Scion xA aero mods

[aerohead]

Originally Posted by MetroMPG

I've always thought this is the case with GM's EV1 as well (Cd 0.195). The rear glass angle is quite a lot steeper than the generally accepted angles on current aerodynamic production cars. It's something like 18 degrees, if I recall (hoping that's right).

The major difference being that the EV1 from the driver's door back to its trailing edge has a pronounced conical shape which interestingly isn't always apparent in profile photos. The plan taper is quite dramatic, and the car lacks the typical defined "shoulders" at the transition from "side" to "top" surfaces. All of which likely lets it get away with a steeper profile taper (roof through to trunk) because proportionately more pressure recovery is happening along the side of the vehicle.

Of course, this is all topside related.

Back to the bottom taper issue: personally, I'll be sticking closer to 4 degrees.

I spent some time with an enlargement of the EV1 speed record vehicle photo which appeared in the cars brochure.
The image is shot telescopically and looks close to true length/dimensions,as we'd see on a blueprint.Here's what I measured with squares and protractor:
(1) The angle described by a straight-edge laid across the roof and tail edge of tailcone comes out at 15-degrees.
(2) The original angle of the backlite is 20.5-degrees.
(3) The angle of the top of the boat tail cone itself is 9.5-degrees.
This racer is lowered although there id no mention whether or not it is "raked".I don't believe that it is.
Where the wind tunnel smoke trace over the backlite of the stock EV1 clearly separates,I suspect the flow completely recovers with the very conservative curvature of the cone,helping towards that 183-mph record speed.

[MetroMPG]

Originally Posted by NeilBlanchard

9 degrees on the sides of the lower part (to keep some width at the back)

Why do you want to "keep width"? For lighting?

If you're going to go to the trouble of making a boat tail, my advice would be to make it as streamlined as possible. You could add other rear-facing lighting further up to get "illuminated width". I will be doing this with mine.

[MetroMPG]

Originally Posted by aerohead

Where the wind tunnel smoke trace over the backlite of the stock EV1 clearly separates

Interesting. I didn't come to the same conclusion, but it's hard to tell from the stills I have. FYI, there are pics of the smoke testing of the car in this thread: http://ecomodder.com/forum/showthrea...olar-7637.html

[SlideWRX]

To pick the angle you want, take a look at page 31 of this pdf, in the General efficiency section sticky:
http://me.dyu.edu.tw/lecture/Vehicle...ign%282%29.pdf
it is ~2mb and slow to load.

It shows that the ideal angle is different depending on how large the tail is. For a short tail (~1.5 ft), the ideal angle is roughly 13-14 degrees. For a *very* large tail, the angle goes up to 20+ degrees!

Food for thought.

Tom

[NeilBlanchard]

Greetings Tom and welcome to EM,

Thanks for posting that -- is this the chart you are referring to?



If the length of the boattail is less than 1/10 of the length of the car, then the maximum/ideal angle is ~14 degrees. If the length of the tail is 1/4 the length of the car, then the angle is ~17-18 degrees. And if the boattail is 1/3 the length of the car, then the maximum/ideal is ~19-20 degrees; and it yields a drop of 0.07+ in the Cd -- if I'm reading it correctly.

So, on my proposal model, the maximum angle can be ~17-18 degrees, since the tail is about 1/4 of the overall length after it has been added.

This chart looks like it might have come from Hucho -- can someone confirm?

[JackMcCornack]

Yep, page 173 in the 4th edition and probably somewhere nearby in the earlier editions. Unfortunately it doesn't tell if the sides taper in as well. The problem isn't as much flow separation if the roof to trunl angle gets too steep, it's more that drag will go up even though (or more accurately, because) the air is following the surface, since as the angle goes up (assuming the flow doesn't separate) the rotors caused by air spilling over the sides overpowers the savings gained by filling in the space behind the car.

[SlideWRX]

Originally Posted by NeilBlanchard

Greetings Tom and welcome to EM,

Thanks for posting that -- is this the chart you are referring to?

Thanks!

Yep, that's the one. I had seen a version of that somewhere and finally found it again in the stickies.

[Jethro]

I don't think the 'angle' is quite right, but you might look at the rear window off a C5 Corvette with the targa top. It would provide excellent visibility compared to other options.
Might you 'countersink' some LED strips into the outermost edges of the car for running / brake lights? I know alot of minitruckers get away with 1 LED strip (about 12" long) on either side as legit taillights.

[3-Wheeler]

Originally Posted by MetroMPG

Why do you want to "keep width"? For lighting?

If you're going to go to the trouble of making a boat tail, my advice would be to make it as streamlined as possible. You could add other rear-facing lighting further up to get "illuminated width". I will be doing this with mine.

Hi Guys,

Does anyone have ideas on how to design-in flush mounted side reflectors, that someone can see when coming up the street past our parked car?

Since the boat tail lights will be mounted somewhat narrower than the widest portion of the car body, I would really like to find side mounted reflectors that work to the rear of the car.

I know Mike on the aerocivic, mounted side lights, but they stick out into the breeze slightly. It would be nice to get lights/reflectors with a low profile.

Jim.

[BamZipPow]

Originally Posted by 3-Wheeler

Does anyone have ideas on how to design-in flush mounted side reflectors, that someone can see when coming up the street past our parked car?

You could use some DOT reflective tape...pretty low profile.