Almost every vehicle faster than a trotting horse is
equipped with some form of suspension. Many have been
developed but all share a common feature, the wheels
are
able to move independently of the rest of the vehicle. To
achieve this trick more than once, every system so far
has
employed
a spring. Wheel hits bump, wheel moves up,
compressing spring. Wheel goes over bump, energy
stored
in spring returns wheel to original position. Simple.
Sadly, springs are efficient, and the above scenario isn't
the full story. Wheel hits bump, wheel moves up
compressing spring, spring exerts upward force on
vehicle... vehicle begins to move upward, wheel passes
bump and spring pushes wheel downward, while the car
is
moving up still. The result is a rather nasty oscillation
and
the vehicle bounces around for some time.
To stop the oscillations, most vehicles use dampers.
Usually this is just a piston with holes in, moving through
oil. This wastes energy, and the oscillation reduces in
amplitude quickly. There are sophisticated ways of
changing damping, lots of different valves, even oils that
change their viscosity in response to a switched
electomagnet.
Springs are less sophisticated, they come in two flavors,
linear and progressive. A typical car will have linear coil
springs* rated at something like 100kg/cm. When
lowered
onto its wheels, the car will sink a few cm into its
suspension travel, partly compressing the springs. This is
an important feature, it gives the car negative travel, so
the wheels stay on the road while traveling over slight
hollows or allowing the inside wheels to stay in contact
with the road while cornering.
There is a problem however, imagine you are driving
around the outskirts of Knutsford, Cheshire, not too far
from Plumley. You find that hump-backed bridge over
the
railway, now, at 32mph you will get a fun roller-coaster
type experience. At 58** you tend to take off***. Now, in
many recently rented vehicles you may hear a "clunk" as
the recently rented vehicle is carefully guided onto the
correct side of the road mid air.
The "clunk" is because the spring is many times more
forceful than necessary to extend the suspension.
Now, the most basic type of spring-damper shock
absorber
looks like this <link>. My solution is to add another
spring,
in this example, below the piston which is just about
engaged at normal ride height. Now, the main spring IS
able to fully extend the piston, but as it does so, the
second spring compresses antagonizing the main spring,
meaning that the effective spring rate of the main
spring
reduces in a non linear manner as the suspension extends
in "droop" i.e. below the normal ride height. This means
that the car sits at its correct ride height with the
springs
forming an energy hill on BOTH sides of the suspension
travel, provided the car is on the ground. Should the car
leave the ground, the suspension will extend with much
less force while being transiently more lenient on
landing.
In addition, this system will also perform a similar role to
the anti roll/sway bar but without the additional
components. The system will need less damping because
the normal ride height is more energetically stable, the
car
will also have a much more stable ride height with
changing loads without the horrid unloaded performance
of
progressive springs.
*linear coil, sew the seeds of confusion.
** National speed limit road, caught me totally by
surprise
officer, nothing to see here.
*** Because the bridge also kinks to the right, and you
can't
turn in the air, the safe thing to do is take off on the
wrong
side of the road.