Reader Comments on Flat Engines

[Hugh Janus]

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Cycle World’s readership continues to surprise. I never expected the growing stream of comments on my recent flat motors story, which discussed engines such as BMW’s boxer twins and the Honda Gold Wing’s fours and sixes. As it happens, right around the time that Max Friz was laying out the BMW R32, several makers around the world, including Mercedes, were making or planning small opposed-twin aircraft engines. The design was clearly in the air at that time, and BMW’s lasting faith in the layout attests to its fitness.

Reader lumpy rightly reminds me that VW and Porsche have built a great many successful flat engines. In their case, cooling is accomplished by use of a large blower that pushes cooling air through the fins of all cylinders equally.

BMW’s R 1200 GS was the first boxer from the brand that featured water-cooling to critical areas of the engine. (BMW/)

In aircraft use, the pressure required to push that cooling air through fins spaced as closely as 0.132 inch (in the case of the forged-and-machined heads of late postwar Wright R-3350s) had to come from the ram pressure of the aircraft’s speed through the air. B-29s really needed to be going 200 mph to ensure adequate cooling. In some cases even that was deemed insufficient, as Germany’s Fw 190 fighter and the US-built six-engined B-36 bomber had engine cooling fans.

RELATED: The Advantages of Flat Motorcycle Engines

Another reader, RZ500, shows admirable compassion for the rear cylinders of the four-row, 28-cylinder Pratt & Whitney R-4360 air-cooled radial (used on the aforementioned B-36, among other aircraft). “How to cool the rear cylinders?” he asks. At one time I had three of those engines in my shop, and can tell the tale. Between each slightly curving array of four cylinders and the next was a diagonal sheet-metal divider. One side of it, acting as the cold duct, brought cold air from the front of the engine, that air flowing sideways through the fins of each of the four cylinders it served. Subsequently, that heated air emerged into the next row’s “hot duct,” which also contained its exhaust pipes. The cold ducts tapered, largest at the front, smallest at the rear, while the hot ducts expanded from small at the front to wide at the rear, ultimately joining to flow out the ring of adjustable cowl flaps. Design chief Luke Hobbs had several alternative cooling schemes thoroughly rig-tested before choosing that as the best.

The frequently commenting Ricardo Juliet called attention to the wonderful combination of a König flat-four liquid-cooled racing outboard powerhead and a Norton gearbox in a rather long motorcycle chassis, ridden by the late Kim Newcombe in the early 1970s. Newcombe and wife Janeen went to Europe in 1969; he worked at König and developed the idea of putting the engine in a bike. Have a look at the heartbreaking 2006 film Love, Speed, and Loss. Newcombe was able to give 500-class-dominating MV and Giacomo Agostini real competition, some of the first glimmerings that two-strokes could succeed in the 500 class.

Our appreciated engineering scholar Basil presents the concept of “the in-line boxer” (q.v.) which avoids the problem of rod offset in a 180-degree flat twin by having three crankpins. The center one carries a single con-rod linked to one piston, while two other crankpins at 180 degrees to the first carry two con-rods linked to the other piston. This places both pistons in the same plane. Longtime BMW riders will remember that, as the original 500 flat twin was enlarged in steps to its present 1,250cc size, its oscillation around a vertical axis through the crankcase (known to me as “BMW buzz”) increased with growing piston weight to the point that the makers now provide a balancer to cancel it.

Erica writes to say that the Bavarian flat twin “feels like riding a tractor.” My chemist friend who values his BMW as a platform for mind-clearing revels in his BMW’s “steadiness,” and was clearly sincere when he said to me, “I just can’t understand why anyone would want any other make of motorcycle.” But Erica clearly does, and sales of such things as blazing 600 sportbike fours revving to 16,000 rpm were lively for years. To each his or her own.

Ed wants us to remember rotary engines, but doesn’t say whether he means Wankel rotary-piston engines or the World War I aircraft rotary radials whose crankcase and cylinders whirled around while their crankshafts were stationary, bolted to the firewall. Why such craziness? Two reasons: First, the whirling engine was its own flywheel; second, cylinder heads whirling about at 200 or more feet per second could still be well-cooled even though early-war aircraft barely had any airspeed at all, putt-putting along at 60 mph.

Saddle Burns writes to remind us that there’s a difference between low center of mass (useful on heavy tour bikes because they try less hard to fall over at stoplights) and mass centralization. Erik Buell had fancy mass properties rigs in his factory to make sure that his bikes could change direction quickly. The analogy is between a 24-pound ladder, 8 feet long, and a 24-pound cannonball, just under 6 inches in diameter. The cannonball has excellent mass centralization!

Reader keechmabreeks would have liked to see mention of Douglas motorcycles, which were twins with one air-cooled cylinder forward and the other backward. And there they were, in the last paragraph of my “flat motors” article.

Douglas oriented the cylinders of its flat twin front to back. (Gérard Delafond / Wikimedia Commons/)

A parting shot from RZ500 calls attention to video of a Short Brothers Stirling bomber, its quartet of Hercules sleeve-valve 14-cylinder radials visibly smoking on takeoff as it drags a troop-carrying invasion glider into the air. Only a part of that smoke came from lube oil: to increase engine power for takeoff, the fuel mixture was enriched 20–30 percent as a means of limiting flame temperature and avoiding detonation (an abnormal and destructive form of combustion) while on high supercharger boost. That extra fuel released the black free carbon you see in the exhaust. You’ll see the same for any of the other great air-cooled radials, roaring and laboring down the runway on takeoff power.

It was similar for air-cooled motorcycle engines, especially in racing. Engines were run one or two sizes rich. Why? Tuners had discovered that although best-power jetting made the first three laps really fast, jetting rich (more fuel in relation to air in the mixture) could lessen the slow-down that resulted from overheating. Fuel-cooled.

The variety of these reader responses gives assurance that none of us ever need be bored, and for that we should all be thankful.

Source

[XTreme]

[boboneleg]

[busabeast]

An interesting read

[Buckster]

7 hours ago, Richzx6r said:

An interesting read

Shut the fuck up.

[busabeast]

1 minute ago, Buckster said:

Shut the fuck up.

Haha is this because you don't understand what it says