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May 03, 2008

Hidden Front Wheels

Donald Pittenger writes:

Dear Blowhards --

In the very real world of engineering, you can't optimize everything. Putting it another way, engineering is a realm of trade-offs, compromises.

For example, instead of text in that comic-strip balloon above your head, we see a glowing light bulb, and your mind is exploding with the word "Eureka!" You suddenly realized that one way to improve gas mileage of automobiles is to streamline the car's body. That comes from reading that aerodynamic resistance, at speed, is a function of a car's frontal area (the number of square feet/meters at the vehicle's largest cross-sectional point) and the coefficient of drag. For a given frontal area, the resistance can be reduced by improving the coefficient of drag by streamlining the car's body.

A brilliant insight, but not exactly new. For example, Paul Jaray was investigating automotive streamlining in the 1920s and took out several patents. The Ill-fated 1934 Chrysler Airflow made use of wind-tunnel tested streamlining in an effort to reduce drag.

One of the ways to cut drag is by eliminating or controlling sources of air turbulence. For instance, projections from the car's body such a rear-view mirrors can create turbulence. Since mirrors are essential to driving safety and cannot easily be eliminated, they are now housed in streamlined shields; when I was a lad, they were the shape of a dentist's mirror, presenting a nearly flat surface to the wind.

Another source of turbulence is holes or gaps in the body surface. The largest such gaps are the wheel wells. Therefore, when engineers and stylists began to think seriously about streamlining in the 1930s, they set about eliminating wheels wells, both front and rear. Let's take at look:


Boeing P-26 "Peashooter"
Reducing drag of wheels was nothing new in the field of aviation. The P-26 fighter, first flown in 1932, was one of many designs that featured streamlined "spats" over the wheels and landing gear struts. This was a compromise. The spats improved streamlining over open struts and wheels, but a better aerodynamic solution was retractable landing gear. But retractable gear were heavy and complicated. So spats were acceptable for P-26s that had a top speed of a little more than 200 mph, but weren't the best solution four years later when Curtiss Hawk 75s could hit 300 mph. Most 75s had retractable gear, but the 75Ns that were sold to Thailand had spats.

Norman Bel Geddes model, 1934
Pioneer industrial designer Norman Bel Geddes created several aerodynamic car designs in the 1930s. The copper model shown above has eight (!!) wheels and a body whose interior might have been a akin to todays' minivans. Note that both the front and rear wheels are covered by fenders.

Panhard "Dynamic" 140 coupe, 1937
If you look closely, you can see that there are three windshield panes, a large one and small curved ones at each side. Panhard called this primitive wrap-around system panoramique; General Motors mass-produced single-pane wrap-arounds starting in 1953-54. What can't be seen due to reflections on the windshield is that the steering wheel was positioned at the center and not on the right or left. And the wheel wells are skirted, though the bottom edges have nice French curves.

Delahaye 165 M - 1939 - body by Figoni & Falaschi

Delahaye 165 S - 1949 - body by Saoutchik
Another pre-war French approach to streamlining (yes, the Saoutchik version was built after the war, but that doesn't matter, as you can see). Probably more style that aerodynamic substance, but note that there are no open wheel wells.

Chrysler Thunderbolt - 1941
A few of these Alex Tremulis-designed show cars were built. Thunderbolts had a metal top that retracted by simply pivoting into a compartment behind the seat. Again, no open wheel wells. The Thunderbolt is noteworthy because it exemplifies the "car of the future" according to conventional wisdom in the late 30s and early 40s. The only missing detail from the typical CW dream car sketch of the times would be a clear, plastic top.

Nash, 1950
The nearest thing to a pre-war "car of the future" to make it into production in America was the 1949-51 Nash. The car is bulky, but an honest attempt was made to streamline it. Front and rear wheels are mostly covered, but not as much as they could be; the skirting does not extent as low as the lowest part of the body below the doors.

