the rear corner boxes actually helps a bit as it partially opens the rear wheel well all the way back to the rear bumper. After this, cutting holes to the rear bumper would allow the air to exit. Of course, the top half of the wheel well is still closed at the rear but because the N2 flares deflect the air quite well, most of the air coming into the wheel wells probably come from underneath the car. Small vertical "walls" along the rear frame rails might also aid in directing the air from the wheel wells and isolating these air flows from the main airflow along the "diffusor".

[NoHachi]

The metal sheeting under the lip is what I meant by the tray. Its the bit making use of the high pressure area there and needs to be sturdy as hell if it wants to have a chance of pulling the entire car down to the ground. Thats why they made it metal and probably did indeed attach it to something solid like the x-member.
You can see from the pictures that the ae86s aren't running any ground effect (no skirts).

Other solutions to cleanup flow that I have seen include mounting the silencer in the trunk (if you have a silencer) etc. With the wheelwell removed and tank moved as well, you should have plenty of space to create a useable diffuser at the rear.