Motorcycle Swingarms

[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/)

The rear suspension of modern motorcycles and scooters is provided by what riders call a swingarm and auto designers term a “trailing link.” The basic idea is that the rear axle attaches to a link or frame pivoting on a horizontal axis. On a motorcycle, this pivot is just behind the engine. The concept is very old; Indian offered a swingarm rear suspension as early as 1912, but it failed to catch on, surely for two major reasons:

1. US highways of the time were primitive, making rigid frames the longest lasting.
2. Not enough was yet known about how much stiffness motorcycle chassis required, so for many years old-timers would intone the mantra, “Nothing steers like a rigid.” It was <a href="https://www.cycleworld.com/stanley-woods-isle-of-man-tt-champion-5-reasons-why/">Stanley Woods</a>’ 1935 Senior TT win in the <a href="https://www.cycleworld.com/tags/isle-man/">Isle of Man</a> that proved it was time to give up rigid frames.

Swingarms and their necessary pivots caught on only slowly; after Woods’ victory on the factory Guzzi, Velocette was an early adopter. Half-measures such as sliding pillar as tried by Norton and many others, Triumph’s dreaded “jack-in-the-box” spring hub, and the short trailing links of the Coulson-B bike allowed rigid frames to continue with such afterthought suspensions tacked on.

The swingarm wasn’t always a common component of a motorcycle, despite Indian offering swingarm rear suspension as early as 1912. (KTM/)

The first swingarms were basically nothing but three pieces of pipe welded together, and desperately needed more stiffness. The swingarm-pivot bolt passed through the middle piece, and the rear axle joined the ends of the two beams.

Single-Sided Swingarms

Back in the early days of motorcycle chassis experimentation, the French fuel company Elf (and, later, Elf-Honda) created a single-sided swingarm to speed up wheel changes in endurance racing; with no axle to pull out, as a stub live axle on its own bearings projected from the single robust swingarm beam, wheel changes could be as fast as for race cars. For whatever reason, this so tickled the public fancy that both Honda and Ducati have offered production bikes with single-sided swingarms. Even the current Gold Wing, far removed from a racebike, offers the feature.

There is little point in making the chassis stiffer in search of greater stability if the swingarm, which is subject to the same stresses as the main frame, is weak. That’s why, very quickly after Superbike racing developed in the US, the AMA (at the time the sanctioning body) declared that “swingarms may be modified or replaced.”

Swingarms and Weave

Why was this? Because stock swingarms remained basically just three pieces of pipe right into the 1990s, and when bikes with such swingarms were run at high speed on big grippy tires, they became unstable. We at Cycle World had our noses rubbed in this when the trick, reinforced swingarm on our mid-’90s Daytona 200 Superbike developed a crack and had to be replaced by a borrowed stock swingarm from a generous racegoer. Our entry then weaved for the whole 200 miles, oscillation of the rear caster at a frequency of two-to-three cycles per second.

When, a few years ago, I asked former GP champion and ex-team manager Kel Carruthers about the weave of Honda’s RC161 racebike, a four-cylinder 250 from the early 1960s, he replied, “Look at how the swingarm attached to its backbone frame.” The backbone was a curved single piece of largish tubing, carrying the steering-head bearings at its forward end, projecting back, then bending down and heading vertically for Mother Earth just behind the engine. A side-to-side hole pierced the backbone at that point and a slender cross-tube was slid in, centered, and welded in place. That was the swingarm pivot! A twin-loop chassis replaced the weaving backbone type next season.

As you can imagine, that slender crosspiece flexed when the rider twisted the throttle, causing chain pull to flex the swingarm to the left, steering the rear of the bike to the right. Or, in bumpy corners, that bendy cross-tube would flex constantly, setting off the weave oscillation. Listen to those 1960s’ Isle of Man sound recordings to hear how many times a minute the bike’s rear wheels left the ground.

Many a classic design reinforced its too-flexible swingarm-pivot region with a pair of diagonal tubes running forward and upward from the pivots directly to the steering head. Two examples are the Seeley-Matchless starting in 1966 and the modified BMW twins built for racing by Udo Gietl of Butler & Smith.

