Dear Reader,
Did you know that once people thought that the world was flat?
And then people thought that the universe revolved around the Earth.
Then people thought that the Earth revolved around the Sun.
And I now know a group of people who now think the world is centred on the Flat Salt of Bonneville…
………………………………….its all a matter of perspective.
In a similar vein explorers can find numerous explanations that describe the forces transmitted through a motorbikes rear swing arm. And there are twice as many descriptions for the optimal setup. None are particularly wrong, what is important is deciding what is right for your bike. Here is the Editors choice.
On the flat lands of Bonneville, high speed stability is essential. There are no discernable bumps, there is no need to achieve tight cornering trajectories, or achieve aerobatic type manoeuvres. The suspension is limited to +/- 1 inch travel and designed so the pilot does not lose either his eyesight or too many of his fillings during the runs. The layout of the bike favours a very long, nearly horizontal swing arm. And the acceleration will not make you lose your grip with the handle bars, nor the deceleration make your eyes pop out, but both of these forces are continuous and belies the energy dissipated by the bike in the quest for ultimate speed. Ie Stability and Safety are key.
So what are the forces that will be transmitted through the rear swing arm, and how will these affect the bikes performance?
As hinted at in the introduction there are numerous ways that these loads have been viewed. None are particularly wrong, but some more relevant than others for the type of bike and riding style being considered. So viewed, as if standing on the swing arms centre of rotation:
First let us understand where these loads are coming from: Put simply….
Gravity
Everyone should recognise this one. The reaction is vertically up from the centre of tyre contact. Viewed from the pivot of the swing arm this is an upward force.
Weight transfer due to acceleration, (or deceleration)
Again, most bike riders will recognise the effect of rapid acceleration, (or deceleration) on their bike, causing an additional downward load on the rear wheel during acceleration (with lightening of the front). And similarly when braking, front wheel dive (with lightening of the rear). This force acts through the bikes centre of gravity, (you know the old f=m.a equation), and produces a couple which reacts with the ground through the swing arm. These are all vertical forces acting again through centre of the tyres contact patch with the ground.
Additional loads caused by aerodynamic effects
Less obvious and only really relevant in high speed operation. Put crudely think of the largest, or fattest vertical section of your bike. Think now of the aerodynamic load acting through its geometric centre. This acts in much the same way as the acceleration forces through the bikes Centre of Gravity; with increasing speed it increases the vertical load on the rear wheel. For further info Silver and Gold members can read Swing Arms 2.
Now lets think of the additional loads and forces the bike generates.
