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

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Joined: Wed Nov 11, 2015 4:04 am
Location: Florida USA

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

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Thanks for the compliments & interest!

As I mentioned in the text of a previous post I'm well aware that the cardboard mock ups of the diffuser section are too big and I intend to cut them down before using them as patterns to fabricate the 1st actual diffuser to test. When I made them the car was 3' off the floor so I used some string to determine the maximum space that appeared to be available (after modifying anti sway bar mounting) and made the cardboard mock up. It certainly shouldn't be that big and would stall. I also made the cardboard versions longer than the actual diffuser will be so that makes them look even bigger.

When these cars were built they mounted the rear anti sway under the bottom of the shock/spring plates. So they probably created a lot of drag. The aftermarket suspension I had in the car previously had a bar that mounted to the floor pan of the body and the axle tubes so it was up higher than the stock bar set up.

The new GEN II suspension from Pro-touring F-body.com has an adjustable rear bar that mounts to the position of the factory mounts on the rear frame rails and under the shock/spring plate also like the factory bar. The location under the shock plates puts the bar in a position that would create a lot of drag without an under tray and limited the area available for a diffuser so I made some new mounts to attach the bar to the axle tubes. Once I can drive the car and test things I may need to modify the upper supports for the bar to change the angles.

Stock rear bar mounting position.

Image

Bar flipped & mounted to axle tubes.

Image

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Joined: Wed Nov 11, 2015 4:04 am
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Re: Splitter/under tray/rear diffuser for full body production car

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e36jon wrote:
Mon Oct 19, 2020 6:37 pm
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
Part of the reason I made the splitter in two parts was so that I could test different sections on the front. One of the things I'd like to test is a curled lip splitter which as you noted would also provide more clearance. The flat one I've shown in pics should have 1 1/4" clearance when the car is at full dive. i checked with no front springs in the car and the sub frame sitting on the LCA bumpstops.

I have considered a horizontal widening diffuser but will start with a simpler to fabricate one and test. I have a lot of things I'd like to try but with limited math skills, and no CFD skills, I figure I'll start with simple concepts, build them and see what seems to work or not. I'd love to find someone learning CFD who would be interested in using my car to learn on.

e36jon
e36jon
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Joined: Mon Apr 25, 2016 1:22 am
Location: California, USA

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

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Thank you for the considered answers. Sounds like you have things well in hand and have built in lots of room to experiment without having to completely start over, which is genius.

You are no doubt aware of the CFD and 'Mantium Virtual F1' threads, so maybe post a 'help wanted' link over there? I think modelling just your splitter + underbody tray + diffuser and the interaction with the tires should be quick-ish for somebody familiar with the tools.

Looking forward to seeing things evolve!

Jon