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This article was first published in 2006.
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In nearly all car suspension modification you’re
not starting with a blank sheet. Instead, you have what the car’s manufacturer
provided – whether that’s a McPherson struts, wishbones or some other design.
Mods might then involve altering spring rate and length, or changing bushes or
dampers. But what if you are starting from scratch – whether that’s in
the design of a small vehicle like a fun go-kart, or in something more serious
like a track-only or kit car? In those cases you have complete freedom of choice
in the design... and then where do you start?
In this article we’ll take a look at the
advantages and disadvantages of different suspension designs.
Non-independent Suspension
The simplest front suspension is a beam axle. In
this design a solid piece of metal (the beam, often forged in an H-section)
connects the two wheels. The wheels are mounted on vertical (or near vertical)
swivels called kingpins, allowing the wheels to be steered. The axle is most
often suspended by a pair of leaf springs, arranged fore-aft as close to the
outermost ends of the axle as possible. Alternatively, a beam axle can be
suspended by coil springs (or a transverse leaf spring) and locating links.
Front beam axles are still used on trucks and
off-road style four wheel drives but their use on cars starting declining in the
1930s.
The advantages of the beam axle include strength,
simplicity (and therefore low cost), and the fact that each wheel remains
vertical at all times - that is, there is no camber change with suspension
deflection.
However, the disadvantages far outweigh the
positives. The disadvantages include a heavy unsprung weight resulting in poor
ride and tyre adhesion over bumps, and the fact that the behaviour of one wheel
can affect the other. When they are steered, the wheels can shimmy and tramp,
and obviously a bump experienced by one wheel is directly transmitted to the
other. Finally, if there is to be sufficient suspension travel, a beam axle
takes up a lot of room.
For these reasons, a non-independent beam axle
design is seldom used on any vehicle – even a fun dirt kart.
Independent Suspension
Swing Arms and Roll Centres
Before we look at the different designs of front
suspension, a word on the two most vital of characteristics of independent
suspensions – roll centre and end-view swing arm length.
The roll centre is an imaginary point about which
the car rolls. The calculation process that’s followed to find the roll centre
varies a little according to the suspension design; this diagram shows the
approach for double wishbones where one wishbone is angled to the horizontal.
The lines of the wishbones are extended until they reach a common point – ‘A’. A
line is then drawn that connects ‘A’ to the centre of the tyre’s contact patch –
‘C’. The roll centre is where this line crosses the centreline of the car –
‘R’.
Because the roll centre is governed by the
geometry of the design of the suspension system, it is different in position
(and often height) for the front and rear suspensions. The roll centre position
also often changes with suspension movement. The amount of body roll that occurs
with a given cornering force largely depends on the relationship between the
height of the centre of gravity and the roll centre. Raising the suspension roll
centre, or lowering the centre of gravity, will decrease roll.
However, while having a high roll centre therefore
sounds attractive, it has significant negatives associated with it. In fact,
most well set up cars run a roll centre at, slightly above, or slightly below
ground level.
The end-view swing arm length indicates the
lateral distance between the wheel and the virtual pivot point (‘A’ in the above
diagram). The shorter the swing arm length, the greater the change in track
during suspension movement and the greater the camber gained during bump. This
camber gain is normally desirable (see the Dynamic Camber Gain breakout) but if
the swing arm is too short, the system starts to act like a swing axle, with
resultant jacking forces. A swing-arm length of 2.5 – 3.8 metres is a good
compromise.
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A sliding pillar (or sliding kingpin) design looks
much the same as a beam front axle but the wheel is allowed some vertical
movement on its steering swivels. This system is characterised by very limited
suspension travel and stiction, resulting in a poor ride over small bumps. Very
few cars have ever been made with sliding pillar suspension.
A swing axle effectively divides a beam axle into
two, placing a pivot on the inner end of each half of the axle. The wheels can
therefore more independently of each other which improves ride, tyre adhesion
and prevents the action of one wheel immediately impacting on the other.
However, a swing axle is about the worse of the independent suspension
designs.
The major disadvantages of swing axles are that
when cornering, the car is lifted via a so-called jacking effect. This jacking
causes positive camber on the outside (ie loaded) wheel. The result is a sudden
loss in traction. Benefits of the swing axle include the fact that the system is
independent and the approach is simple and so cheap.
Swing axles have a high roll centre and a swing
arm length which is short – in fact, as long as the swing axle itself.
