The Aerodynamics of Two-Wheeled Speed
THE AERODYNAMICS OF TWOWHEELED SPEED
RICHARD C. RENSTROM
THE YEAR OF 1965 was monumental in the annals of record setting at the famed Bonneville Salt Flats. Huge jetpowered cars roared across the white expanse at over 600 mph; big supercharged Chrysler V-8 streamliners made 200 mph look puny; and even 175 mph was pegged back by unsupercharged 15 cubic inch motorcycles.
Yet of all the records established last year, one of the slowest turned out to be the most incredible. The day was October 23rd, and the machine was a small Kreidler — a streamlined two-wheeled jewel that came all the way from Germany. The maximum recorded speed of 140 mph is not so astounding. What is remarkable is that the record setter was powered with only a three-cubic-inch engine — just one piston with a diameter of 1.6 inches operating on a stroke of only 1.5 inches. It all fits quite neatly in the palm of your hand, but Rudolf Kunz nevertheless pegged back a 130.88 mph record with it.
What is even more significant about the little Kreidler is that it epitomizes the very fascination of the modern motorcycle—the truly remarkable performance that can be obtained from small, but well-designed engines. The only problem with this fascination is that the engine usually receives all the credit, while very little heed is paid to what goes around the powerplant. In the sphere of speed records this is particularly true—the fellow who built up the engine gets all the credit and the chassis designer is never mentioned.
Oddly enough, the world motorcycle speed record holder in 1930 was developing a great deal more horsepower at 150 mph than does the current 230 mph titleist. J. S. Wright’s old l,000cc J.A.P. supercharger was putting out something like 84 hp and the current champ is obtaining only a little more than 70 hp from his 650 Triumph engine.
The difference in speed between these two bikes is due not to the engines, for the former was by far the more powerful, but rather to the science of aerodynamics. For it is the science of air flow, more than the science of engine technology, that is responsible for the increase in speed of our record bikes—and this development makes for an interesting study.
The best place to begin is with the most basic—the “naked” or unstreamlined bike. The motorcycle, as opposed to a car, or airplane, possesses a poor shape for air penetration. From the front, a bike with a rider is a mass of irregular shapes that form many pockets of air, all of which cause air drag. Up to 50 mph it isn’t too important, for it takes only about 5 to 8 hp to overcome both the rolling and wind resistance. But from 50 mph up, the wind resistance begins to absorb increasing amounts of horsepower, until at 75 mph it takes something like 18 hp and by 100 mph, requires around 30 hp.
This problem is due to the fact that air resistance increases as the cube of the speed, while rolling resistance has a constant rate of increase. The end result is that an unstreamlined 500cc road racing bike must develop nearly 78 hp to achieve a speed of 150 mph—and there just aren’t many bikes around developing that much power! By studying the accompanying chart, which is for an unstreamlined 300to 325-pound road racing bike with the rider “tucked in,” it can be seen that an increase in speed from 130 to 143 mph (10 percent) would require an increase in power of nearly 30 percent.
To apply this air drag factor to an unstreamlined speed-record bike, let’s go back to 1930, when J. S. Wright was wringing 84 hp out of his J.A.P. Wright’s V-twin was a bit larger than a standard road racing bike and it had a frontal area of about six square feet with an air drag factor of about .0008. With his 84 horsepower he was able to achieve a 150.736 mph record.
The Germans were hard at work, though, as they had plans on capturing the record from Wright. The new contender was a BMW, and the blown 750cc engine developed 74 hp—10 less than the J.A.P. At this point, a great milestone in motorcycle aerodynamics was recorded when the BMW appeared with a sheet-metal paneling over the central section of the bike. With a slightly lower air drag factor of .00075 and a frontal area of about 5Vi square feet, Ernst Henne was able to push the world’s speed mark up to 159.0 mph, which was the first significant attempt at decreasing the air drag factor with any form of streamlining on a bike.
Then followed a period when brute horsepower did battle with the infant aerodynamic science. Aerodynamics came out on top. The ultimate in horsepower was Eric Fernihough's l,000cc J.A.P.-powered Brough Superior. The big V-twin had a supercharger and it ran on straight alcohol —developing 120 hp. The only concessions to improved air penetration were the small shield to break the blast of air around Eric’s face, and a tail covering. The engine had the necessary urge, however, and a 169.786 mph record was put on the books.
Germany responded in 1937—with more aerodynamics and a smaller engine! The new BMW speedster had a 500cc supercharged engine, but the bike was covered with a full shell that had only the rider’s head sticking out. The opposed twin engine made for a fairly wide shell and the frontal area is estimated at six square feet with an air drag factor of about .004. With this much lower air drag, Henne was again successful and he boosted the record on up to 173.88 mph—despite having some 42 less horsepower than Eric Fernihough. Henne’s achievement made the point well; streamlining was a necessity for motorcycle
speed-record setting.
It was not until after the war that any more successful attempts were made to snatch away Henne’s record. Again it was Germany, but this time it was the NSU concern who supplied the machine. NSU had two bikes, one with a 350cc engine and the other with a 500, and both were supercharged vertical twins. The 350 developed 70 hp and the 500 pressed 105. The two bikes were actually the pre-war road racing machines with a full streamlined shell.
With an air drag factor of about .0004 and a frontal area of about six square feet, the two models were capable contenders. The actual runs were made on the MunichInglestadt Autobahn in 1951, with the 350 cc model returning a 173.3 mph speed and the 500 setting a 180.065 record.
