EV build aero.

[Xist]

Originally Posted by mistareno

If I follow the template as strictly as possible and chop off the tail where legislation dictates, I end up with the image below, which is similar to the outline of the design posted by Xist

Botsapper attached them, I just hit the image button so people could see them easier.

[aerohead]

Originally Posted by mistareno

If I follow the template as strictly as possible and chop off the tail where legislation dictates, I end up with the image below, which is similar to the outline of the design posted by xist-

Probably good because it has more space and will be easier to build.

A quick rough shot from the rear (it doesn't show curvature, but is probably how the basic 'frame' will look.

The folks down at the local SHELL station probably won't like it,but you'll be a hero around here!

[mistareno]

Originally Posted by Xist

Botsapper attached them, I just hit the image button so people could see them easier.

Ooops, sorry. Thanks Botsapper.

[mistareno]

Bit of a thread resurrection here.

I eventually purchased a 1999 Ford Transit RWD cab chassis that I will use for the basis of the project.

It has a more angular front than the Kia I was looking at, which doesn't line up quite as well with the template, but the Kia was pretty blunt in plan view.

FWIW, it has had a Buick 3.8 V6 conversion and is quite light when naked but has a 2.5 tonne payload with an open chassis that will allow easy mounting of batteries above the prop shaft.

I've overlaid the aero template but am unsure how flat the roof would have to be before I start the teardrop?



If I can move the template further forward, then I can probably extend the template to the 22degree line and still fall within the overhang limits.

[ChazInMT]

Hey Mistareno, the 22° angle is not the goal. If your vehicle is not long enough, you cut off where ever you are an it'll work well. If you make the 22° thing your goal, you'll probably end up sloping too quickly and end up with a less efficient design, that you can't put as much stuff underneath, sort of a lose-lose deal. I looked at what you've drawn with a template overlay and it looks about perfect if you follow the template. The top line you have drawn is at about 12° overall slope, right in the sweet spot for best efficiency.

If you wanted to raise the back up about 5 inches (red line) and drop 10° (blue line) overall, this would only affect the Cd very slightly but I think it would make your Lift Coefficient drop a lot making the vehicle feel much more solid on the road.



I base what I'm saying on these charts which were in a detailed paper on pickup truck aerodynamics.

[mistareno]

Sorry, I think I was a bit unclear with regards to the 22 degree thing.

I wanted to know if it would be beneficial to 'slide' the template further froward so the next line on the template (the 22degree line) was effectively the rear of the truck. It would mean a shorter flat section over the roof to teardrop transition.

This would give a smaller cross section at the rear but would reduce load space.

I must admit, I hadn't really considered lift as an issue as when its converted to EV I thought the weight would negate any aero attributes but as I planned to test it with the existing IC engine, the rear is rather light when unladen...

[freebeard]

Keep in mind what The Template looks like in cross-section—a half circle that is twice as wide as it is tall. The slope assumes both sides taper as much as the top

The rear edge of the cab is basically a rectangle that is as wide as it is tall. Since there is twice as much taper in the sides, you wind up with a vertical trailing edge.

These are full bodies, instead of half-bodies; but include the squircle, half-way between the square and the circle:

[ChazInMT]

Yeah, I know what you meant. You wanted to get the last bit of the boat tail sloping 22°, again, that is not the goal, it is a result of a vehicle being long enough to reach the 22-23° slope at the end. If your vehicle can't be that long, you are faced with having to make a compromise. The place to make this compromise is certainly making it shorter in back as has been shown time and again.

You make the biggest gains by staying at or a bit above the template line until you run out of vehicle to support the curve. The red line I drew above has a final angle of ~18° at the back. If you made a 22° overall angle boat tail, it would be a disaster aerodynamically.

You really really want to make the shape at the front of your add on to be as close to the curve as possible. Making your rear cross section smaller would be a bad thing here, I know it is counter intuitive. The goal is to upset the air as little as possible as you drive through it. By following the template to the end you do that, if you tinker with it, you will invariably create more turbulence which will erode your Cd gains even though you have made the back as small as possible.