Splitter/under tray/rear diffuser for full body production car

[NOT A TA]

I've been a lurker here for many years and finally signed up as a member a couple years ago thinking I'd get involved with the KVRC but decided it was waaay beyond this old mans computer skills. So now I just follow the current MVRC thread and others.

I've been working on an under tray for one of my own cars and figured I'd share it here since there seems to be quite a few people here with an interest in race car aerodynamics. For me this is a learning experiment, not trying to "win" anything. The car gets used for road tracks, mile land speed race events, drag race test and tunes, and driven on the street. Road courses are my favorite activity and the tray system design I'm currently building will hopefully decrease lift/increase down force. You could think of my car as something like a (really old) time attack car or hill climber but with no rules to adhere to.

There's a couple twists to what I'm doing that are different than the typical race car set up. The splitter is hinged to be able to be used as an expanding or contracting diffuser, the whole tray system is suspended from the car, and the space between the tray and car body is open on the sides with an additional opening in the center of the rear diffuser.

While there's a lot of folks here with the knowledge and skills to model, mesh, etc. and then run CFD programs to determine a more optimal design, I can't. However in less time than it would take me to learn one step of that process I can fabricate a crude under tray system to test. And since I'm not trying to win anything but could learn a lot I decided to just jump in and build my vision.

So I'll end this first post with a couple pics of the car I'm building the contraption for and a quick sketch of the concept. I'm getting old, downsizing, and sold my drafting table etc. several years ago so I can't even post a Newey type drawing.

[NOT A TA]

So moving along I'll start at the front and work my way back. Keep in mind this version is only to be used for testing purposes so it's a bit more complex, crude, and heavy than the final pieces will be.

The splitter section is built in two pieces so I can quickly swap the front section between track sessions to test various lengths. This is also why the front section is made of inexpensive plywood and the hidden part is overbuilt. I can try various front sections that protrude further forward or out to the sides and some versions may have fences or other things also.

The splitter section hinges at the rear edge about where the radiator is located allowing the splitter section to tip up or down. This will allow it to be expanding or contracting. With cables supporting it there's also the possibility that it could move up if it was to hit corner curbing or other obstacles however that also opens the possibility of fluttering or other unwanted characteristics which I won't find until testing. I can always use solid supports in place of the cables if necessary.

The splitter is height adjustable (as is the rest of my design) so it can be raised and lowered to hopefully allow tuning for different tracks, conditions etc. This adjust-ability also allows the entire assembly (or sections of it) to have a rake independent of the rake of the actual car itself.

The plywood in front of the tires will be cut short enough so that if the splitter gets broken the cables will keep it from going under the tires and it (or pieces) would just drag on the pavement. The supports at the hinge point have a break away point so if something happens to the splitter hopefully further damage would be reduced.

The core support area in front of the front wheels has all been sealed so the only air allowed through is for engine cooling and the brake ducts. Old cars like this had a lot of gaps where air could get through and raise under hood pressure while reducing the radiator's cooling capability.

Any comments or questions are welcome.

[NOT A TA]

The tray under the engine compartment is all made of aluminum because of heat from low headers. The tray can be raised till it hits the engine crossmember and is supported at the rear by a large horizontal hinge using a similar vertical tube within a tube set up with the outer tube welded to the sub frame in front of the firewall as seen in pic below.



The next section is a short section between the firewall and the A pillars. It's all aluminum due to the headers and the rear of this section is supported with the same type of hinge with the vertical supports sliding up into tubes which go through the floor of the car. Here's a couple pics showing how the vertical supports function and last pic gives an overall idea.

[Singabule]

Pretty good, i remember top gear episode of 24 hours silverstone, and jeremy lost his front splitter and the car balance turn to worse and destroy his tyres quickly. With correct design you may gained up to 300kg of downforce from splitter and diffuser. However when you use aluminium frame, dont expect you will gain much lap time since its heavy. How much weight of your floor?

[NOT A TA]

The whole prototype from splitter to diffuser should be a little over 100 lbs with all the supports and hardware. The prototype is for testing purposes and if all works well I'll build a much lighter version as a final assembly.

The section under the passenger compartment in pics below is the heaviest section at 30 lbs. The front is to the left side. The notched out area on the right is where the diffuser fits in. Center white section is corrugated plastic and the aluminum sections are where the side pipe exhaust is.

