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Old 06-11-2008, 09:14 PM   #1 (permalink)
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Lightbulb Scion undertray.

I've been trying to work out in my head how to do a rear undertray for the xB, which would, in gross ouline, translate to the xA (being that they are substantially the same car).

I propose having a small section that 'floats' with the rear axle. At normal ride height, it would be approximately flush with a big section under the body, and the giant cavity between the axle and bumper. All of these sections would overlap like shingles on a roof- it wouldn't be perfectly smooth across the length of the car, but it would be a good compromise. If the car got put up on a lift, it wouldn't get damaged by the suspension sagging, for example. The floating section would be attached close to the

Does this make sense? I realize that it sounds kind of convoluted.

A front section, and center section would be pretty simple, really. I have a fiberglass body kit that has a substantial lip around it's perimeter (except on the rear bumper, oddly) that would make for excellent attachment points, assuming I can concoct a fastening scheme that wouldn't be a complete PITA to remove for service.

Now that I think of it, this general design would translate to pretty much every car with a torsion-beam rear end.

Here's a shaded picture. The red part would be the big part of the pain under the bulk of the body. The green part represents what the floating section I propose would cover. Behind that would be a smallish rear section that I did not shade.


Now, I have to go get my car off of the ramp.

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Old 06-11-2008, 09:56 PM   #2 (permalink)
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Hello,

It looks like the plastic rear bumper fascia on the xB hangs down all by itself? Could it be cut off higher up; to reduce the sail-effect? (On the xA, there are a couple of tabs that extend over to the metal frame, to brace the plastic a bit.)

Or, would the pan extend back to be flush with the bumper fascia?
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Old 06-11-2008, 10:05 PM   #3 (permalink)
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It does hang down like that. It would be tempting to cut it off, but there are tow hook brackets (not visible) and some bizarre metal tab hanging down (to the right of the muffler-- I think the stock rear bumper lip attaches to it-- it would be a good place to mount truck nuts), so I think it might just be easier to go around those.

So, yes, the idea would be for a pan to go all the way back to the bumper. Other than the junctions between the various proposed sections, it would be very flat across the entire car.

I hope some of the folks here with full belly pans chime in, and explain how they got around their axles. It just seems to me like covering it up without providing for suspension travel would be asking for trouble.
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Old 06-11-2008, 10:14 PM   #4 (permalink)
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Hello,

I have a concern about a totally flat belly pan: that it would induce lift and therefore (possibly) making the vehicle less stable. All the cars that I've seen with purposeful aerodynamic rear ends have the bottom surface slope upwards towards the back. I think this helps evacuate air out the back, and reduce the pressure, and it might even induce a little downforce; making it more stable.
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Old 06-11-2008, 10:28 PM   #5 (permalink)
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Good point. However, doesn't the venturi effect some into play with a narrow, smooth gap at speed, generating downforce? Either way, I can't imagine lift being a huge issue at speeds at which sensible people drive on public roads.

The xB already has goofy handling at highway speeds anyhow, with the combination of it's short wheelbase, narrow track, short gearing, turbulence-generating shape, and a seating position that makes the center of gravity feel higher than it actually is (since the seating position is pretty much above it).

I'm no engineer, so I don't mind being corrected in the admittedly likely event that I'm mistaken.
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Old 06-11-2008, 11:16 PM   #6 (permalink)
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Hi,

I don't think the Venturi affect applies here -- that would be for when the air is in motion and the object is stationary. This is the object is in motion through the air -- and like an airfoil wing, with a flat bottom and a longer air path over the top, you get an increase in pressure under and a decrease in pressure above the vehicle = lift.

The slope up to the rear decreases pressure (because there is greater volume for the air to be in), and this would produce some small bit of downforce -- or at least less lift.
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Old 06-12-2008, 12:11 AM   #7 (permalink)
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That plastic hang-down bit on the xB's rear bumper pops right off!!

