GM: Chevy Volt's Cd is 0.28, Prius is 0.30 (tested in multiple tunnels), EV1 is 0.21

[Frank Lee]

I strongly doubt this aero wheeltracks idea has merit. How often does a vehicle operate in 0 yaw? Almost never. Thus for all practical purpose every 4 wheeled vehicle presents 4 wheels to the wind.

[aerohead]

Originally Posted by jamesqf

This is a good point. In these parts, it's not at all unusual to have a 40 mph or so crosswind - and anyone who's flown small planes into tight fields knows that a bit of slip is a darned good way to bleed off altitude/airspeed in a hurry. So I wonder what that 0.28 Cd straight into the wind becomes when the effective wind's at 30 degrees?

If the relative wind is 30-degrees off to one side of the vehicle,and as some of the side of the car is now impacting the airstream,the Cd based on frontal projected area goes right out the window,and all the body curvature which governed separation @ 0-yaw,is oriented completely different.
I'd done some crosswind testing in New Mexico and noodled with force vectors as one might in Statics.It was an interesting thought experiment but I never had any confidence in the "science" of it.
I suspect that these conditions are best investigated in a tunnel.
Santa Claus,are you listening? We've all been very good!

[orange4boy]

I think the EV1 had a narrower rear wheel track so they could taper the body more. I read that somewhere. Can't remember where.

I think Franks' right about 4 wheels to the wind. That's why wheels have such a big aero penalty. Lukes' landspeeder has the right idea. After the aft body, the wheels are the biggest deal hence the foul call on GM's 17" wheel Prius comparison. And foul on Toyota's same CD for all models. Lot's of foul to go round. Drumsticks for everyone!

[aerohead]

Originally Posted by orange4boy

I think the EV1 had a narrower rear wheel track so they could taper the body more. I read that somewhere. Can't remember where.

I think Franks' right about 4 wheels to the wind. That's why wheels have such a big aero penalty. Lukes' landspeeder has the right idea. After the aft body, the wheels are the biggest deal hence the foul call on GM's 17" wheel Prius comparison. And foul on Toyota's same CD for all models. Lot's of foul to go round. Drumsticks for everyone!

I believe I've got an overhead shot of the EV1 in the Phil Knox aerodynamic photos album which shows the plan taper.

[thatguitarguy]

Here's one:

[jamesqf]

Originally Posted by aerohead

If the relative wind is 30-degrees off to one side of the vehicle,and as some of the side of the car is now impacting the airstream,the Cd based on frontal projected area goes right out the window...

So some out-of-the-box thinking... Who says the wheels have to be in line with the body? Let them go straight down the road, while the body crabs to face into the relative wind.

Back when I was logging, we used to run a loader (ex military, IIRC) that would do just this: all four wheels could be turned about 30 degrees to the body while still staying parallel to each other, for getting in & out of tight spots.

[thatguitarguy]

Diminishing returns? Weight and complexity for how much aero advantage?

[NeilBlanchard]

Driving such a machine would be a massive problem on narrow roads, with other cars in adjacent lanes, etc. On my xA for example which is ~13 feet long and ~6 feet wide, then a 30 degree swing in either direction would be making the total width well in excess of 10 feet! (I drafted this in DataCAD to check...)

Such a system would have to be automatic, because the driver already has to think about enough, and could not manage very well, if not actually harm the efficiency?

If it was automatic, then the driver would not know from one moment to the next how wide their own vehicle would be! And, the computer controls, sensors, adjustment mechanisms, and the power use of these, etc., would make it tough to justify, IMO.

*+*+*

OTOH, I think that similar to what Citroën has done, I think that an integrated ride height and leveling suspension would be feasible and beneficial to the aerodynamics of the body. I also think that such a system could be used to (re)generate some electrical power, to remove the need for an alternator, as well. I'm trying to work out whether an air or electrical or hydraulic suspension system would be the best way to go. (Remembering too, that I would also like to have non-inflating tires and rigid wheels that yield as low rolling resistance as possible.)

[jamesqf]

Originally Posted by NeilBlanchard

Such a system would have to be automatic, because the driver already has to think about enough, and could not manage very well...

Maybe for the stereotypical American driver, who's fully occupied with his/her cell phone &c. I wouldn't think it would be more difficult than keeping a small plane coordinated: takes a bit of time to learn, keeping your eye on the ball (or the string if you fly sailplanes), but once learned it's like riding a bike.

As for road width being a factor, it depends on the car. My Insight is not all that much longer than a Hummer is wide :-) And if the vehicle is an Aptera-like tadpole, there's even more room.

[gone-ot]

...if the railroads of the world can "live" with the widths of two Roman horses a$$es for 2500 years, maybe car widths are too wide?