Motorcycle Suspension Part 3 – Compression and Rebound

Previously, we looked at the role that springs play in motorcycle suspension.  Suspension is comprised of a spring and a shock absorber.   The shock absorber’s role is to limit the rate at which the spring compresses and expands.
Ideally, a motorcycle wheel should not lose traction with the road surface.  On a perfectly smooth surface, this is not hard to achieve!  For the wheel to grip properly, it must be pressed to the surface with some degree of force.  Luckily for us, the weight of the motorcycle provides us with this downward pressure.
Suppose for a moment we have a motorcycle without any form of suspension:  Not only is there a lack of springs and shock absorbers, but the tyres themselves provide no flex.   Once we are moving forward, the motorcycle has inertia in this forward direction.

Bike moving forward

When this motorcycle hits a bump, there is no way of “absorbing” the bump.  As a result, some of the horizontal forward inertia changes to vertical inertia.

Bike moving over bump

Once the bump “ends”, the ground “falls away”.  The motorcycle being subject to gravity loses its vertical inertia and returns to the ground.  The vertical velocity is not overcome instantaneously. Rather, it is subject to the laws of gravity.  Depending on the speed of the motorcycle and the size of the bump, the wheel(s) will remain in the air for some period of time.
With the benefit of suspension, not all of the motorcycle needs to be subjected to the vertical inertia caused by the bump.  If the bump is small, the springs may absorb the bump.  As we saw previously, the spring will “want” to return to its state of rest.  The spring will expand in whatever direction cannot suppress the force that the compressed spring now has.  If the bump has ended, the spring will likely expand downward, otherwise the spring will expand upward.  The spring extending upwards is effectively transferring the vertical inertia to the rest of the motorcycle.
As noted previously, the spring continues to oscillate shorter and longer of its “at-rest” state, until all its kinetic energy is expended.  The suspension unit helps regulate the extending and compressing of the spring, by forcing oil to pass from one area of the suspension unit to another.  The oil can only pass through small valve holes.  Because a liquid cannot be compressed, the oil passes through the valves at a set rate.
Adjustable suspension allows the size of these holes to be altered, allowing a faster or slower rate of oil transfer.  Different systems handle the transference of oil in different directions.  That is, when the shock absorber is being compressed, oil is transferred by one set of valving, and when the suspension expands (or rebounds) a different set of valving operates.  The first set of adjustable valving gives you the “compression” setting, and the return valves gives you the “rebound” setting.
In recent years, suspension has been further refined by having two lots of compression valves.  These are known as “high speed” and “low speed” compression valves.  This “speed” refers to the speed that the suspension compresses.  Fast compression is needed to handle surface irregularities (i.e. bumps) whereas “handling issues” arise from slow compression motion.  Unfortunately, I have not had the chance to study suspension with this feature, so how the shock absorption works and which valving is used is as much a mystery to me as the next person.  (Unless they happen to work for Ohlins).
As you may well be aware, oil viscosity affects its flow rates.  A more viscous oil will therefore be slower to transfer through the valves than a “light” oil.  Oil has a tendency to become less viscous as it heats up.  Forcing oil through small valves tends to lead to the oil heating up.  (The kinetic energy of the oil movement is exchanged as heat, due to the friction caused by the fact that the oil cannot move freely.   This of course affects the compression and rebound rates of the suspension.  The bumpier the surface, the more this becomes an issue.  This is an unavoidable fact, but that does not make it desirable!

To combat this, suspension units have been designed to carry as much oil as possible.  More oil, means more oil to heat, meaning more energy required to do so.  A bigger oil reservoir also means a larger surface area in contact with the oil, drawing excess heat out of the oil.

That is about it for how the parts of suspension fit together.  How to make the most of the suspension you have, is a story for another day.

Photo attribution: courtesy of Nathan Wells.