The Evolution Of Rear Suspension

[Hugh Janus]

That’s the swingarm pivot at the top, with the axle holders at the bottom. Each beam of this Suter arm (provided by Graves Motorsports) is vertically deep to provide stiffness against twist. But each beam is thin to provide some flex that can keep the tire hooked up midcorner. The shape is ­organic to prevent cracking. (Jeff Allen /)

The first motorcycles had no formal suspension, front or rear. ­Suspension allows the wheel to independently move up and down over bumps, supporting the chassis and rider through a spring or other elastic element without transmitting all its motions to the chassis.

Front suspension came first out of necessity—to prevent larger bumps from wrenching the bars out of the rider’s hands. The rear wheel, passively following the front, could get by as on any bicycle—by being rigidly attached to the frame.

RELATED: Suspension Setup Guide

Because rigid frames give a hard ride, two solutions beckoned. One was easy: to support the rider’s seat on springs. The other—to free the rear wheel to move up and down over bumps—proved difficult.

Many rear-suspension schemes were tried, but as the bicycle’s original “diamond” frame was already too limber for the drive-chain pull forces of the larger early engines, adding even more flex in the form of a rear suspension produced wobbling and could even run the drive chain off its sprockets.

Sign up here to receive our newsletters. Get the latest in motorcycle reviews, tests, and industry news, subscribe here for our YouTube channel.

As always happens in the early days of a technology, there were plenty of ideas. In 1913, Indian offered a swingarm rear suspension, controlled by quarter-elliptic leaf springs above. Because it was poorly supported at its pivot, it was too flexible laterally and didn’t sell.

In 1919, the British Coulson-B motorcycle offered a rear suspension consisting of short trailing links attaching to pivots where a bicycle’s rear axle would normally be. It improved comfort, but the short links and axle bolted to them lacked twist rigidity, letting the wheel tilt in the frame, sapping rider confidence.

Engines were gaining power ­rapidly now, and the effects of bump impacts grew as the square of the bike’s speed.

The two Guzzi brothers, Carlo and Giuseppe, responded in 1928 with a true swingarm: two beams that went forward from the rear axle to a connecting tube ahead of the tire, braced beneath against twist as modern swingarms are, and mounted to pivot between plates that bolted to the back of the engine. This was first offered on Guzzi’s 497cc Gran Turismo production bike.

When we see techy looking items like this KTM 1290 Super Adventure R swingarm, we can’t know for sure how much is styling exercise and how much is backed by dynamic FEA analysis and hours of prototype testing. Probably both here. (Jeff Allen /)

In 1929, young Phil Vincent—having bought the failed HRD ­company—began to build bikes with a top-braced triangulated swingarm rear suspension.

An alternate approach to the problem was the sliding pillar, or “plunger,” rear suspension. Sprung axle carriers moved up and down on tubular guides mounted at the rear of the frame, one per side. This system provided limited travel but was widely used between 1930 and 1950.

Throughout this time, veteran riders continued to assert that “nothing steers like a rigid,” and sadly, they were right.

The crucial moment came in June 1935, when Stanley Woods won the Senior TT at the Isle of Man on a Guzzi with swingarm suspension. This couldn’t be ignored—even Norton race boss Joe Craig stated publicly that Woods’ stunning last lap could not have been achieved on a rigid. The following year, Velocette put three-piece swingarms (two beams splined to a single, pivoted cross-tube at the front) on its factory TT bikes.

Here’s a single-sided swing-arm executed by BST in super-stiff carbon fiber. Bearings for the stub axle fit inside the big ring. (Jeff Allen /)

Such simple swingarms, even when welded together rather than keyed or splined, remained less stiff than frames as a whole. Yet when strongly supported between the two frame loops of the 1950 Norton “Featherbed” frame, they worked better than anything previous. It was clear that the swingarm, supporting the bike through twin oil-damped rear shocks, was the future.

The twist-resistant ­triangular bracing of Vincent’s original ­swingarm returned as Yamaha’s “Monoshock” rear suspension of 1973. Twin shocks were out; single-shock suspension was in. As roadracing bikes passed the 100 hp level, sanctioning bodies acknowledged the continuing weakness of stock three-tube swingarms by allowing them to be “modified or replaced.” On racing and sportbikes, three-tube construction was displaced by bridgelike structures or by welded box fabrications. ­Single-sided swingarms of large cross-section appeared.

Composite prototype swingarm from BMW is meant to show what’s possible. Not on a production bike yet, but... (Jeff Allen /)

Direct connection to a single spring/damper suspension unit was replaced by linkages making possible any desired spring-and-damping curve. The high multiplied forces in such linkages now had to be carried on rolling bearings to preserve smooth action.

In the new century, swingarms came to resemble aluminum tents. Side beams grew in height and shrank in lateral thickness, combining high twist resistance with lateral flexibility that allowed such arms to act as “sideways suspension” at high lean angle.

As lateral flexibility was increased, the by-now-usual aluminum material began to crack, in some cases forcing designers to experiment with more-fatigue-tolerant carbon fiber. A practical limit to side flex was reached in 2018, when Honda’s MotoGP bikes occasionally ran their chains off.

Each new solution reveals fresh problems. What next? Could laterally flexible wheels supplement the suspension action of tire sidewalls? Is there a role here for sensors and electronics to keep chain sprockets aligned? Development never ends. n

Source