The Long and Short of Connecting Rods

[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/)

Ever hear the term “rod ratio”? It’s the number you get when you divide a connecting rod’s eye-to-eye length by the engine’s stroke. The most common ratios are in the range of 2.0 to 2.2. While hardly a hot-button issue for most of us, rod ratio is still a topic of heated debate among a small group of engine builders.

Here’s why. When you consider the relationship between crankshaft rotational position and piston motion, there are two controlling variables. One is the height of the crankpin above a horizontal plane drawn through the crankshaft centerline. Its rise and fall as the crankshaft rotates is the classic smooth sine wave. If the connecting rod were infinitely long, the piston’s rise and fall in the cylinder would also be described by the classic sine wave. And piston motion in the curious “Scotch yoke” mechanism is just that; more on this shortly.

Since practical engines can’t be infinitely tall, how do we decide how long to make real connecting rods? The shorter the rod, the greater the angle through which it swings; its small end is traveling in a straight line up and down along the cylinder axis, while its big-end revolves with the crankpin. The larger this rod angle, the greater the side thrust the piston exerts against one side of the cylinder when combustion-gas pressure acts on the piston crown. In earlier times, when motorcycle engines were air-cooled and their parts ran hot, and when oils weren’t as good as they are today, too short a con-rod, operating at too large an angle and generating too much side thrust, could result in piston, ring, and cylinder scoring.

Which rod is longer? That big Pratt & Whitney R-2800 master rod tapes out at just under 18 inches and weighs over 18 pounds. Its rod ratio is 2.15, while the little forged-aluminum Briggs & Stratton part weighs just ounces, and returns a 1.94 rod ratio. (Mark Lindemann/)

But trying to reduce or eliminate side thrust by making the con-rod really long required a taller, heavier engine. Also, making the rod longer increased its weight, adding additional inertia load to the already hard-working crankpin bearing.

Result? Compromise—rod ratios between 2.0 and 2.2 proved short enough to avoid making engines ridiculously tall and rods too heavy for their bearings, yet were long enough to avoid the high side-thrust forces that scored cylinders.

Cylinder Offset

Another way to reduce friction from con-rod side thrust is to offset the cylinder in such a way as to stand the con-rod up straighter during the power stroke. This technique, originated at the very beginning of the 20th century, was later used by certain Supersport engine builders, and is now incorporated in many production engines. If your bike’s engine rotates forward (same direction as the wheels), then on the power stroke the rear cylinder wall is the thrust surface, so you would offset the cylinder a few millimeters toward the front.

Engine Height

Back in the 1920s when large air-cooled radial aircraft engines were being developed, a major concern was engine diameter, which increased frontal area and aerodynamic drag. But when the US Navy insisted that its P-1 prototype radial of 1923 be given short rods of a 1.6 ratio, the resulting engine was excessively rough-running.

Why? As the rod’s big end swings from side-to-side twice per revolution, it imposes a height variation on the piston (the farther the rod swings, the lower the piston’s position in the cylinder). This creates a secondary shaking force, and it was this secondary vibration that rattled the Navy’s P-1.

This experience caused subsequent big radials to be given larger rod ratios—for example the long-serving P&W R-2800 had a 2.15 ratio.

There are, however, many engines operating successfully with smaller rod ratios. One from the 1960s was Oldsmobile’s big-block 455, an engine which powered the Olds 442 of the “Supercar” era. Its short rods (1.65 rod ratio) allowed this otherwise large engine to fit between the suspension towers of compact autos. Since it was water-cooled and better lubricated, cylinder scoring was not a problem.

The marine engines powering container ships are already tremendously large, so to avoid losing too much valuable cargo space they are given quite short rod ratios. This presents little problem because engine speed is so low (typically 80 rpm!) and side thrust doesn’t act on the pistons but on a pressure-lubricated crosshead like that of a steam railway engine.

While waiting my turn to speak with the late John Britten, who in revolutionary fashion introduced us all to powerful twins, I watched and listened as he and the late restoration machinist Homer Knapp pulled fascinating things from their pockets and talked about them. When Knapp asked how Britten had chosen his rod ratio, he pulled out a con-rod, saying, “Here’s a rod from a Cosworth DFV engine and it has a rod ratio of 1.8. I reckoned I could do worse than start there.”

But then it turned out to be common F1 practice to use shorter rods to make engines fit between under-chassis venturi tunnels. Later, in the 20,000 rpm V-10 era, F1 rod ratios would expand to 2.5, mainly to allow clearance between piston skirts and crank counterweights in extremely short-stroke engines.

Where’s the Controversy?

Up to this point it sounds like rod ratio may be more a matter of convenience than a significant performance variable. So where’s the controversy?

Some engine builders argue that using shorter con-rods increases the time the piston remains in the vicinity of BDC (the BDC dwell time), thereby providing more time to complete the intake process.

If the con-rod were infinitely long, piston acceleration at TDC and BDC would be equal, and so would be the dwell times around TDC and BDC. But with practical rod lengths the piston’s acceleration is much greater at TDC than it is at BDC, with the result that the piston spends more time in the lower half of its cylinder than it does in the upper half. Yet if we pull out a standard table of piston travel versus crank degrees for various rod ratios, we don’t find significant differences in the amounts of time the piston spends, for example, between 20 degrees BBDC and 20 degrees ABDC. Comparing rod ratios of 1.75 and 2.25, the differences are of the order of one-tenth of 1 percent.

Because the controversy still simmers and bubbles, Circle Track magazine built two small-block V-8s—one with conventional-length rods, the second with short rods—and held other variables such as compression ratio the same between the two. In dyno testing, the long-rod engine outperformed the short-rod engine. This was attributed to the difference in con-rod side thrust. Similarly, when a friend opened the Superbike race kit he’d ordered for his Yamaha FZR750, it contained longer-than-stock con-rods.

Still, we must respect an experienced engine builder who swears by short rods for certain applications and the success that often goes with it.

The Scotch-Yoke Engine

Now, as an extreme, let us consider the venerable Scotch-yoke engine. In such a design, a single rod joins the pistons of a flat-twin whose cylinders share the same axis. At the rod’s midpoint is a block with a transverse slot in it, milled at right angles to the rod. In this block moves a rectangular slider bored to fit over the crankpin; the pistons move up and down purely as the height of the crankpin, in true sine-wave fashion, giving equal piston accelerations at top and bottom center, as well as identical dwell times at top and bottom center.

Proponents of this design believe that because its pistons remain close to TDC longer, more of the charge will burn at the highest compression ratio, thereby increasing thermal efficiency. But one can also see that spending longer near TDC might increase heat loss before piston motion can significantly expand the high-pressure gas on the power stroke. To my knowledge, no one has run back-to-back high-performance testing of similar engines, one with conventional con-rods and the other with the Scotch-yoke design. One likely reason why is that the Scotch-yoke design is not well-suited to operation at higher engine speeds.

The short-rod builders believe that with the appropriate cam timings there is a gain. We’d all like to know more.

Source

[XTreme]