You take out a couple of nylon fasteners, and the whole thing pops off the car. I know because I ran over a deer. The only damage to the car (besides hair and blood everywhere) was the rear bumper. Two support tabs broke off and the lower portion of the bumper popped off clean. It was pretty easy to put it back on - the support tabs just hang there, but the bumper is pretty secure anyway. A really great design by Toyota. I was very impressed.

I have a picture or two around somewhere, but the portion that comes off is separated from the upper bumper by a clean, straight seam.

It would be trivial to take off the lower portion and fair it out with plastic.

DO IT!
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Old 06-12-2008, 12:39 AM   #8 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
Hi,

I don't think the Venturi affect applies here -- that would be for when the air is in motion and the object is stationary. This is the object is in motion through the air -- and like an airfoil wing, with a flat bottom and a longer air path over the top, you get an increase in pressure under and a decrease in pressure above the vehicle = lift.

The slope up to the rear decreases pressure (because there is greater volume for the air to be in), and this would produce some small bit of downforce -- or at least less lift.
Ok, I have poor recall of high school physics (damn you, biology degree!). Still, I would imagine that the underside of a car, particularly if turbulence is kept low, to induce low pressure, and thus some downforce, rather than lift.

Quote:
Originally Posted by millenniumtree View Post
That plastic hang-down bit on the xB's rear bumper pops right off!!

You take out a couple of nylon fasteners, and the whole thing pops off the car. I know because I ran over a deer. The only damage to the car (besides hair and blood everywhere) was the rear bumper. Two support tabs broke off and the lower portion of the bumper popped off clean. It was pretty easy to put it back on - the support tabs just hang there, but the bumper is pretty secure anyway. A really great design by Toyota. I was very impressed.

I have a picture or two around somewhere, but the portion that comes off is separated from the upper bumper by a clean, straight seam.

It would be trivial to take off the lower portion and fair it out with plastic.

DO IT!
I know it comes off, but there are a lot of metal bits hanging down between the axle and the bumper (again, besides the muffler, there are tow hooks and that wierd tab that's about 6" in front of the bumper in the center of the car). I would imagine that the simplest solution would just be to cover it all up, even if it's not the aerodynamic ideal. Cutting hunks off of my car's unibody isn't something I want to toy with.
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Old 06-12-2008, 12:53 AM   #9 (permalink)
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Quote:
and like an airfoil wing, with a flat bottom and a longer air path over the top, you get an increase in pressure under and a decrease in pressure above the vehicle = lift.
I must disagree. A wing with a flat bottom may generate lift from the bottom, but that is only if it has a non-horizontal angle of attack.

The length of the air path is not relevant, even though that is the way many people explain it. Lift is caused by curvature of streamlines. In any curving streamline, the air pressure is lower on the inside of the curvature, and higher on the outside of the curvature.
This paper explains the idea, and it is permissible to glaze over the math.
http://ocw.mit.edu/NR/rdonlyres/Aero...0/4liftgen.pdf
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Old 06-12-2008, 06:48 AM   #10 (permalink)
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Hello,

Quote:
Originally Posted by ttoyoda View Post
A wing with a flat bottom may generate lift from the bottom, but that is only if it has a non-horizontal angle of attack.

The length of the air path is not relevant, even though that is the way many people explain it. Lift is caused by curvature of streamlines. In any curving streamline, the air pressure is lower on the inside of the curvature, and higher on the outside of the curvature.
I think that you are misreading the paper. If it was only the angle of attack, then all wings would push the airplane down to the ground, since the top is angled more than the bottom. Also, increasing the angle of attack increases drag.

It is the low pressure developed above the wing (car), and the higher pressure developed below the wing (car) that lifts it.

Think of the Stanley Steamer that set the land speed record on the beach in Florida: it flew up into the air! It's bottom was virtually flat, and there was no real angle of attack, either:

Obviously, 120+mph is faster than we're contemplating, but the principles are the same.

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