BSA Bantam and Harley-Davidson Hummer - CLASSICS REMEMBERED

A good design is a virus

bsa bantam d1 static side view
BSA Bantam D1By Kirc - PHOTO, CC BY 3.0,

Why would Cycle World even mention 70-year-old motorbikes that produced between 3 and, on a good day, 4.5 hp? These bikes represented a revolution in the making—the spread of DKW's original RT-125, whose flat-topped piston was a huge reliability improvement over the heat-gathering "deflector" pistons of conventional two-strokes.

DKW in 1929 acquired the patent for a new form of scavenging developed for Diesel engines by Adolph Schnuerle. Before this, the simplest two-strokes placed intake and exhaust ports opposite each other in the cylinder wall, with the exhausts opening 10 to 15 degrees early. To prevent fresh charge from just blowing straight across the piston and out the exhaust, a fence or deflector was cast into the piston crown, forcing the incoming fuel-air charge to flow upward then across the underside of the head and down the exhaust cylinder wall in a looping flow path. The idea was that during the time taken by the intake flow to traverse this loop, the exhaust process would finish and the rising piston would close off the ports, trapping the volume of fresh mixture still in the loop.

The problems were obvious. The presence of the piston deflector limited compression ratio and its product, torque, and its extra area took up heat from combustion, making the piston hard to keep cool enough not to initiate detonation and seizure. These problems limited power and compromised reliability, such that the search for two-stroke power in the 1920s and ’30s turned to such complexities as two opposed pistons in one cylinder, controlled by two cranks, or two parallel cylinders sharing a common combustion chamber, plus the use of an external scavenge blower in piston or rotary form. Such engines were neither simple nor cheap.

This explains why DKW management saw value in Schnuerle’s scavenge scheme. A single exhaust port was located in the cylinder wall with its lower edge at the level of the piston crown at bottom center. On either side of that exhaust, and angled away from it, were two somewhat lower transfer ports supplied from the crankcase (the underside of the piston acted as a scavenge blower). The flow path of fresh charge entered the cylinder through these transfers, flowed across the piston crown to the non-exhaust cylinder wall, flowed up it, across the underside of the cylinder head, and down the exhaust-side cylinder wall to the exhaust port. This achieved good separation of the intake and exhaust flows as effectively, as did the use of a piston deflector, yet eliminated its problems. Now compression could be increased while piston heating was reduced by at least one-third.

1950 dkw rt-125 static 3/4 view
1950 DKW RT-125CC BY-SA 2.0 de,

Every time I pour cream into coffee I do so at the edge of the cup. At first the cream just disappears into the black liquid, but a second or so later I see it rise into view opposite where I am pouring; it has performed Schnuerle’s loop.

In the late 1930s DKW put its simple, cheap RT-125 single into production with Schnuerle scavenging. It was a great success, both as transportation and as a learner-bike whose riders might later buy and ride something more interesting. The activities of German armed forces carried these useful machines over much of Europe.

At the end of WWII the Allied Control Commission or some such organization widely distributed RT-125 drawings as "captured enemy technology" and they, plus the acute need for transportation in a half-destroyed world, made it the most-copied motorcycle design of all time. RT-like 125 singles were produced by MV and Mondial in Italy, by Honda, Yamaha, and Suzuki in Japan, and by BSA in England and Harley in the USA. As a starting point for racing development the RT was without compare, being the seed from which Daniel Zimmermann grew the successful ZPH, which in turn was confiscated by East German strong-arm types and handed over to IFA (he was told to go race outboards). At IFA (later renamed MZ), the ideas of many would be integrated by Walter Kaaden (arriving in late 1952) into the modern two-stroke engine that with systematic development dominated GP racing for a quarter-century.

Universal English engineer Bert Hopwood regarded BSA’s version—the Bantam—as a valuable low-priced learner bike and deplored its discontinuance in 1971.

In the fall of 1959 I bought a used D1 Bantam for $140 as my first motorcycle. I knew it was the lowest of the low, regarded by all proper motorcyclists with contempt. It featured a rigid rear frame, no-damping front fork, and three-speed transmission with cork-insert clutch (clutches are better now, but Pol Espargaro smoked the clutch of his KTM MotoGP bike last Sunday in Texas). Yet it would jolt me along at 45 mph, as advertised. I "sent away" for a copy of the Floyd Clymer pamphlet "Souping Two-Stroke Engines for More Power and Speed," reprinted from CYCLE magazine. It cost $1.