[NOT A TA]

Moving along to the rear, here's a few pics of the cardboard mock up of the basic diffuser section. This is a leaf spring stick axle car so the center section has to clear the differential and the lower sections on the outside have to clear the spring plates and shock mounts. I want to put the car on the ground and test fit the diffuser before making it out of different materials or deciding if I want to make anything behind the rear tires. Total width of the diffuser is 48" and length from the flat floor section to the rear of the diffuser is about 85" while the height at the rear openings is 19". The sections are divided by strakes with the side sections 5" wide, the wide sections 14" each, and the center section 10". Final height and length will probably be slightly less as I made the cardboard ones a little big so I can cut down to size while test fitting. Yes, the upper part of the center 10" section is open to air flowing between the car floor and the under tray assembly. The pieces of cardboard on the rear of the mock up diffuser are just to give the cardboard support and the rear openings between the strakes will be open of course on the final assembly..

[NOT A TA]

Pic below gives you an idea of what the rear of the car looks like. The diffuser will stick out past the body work and the top will be about bumper height.

[djos]

Cool project, I'm wondering how I missed the topic until now!

[rjsa]

That's just too cool.

[Callum]

This looks fantastic (the car is lovely). Are you planning on sealing the sides of the undertray somehow? How's the project coming along??

[NOT A TA]

Thanks for the compliments! Although I check new posts on this forum almost every day I must have missed the 22nd last month.

I am not sealing the sides of the tray up to the body and there will also be an opening in the center of the diffuser that will allow air between the tray and car body to escape. Everything from the splitter to the front wheels is sealed so air at the front will go through the radiator and brake duct openings, all other air will go over, under or to the sides of the car. Not sealing the tray to the body on the sides allows me to easily change the gap between the pavement and tray system and change the rake of the tray (or sections of it) independent of the ground clearance or rake of the car itself. Not being sealed to the bottom of the car will (hopefully) also help with removing engine, transmission, and rear end heat without the use of additional coolers.

With all the aero guys who frequent this forum I expected at least one person to mention that the cardboard mock up diffuser looks too big (which it is). I made the mock up to the limits of my space available which came out to an area ratio of about 7.3 with the front of the diffuser 3" off the pavement. I made it to the limits expecting to cut it down to a ratio of less than 5 and then I can use it as a pattern for a diffuser I'll be making out of plastic for testing once I put the car back on the ground. The tray system is just one part of a complete bare metal rotisserie restoration/rebuild with a lot of upgrades so I've also got a LOT of other work going on. I just installed complete new suspension with increased spring rates that will hopefully take advantage of the reduced lift I hope the tray system will provide. So I will have it back on the ground soon to mock up the whole tray system at what should be pretty close to final ride height, at least close enough to cut down the cardboard diffuser and make the plastic one..

[NOT A TA]

Been a while since my last post in this thread. I have not given up on the project. Had a heart attack followed by other health issues that slowed me down a lot for over a year then more heart events. However I did get the car back on the floor and rolled it out where I could slide all he sections under so I could weld the last support to the frame in the engine compartment area. I also got a carbon fiber wing from a buddy. It was made for a Noble by ReVerie and although it's not the latest greatest swan neck mounted 3D design I'll play with it. Will need to make new supports for it.

[e36jon]

Welcome back! Glad your time away from the forum wasn't permanent...

I really appreciate your project as it's something I've always wanted to do myself. If I may ask a few questions?

- Your splitter is awesome. I have noticed that many cars are moving away from a flat design to one that has some curve to it, in the forward direction (See pic below). I think these curved splitters help cure the choked-flow effect of the flat splitter when suspension travel causes it to scrape and close off all under-car airflow. Have you addressed this already?



- The diffuser exits are massive! I assume you are able to go under your rear axles? Did you ever consider expanding horizontally as well, to limit height for better clearance of components at the rear? Apologies for Ford in the pic, as well as it not being a perfect view to illustrate the point...



Cheers,

Jon

[Stu]

What an awesome car! Retro time attack.
Many years ago I thought of doing something similar to a Mini, it seems to have a floor shape to suit this....

[Jolle]

With these giant and long diffusers it might happen that the center of downforce will be to much forward. The flat area under the seats and in between the front wheels will do the work, the area under diffuser itself, not so much.
Due to the shape and the “dirty air” from the front wheels, it could be that a smaller diffuser would generate more downforce.

Basically, focus on the floor where you have the full length of the car. Raise the splitter in the middle slightly (like on that Ferrari, some streaks between the front and rear wheel to clean up the tire squeal and see if you can vent some pressure from the lop of the wheels.