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Old 02-16-2021, 09:41 PM   #111 (permalink)
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Actually, what would you guys think about me cutting further up into my rear bumper so that I can slope the underbody paneling further up and reduce the size of the wake at the rear? I’m thinking the air won’t really cling to it even with a diffuser. Between me being clumsy when reversing and the ugliness of it, I’m going to do everything but a boat tail.

If it’s unclear what I’m asking, I can draw up a picture of some sort to explain.

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Old 02-16-2021, 10:37 PM   #112 (permalink)
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It all starts at the stagnation point and depends on everything after that.

Make it crushable and you can use it for a curb feeler.
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Old 02-16-2021, 11:26 PM   #113 (permalink)
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That’s...interesting looking.

Would fill right up tight with snow, lol. I’m thinking of going even higher, but definitely not as long. I’m sure that will be well past the point of diminishing returns, though.
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Old 02-17-2021, 03:06 AM   #114 (permalink)
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It depends on how far forward you can start. It would need to be a flat surface from prolly the rear axle line. I'd be surprised if there isn't a gas tank or muffler protruding if you raise the rear bumper more than one or two inches.

The example from my albums has excessive fences, three is enough. If snow buildup is likely, consider a time attack racer approach.


https://frontstreet-wpengine.netdna-...0-1180x788.jpg


https://www.stanceworks.com/wp-conte...ow-springs.jpg
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Old 02-17-2021, 09:40 AM   #115 (permalink)
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The gas tank is about the same height as the cross-beam that holds the two suspension arm pivots. They drop down a bit further than the rest of the underbody. I don’t think I’ll take it out nearly as far as those cars but I’ll see what I can do in the future. The exhaust would be on the right side, but I can modify it if need be. It turns around the gas tank, comes up over the suspension arm, and then juts out the bottom of the bumper. I’ll probably be replacing the fart cannon with a smaller, lower profile custom resonator, so that will help either way.

Is it important that the diffuser carries so far outward of the rear?
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Old 02-17-2021, 10:55 AM   #116 (permalink)
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No it isn't critical, the diffuser only needs to go to the bumper edge, but the further out it goes the better the downforce. There is also some potential benefit in aerodynamics extending it further but that really depends on far too many factors for me to give recommendations on how far because I don't know what the research says.

Things like spoiler positioning (if any), roofline shape, diffuser angle, expansion ratio all can affect how far out may be optimum for drag or downforce.

But if you are worried about ground clearance then just go to the bumper, no side fences needed.

Last edited by AeroMcAeroFace; 02-17-2021 at 11:09 AM.. Reason: Read some of the rest of the thread, tailored the answer a bit better
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Old 02-18-2021, 10:22 AM   #117 (permalink)
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I’ll do just that. Side fences aren’t worth it?
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Old 02-18-2021, 10:57 AM   #118 (permalink)
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Quote:
Originally Posted by JacobLeSann View Post
Iíll do just that. Side fences arenít worth it?
For low angles they aren't essential, if the angle is very steep, side fences are more useful. If the angle is shallow then side fences aren't essential, but may have some limited benefit, this video from a former contributor to this site may be of use.

In very simplistic and generalised terms, there are two types of diffusers, low angle (0-12 degrees) and high angle (20-30 degrees), low angle has attached flow all along it and high angle which has vortices and separated flow which the fences and strakes help to control. Low angle is best for economy, high angle is best for downforce.

That is a generalisation though and exact angles, ground clearance, interference from tyres/suspension/exhaust may change some things. Anyone who tells you there is a generic optimum angle or formula is lying, the actual optimum for your car depends on so many things and may also be completely impractical to build or drive.
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Old 02-18-2021, 12:24 PM   #119 (permalink)
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Thanks for sharing that. I guess I’ll keep it low, then. Essentially just remake my old one.

To anybody lurking and interested in an underbody panel, be sure to have a frame or at least some sort of strut over open areas. Also, it’s much easier to keep the ends underneath the lip of your bumpers, with removable screws instead of rivets. My rear panel was the one without any backing to fill the gaps between the trunk floor and itself, and it’s completely disappeared. If you’re a good driver on perfect roads, don’t worry, but in anything worse it’s best to put in the effort.

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