Motorcycle 5

A motorbike frame is, in it's simplest form, a way of connecting the steering head in the front with the rear wheel. The steering head is described as the part that accomodates the front wheel steering and all it's relevant forces. To test the stability of a frame, one could, e.g., attempt to twist the front wheel out of alignment compared with the rear wheel, the same as one does with motor cars. The values would be shown in Newton per degree (°). So, the motorbike frame must be resistant against buckling. Of course, it must also absorb all the other forces working on the front- and rear wheel as well. If it is also able to accomodate the engine, the tank and the seat, one could be content with it.

The impact from uneven parts of the road, should as much as possible, be filtered from the rider and passenger. This is why the front fork is fitted with shock absorbers. For a long time in the past, most motorbikes had a rigid frame (see picture 1) which held the rear wheel and well-sprung seats took care of the rest. Later they were tube-guided (see picture 2) and still later, the whole swinging arm system was given shock absorbers (see picture 3). This was sometimes done by two rigid trailing arms linked with each other (see picture 4) or also by having only one swinging arm (see picture 5), indeed, this was generally somewhat more massive. Of course, in the event of having to remove the rear wheel, the latter proved to be a great advantage.

Staying with the subject of the actual frame, the problem which then arises, is the supporting the rear suspension. In the picture you can see the support against the middle part of the frame, this however, requires a solid construction, which includes at least the seat. Someway or another, it also has to continue to the tail-light and number-plate. After the swinging arm bearings at the bottom rear, the upper- and lower parts of the machine can no longer support each other. In comparison with the unsprung version, the weight, first of all increases.

The single-swinging arm is of course, generally heavier than the twin-arms. This is why the latter is preferred for racing machines. Here you can see a good example of how the swinging arm is connected to the frame and leads directly to the steering head. Distinctive twin-arms also offer the possibility of using a single, central shock absorber (see picture).

Should the bike be designed to carry two people, this area has to be particularly stable, unless the pillion seat is unsprung and placed directly over the rear wheel. This wasn't at all unusual back in the old days. At the rear, the upward sloping and extended part of the frame is strained even more when an additional topcase is mounted.

It was much more simple for the motorcycle constructors in the past (see picture). The frames and the pivot construction of the swinging arm was pretty much the same, regardless of of the manufacturer. The follow-up to the above motorcycle, which can be seen below, presented a particular challenge. In this case, the weight of the machine had to be kept under a certain limit, although there were even special training programms to learn how to pick the monster up if it should fall over. 12/13