The Motorcycle Is A Collection Of Springs

[Hugh Janus]

Springs, glorious springs! The spring in my grip exerciser broke in fewer than 15,000 cycles, but 15,000 highway miles can flex a bike’s valve springs a million times. Quality must match the application. (Jeff Allen /)

The word “spring” usually applies to an intentionally created elastic element, such as the helical coil springs used in vehicle suspension or engine valve trains. It can also apply to the compressibility of gases, as in the pneumatic valve springs used in MotoGP, or in the MX “air suspensions” that flower every few years. Rubber in torsion was a common form of spring in 1960s British motocross bikes. Spring can also describe ­structures never intended to be flexible, as when former Ducati engineer ­Corrado Cecchinelli explained to me that certain riders mistakenly feel flex in footpeg brackets as loss of tire grip.

Structures acting as springs became a big subject during the decades when chatter set the upper limit of chassis performance. I was shown a Yamaha 750 Superbike on a two-post hydraulic shaker rig as the operator Rob Tuluie (Makers, Issue 3, 2019, CW) made a frequency sweep. At around 22 Hz the front wheel essentially disappeared, so large and rapid was the fore-and-aft whipping motion excited in the springy fork tubes and steering head. When in 1993 Yamaha again stiffened the chassis of its two-stroke YZR500 GP bike, rider Wayne Rainey characterized the result as “chatter, hop, and skating.” Since that time, intensive work has gone into making chassis and swingarms into functional suspension springs, their deliberately ­created lateral flexibility improving the ability of the tires to follow irregular pavement at high lean angle, rather than skip rigidly from crest to crest, losing grip each time.

Conventional steel-coil springs only seem straightforward. When former US Honda racing chief Gary Mathers actually measured the spring rates (rate is measured as pounds of load required to produce 1 inch of spring compression) of the race team’s color-coded suspension springs, he found wide deviations from nominal values.

During the 1920s, the quality of steel spring wire in engine valve springs was so far behind the vigor of valve train dynamics that the best protection against spring breakage was statistical—to provide multiple springs per valve (as many as 16) in the hope that not all would break. Velocette engineer ­Eugene Goodman used a strobe light to reveal spring motions. The sudden acceleration as valve lift begins can generate a Slinky-like wave that bounces many times between the spring’s ends at every valve event. The result is abnormally fast accumulation of stress cycles—and early fatigue failure. The hard vibration of my 1973 Kawasaki H2-R excited similar wave action in its suspension springs, causing one to break at Laguna that year. In valve trains, three cures were developed:

1. Coil-to-coil friction between nested spring pairs damped out wave action.
2. Progressive-wound or conical “beehive” spring shapes outfoxed wave action by having no single resonant frequency.
3. Fatigue properties of spring wire were hugely improved by techniques such as vacuum remelting (to evaporate oxide contaminants) and surface compression by means of shot-blasting.

Because steel is heavy (density 7.8), designers sometimes turn to titanium (density 4.5) wire to reduce spring weight. Not so fast! Because spring winding damages the titanium wire’s surface, this damaged layer must be removed by acid etching to prevent early failure.

An Öhlins shock spring, made in a wide range of stiffness, measured in pounds-per-inch or kilograms-­per-millimeter. Which one is just right? (Jeff Allen /)

Because in high-rpm engines the flexibility of parts can lead to cam and crankshaft windup and valve bounce, it has become ­customary to consider the flexibility and ­natural frequencies of all highly stressed parts. As a test, I once assembled a race engine with a very close squish clearance (piston-­to-head) of 0.018 inches and was rewarded upon teardown after operation by the appearance of bright zones on the piston crowns, indicating that they had touched the head at peak revs. At such stress levels, every part—including the con-rods—becomes a spring.

Steering a motorcycle takes place through a stackup of springs, and the speed of response depends on their collective stiffness. I apply pressure to the bars to turn the front wheel, but because the wheel is a heavy gyroscope, it resists, forcing me to “wind up” the fork somewhat with steer torque. As the wheel begins to steer, the tread rubber rolls in a slightly different direction. That pulls sideways on the front end of the bike by first distorting the tire’s carcass to transmit the new force to the wheel rim, then deflecting the fork tubes slightly and finally the steering head. As a former Cycle World off-road editor once noted: “I tested two 125 MX bikes of different makes with identical rakes and trails, same tires—everything. They should have steered the same. But one bike had right-now steering, and the other had serious steering delay.”

This is a fork spring. Note how the end coil has been closed and ground flat to rest squarely. When you think of it, a coil spring like this is just a torsion bar rolled up into a more convenient package.­ (Jeff Allen /)

The spring stack was different in the two cases—handlebar flex, bar mounting, fork twist, tire stiffness, wheel flexibility, axle bending, fork-tube deflection, and steering-head stiffness. Add all these springs to get steering response.

Thinking of the motorcycle as a collection of springs has brought many useful insights.

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