Cooler Heads Prevail

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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/)

As I write this, summer 2021 officially starts in just a few days, yet much of the US is already coping with high temperatures, some record-setting. Add a warm wind, and it can be like standing in front of a blow-dryer. If you don’t look out, you’ll be reduced to so much beef jerky with a helmet strapped on. And if you think you’re suffering, think about your motorcycle’s engine.

When it comes to those engines, I’m going to define cooling interference as any running problem arising from heated air. For motorcyclists, that air is generally heated in two ways: having passed over an air-cooled engine’s cylinder fins, or through or a water-to-air heat exchanger—its radiator.

AJS’ air-cooled 500cc V-4, designed in 1935, provides us with an instructive example. The two rear cylinders “cooled” by air preheated by the two front cylinders. AJS recognized the problem, and to throw those cylinders a life preserver, they ran a lower compression ratio than the fronts. Looking for performance in the 1936 TT, the bikes got an aluminum top end and superchargers. And while their top speed was good, due to the low compression they needed to avoid detonation, they accelerated poorly. Cut to the chase: Because of the cooling/compression compromises, the supertrick V-4 was only slightly faster around the circuit than the company’s pedestrian 350 single! Redesigned for 1938, the bike overheated in the TT and retired. Given water-cooling for 1939, the bike (now a hefty 405 pounds) was fast, but now lacked stability. Time to move on.

Engine Architecture and Cooling Compromises

Even earlier were the air-cooled Douglas flat twins. Think of a BMW engine, turned 90 degrees, where one cylinder points forward and the other backward. Maybe you see the problem already: the crankcase and magneto block cooling air to that rear cylinder. Or how about Ariel’s famed Square Four? So long as neither engine had to give much power, all was well. Efforts to get more resulted (as by now we’d naturally expect) in the rear cylinder(s) overheating. Douglas had a brief period of TT racing success in the early 1920s, but as the better-cooled upright singles began to develop more power, Douglas was left behind. The twin-crankshaft Ariels were admired not for their blistering track performance, but for their smoothness and ability to drag heavy sidecars down to Bognor Regis on the south coast of England.

No less than the respected Phil Irving provides the next example. In his autobiography he describes the popular Vincent V-twins, which he co-designed, as being suitable at most for spirited riding on public roads, but disqualified from racing. Why? The difficulties of cooling the rear cylinder with the hot air streaming off its partner up front.

In the 1950s Guzzi occasionally suffered cooling interference of a different kind. We’re not talking about the twin-cylinder Guzzis you think of today, but about their horizontal single. Its problem: Its head (the hottest part of the engine) was cooled first, heating the air which then streamed back to “cool” the axial (parallel to the cylinder axis) finning on the cylinder. Piston cooling is the second-most-severe problem air-cooled engines face; consequently even the well-looked-after Guzzi racing singles suffered occasional seizures. (What’s an air-cooled engine’s most severe problem, you ask? Cylinder-head creep, distortion, and cracking, especially around the exhaust-valve seat.) Still, the occasional seizure did not prevent Guzzi from winning five consecutive 350 world titles, 1953-57.

How did air-cooled radial aircraft engines cope with such problems? First, each cylinder received its own fresh, cold, cooling air—no cylinder or head was bathed in air previously heated by another cylinder. This was true even with Pratt & Whitney’s famous R-4360, a 28-cylinder engine affectionately called the “corncob” thanks to its four-row construction. And second, close-fitting sheet-metal baffles clamped against the fin tips forced cooling air to flow through fin space all the way from the front of each cylinder, around the sides, and to exit through a tapered axial slot at the back. (Ed: Champion’s legendary Arnold Frank, a wonderful man who taught so many of us so much, would regale us with stories of changing all 56 plugs in these formidable temples of internal combustion.) These were fins designed for function, not fashion.

When Dick O’Brien and Jerry Branch were developing Harley-Davidson’s never-raced study project called “the Midget,” wind-tunnel personnel at Caltech persuaded them to try aviation-style ducted cooling with such baffles. As the bike moved fast enough to push air through the system, it worked great; at lower speeds it was another story, and overheating was immediate.

Air-Cooled Engine’s Hybrid Cooling Model

Air-cooled bike engines cool in a hybrid manner. During the few seconds when you’re using high power (think straightaway), most of the generated heat doesn’t immediately dissipate through the cooling fins—most of it is absorbed and stored in engine structure, and the engine’s temperature rises. Then comes a corner, and part-throttle operation. Now that heat has time to disperse into the surrounding air. Of course O’Brien knew this, and that’s why, when Piet Zylstra was preparing the original drawings bike we know as the XR750 back in 1972, O’Brien told him, “I want an inch of goddamn aluminum on top of those combustion chambers!” Obee was nobody’s fool—he wanted that metal as a heat sink, a sort of thermal 401(k) where he could deposit heat during high-power operation and pay it back into the atmosphere later.

Getting closer to our own era, cooling interference took another form. Hot air would pass through a bike’s cooling fins or radiator, and then the carburetors would breathe it back in. None of us ran airboxes in the 1970s—they added weight, complexity, and complicated jetting changes. I considered this problem in 1972 after seeing the three VM35 Mikuni carbs in the hot airflow of my 750 Kawasaki’s wall of cooling fins. I wanted to protect those carbs in a box, supplied with cool air, but had more pressing duties. Yet the problems remained:

1. We were losing horsepower, because the hot air reduced air density.
2. We had to lower our compression ratio to prevent detonation induced by the hot charge air.
3. How could we design an airbox that didn’t obstruct the cooling air flowing through the engine’s fins?

Airbox Benefits

In time, the two-stroke GP racers tackled these problems, but for many years it remained an article of faith that as soon as you took delivery of your new sportbike you had to toss its restrictive air-filter case and replace it with higher-flowing sock filters. Five horsepower, free and easy!

We didn’t realize what we were giving away in the bargain. In 500 GP, bikes progressed from no airboxes, to airboxes which merely kept hot air out of the carburetors, to full fruition using the airbox as an intake resonator, also able to recover valuable intake pressure with forward-facing ram-air ducts.

Imagine how surprised the ardent Supersport racers of the early ’90s were when, upon discarding their airboxes, performance fell flat. They had lost the extra horsepower that the newly discovered airbox resonance generated. Quick! I hear the garbage truck coming! Jump in the dumpster and rescue that airbox!

Right now bike engines have lovely intake airboxes supplied with cool air by a forward-facing intake. That’s real progress. But the engines themselves still hunker behind a coolant radiator jammed between the aerodynamically sloppy front wheel and the hot exhaust pipes. No diffuser leads air efficiently to that radiator, and the hot air streaming from its rear face is heated even further by the exhaust pipes, losing all its energy as it stumbles along trying to find an exit through the hot-air outlets in the fairing sides. Plus, those side outlets effectively increase the “aerodynamic width” of the bike, thereby increasing drag and reducing top speed. Why?

Because the outflowing hot air has lost the velocity it had as it first approached the front of the bike. Now its outflow forms a low-velocity obstruction, which deflects free-stream air flowing around the sides of the fairing. But because bikes are already so densely packed, there’s no room to duct spent cooling air to the low-pressure zone behind the bike.

Problems never end. Fix one and discover three new ones. That’s life.

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