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Old 05-31-2015, 10:29 PM   #1 (permalink)
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Whole Wheel and Tire Fairings

I decided to put this in a new thread instead of burying it in my existing thread since I wanted to drum up some discussion and this topic isn't specific to my vehicle. Lets talk about wheel and tire fairings. To clarify, when I say fairing I am referring to structures fore and aft of the tire designed to prevent air from impacting the tire directly, not smooth wheel covers or wheel well covers. I know there are some disagreements on the exact name of such a device.

I have been doing some rough design on some wheel and tire fairings for my Tacoma. Not the little “air tabs” manufacturers are putting on vehicles, full on “deflect most all the air from hitting the face of the tire” fairings. My plan is to build rigid frames to hold conveyor belt material from the top into the desired fairing shape down below. This is not a new idea, like many things I have seen it suggested on this site before.

I have had trouble finding any good information on this subject (tire fairings in general), just picking up pieces here and there. Here is what I think I know. The air at the front under the car is flowing at an angle and not straight back (i.e. trying to flow out the sides), which the angle is probably around 30 degrees in the very front. This means the very front fairings need shaped more to the inside of the vehicle to deflect this angled air.

The air at the back of the front tires is still at an angle, but the angle is probably less now. I would think a boat tailed' type fairing would be appropriate here (with small curves on leading edges). I also thought the outside should be roughly straight back, and any boat tailing of this section should happen on the inside face, due to the angled air flow.

The air at the back wheels is probably more straight back, or at an angle low enough to not effect the design significantly. This leads to a more standard straight on semi-circular front fairing. I was thinking the back fairing should have the inside wall close to straight back, and the outside wall should have angling like a boat tail, so the exact opposite of the front fairing.

I have attached my very rough/preliminary designs to hopefully get some feedback. Note my sketches show drivers side details, passenger side would be mirror imaged.

I have been designing the front like a semi-circular curved leading edge, and the back like a mini-boat tail. Is this the correct way to design these items? Are their any established ratios or curves to use for wheel fairings? Any thoughts on minimum or maximum distances between the tire and the fairing? Any thoughts on sizes of the fairing in relation to tread widths and if any offsets are needed to the inside or outside?

Any yes, I know I really need a wind tunnel to actually know anything for sure about this. And yes that really isn't an option so I am really just looking for any helpful information anyone might have.

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Old 06-01-2015, 05:47 PM   #2 (permalink)
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wheel fairings

*Adding wheelhouses and wheels to a low drag body can double the Cd.
*Fairings can help 'integrate' the wheels into the body and lower drag,closer to a non-wheeled body.
*Goro Tamai,of MIT's solar race car program performed wheel fairing wind tunnel tests,and asserted that their benefit was so great,even when frontal area was increased by them that you'd want to have them.
*They need to be as large (gentle) as you can make them to mitigate harmful pressure spikes which can trigger separation.
Here are some examples of Cd 0.10 territory cars and their fairings






These on the Template respect the SAE approach/ramp/breakover angles for clearance


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Old 06-01-2015, 07:16 PM   #3 (permalink)
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Quote:
Originally Posted by aerohead View Post
They need to be as large (gentle) as you can make them to mitigate harmful pressure spikes which can trigger separation.
Is that for true laminar flow, which we aren't going to get on the street anyway?
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Old 06-02-2015, 02:41 AM   #4 (permalink)
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Here's my version of the picture you attached.



It would use a U-shaped metal strap with the conveyor belt riveted on only along the sides (to improve flexibility?) I agree the angles should reverse front and rear.

An interesting question is whether the 'grain' of the belting should run lengthwise or across the part.
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Old 06-02-2015, 08:54 AM   #5 (permalink)
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Quote:
Originally Posted by freebeard View Post

An interesting question is whether the 'grain' of the belting should run lengthwise or across the part.
That had crossed my mind. I reckon Phil would be the guy to answer it. Inside my head there are advantages both ways, but I don't know enough to say which answer is right, or if there's a threshold at which one becomes better than the other.
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Old 06-02-2015, 10:39 AM   #6 (permalink)
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i feel like conveyer belt would buckle, esp if left open in the back. a lot of force is applied from a lot of angles that won't be controlled unless other mods are in place.
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Old 06-02-2015, 03:52 PM   #7 (permalink)
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laminar

Quote:
Originally Posted by Frank Lee View Post
Is that for true laminar flow, which we aren't going to get on the street anyway?
For real cars the boundary layer will be fully turbulent,but the outer,'inviscid' flow along the fairings will be laminar.
On the solar racers they may have a laminar boundary layer up to the 1st minimum pressure point,then the boundary layer must transition immediately to a turbulent boundary layer,which is good,as without the TBL,they'd have separation right there.The flow may be so clean that the rear fairing face also see's some laminar boundary layer.
The solar teams appear to be obsessed with 'laminar' wing sections.
Honda's 'laminar wing'-based solar racer had higher drag than their 'flattened-torpedo'-based cars.
Delft is also using 'laminar' profiles with the thickened portion well aft on the body,and zero transverse cut lines on the body until after the 'bulge',which moves the separation point ridiculously far back on the body.If they get any crosswind it kills the 'laminar' ability of the shape.NASA calls it crosswind contamination,or transverse contamination to the boundary layer.
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Old 06-02-2015, 04:14 PM   #8 (permalink)
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grain

Quote:
Originally Posted by elhigh View Post
That had crossed my mind. I reckon Phil would be the guy to answer it. Inside my head there are advantages both ways, but I don't know enough to say which answer is right, or if there's a threshold at which one becomes better than the other.
I've never worked with the material,and I wonder if the big challenge is how to get it to hold any shape,other than flat when it's bent into a curve.
I'm gonna use sewn leatherette over foam rubber,attached to a plate which catches the top of the belly pan,allowing the fairing to descend down through the hole.
If hinged at the front,a ground strike will simply lift it into the void above the belly,and then descend once more as the hazard passes.
The rears can be made to follow the rear suspension,lifting them if a hazard comes along.
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Old 06-02-2015, 04:27 PM   #9 (permalink)
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I don't have a sample of the material in hand; but I have worked with conveyor belts.



A lightweight belt is good for bales of hay (ask me how I know). The belt used in quarries for crushed rock would be more substantial.

As an alternative, I propose a section Saw-z-Alled out of a car tire. It's already the right shape.

I like the idea of the passively hinged fairing. In the rear I can see options for longitudinal leaf springs or trailing arms, but I'm not sure about double A-arms.
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Old 06-02-2015, 06:56 PM   #10 (permalink)
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Thank you everyone for your input and feedback.

I have previously sucessfully used conveyor belt (baling belt from TSC) for my front air dam. (http://ecomodder.com/forum/showthrea...tml#post453541) It works very well and is quite stiff when it is curved in one dimension and rigidly fixed at the top. It does not deflect even with my relatively minimal curve on my air dam on highway/interstate speeds, so in even sharper curves I would not worry about wind deflecting the fairing. The conveyor belt does have enough give that it would give if it contacted a rigid object to deflect and bounce back, so no real need for hinges.

I did some rough sketching in Sketch-Up, and attached some renderings of a front tire and fairings. Hopefully this will be more clear than my poor sketches in the first post.

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