Roland Pike, justly admired for his systematic development of BSA’s versatile "Gold Star" four-stroke singles (350 and 500) clearly stated that he did not like two-strokes.

“The noise they made was dreadful and so was the oil they blew all over the test shop.” When BSA took an interest in trials, “Reg Wilkes worked a couple of weeks to get 5 to 6 horsepower over a wider range.”

harley davidson hummer static side view
Harley-Davidson HummerImprobcat at English Wikipedia [CC BY-SA 3.0 or GFDL], via Wikimedia Commons

Harley-Davidson's version, a mirror image of the RT to put the shift pedal on the American side, entered production in late 1947 as Model 125, making 3 hp. How is it possible to make so little power? Its two vaguely oval transfer ports measured 12 x 17mm—about the section of my little finger.

English enthusiasts immediately went racing with tuned Bantams, creating the Bantam Racing Association, which persists to this day.

When BSA wanted more power from the Bantam, Pike suggested they draft in a proven Bantam racer, Hermann Meier (who would later design Royal Enfield’s 250 racer GP5, of which maybe 30 were produced in 1965). Meier had shown in Floyd Clymer’s book how to shrink a deeply finned aluminum "muff" onto the OD of a cut-down iron Bantam cylinder to improve cooling, keeping pistons alive at higher powers. Other mods included moderate porting, copiously finned head, a bigger carburetor, various kinds of exhaust pipe, and a “stuffed” crankcase. The result was 10 hp and a claimed top speed of 80 mph. Pike remarked that, “He doubled the power of the Bantam in a few months,” and was then called upon to design 150 and 175cc versions. Pike did, however, see to it that Meier’s enterprise was moved away from his race shop, taking its “dreadful noise” with it.

With the aid of my $1 tuning book and some manual labor, I soon had my Bantam keeping up with a high-school friend’s Model-A Ford. Going like 60.

In 1953 Harley puffed its 125 up to a 165. In the story "165 Harley Hop Up," by Earl Bolen, one could learn "the San Diego formula for a 12 horsepower H-D 165," developed by Jack Catrell of the storied Leonard Andres Harley shop in San Diego, and ridden by Bob Munson (his name was later borrowed for a character in the TV series Sons of Anarchy).

If all you had was the Hummer 125, speed and power were still within reach thanks to ML Mason's CYCLE article "Six Horses for Six Bucks." Carrying forward this tradition of "power to the people" through the dissemination of secrets previously known only to wizards in tall conical purple hats with stars on them was the late Gordon Jennings, whose magazine articles and "Two-Stroke Tuner's Handbook" did so much to populate the US with accomplished two-stroke sub-wizards. Much of the tuning he revealed could be carried out with nothing but files!

Why was it so hard then when now it’s so easy? We know that when the FIM’s 125 GP class ended, front-row bikes were making 55 hp at 13,000, 18 times more than a postwar RT knockoff. Instead of 1,000 monkeys at 1,000 typewriters, trying to reproduce Shakespeare’s sonnets by pure chance, we saw a small number of hard-working practical engineers starting with an equal number of RT-125s or Bantams or Hummers or equivalent.

Daniel Zimmermann brought ideas together and achieved workable con-rod bearing life at 10 hp and 8,000 rpm. Walter Kaaden added to this Erich Wolf’s bi-conical exhaust pipe combined with ever-taller exhaust ports, taking power to 20 hp (1958) and then 25 (in 1961), eclipsing the best four-strokes of his time in power if not always in reliability. Incremental development by Japanese and European engineers (think of Jan Thiel, who retired from Aprilia) did the rest.

At Honda in the 1980s and ’90s there were two essential men in NG Block, one of them known only as “Lobsterman,” who used die-grinders, exhaust pipe sections, and a mass of experience to transform what the graduate engineers drew into Grand Prix winners. What were their methods? CFD? Time-area analysis? Laser interferometry mapping? No, they did what we all did in those days; they tried stuff. Thiel has described preparing and testing 40 different cylinders in the same manner. Yet Adolph Schnuerle, had he been living in 2011, would clearly have recognized his basic scavenging concept in Aprilia’s last 125 GP engine.

How many RTs, Bantams, and Hummers were produced? No one seems sure—maybe 100,000 RTs prewar and 300,000 after, plus a quarter- to a half-million Bantams. So, with all the copying that went on (or we can be charitable and call it “cultural diffusion”) surely over a million. That’s nothing compared to the millions and millions of Honda step-throughs, but a mass market has to start somewhere, from something.