Norton, in its tremendous racing history from the 1930s to the early ‘60s, long relied upon a basically single-plane chassis called “the garden gate” because, like an actual garden gate, although it was built to be rigid, it could swing to and fro. After 1935 it was given a sliding-pillar rear suspension, but it still retained its qualities of:

1. Self-steering the rear of the bike through flexure
2. Suddenly “snapping like a carrot” because many of its tubes were loaded in simple bending

Norton, Cobas, Tamburini: Breakthrough Frame

Therefore when in 1950 the Irish McCandless brothers, Rex and Cromie, both experienced racer/builders, designed an entirely new twin-loop chassis for Norton’s racing use, they placed the frame loops far enough apart at the rear to allow the swingarm to fit between them, giving the swingarm pivot the maximum resistance to flex steer. This would be one of the three most-copied motorcycle chassis ever, with the others being Antonio Cobas’ twin-aluminum-beam chassis of the 1980s and Massimo Tamburini’s “trellis” chassis made as a triangulated steel-tube truss structure.

Quite another system of locating the swingarm pivot was tried in the decade after World War II. Vincent pivoted its triangulated swingarm on the back of its engine’s crankcase, and so did Guzzi for its fabled 500cc V-8 GP bike. Honda would revive the idea at the turn of the century as its “pivotless frame.”

Swingarm Bushings and Bearings

At first, swingarms pivoted on simple bushings. Some of those bushings were brass or bronze; Norton used bonded rubber “Metalastic” bushes; and Yamaha production racers arrived with pinkish phenolic plastic swingarm bushes that looked as if made from compressed peach pits.

In the 1970s, when various forms of single-shock rear suspension replaced twin-shock designs, the former’s very high leverage ratios, with or without linkage, exerted pivot forces that caused simple bushings to stick-slip, interfering with smooth suspension action. High-capacity rolling bearings replaced the bushings.

Those leveraged high forces could also deform structures. In one case, the anchorage of a single shock to a robust-looking frame cross member above the swingarm pivot flexed it enough to pull the frame’s uprights inward, pinching the swingarm between them enough to cause jerky motion. (The uprights are the parts of a dual-beam chassis that turn downward behind the engine and are pierced to carry the swingarm pivot shaft.)

Because the motorcycle’s basic design has changed slowly over years, it’s tempting when adding something new, such as single-shock rear suspension, to assume that the basic structure will remain adequate. No! New loads and new structures require a fresh stress analysis.

Squat and Swingarm Angle

During the many years when a 40 hp motorcycle was considered powerful, the details of rear suspension made little difference. Bikes worked OK with swingarms that drooped a bit to the rear, or were horizontal, or that even exhibited negative droop. But when the first 100 hp racebikes arrived in the early 1970s there was trouble. As anyone can see in daily traffic, when cars accelerate they transfer weight from front to rear, causing the front to rise and the rear to squat. A motorcycle’s short wheelbase intensifies this, and is complicated by other forces generated by chain drive to the rear wheel. Because the top run of the drive chain is at an angle to the central plane of the swingarm, chain tension generates a downward tangent force, tending to oppose squat.

Until someone went through the arithmetic to resolve these forces, strange things happened when riders throttled up to exit corners. Worst was squat, pulling weight off the front until it began to push. Alternatively, with a higher swingarm-pivot position, chain force could cause the opposite—extension of the rear suspension until it topped. We now know that many clever people got to work on this problem, among them dirt-tracker Mert Lawwill, and for pavement applications the late Warren Willing. For others, rear-suspension weirdness was just a mystery, and their “solution” was to immobilize the rear suspension with a very stiff rear spring. That sacrificed mechanical grip (the stiffer the suspension, the less well it stays in contact with pavement irregularities), but with enough stiffness the bike no longer headed for the outside when the rider twisted the throttle.

Öhlins internally circulated its analysis of this problem, and Australian Greg McDonald offered a commercial solution via his aviation-inspired Computrack teletheodolite chassis-measuring scheme. American Honda, during its time of AMA Superbike dominance, had its own in-house Computrack setup. More than one Japanese sportbike maker began to offer its wares with adjustable swingarm-pivot height. It took time for manufacturers and race paddocks to digest the new analysis, but the benefit to riders has been rear suspensions that continue to function in midcorner, suffering neither squat-and-push nor topping.

Source

[Buckster]

Does @Slowlycatchymonkey write this shit or what?