Safety Officer’s Report:                           Steve Coomer, Safety Officer

Suspension & Steering Basics, Part III

In our last issue I presented information on telescopic front forks, why they work, and how rake angle can affect their performance. This issue, I will discuss externally sprung forks, the various types that evolved, and why they are less affected by rake angle than their wet descendents. Our next issue will discuss the evolution of rear suspensions.

Externally sprung suspensions have been used since around 1908. The ‘leading link’ and ‘trailing link’ designs are the most recognizable, while the ‘sprung girder’ is the lesser known of the three. A ‘leading link’ springer is one where the axle connection is forward of the weight bearing (load) pivot point on the rocker plate. Conversely, the ‘trailing link’ has the axle located to the rear of the load pivot. Trailing link springers came in two varieties: the coil spring variety we are used to seeing on the modern springer and the leaf spring design common to the Sears, Excelsior, and Indian motorcycles of the past.

The versatility of the traditional springer has allowed it to remain popular while the trailing link version and the girder faded into history by the late 50’s. The girder front end saw a brief resurgence during the mid-to-late 60’s when it was the front end of choice on choppers. This was due to the suspension geometry remaining constant regardless of rake angle. The springs connected the moving portion of the girder to the neck post section by means of four parallel pivot arms. This maintained a uniform load on the neck bearings at all times. The springs were anchored to the upper neck section and the lower girder cross brace which allowed for uniform suspension at any neck angle. Girders were used most successfully on DKW’s of the ‘30’s, Vincents and Indians of the 40’s and early 50’s, and the Harley S-125 from  ‘47-’51. The machining and fabrication costs for the various parts signaled the death knell for this design. As girder parts became harder to find even the chopper builders moved away from the design and began experimenting with long, narrow springers.

Springers work similar to their wet cousins, albeit in a more visible manner. When you hit a bump, you can see the springs react to the loading and unloading of the axle. While all springer forks are similar, the major differences are with the mechanical linkage required for the springer to work: The rockers connect the weight bearing rear legs to the sprung front legs and establish the offset for the desired ride and steering effects. The connection points are four bolts within bronze bushings(on vintage, pre-’57 units) that allow for the rockers to transmit axle motion through the front legs to the springs. Except for replacing the bronze bushings with rocker bearings with the 1989 reintroduction of the springer, the design is essentially unchanged from the originals dating back to 1908. Early models from ‘37-’47 used a friction lock to accomplish additional damping and ride control, while a shock unit became available as well from 1946-1957.

The rear legs of a well-designed springer are elliptical in cross section and taper as you get closer to the rocker pivot. The decreasing elliptical cross section provides a uniform rigidity across the entire length of the fork leg, resulting in less flex and better neck bearing wear. This is an important contrast to most of the aftermarket springers, which are made from bar stock of uniform cross section. This uniform cross-section results in severe loading at the lower steering head bearing and neck plate due to the overly rigid lower portion of the leg length.

The front legs are connected to the springs at the top and the rockers at the bottom. They provide the suspension action and are usually round from the forging at the top to the rocker pivots at the bottom. The rear leg forging separates the spring sets allowing the lower spring set to support the weight of the bike while the upper set provides damping and rebound control. By redesigning the rockers to reposition the axle ( adjusting trail ), any rake angle can be compensated for as long as the travel of the front legs is kept parallel to the rear legs.

Regarding maintenance, it is imperative to follow the service intervals for all springer front ends. Even if you are equipped to perform the work yourself, you are better off trusting the required maintenance to a qualified mechanic. The 4 rocker bolts are all that separate you from an interesting opportunity to pole vault on your bike should any of the pivot bolts shear due to incorrect adjustment or excessive wear. Once the rear legs hit the pavement you learn to fly as your bike catapults you onto the roadway.

SHOCK! Next issue we finally get to the back of the bike and discuss the virtues of SofTail suspensions, standard swing arm with coil-over shocks, and plunger units. Until then, enjoy the ride…