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Old 02-04-2012, 09:37 PM   #271 (permalink)
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Okay, here is just about the lowest drag car that I know of -- Cd is 0.14:


(click on image for link)





Here's the best views of the back that I can find that shows the height of the underside and the small rear fascia. The rear wheels are surrounded by strakes.




Awesome!

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Old 02-04-2012, 11:15 PM   #272 (permalink)
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Fascinating pictures Neil. The rear work is very interesting. Tires are very narrow and look to be inflated at 80-100 psi. (little contact patch showing). The rear slop is steeper than Phil would approve ;-)

Looks like a German license plate.

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Old 02-05-2012, 09:16 AM   #273 (permalink)
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Yup, it is a German company SolarWorld AG and Bochum University. The lowest part of the body (other than the wheel strakes) is between the front wheels, so the slope upward is pretty close to ideal. I think they need a crisper Kamm edge on the outside rear fenders (near the fellow's hand closest to the camera in the last photo). This looks a bit too rounded to have a clean flow, and it might have turbulence. Also, the rear wheel strakes look wider than they need to be? Or, maybe they have four wheel steering to reduce the turning radius; since the front wheels may be limited by the fenders?
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Old 02-05-2012, 11:12 AM   #274 (permalink)
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Excellent looking car.
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Old 02-05-2012, 01:12 PM   #275 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
Okay, here is just about the lowest drag car that I know of -- Cd is 0.14:
I love it, however:

1. I was expecting more of a tapering teardrop on the aft canopy.

2. Also the rear corners are curved inward without a clean edge for the air to separate from.

3. It sits higher than what I would assume is optimal, with no chin spoiler, air splitter, air curtains or other new fangled things.

4. Rear deck lid as with the rear corners looks like it would allow messy air patterns to form.

Below is what I would have assumed would be better (lowered car):
Automobile 2 - Odds and Ends pictures by kach22i - Photobucket



If someone could number by number correct my assumptions (and explain why), I'd appreciate it.

Oh, #4, trying to show similar to the below, but with sides also boxed.
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Old 02-05-2012, 01:46 PM   #276 (permalink)
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I suspect that this vehicle's design speed is in the 25-30 mph,
50-60 kph, range.

I wonder if the tricks that we associate with low drag at typical
highway speeds of 60-70 mph, 100-125 kph, are effective or
necessary at those speeds.
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Old 02-05-2012, 02:53 PM   #277 (permalink)
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Kach22i, I largely agree with your points -- the rear fenders in particular are too rounded I think. They need as much surface area on the top for PV cells. I wonder if the rear panel could be redesigned with slightly raised 'V' ridges running longitudinally, that could add more surface area and increase the number of cells that are facing the sun at any given moment?

The Cd could probably be improved by having a "crisper" Kamm back on the rear fenders (around the taillights) and if the rear wheel strakes were narrower and fully tapered to an edge. The windshield could be pushed even more forward, and this could add more PV cells up front. The front of the ideal template is much more blunt, and this helps develop the localized higher pressure which increases the "spring" so the air then returns better by staying attached at higher angles, than with a "pointier" front.

With a smooth underside, I don't think a splitter / dam would help, and there is an optimum ride height -- the Schlörwagen was tested for this (this is in Hucho). Too low and you compress the air under the car (adding lift and drag) and too high and you add more exposure to the wheels. This Bochum car has the height about right I think; though it could go for a lowered stagnant point instead.

I think the underside is already as steep as it can be, and ditto for the top side. The rear wheel track could be narrowed, though -- this would possibly be another tenth or more?

Also, I'm wondering if having a small slightly protruding "nose" has a benefit? This is subtlety different from the whole front being pointy, I think. I'm thinking of the dynamic that happens when you put a sphere out in front of the front of a car -- it gets the air flowing in ways that are not necessarily intuitive. The early models of the Boxfish / Bionic have this, and so does the Prius and the Edison2.
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Old 02-05-2012, 03:13 PM   #278 (permalink)
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The "excessive" rounding behind the rear wheels was the second thing that stuck out to me.

The first thing was how far the roof extends in front of the driver's head- I understand they wanted the space for cells, but out in the real world, the driver will not be able to see traffic lights, I'd bet on that. Heck, I've driven a Solstice and sat in a new Camaro and I couldn't see traffic lights unless I scrunched waaay down. I would NOT push the windshield even further forward for this reason; another reason would be the "back seat driver" feeling you get when the windshield is unnaturally far away, like in the dustbuster GM vans. I suppose you get used to it, but GM had a real problem with people damaging their vans' bodywork because they had no sense of where it was in parking maneuvers.

The third thing I noticed was that the skirts appear to be fixed, leading to the same problem the Schlorrwagen had, that of more frontal area than necessary. It could probably be slimmed down a bit with articulated front skirts.

Not worried about the trailing angle of the roof- perhaps it could be a bit steeper for aero, but they don't want that for better angle of cells to sun?
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Old 02-05-2012, 06:10 PM   #279 (permalink)
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The rear slope is 15 degrees, give or take a bit. Probably fine, and the smoke makes it look ok.

Looking at some of the pictures, I think it has 4 wheel steering. Like Frank, I think they would have produced a more narrow car by using articulated wheel covers in front, then they could have narrowed the back a bit more. The best template is a template of rotation. Currently the sides looka bit flat and the Kamm area is badly handled. It would be very interesting to see some smoke testing of the back area both plan and profile.
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Old 02-05-2012, 06:47 PM   #280 (permalink)
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I'm guessing the rear is flat because with the well-curved edges of the roof, it would be difficult to bend the solar cells over a compound curve. The Aero Civic seems better executed aero wise. I wonder if it's the front end that's holding it back?

The rear of the strakes and whatnot actually looks like it might have attached flow. The surfacing is sub-par though. But I can't complain. I'm not much better than 0.3 Cd.

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