Wire-Spoke Wheel Evolution

[Admin]

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/)

Today we think of wire-spoked motorcycle wheels on streetbikes as mainly a customizer’s theme concept, useful for invoking our feelings for the graceful past (i.e., before the present cast-wheel orthodoxy).

I recently received a copy of a 1923 SAE paper titled simply “Wire Wheels,” which spells out some of the developmental difficulties that had to be overcome before wire wheels for automobiles could beat out the competition from wooden-spoked “artillery wheels” and the more modern pressed-steel “disc wheel.” At the time, wire wheels were well established as correct for bicycles because of their lightness and durability—at least with the light loads bicycles carried. The paper’s author, W.W. Davison, had an axe to grind, as he was factory manager for the Buffalo-based Wire Wheel Corporation of America.

The Beginning of Spoked Wheels

The first wire wheels for cars failed because their design was inappropriate. Their spokes were of the simplest construction, still found on many bicycles today, and the wheels failed under the much greater weight of autos. The end of the spoke that laces through the wheel hub flange has a button head to prevent it being pulled through the hole in the flange. The other end is threaded for a nipple (usually brass to prevent its rusting solid to the spoke), which secures the spoke in the wheel rim.

Related: Why The Wire-Spoke Wheel Remains In Use

In automobile wheels, such unsophisticated spokes quickly failed, breaking in the places made familiar to us from fastener breakage in motorcycle applications. Parts break where stress is concentrated, such as at sudden changes of cross-section or at notches. In threaded fasteners this is most often at the root of the last thread. Early wire auto wheel spokes broke either there or at the button head, where the small diameter of the spoke became the much larger button.

Butted Spokes

The proven way to prevent this is to “butt” the spoke; to enlarge the diameter of the spoke in the regions where the failures occurred. Why not enlarge the whole spoke end to end? There are two reasons:

1. All wheels flex, including the carbon fiber wheels now being made for motorcycles. The larger in diameter the whole spoke becomes, the greater the stress in the spoke as this flex occurs.
2. The larger the diameter of the spoke, the more tension it takes to elastically stretch it. Some stretch is essential, for without it, when stress is applied to the wheel, spokes will loosen, allowing them to “work” in the spoke flange and where the spoke nipple bears against the inside of the rim. Better to have enough stretch in all spokes that the stresses of normal operation cannot make any of them loose.

The paper’s author describes building a wheel-testing machine capable of applying repeated torques through the wheel, equivalent to those occurring in vehicle starting. They found that with the best spokes then in production, spokes began to break after just 6,000 torque cycles.

An illustration shows the stages of spoke improvement that followed—from the plain spoke with button end to a double-butted spoke. These were able to survive as many as 100,000 cycles before failures appeared.

As there was still cracking and failure where the inner butted part of the spoke swelled quickly to form the button head, they put in its place a longer, hornlike taper (they called this a “bung head”) as a means of avoiding the stress concentration at the button. With a 25-degree bung-head spoke, up to a million and a half cycles could be endured. Later and even longer-lived designs continued the taper almost to mid-spoke.

Spoked wheels can still be used on high-performance motorcycles; the MV Agusta Dragster RR SCS has 140 hp and weighs 385 pounds—much more than my 1965 Yamaha TD1-B. (MV Agusta/)

Just now I ran up to the shop to look at spokes. Those of my 1965 TD1-B are just glorified simple bicycle spokes, button-headed and not butted at either end. Why? Because on that light motorcycle they were adequate. In my spoke collection I could find only a few that were butted. That tells us how the more aerodynamic spokes of high-end racing bicycles can remain durable even when much lower spoke counts are used—by greatly reducing the concentration of stress in spokes, a single carefully designed spoke can do the work of two or three simple spokes.

Related: Why Do Some Motorcycles Have Wire-Spoke Wheels While Others Have Cast Wheels?

One quality of wire wheels was revealed to me by something that happened during the era of US factory Supersport competition. One major team encountered chatter they just could not overcome, despite careful research using one design change at a time to components such as fork tubes, steering stem, fork crowns, and so on. But when they tested with a wire-spoked front wheel, the chatter disappeared. Some unknown combination of flexibility and damping just didn’t chatter. Therefore I am careful not to regard wire spokes as obsolete.

Wheel Stiffness and Its Influence on Handling

It is also interesting that at the highest levels of motorcycle racing, riders are given a choice among cast or forged wheels of various stiffnesses. When the first carbon fiber wheels were made and tested, riders rejected them as too stiff. Up to that point, engineers had assumed that rigidity must be the major reason to use one-piece wheels. Not so!

Today a great many of the motorcycle’s parts are recognized as having influence upon handling through either their flexibility or their damping. More than one make of bike has been found to behave better in corners if engine mounting bolt tension was reduced. Why? Perhaps because lower bolt preload allowed those engine-to-chassis points to move, generating a frictional or damping force.

The last word on all this has yet to be uttered, because again and again over the years, top racing teams faced with new tires of unknown properties have been unable to make science cough up successful chassis solutions, even when assisted by high-speed computing. This remains very much the case in the present MotoGP season. It still comes down to real-world testing, supervised by people—not necessarily engineers!—with the curiosity and imagination to find patterns in masses of data.

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[Gerontious]

Love the small point that in a history of the evolution of spoked wheels. The only photo is of a Kineo wheel. A spoked wheel that is both unorthodox and unique. And that, fairly important point is never mentioned. Fitted to the MV mentioned in the article and from which bike the photo is taken. Whereas standard spoked wheels are relatively cheap to produce, the Kineo is not. Mostly because unlike all other spoked wheels it has a solid rim.  And is made specifically for tubeless tyres.