1987 GM Sunraycer solar car, Cd 0.089

[aerohead]

https://calteches.library.caltech.ed.../MacCready.pdfIf this links, it will get you to an article about the AeroVironment/ GM Sunraycer development.
On page-5, you'll see an incomplete image of the 1/4-scale wind tunnel model they used at the GALCIT tunnel at Cal Tech University.
As of 1987, this model registered the lowest drag of any land vehicle ever measured, @ Cd 0.089.
In the photograph, you can just make out the full wheel fairing at the front wheel, part of a complete package.
Concerns over crosswind gust stability of the 573- pound ( 260.4-kg ) total weight car led to removal of the entire wheel fairing package, which degraded the drag to Cd 0.125 @ zero-yaw.
The car was later yaw-tested for SAE crosswind-averaged drag, with Cd 0.147.
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The wheel fairings reduced drag by 36-counts ( 28.8% ), an approximate 80% drag reduction for the exposed wheels.
In Goro Tamai's book, 'The Leading Edge,' he suggests that full, swept fairings could account for up to a 70% drag reduction of the wheels.
Sunraycers' wheels are more exposed than many solar cars that would follow after 1987.
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Dr. Paul MacCready, who ran the program, estimated that a modestly-powered, internal combustion variant of Sunraycer would return 400- miles per gallon, highway.
At 90- km/h, Sunraycer's total road-load power requirement was 2,000-Watts ( 35.76- Wh/Mile ), at Cd 0.125, 14.222 square feet frontal area ( 1.321 meters-squared ), and Coefficient of rolling-resistance of Cfrr- 0.00653.
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[freebeard]

TIRES
The race did demonstrate one "flaw" in Suraycer. We had only three flat tires during the race. We should have had more. If we had used tires with thinner tread, the lessened friction would have saved a hour over the 45-hour race. more than compensating for the extra 15 minutcs[sic] or so that would been lost handling another dozen flats. Stopping, changing a wheel and re-accelerating cost about two minutes...while the extra charge accumulating from the sun the interval could later be used to make up some of the time lost.

Interesting strategy.

[cRiPpLe_rOoStEr]

Originally Posted by aerohead

Dr. Paul MacCready, who ran the program, estimated that a modestly-powered, internal combustion variant of Sunraycer would return 400- miles per gallon, highway.

Makes me wonder if the engine out of some small Honda motorcycle could qualify as such "modestly-powered".

[Piotrsko]

Modestly powered in this case probably means perhaps a limit of less than 5 hp.

[cRiPpLe_rOoStEr]

Originally Posted by Piotrsko

Modestly powered in this case probably means perhaps a limit of less than 5 hp.

I guess around 7hp would be enough to provide a comfortable cruising speed for real-world driving conditions.

[aerohead]

Originally Posted by cRiPpLe_rOoStEr

Makes me wonder if the engine out of some small Honda motorcycle could qualify as such "modestly-powered".

The water-cooled engines would have a BSFC advantage. The HONDA Shadow, 700cc, V-twin, @ 57-hp, 6-speed, would make a Sunraycer-ICE variant like a Le Mans racer.
I started down that road, got about $1,000 in parts collected, then after much meditation, decided that it would be a death trap. Where I live, anyway.
There just doesn't seem to be any right to life in North Texas traffic. And no law enforcement.
The car should have at least the performance of a 250 cc motorcycle. Ability to accelerate out of a dangerous situation, when braking won't save you.

[cRiPpLe_rOoStEr]

Originally Posted by aerohead

The water-cooled engines would have a BSFC advantage.

Sure, and maybe could allow minimum impact to the aerodynamics.

[aerohead]

Originally Posted by cRiPpLe_rOoStEr

Sure, and maybe could allow minimum impact to the aerodynamics.

Yes, that's my conclusion.
The radiator and ductwork could be 'off-shored' anywhere inside the body, unlike an air-cooled engine.
If one was going to pull off a 2%- drag cooling system, liquid-cooling would seem the better choice.
The drag reduction itself, would reduce the heat flux required to be rejected from the engine/ cooling system, a positive feedback.
And the BSFC issue, another incentive.

[cRiPpLe_rOoStEr]

Originally Posted by aerohead

The radiator and ductwork could be 'off-shored' anywhere inside the body, unlike an air-cooled engine.

I guess you're consider NACA ducting to provide air to both exchange heat with the radiator and to the intake charge. I was considering the fitment of either one or more radiators to the outside, while keeping the engine better enclosed, just like some Russian aircraft which had a water-cooled engine instead of an air-cooled and were supposed to be less prone to thermal shock damage while descending too fast.