How Structural Stiffness Has Given Us More Responsive Bikes

[Hugh Janus]

Modern motorcycles with nearly flex-free front ends thanks to upside-down forks held by triple clamps with large clamping surfaces. (Jeff Allen /)

This past week I was forcefully reminded of how much motorcycle structure has changed during my time in the sport when a friend arrived one afternoon on a new 790 KTM. As I used to do when I was a tech inspector 30 years ago, I took that bike’s front wheel between my knees, grasped the bars, and tried to twist. 

Nothing. No perceptible movement. It was as though that motorcycle’s front end, made up of many parts clamped together by bolting, was welded into a single rigid unit.

That brought to mind words from CW’s former dirt editor, Jimmy Lewis, who described test-riding two different brands of 125 MX bike with near identical steering geometry and wheelbase. One of them responded to steering in a modern, right-now fashion, while the other seemed languid and slow, as if asking out of the corner of its mouth, “You want something, buddy?” before responding to control input.

And that in turn brought to mind a conversation with Aprilia engineers last month, in which one of them reminded me that when engines are made smooth by use of internal balance shafts, there is no longer any need for such beastly antique compromises as rubber-mounted handlebars (like having a rubber steering-shaft in a car!) and squidgy-feeling rubber-mounted footpegs. Look back in time, next time you find yourself surrounded by other people’s bikes from past eras, and see how much rubber had to be placed between the rider’s extremities and the buzzy unbalanced secondary vibration of older inline-fours or the orbital gyrations of unbalanced Vee-twins.

In 1969, we opened Arlington Motor Sports just outside of Boston, offering Kawasaki and Triumph. I had known long before from experiences with British chassis that fork stiffness was not a strong suit with Triumph. I remembered in particular a 500 twin whose bars could be twisted several degrees each way without moving the front wheel, and they would stay there when released. Stunning lack of stiffness! What it meant was that when you initiated a turn, actually creating steer-angle at the front wheel had to wait for the process of wind-up as you turned the bars, the fork tubes slipped in their crowns, and not much happened.

In 2003, I attended World Superbike at Monza, Italy, and discovered fascinating things in a shed located on the property—old gents in pale-blue Gilera factory mechanic coveralls, looking after a priceless collection of Gilera road racebikes from the 1950s.

The stiffness of a telescopic fork is strongly dependent on its tube diameter, which is why today the larger female tubes are at the top, where the greatest bending moment (leverage) exists, and the smaller tubes that slide within them connect to the axle below.

Riders complain when bikes don’t suit them, so fork tube diameter has steadily increased from the 32mm upper-inner tubes of the 500cc four Gileras I saw that day. Those bikes were having to compete with the factory Norton singles ridden by the late Geoff Duke, built into the McCandless brothers’ revolutionary “Featherbed” twin-loop chassis. Despite being seriously down on power in comparison with the higher-revving Gileras, Duke missed being champion by only one point in 1950, winning three of the six GPs that year, then thrashed the Gileras in 1951, 35 points to 31, again winning half the races.

Multiple pinch-bolts and large clamping surfaces on large-diameter female fork legs resist the twisting forces input by the rider and road. (KTM/)

The Italians rightly concluded that something other than raw horsepower was necessary to win championships, and set about “Norton-izing” their chassis (they also took care to hire Geoff Duke). To remove “Triumph twist” from their floppy telescopic fork, they built upper and lower fork crowns into a single rigid box, welded from sheet metal. Since then, fork tube diameter and the rigidity with which the tubes are held in the fork crowns have increased steadily. Have a look at where the lower fork crowns of big sportbikes grasp the tubes and you will find as many as three pinch-bolts per side, instead of the feeble single bolt of early designs.

When I was a lowly student I once took the bus to go visit my parents. After gliding smoothly along expressways, the bus turned onto secondary roads as we neared my destination. At each pothole or pavement transition we hit, the chassis of that bus responded with a harsh “wubba-wubba” vibration that really held my attention. Some 20 years later, encountering a Corvette engineer at Daytona, I asked him about wubba-wubba.

“What you were feeling, we call the corner frequency. When a wheel attached to a flexible chassis hits a bump, the suspension is only part of the motion that results. The rest of it is deflection of the frame at that corner, which because the frame is a spring without a damper continues as the vibration you felt on that bus. It has a specific frequency because the mass in motion and the spring constant of the frame combine to create it.”

Robin Tuluie, the motorcycle enthusiast and builder whose analytical interests took him to the heights of F1 engineering, once showed me some of the vibratory modes of a motorcycle that was set on a two-post shaker at MTS Corp., near Minneapolis. As he excited the bike with a frequency sweep, first one and then another mode would appear, the affected parts moving so rapidly that each in turn became a blur.

Modern motorcycles like my friend’s new 790 have right-now responsive handling because riders never stop complaining and engineers never stop trying to shut them up with better and better solutions, from Gilera in 1950 to the present day 70 years later.

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