That is it for Aptera

[Ken Fry]

Originally Posted by kach22i

I remember way back when I first saw VW Rabbits and Mini's racing. They were all 3-wheelers in the turns.

Or even 2.25 wheelers.

[drmiller100]

Originally Posted by Ken Fry

Actually, there is no intrinsic benefit to three wheelers for drag reduction. .

I think about things differently then you do. I think I found a significant advantage in drag for 3 wheels vs 4, but I need to keep working on the prototype.


I agree it is a LOT easier to build and title a motorcycle then a 4 wheeler.

[drmiller100]

Originally Posted by Ken Fry

Actually, there is no intrinsic benefit to three wheelers for drag reduction. .

I think about things differently then you do. I think I found a significant advantage in drag for 3 wheels vs 4, but I need to keep working on the prototype.


I agree it is a LOT easier to build and title a motorcycle then a 4 wheeler.

[dcb]

if it is teardrop in a top view, the tadpole configuration makes a bit more sense too.

[Ken Fry]

Originally Posted by drmiller100

I think about things differently then you do. I think I found a significant advantage in drag for 3 wheels vs 4, but I need to keep working on the prototype.


I agree it is a LOT easier to build and title a motorcycle then a 4 wheeler.

The Aptera claim was something like this: "...and of course three wheels have less rolling resistance than four." That's incorrect, unless they are saying that the vehicle is lighter, and therefore has lower rolling resistance.

Three wheelers (aside from the side car arrangement) pick up the additional frontal area of the third wheel, whereas the rear wheels of a four-wheel car are in the wind shadow of the fronts.

[Frank Lee]

Originally Posted by Ken Fry

Three wheelers (aside from the side car arrangement) pick up the additional frontal area of the third wheel, whereas the rear wheels of a four-wheel car are in the wind shadow of the fronts.

Yeah, in theory. As a practical matter I'd say vehicles operate in cross-wind almost all the time, so hardly ever would the rear wheels on a quad truly be shrouded by the fronts.

Look at a trike and a quad at about a 15 degree offset from the front. I see more shadowing with the trike.

[jakobnev]

Originally Posted by kach22i

I remember way back when I first saw VW Rabbits and Mini's racing. They were all 3-wheelers in the turns.

And they were like the most brilliant 3-wheelers ever, because the third wheel was always on the outside corner, where it provided the most stability!

[NeilBlanchard]

The lowest drag 4 wheelers have narrower rear tracks, anyway. The Cd *far* outweighs the frontal area, especially when the Cd is is as low as possible.

Ken is correct about the rolling resistance of the tires -- if the weight is the same, then it doesn't help to have 3 wheels instead of 4. I think there may be a small difference in the *bearing* friction, but that is definitely easily swamped by even the smallest difference in aero drag.

Back on the topic of the Aptera -- I think the most important concepts it illustrates is taking an "ideal" central shape, and then adding the wheels to that. The Edison2 VLC is a 4 wheeled example of this approach.

Another aspect of the Aptera that is interesting is the "beaver tail" -- this provides visual width, but the doesn't take advantage of the taper that is possible on the sides. The top and the sides of a vehicle can have greater angles on the closing slope; whereas the bottom is more limited in the angle by the fact that the air flow back underneath is limited by the ground itself.

The Edison2 VLC is a "fish tail" and since it has 4 wheels the width is provided by the wheel fairings. By the way, the "old" SAE Cd number for the Edison2 VLC is 0.145, so that is what we would compare to the Aptera numbers. Though, the Edison2 number is "real" and measured in the GM wind tunnel, while as far as i know the Aptera number is from CFD. The current SAE standard Cd of the Edison2 VLC is 0.161, and the frontal area is 1.7082 square meters.

On both the Aptera 2e and the Edison2 VLC, I am interested in the interaction of the air flow between the multiple shape volumes. When the Aptera 2e went to the front wheel drive, the wheel fairings got a lot closer together, and this along with the lower underside may well have account for much of the increase from Cd of 0.11 up to 0.15. (The other obvious change is the exposed unfaired drive shafts.)

That brings up another interesting study: the Aptera 2e uses elongated 'A' arms which are shaped like airfoils so on the spinning drive shaft has significant drag. The Edison2 VLC has fixed struts which are fully faired, so if you think about it you are left to wonder how they have any suspension travel on the wheels. They do this with compact trailing arms on the front (which also have to pivot for steering) and with trailing arms in the rear that house a chain final drive.

Another aspect of exterior faired wheels that I didn't fully appreciate until I saw the Aptera and the Edison2 in person, while they are being driven; and that is the room required for the steering pivot on the front wheels. The trailing taper on the wheel fairing has to stay clear of the main chassis, and they stick out on the outside of the turn.

[Ken Fry]

Originally Posted by jakobnev

And they were like the most brilliant 3-wheelers ever, because the third wheel was always on the outside corner, where it provided the most stability!

These were the first true "smart" cars. Another feature they both shared: when a tire went flat, it only became flat on the bottom! This feature has become so common and widespread that we sometimes forget to thank these pioneers.

[Ken Fry]

Originally Posted by Frank Lee

Yeah, in theory. As a practical matter I'd say vehicles operate in cross-wind almost all the time, so hardly ever would the rear wheels on a quad truly be shrouded by the fronts.

Look at a trike and a quad at about a 15 degree offset from the front. I see more shadowing with the trike.

You're like many pilots, who are always flying into a headwind and for whom every landing is a cross wind landing.

Assuming 60 mpg, where aero drag predominates, then 15 degrees means a 90 degree cross wind at 16 mph. The national average wind is 8 mph. Wind direction and speed and vehicle directions and speeds are always changing. Therefore, streamlining (the portion focused on drag reduction) makes little to no accommodation for relative wind direction, although studies have been done to evaluate such things. It's hard enough to just get the car working well in a wind tunnel with straight-on flow.

(I'd guess that the national average car while at highway speeds, spends 10% of its time with the apparent wind coming from more than 6 degrees off the nose, and 90% of its time with angles less than that. I've driven in a lot of snow storms, watching the apparent angle-of-approach of snowflakes.) (Craig Vetter seems to worry about 30 mph headwinds.)

Vehicles are, however, designed aerodynamically to deal with crosswinds from a vehicle stability standpoint. Its better to have the "sail area" in the back rather than in the front if stability is what you are looking for. In general it is also better (for stability) to have a boxier shape