PS reader question about motorcycle technology

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PS reader question about motorcycle technology
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PS reader question about motorcycle technology

PS reader question about motorcycle technology
Beam or banana swing arm?

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What exactly is the difference between a beam swing arm and a banana swing arm? And what are the advantages and disadvantages of each type? Motorcycle whisperer Werner “Mini” Koch explains.

Werner Koch

06/16/2016


PS reader question about motorcycle technology


fact

The voluminous front silencer of the Yamaha YZF-R1 leaves no space for a beam, which is why you put the stabilizing triangle on top.

PS reader question from Hans Bernhardt: My technical question relates to the swingarm of superbikes. I noticed that all the machines in Moto2 and MotoGP have deep-drawn arms. This swingarm is also used in the Superbike World Championship, with the exception of the Ducati Panigale. My Yamaha YZF-R1 RN22 and its predecessors also have a swing arm with a support. The current R1 has a so-called banana swing again. What are the advantages or disadvantages of both types? Or is that just a question of cost?

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PS answer: Whether the rear swing arms carry the stabilizing triangle above or below is almost irrelevant in terms of cost and weight. However, there are several reasons why designers are currently focusing more and more on the “underlay”, especially in racing.


PS reader question about motorcycle technology


2snap

With the old RN 22, there was enough space thanks to the underseat silencers.

On the one hand, the free space under the bench is getting smaller and smaller because the MotoGP teams are increasingly stashing fuel there. On the other hand, the lever for the suspension strut can be set far down on the beam swing arm. This also lowers the articulation point for the upper suspension strut eye, a circumstance that enables a more rigid and deeper connection to the frame because the forces on the suspension strut are introduced into the frame in the area of ​​the rigid engine suspension.

The great advantage of both designs is the good flexural rigidity combined with low weight due to the large space in the area of ​​the strut deflection. This is where the highest bending forces occur and are absorbed by the massive but light upper or lower beams.

Space-creating banana swing

There are also advantages in terms of flexural rigidity and the desired flex of the rocker arm. Due to the bending process, the upper parts are compressed under high load; if they run below the swing line, tensile forces act on the material. These tensile forces are easier to calculate than compressive forces and allow a defined deformation, which can play a major role in the increasingly important flexibility of the chassis components. Because in a fully inclined position, i.e. more than 60 degrees with the Moto2 and MotoGP machines, a shock acts more in the transverse direction to the swing arm than in the originally intended spring direction to the motorcycle vertical axis. And it is precisely at this moment that the tire sidewall and the swingarms have to step in as spring elements.


PS reader question about motorcycle technology


Kirn

In two-stroke engines, the one-sided curved banan rocker provided the necessary installation space.

In the case of series machines, on the other hand, the space available clearly plays the most important role, since, for example, many athletes are currently no longer able to comply with the noise regulations without a front silencer. The larger the front silencer, the slimmer the tailpipes. So you try to stow the large front silencer between the oil pan and the rear wheel. This can usually no longer be done with a beam.

The only task of the banana wings was to create enough space for the side exhaust systems. The four-cylinder two-stroke engines in particular, with their voluminous exhaust pipes, relied on the upwardly curved swing arm bars, but the asymmetrically curved swing arm also provided four-stroke engines like the Yamaha YZF-R6 with sufficient lean angle without restricting the exhaust gas flow.

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