Springs
Springs come in many shapes and mediums, but essentially work in the same
way, they deform when stress is applied and return to normal when stress is
relieved. Over the years this deformation has been achieved by a variety of
methods
Leaf Springs
Because it was already used in the horse drawn carriages, the leaf spring has had a
heavy usage in the automobile suspensions.
The original C-shaped spring was
strengthened by the addition of steel strips. At first, the elliptical
leaf spring was employed. It was made of two long, curved leaves or blades
pivoted together at their end to form an ellipse.
They were stregthened at their
mid-point by additional leaves whose lengths decreased as they were located
farther from the main blade. This arrangement was designed to provide the
maximum strength with adequate deflection for given loads.
As the loads increased, the
elliptic spring was replaced by the semi-elliptic, smaller and stronger than the
first one. It had its ends secured to the chassis and its centre to the axle
A new problem appeared : as the spring was flattened, its length increased which
was not compatible with a strong contact with the chassis. To overcome this, one end was bolted to the chassis while the other was allowed to slide
on the frame or pivoted to a shackle.
Semi-elliptic spring with double
shackles, Single shackle semi-elliptic spring
Two further forms of leaf springs
were designed : the quarter elliptic and the three-quarter elliptic. The
three-quarter elliptic was particularly innovative as it was the first two-rate
spring : it was a semi and a quarter spring joined at their pivot so as the
quarter was much stiffer it could deal with heavy shocks, leaving the weaker to
deal with small irregularities of the road surface.
Quarter-elliptic spring
The further developements in the
domain of leaf springs tended to the use of fewer, shorter and wider leaves
which were initially curved to a greater radius to provide a spring where the
number of leaves in stress increase with the deflection.
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The advantage of the leaf spring method was that it provided a secure
location for the axle, using other styles like coils means that the axle must be
located seperately hence increasing the number of parts hence more cost.
The disadvantages include crude performance and a lack of flexibility in
suspension design |
Consider this many off-roaders still use cart springs (more formally known
as semi elliptic leaf springs) which were in use before the
invention of the internal combustion engine! Remember those cowboy flicks with
horse drawn wagons? Observe the springing used in those wagons and see how
similar they are to the springing used on a typical off-roader. There is no
difference at all!
In those days, this springing arrangement held an important property: the
deformation of the leaf spring meant that the individual "leaves"
rubbed against one another thus providing a crude form of damping action. In
any kind of suspension system, the compression of a spring requires a damping
force so that when the spring rebounds it will not oscillate indefinitely.
Friction based damping is crude because it is difficult to control the damping
properties of the system. In those days (over 100 years ago, that is) damping
was provided by the friction inherent in the leaf spring design since there
were no other means to do so.
The invention of the familiar telescopic damper (or "shock
absorber") has rendered friction based damping completely obsolete. In
fact, friction is regarded as a totally undesirable property in suspension
design today and great pains are taken to eliminate friction completely. The
ubiquitous coil spring can be easily seen to have practically no
friction in its action and is ideally suited to working cooperatively with the
telescopic damper as part of a modern suspension system. The coil spring can
also be designed to work with far greater values of suspension travel than the
leaf spring ever can.
Ideally a modern damper should be soft on compression to allow the spring
to compress and thus absorb the shock, and firm on decompression so the
natural oscillating motions of the spring is rapidly halted, thus eliminating
floatiness or bouncing. These characteristics provide what is perceived to be
a smooth ride.
A leaf spring has roughly the same amount of friction on both compression
and decompression. Friction in any suspension system inhibits the compressing
of a spring, which leads to more road shock being transmitted into the cabin.
This characteristic leads to what is perceived to be a rough ride.
Twisting Springs
In this category, we
find the classical coil spring and the torsion bar while the leaf spring
reacts to loads by bending, other automobile springs such as the helicoidal
spring and the straight torsion bar, react by twisting. When a coil spring
deflects along its axis, the angle between the coils increases or decreases by
twisting, or torsion, of the coil material.
The above mentioned torsion bar is effectively a coil spring - straightend.
These springs were used in the
early days but came into much greater use with the development of independant
suspension systems, because with such systems, the axle location ceased to be a
spring function.
Rubber Springs
The use of rubber goes
back to 1847 when de Bergue fitted a cart with springs comprising
alternate discs of rubber and steel. Springs of this type where interesting
because of the increase of their rate with deflection but they required a
mechanical location for the moving elements.
The work of A.E. Moulton
and Dr A. Boschi has resulted in the invention of a rubber spring where, thanks
to a hollow conical shape, the rubber was in both compression and in shear so
they are stable without any external locating means. They could be designed to
give a low rate at mid-range and an increased rate on compression or expansion.
The Mini used Rubber spring to great effect, and other than a short daliance
with fluid springs bounced around on rubber merrily for 40 years
Pneumatic Springs
George
Stephenson, famous for his railway engineering career, Invented the gas
springing system with a design in 1817 for the Kilmarnock and Troon
Railway
But their first use in
motor vehicles had to wait until 1896 when J.R. Heath placed annular air
bags round the axle ends.
Five years later, M. A.
Yeakley devised a vehicle on which the four wheel were independantly sprung
by air acting on pistons connected to the axles.
Automatic levelling was
devised in 1910 by P.H. de Saint-Senoch but these two systems have never really
been used and people had to wait the Citroen DS 19 in 1955 and its
high-pressure air springs to see Self leveling in action.
Citroen's Auto-levelling system shown on DS 19
Hydralastic is a
low-pressure pneumatic spring created by B.L.M.C.
In the sixties British leyland developed Hydralastic suspension famed for its
ride but rather plagued by leaks. It found it way onto the mini and the at the
time very advanced Austin/Morris 1100 and 1300
A variation Hydragas. It
has been used since 1973 on such vehicles as the Austin Allegro and provided a very
smooth ride .
More susprisingly it can still be found on a Modern MGF sports car.
Hydragas Unit (1973)
Citroen's pneumatic springs
Citroën's hydropneumatic suspension uses nitrogen gas under pressure in small metal spheres sealed by
flexible rubber diaphragms. A piston acts on the diaphragm through a column of
hydraulic fluid , the length of which is
increased or decreased by an engine driven pump to maintain desired ride height
and prevent bottoming out.