been watching a ton of new car videos lately, seems like all cars these days underneath are completely blocked off with a flat black plastic (as opposed to just having an undertray in our cars. is this better for aero, and do we have a similar type of solution?

 

The way I think about it (and I could be 110% wrong) is not that the air is rushing under the car but the air was relatively motionless compared to vehicle speed. So the car is rushing over the air and dragging it along. The more you disturb the air the more energy you give it which in effect adds pressure. And the bottom of a car a lot of area to push against.

And with a diffuser you arent trying to great a negative pressure zone, you're trying to remove that energy or redirect it to help the upstream air, or the air under the car from gaining energy.

 

Yes, it's a fairly decent gain as far as aero goes. Think of the front end of a car as a wedge, as it moves along it will naturally force oncoming air above it and below it. From there it's the normal stuff commonly brought up, velocity and pressure. The less air that goes under the car, the faster it can go and the lower the pressure will be. On a similar note, the less the air can expand and the less things for it to run into on the underside of the car the faster it will go and the lower the pressure will be.

OEM flat underbodies are mostly done to reduce drag which helps fuel economy but doesn't inherently create downforce. On performance applications (like a GT3RS) the underbody is used to control wheel well wake flow streams which makes all of the upper body aero more effective.

 

People call OE rear bumpers 'the parachute'. It does look bad when you go under there see how effective that area (and every vertical surface) could be in slowing the car down

 

i was watching a video from the car care nut, and i forget which car he was looking at but they had the same thing regarding the rear bumper parachute. there were 2 attachments, one on the rear control arm, and one in front of the bumper to make sure the air went under the bumper instead of into it.

why am i thinking about aero? cuz formula 1 is about to start, lol

 

This is much better for aero. The objective is to get as smooth (laminar) and undisturbed flow as possible which will move faster than heavily disturbed flow. The faster air has a lower pressure which isn't a huge difference measured in psi (~ -2psi) but it's acting over a large area.

The diffuser keeps the flow attached in the back and more-or-less gently slows it down and let's it separately cleanly at the back edge into the relatively slow air behind the car. Again, this is to keep air velocity high under the car and it can literally affect the airflow and down force generated all the way to the front of car.
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​​​​​​It's this more sophisticated approach to airflow management is why manufacturers don't just create an airdam at the front of the car like NASCAR anymore. Even mitsu's aerokit for the Evo 9 was just an air dam. It basically stalls the airflow under the car which is better than letting it experience the bad airflow characteristics under the car. But not better than flowing air through a smooth underbody, in which situation you actually want to flow air over that aero surface to reap the benefit. That's why F1 cars don't use airdams in the front or purposely try to block airflow (and stall) their underbody diffuser.

​​​​​Closest I've seen to smooth underbody kits is carbon/kevlar underbodies by some European rally outfits.

 

im gonna have to take a look under there, it can't be that hard to cover up most of the body with some plastic panels. example: bottom of GR corolla:

 

Yeah... I had thought about 3d scanning the bottom of my EVO X... and then printing the needed standoffs to mount ALUMALITE which I can get from the local sign maker business. I used the stuff for my ductwork for the front heat exchangers and it is like MAGIC for the price.

 

Just remember when making under trays there should be nothing sticking out (like bolt heads) above the surface. It should be sunken in like that Corolla kyoo posted. That's cause on small pits like that the air does a pretty good job stagnating in there and the airflow can skip over it.

I was eventually going to do alumalite too. Alumalite front under tray, and then alumalite barge boards that extend to the central tunnel, leaving exhaust uncovered. The biggest issue I've spotted is how to smoothly transition over the rear subframe to a rear diffuser.

 

How do you cut those Alumalite panels cleanly?

 

guys new cars having flat bottoms etc has nothing to do with aero its about fuel efficiency.

But yes it can also have an effect on aero also. I havnt seen any complete off the shelf units available for an evo. you can buy front undertrays/ splitters and rear diffusers but i havnt seen anything off the shelf for in between

 

The increased fuel efficiency is due to improved aerodynamics lol

 

If I remember right, I used a jigsaw with a fine metal blade.

 

I flat bottomed the front end of my car from the splitter to the firewall and the change was actually noticeable at speed. A couple of years ago I made a panel to cover the huge gap where the rear bumper is and didn't notice anything.
That's my scientific interjection hah.

Another strange thing with the front flat bottom is when idling for a while and then taking off I notice the coolant temperatures will drop lower than usual for a moment before returning to the baseline.

 

I just did an oil change on a 2021 Bronco 1.5L this weekend and had a nice flat undertray. even had a little upward scoop to cool the transfer case. Engine looked brand new clean after 35k.