Hollow Connecting Rods

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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/)

We all know that a rod and a tube, each having the same weight per foot, have similar tensile strength, or resistance to being pulled apart. But the two forms have very different abilities to resist compressive buckling or bending. Here the tube wins; by moving its material away from its centerline, it gains increased “leverage” to resist that buckling or bending.

In today’s familiar auto and motorcycle engines, connecting rods are engineered to resist that same bending and compressive buckling in a simpler way: by giving the rod’s shank an I-beam section. This is the preferred shape because it is easily cast or forged in a two-piece die.

Typical connecting rods use an I-beam design to resist bending and buckling. (KTM/)

The Wright Brothers’ Hollow Con-rods

But the I beam is not the only possible solution. Have a look at the Wright Brothers’ post-1903 upright engines and you will see structurally elegant con-rods with hollow, tubular shanks made by joining each rod’s big and small ends with a brazed-in-place length of tubing. This made intuitive sense: As the rod’s big end whirls in a circle with its crankpin and the small end moves up and down with the piston, the rod’s shank is subjected to a bending force as it swings from side to side.

When I asked Honda engineer Yuzuru Ishikawa, large project leader for the 2020 CBR1000RR Fireblade, why he gave its connecting rods a broad shanked design like an F1 engine’s, he demonstrated with his hands and said, “To stop them from flopping.”

In other words, he was increasing their resistance to lateral bending. Why? The driving force behind that model was not so much market demand as a desire to win World Superbike races, an application where the necessary 16,000 peak rpm definitely required what we may call “flop resistance.”

In the interwar years, hollow connecting rods for racing use were made by forging them in one piece and then drilling the shank hollow from below, an improvement over the Wrights’ technique. Although long used in the Offenhauser engines of US Champ Car racing, such hollow rods disappeared when it proved difficult to eliminate machining marks as stress raisers inside the shank.

New Questions, Old Answers

When a new need appears, rediscovering an old idea may provide the solution. Honda, during the 19,000-rpm era of F1 racing, began to notice that engine valves were hitting the edges of the clearance cuts machined into the piston crowns. Further investigation showed the cause was neither inaccurate machining nor side-to-side vibratory flexing of the valves. That left the strange conclusion that the pistons were somehow rotating back and forth (in the neighborhood of five degrees) around each cylinder axis. Bizarre!

It turns out that the very same problem occurred decades earlier when Allison, in Indianapolis, was developing its V-1710 engine, a liquid-cooled V-12 that would power such World War II aircraft as the P-38, P-39, P-40, and early P-51.

That engine’s crankshaft was quite long, making it especially vulnerable to torsional (twisting) vibration. Its length resulted from the crank’s six crankpins and seven main bearings. When in early development the engineering team saw some connecting rods fractured up near the small end, and they realized that the crankshaft’s torsional vibration was causing the crankpins to cyclically go in and out of parallel with the crank centerline. This transmitted into the con-rods as a rapid back-and-forth twisting. When engine speed coincided with the natural frequency of the piston’s oscillating back and forth atop the rod, the amount of elastic twist in the rod could build up to several degrees—enough to quickly fatigue and break the rod shanks.

Allison’s response was to stiffen the rod shanks just enough to prevent their twist frequency from coinciding with crankshaft torsional motion—problem solved.

Honda’s Hollow Rectangular Rods

Because Honda’s V-10 F1 engine had a much wider rpm range, its problem was less easily sorted out. It decided to stiffen the con-rod shanks changing their form, fabricating them as hollow rectangular boxes.

It’s easy to see how they could machine a titanium billet or forging to give them three sides of the box. But how to attach the fourth side? Welding was not a candidate; con-rods in a 19,000-rpm V-10 experience too many fatigue cycles per second. So Honda decided to use diffusion bonding. In a vacuum and at a controlled high temperature, precisely prepared surfaces are pressed together at extreme pressure. Under the right conditions, increased atomic mobility allows atoms from the two originally separate parts to interdiffuse and gradually become one. This is the same method employed to fabricate the wing carry-through structure of the Rockwell B-1B bomber. Diffusion bonding can even be used to join dissimilar materials!

Evidently using this process allowed for a con-rod interior-cavity surface adequately free of stress raisers. Result: a rod shank too stiff to allow resonant buildup of piston rotational oscillation. Once again, problem solved.

As I’ve observed before, sometimes the most difficult engineering problems are actually social. For example, F1 engineering is a perilous business because, as in most other forms of racing, management’s most prized asset is parity, or performance equality between the teams. Sanctioning bodies are determined to keep any one team from dominating the results and turning the race into a boring, repetitive spectacle. This produces a steady stream of fresh bans on this, that, and the other technology. Under one such rule, the concept of a hollow con-rod shank was limited to a maximum diameter of a single 2.0mm (0.078 inch) oil hole, drilled from the big end’s bearing shell up to lubricate the wrist pin.

That’s racing.

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