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Old 11-26-2019, 12:59 PM   #1 (permalink)
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Question For those with pickups or similar with full belly pans...?

For those of you who have built a full belly pan for your pickup or similar large vehicle, have you blocked out the rear wheel wells on the inside where the gap between the bottom of the vehicle and the top of the belly pan is sealed off? Or do you leave this area open to let air escape?

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Old 11-26-2019, 01:45 PM   #2 (permalink)
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Heat management is nontrivial when belly-panning a truck/SUV/Van. The biggest worry I have is about trapping heat from the exhaust, creating an oven. I don't pan under my exhaust for that reason.

In the area you are concerned with, most of these vehicles have a solid/live axle in this area. On my 4Runner, my plans are to omit the belly pan at/under the axle, restarting the belly pan immediately after the axle to cover the spare tire with a radiused leading edge. This is the next piece I need to build for my 4Runner.

I have seen some designs that plate under the axle using flexible materials, but trying to also seal that on the sides would be very difficult. Also many vehicles of this sort have exhaust traveling up and over the axle, so I wouldn't suggest putting a pan under or beside that.
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Old 11-26-2019, 06:36 PM   #3 (permalink)
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This example isn't a truck, but the side plates on the diffuser bell outward the width of the rear tire, separating the wheelwell turbulence and forcing it outward instead of mixing with the underbody flow. The bell shape accelerates the underbody flow irrespective of how smoothened it might be.
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Old 11-27-2019, 12:04 AM   #4 (permalink)
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Quote:
Originally Posted by freebeard View Post
[IMG]The bell shape accelerates the underbody flow irrespective of how smoothened it might be.
The bell shape (which I'm not seeing; the plates look parallel to me, considering that the rear body tapers inward after the wheels on a C4), along with the upward slope under the body, forms a divergent duct with the ground--which will decelerate airflow, not accelerate it (Flow continuity principle at work, and exactly the opposite of what air curtain ducts do. There, the ducts are convergent, which accelerates the flow into a fast-moving sheet of air across the wheel opening).
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Old 11-27-2019, 08:11 AM   #5 (permalink)
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A pan to the front edge of the drive axle sounds reasonable.

We’ve somewhere the comment that expanded metal sheet has “enough” to qualify in this job.

I’ve been thinking that conveyor belt air dam and side skirts is the more easily attained aero mod.

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Old 11-27-2019, 01:32 PM   #6 (permalink)
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Quote:
The bell shape (which I'm not seeing; the plates look parallel to me, considering that the rear body tapers inward after the wheels on a C4), along with the upward slope under the body, forms a divergent duct with the ground--which will decelerate airflow, not accelerate it....
I guess I confuse easily. There is some advantage to a divergent duct isn't there? Trading velocity for pressure going into the wake?

I wish I had taken more pictures. The shape isn't obvious from that angle. It curved from close inside the wheel to the outer face.

This divergent bell was tested in CFD by another member of this forum. I may have time to find that later.
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Old 11-28-2019, 12:29 PM   #7 (permalink)
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Diffuser vanes are something I've considered adding in the future. But that's not the area I'm concerned about currently. I guess I didn't explain very well. So here are a couple of pictures about the area I'm talking about. In these pictures behind the tire at the top is the inside of the wheel well. On the bottom left, you can see the framing and coroplast of my belly pan which is attached to the bottom of the body panels. I'm trying to decide if it's worth it or needed to extend the wheel well walls down to the level of the belly pan...as much as can be done around the suspension. Just wondering what others with larger vehicles and gaps like this have done. Thanks!
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Old 11-28-2019, 03:13 PM   #8 (permalink)
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Quote:
I'm trying to decide if it's worth it or needed to extend the wheel well walls down to the level of the belly pan.... Just wondering what others with larger vehicles and gaps like this have done.
You're the one to decide if it's worth it.

If someone with a larger vehicle that has done something chimes in, then great. But aero principles are scalable, the wheelwell is a localized feature. The solution will depend on the use case and [$$$]. A lateral wall will be more disruptive than a longitudinal one. The 'Bonnevette' had a curved plowshare shaped one.

The bottom of the fender could have speed holes in it.

Alternatively, you could fabricobble a fender skirt.
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Old 11-28-2019, 05:43 PM   #9 (permalink)
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Quote:
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I guess I confuse easily. There is some advantage to a divergent duct isn't there? Trading velocity for pressure going into the wake?
I believe so. If you dig up any more pictures of the Corvette, I'm curious to see them. One of these years I'll make it out to Bonneville....
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Old 11-28-2019, 10:42 PM   #10 (permalink)
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If I were to make it back, that car prolly wouldn't be there, so here's another example. The front fender of the Aston Martin Valkyrie.


https://sf1.viepratique.fr/wp-conten...o-geneva-1.jpg

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