Ford Probe V experimental car - 1985
Ford built a series of experimental cars in the late 70s and early 80s to see how low they could drive the coefficient of drag on a workable automobile. The fifth version, shown above, attained a coefficient of 0.137. To put that in perspective, a typical post-World War 2 American sedan had a coefficient of around 0.5 and a well-streamlined car of today might boast 0.3. Again, the wheel wells are covered.

So if you want to reduce aerodynamic drag on an automobile, a nice thing to do is get rid of open wheel wells. But then, why was the Nash one of the few production cars to have all wheels skirted? Why don't Toyota Priuses and all other cars in showrooms today have fully skirted wheels?

The answer is that covering the wheels creates a number of disadvantages -- apparently enough that any aerodynamic benefits are more than canceled out. One disadvantage is that skirted wheels are difficult to work on. Changing tires and putting on snow chains is troublesome enough without having to deal with a small wheel well opening. Another problem is that snow or mud can be more easily trapped in the wheel well if it is enclosed. Perhaps the most important problem is related to the fact that front wheels need to pivot for steering. Covered front wheels normally result in a wide turning radius -- a great inconvenience at times. Today's cars have a "wide track" stance, where the wheels are closely aligned with the sides of the body. This lessens the chance for roll-over, an important safety consideration. Covered wheel wells would require either a narrower track or side bulges over the front wheels; neither solution is a happy one.

Oh, and there's the aesthetic point that wheels are nice, functional objects and deserve to be shown.

I'll close by suggesting that an extremely tough future government-imposed gas mileage regulation might force car makers to cover wheel wells regardless of their disadvantages.



posted by Donald at May 3, 2008


I'm no engineer, but these cars must have had lousy turning radiuses (radii?). Those closed fenders would have to restrict how much your front wheel could turn, no?

Posted by: Charlton Griffin on May 3, 2008 11:32 PM

Ignore my comment. I see you covered it in the body of your remarks.

Posted by: Charlton Griffin on May 3, 2008 11:33 PM

" concluded one Medhurst, whose New System Of Inland Conveyance, a rail car proposed in 1827, would have been "tapered at both front and rear ends to move through still air or head winds with a minimum of resistance"....Readers of Scientific American must have received a jolt when they looked through the issue of January 11, 1913, and found an artist's conception of "car of the future" that "looked more like a submarine boat...than a carriage". In 1911 W.G. Aston argued that "low wind resistance goes hand in hand with good appearance". Expanding on that statement, he claimed that "air behaves in certain circumstances much the same as the aesthetic sense behind our optic nerves, and it exhibits the same reluctance to adapt itself to bluff surfaces, sharp corners, ugly gaps and ungraceful...curves as does our vision - if properly trained"."

From The twentieth century limited, by Jeffrey L.Meikle, chapter "The Practical Ultimate".

One of my favorite books.

Posted by: Tatyana on May 4, 2008 10:28 AM

Cue up Eric Burden and War's "Low Rider" and take that turquoise-baby out for a spin.....LOL

Posted by: miles on May 4, 2008 2:34 PM

Those are some zany, if sometimes glamorous, vehicles!

How important is streamlining on cars really, from a functional p-o-v? Does a really sleek design result in a few extra mpg's? Anything more significant than that?

Posted by: Michael Blowhard on May 5, 2008 2:36 AM

Michael -- Aerodynamics do indeed help some. I don't have the stats in my head, but my understanding is that, assuming average conditions, a well-streamlined car will get 1 or 2 mpg better than a boxy car. That doesn't sound like much, but it was a big deal for manufacturers, because it was a lot cheaper to streamline their next body re-do than do a lot of re-engineering to eke out the same improvement to keep Uncle Sam happy. (Cheaper because they have to come up with new body designs periodically anyway. That's part of the story behind the original Ford Taurus of the mid 1980s.)

Posted by: Donald Pittenger on May 5, 2008 10:17 AM

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