Not satisfied with their progress, the NSU factory was hard at work on a totally new concept in two-wheeled speed. Working with full factory support, Gustav Baumm developed his new projectile in the Stuttgart University wind tunnel. The salient feature was a long low cigar-shaped chassis that had the rider lying down with the engine behind him. The air drag factor with this new shape was very low, estimated at about .00016, and with a frontal area only 65 percent of the larger NSU speed models.
Tn 1954, Baumm shook the motorcycling fraternity with a whole galaxy of new records. It was not so much the 93.5 mph clocking with a 50cc engine, the 111 mph speed from a lOOcc powerplant, or the 135 mph attained from the 125cc; but rather the manner in which he accomplished these records. Gustav used strictly standard engines for his record setting, thus clearlv demonstrating the advantage of his sleek little projectile.
Across the seas in America, the seeds were sown for a similar machine, and the following year a Texan, Johnny Allen, scorched across the Bonneville Salt Flats to a new world’s fastest at 193.7 mph. Allen’s Triumph-powered record breaker had a chassis very similar to the Gustav Baumm machine, except that it was a little larger and had a slightly less efficient aerodynamic shape.
Germany wholeheartedly accepted the challenge, and in 1956 they traveled all the way to Bonneville with a comprehensive range of machines. The 125cc and 250cc models were the little Baumm streamlined “cigars” with the unsupercharged 20 and 42 hp gasoline-burning road racing engines. For the larger 350cc and 500cc blown twin-cylinder engines, the full motorcvcle frame and shell were retained. The shell had been slightly improved by elongating and lowering, and the air drag factor was computed at about .003, with six square feet of frontal area.
With a snarling 75 and 110 hp from the two larger engines, NSU was confident that they could gain back the world’s fastest title. They did, too, with the 500 becoming the first to exceed the magical 200 mark with a 210.64 mph record, while the 350 clocked 189.5 mph.
Even more amazing were the smallest Baumm machines with the less powerful engines. The little I25cc model churned out no less than 150 mph—and on only 20 hp. The 250cc model fared less than well, as one of the freak gusts of wind that occur at Bonneville upset the machine at speed. The 250 did get in a one-way clocking of 180 mph, though, which demonstrated the advantage of the cigar chassis.
About the only thing that can be added to this story is the current Dudek and Johnson’s 230.269 mph “world’s fastest” record with a 72 hp, 650cc, fuel-burning Triumph engine. The current record holder still uses the cigar chassis with an air drag factor of about .0002 and a very small frontal area. Then, of course, there is last summer's 50cc Kreidler mark, previously mentioned at 130 mph, which made 15 hp look mighty potent.
But what about other phases of motorcycle activity? Has the science of aerodynamics been utilized here? Certainly. In road racing it is the most obvious. Attempts at getting some extra speed out of the European Grand Prix machines started in the early 1950s, when gas tanks began to get “notched” for the rider’s arms. A modest little “head" fairing was next, and the idea of breaking the air flow around the rider’s head was good for several miles per hour.
By 1953, racing machines were using the now common dolphin fairing, and by 1954, the full dustbin fairing was in use. Then, in 1958, the FIM reduced the allowable streamlining to the dolphin-type, which is so popular in road racing today. While a great amount of air-drag factor data is not available on these racing fairings, there has been a sufficient amount of research conducted to give some idea as to the increase in speed. The “head” fairing is known to have given about a 3 percent increase in speed, a dolphin about 6 percent, and the full dustbin around 10 percent.
For a road bike the advantage is even greater, because a road bike is not nearly so well “tucked in” as is a genuine road racing machine. Tests have indicated, for instance, that a well designed sport's fairing will give an increase in maximum speed of about 10 percent—that is, if the gearing is raised to take advantage of the improved air penetration. To obtain a 10 percent increase in top speed without a fairing would require about a 30 percent increase in horsepower—and that would take some doing on a road bike'
So, this is the story of motorcycle aerodynamics, from record machines to road racers, and fairings for standard road bikes. It is obvious from studying the air drag data that the engine isn't nearly as important as many people think—at least as far as record setting is concerned.
For instance, what would happen if NSU were to mount their blown 500cc twin (which now develops 120 hp) into one of their Baumm machines? The 42 hp 250 obtained 180 mph in the sleek little shell—so what would 120 hp do? On paper it would exceed 300 mph, and who knows, maybe that will be the next record. At any rate, the next time a speed record is set, please doff your hat to the fellow who designed the shell. He’s a pretty important man these days. H
USEFUL FORMULAS FOR SPEED CALCULATIONS-----
The power required to propel a motorcycle depends upon two main factors—air resistance and rolling resistance. The following formula will compute the air resistance, and good rule of thumb for computing rolling resistance is to add two percent of the total weight to the air resistance total.
Air Resistance (pounds) = A V2C Where A = The frontal area in square feet
V = Velocity in feet per second C = A constant called the “drag coefficient.”
The horsepower needed to overcome this resistance can be computed from the formula:
Resistance X feet per second
For a chain-drive power transmission system you can figure a 96 percent efficiency. Therefore, the engine horsepower must be approximately 4 percent greater than the rear wheel horsepower.
Typical Frontal Areas:
125cc road racer 4 to 4Vi sq. ft.
500cc road racer 5 sq. ft.
Fully streamlined 6 sq. ft.
shell (NSU 500)
Baumm or cigar shell 3.9 to 4.5 sq. ft.
Typical Air Drag Factors Rider sitting up on
road bike .0010 to .0012
Tucked-in on road
racer .0008
Fully streamlined shell .0003 to .0004 Baumm or cigar shell .0016 to .002
