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  s 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  s 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 click on the advanced nearly secret  information link.

Now lets think of the additional loads and forces the bike generates.

Chain loading

The bikes engine produces (hopefully), enormous amounts of power that is translated into a pulling force on the tight-side of the chain, which drives the rear wheel.  This force acts through the centre line of the tight side of the chain.  This direction of force is typically offset from the centreline of the swing arm, and is also not parallel with the centre line of the swing arm.  Thus depending on the geometry used, forces acting through the chain cause the end of the swing arm to either rise or fall relative to its centre of rotation.

Chain pull typically causes an upward force on the wheel end of the swing arm, relative to its centre of rotation causing the squat of the rear wheel under acceleration. (ie chain is above the arms pivot point)

But if the centre of swing arm rotation is above the chain pull force,  it can also cause a downward force on the end of the swing arm that extends the rear wheel downwards that would cause the rear of the bike to lift.

If the chain force acts approximately through the centre of rotation of the swing arm, and the angular difference between the centre line of the swing arm, and the chain is slight, there is little net upward or downward force!

Tyre forces

Now let  s consider the forces generated by the tyre.  The tyre on contact with the ground generates a horizontal force.  This acts through its hub pushing the bike forward. (or restraining it when the brakes are applied).

The best analogy that I  ve come across is to think of the swing arm similar to a ladder resting against a wall.  If the ladder is at 45 degrees.  Then a horizontal push, towards the wall at the bottom of the ladder would cause the wall end to go up.  Relative to you on top of the ladder, (or at the centre of rotation of the swing arm), the bottom of the ladder has been extended downwards, (causing the rear of the bike to lift).

If the ladder was below horizontal, a horizontal push towards the wall would cause the wall end of the ladder to go down.  Relative the wall end, or swing arm pivot the rear goes up causing the bike to squat.

There.  All we need to do now is add up all these forces to obtain the net force on the swing arm for any instant in the bikes run.

Then we need to consider some dynamic loadings, and put all these loads together to see their effect on the bike, and the selection of other suspension components to suit the bikes intended performance.

……. For the second exciting instalment….members only…. click here.