custom plunger framed wtf?!

Another dumb idea was the rediscovery of the plunger-style frame in the late '60s. The plunger frame was a detour on the road between the rigid frame of the pre-WWII era and the twin shock swingarm frame that replaced it. In the plunger design, there is no pivoting rear fork. Instead, spring boxes are positioned in stirrups welded to the rear frame loop. The rear axle is carried in an eyelet mounted in the middle of the spring box. This arrangement allows the rear wheel to move a limited distance in each direction, which provides some rear suspension. The manufacturers liked the plunger design because it could be readily-read that as cheaply-adapted to existing stocks of rigid frames.

The problem was that the plunger was a flawed design right from the start. First, the basic design destroyed what had been the rigid frame's strong point, namely freedom from flex. Second, it allowed the rear wheel to twist sideways as it rose and fell. And third, because the rear wheel traveled in a straight line rather than in an arc, as it does with a swinging arm, the chain tension varied greatly at the limits of its travel. After a few short years the OEM manufacturers discarded the plunger idea in favor of the far superior swingarm. Why some customizers thought the plunger idea was worth re-exploring a few decades later has always mystified me.

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