Swing axles have been used on a number of car
designs, although usually on the rear rather than the front. The pictured
Lightburn Zeta is one of the few cars that has run a front swing-axle design.
Early Volkswagen Beetles and the Corvair used swing-axle rears.
As the name suggests, a trailing link design uses
two longitudinal links to support the steering ball joints. The wheels are
pulled over bumps – the links are in tension when the wheel encounters a
resistance. Early Volkswagen Beetles use trailing link front
suspensions.
Advantages of trailing link designs are that the
system is compact (especially if torsion bar springs are used) and can provide a
good ride. However, when subjected to lateral cornering forces, the arms tend to
bend so must be made strong to resist these forces. The camber of the tyres also
changes as the vehicle rolls in cornering, reducing adhesion. Finally, there
must be plenty of strong chassis or bodywork directly ahead of the front wheels
to carry the mounting points of the trailing links.
Trailing link suspensions have an infinitely long
swing arm length and a roll centre at ground level.
While now very long in the tooth, Volkswagen
trailing link front suspension can still be found being used in some kit cars
and specials, primarily because of the huge number of Beetles that were produced
with this design and the way in which the Beetle chassis can be adapted to other
bodies.
This design is one of the most common used today
on cars. In this approach the damper and spring are usually integrated into a
near-vertical assembly with a ball joint placed at the base of the strut and
another bearing placed at its top. The whole damper/spring assembly turns with
the steering, with the wheel mounted on a stub axle that’s connected to the base
of the strut. The upper end of the strut bolts straight to bodywork and the
lower part of the strut is located by (usually) an anti-sway bar and a link.
Advantages of the design include a long swing-arm
length and a roll centre that can be at ground level. The wide-spaced mounting
points also make the design very strong. However, the presence of the
spring/damper and steering knuckle in close proximity to the wheel leaves little
room for large tyres without the tyre being well outboard of the point at which
the steering axis would touch the road. This results in an increased scrub
radius, resulting in more unwanted feedback through the steering wheel and
potentially more nervous behaviour under brakes and over variable road surfaces.
However, the most significant negative is that the system cannot be set up to
provide much increased camber on compression, resulting in a camber loss in
cornering.
In this design wishbones – or A arms – are used
top and bottom to support an upright to which the wheel is attached. The broad
lower base of the arms connect to the frame while the ball-joints are mounted on
the apex of the arms. When the arms are of equal length and mounted parallel to
each other and to the road, the swing-arm is infinitely long and the roll centre
is at ground level.
That combination of factors sounds very good, but
the disadvantage of such an approach is that there is no camber gain when the
body rolls in cornering – in fact, as shown here, the tyres can go into positive
camber. Advantages include strength (the arms are in compression when lateral
cornering loads are applied and the wide base of the arms spreads the loads)
giving better dynamic location of the wheel on rough roads and when cornering,
and good ride.
If equal length double wishbones are altered so
that the upper wishbone is shorter, the advantages of an equal length wishbone
system are retained but in addition, camber gain can be created during bump –
and so camber of the outer (loaded) tyre during cornering. Furthermore, by
changing the angles and lengths of the arms, it is possible to change the amount
of camber gain during deflection, and also alter the roll centre position and
swing-arm length.
Disadvantages include complex construction, a
large number of bushes and/or ball-joints, and critical geometry of the mounting
points.
Dynamic
Camber Gain
Car
tyres work best when they are vertical to the road ie have zero camber. If the
suspension maintains a constant camber in compression, when the car rolls (ie
the outside suspension is compressed and the inner extended), there will be an
effective camber loss of the more heavily loaded outside wheel during hard
cornering.
For
example, if the static camber is zero and the car rolls by 3 degrees, there will
be a positive camber gain of 3 degrees of the outside wheel. It’s for this
reason that the static camber is normally set negative (ie the wheels lean in at
their tops), resulting in a vertical tyre when in full roll. However, using lots
of static negative camber can cause straight-line tyre wear and instability
under brakes. A better way is to use a suspension design that adds negative
camber in bump.
An
unequal length double wishbone suspension will cause the wheel to increase in
camber (ie have a negative camber gain) when the suspension is compressed. This
dynamic camber gain can be used to effectively offset most or all of the camber
loss caused by body roll. For example, a suspension may be designed to add 1
degree of negative camber for every inch of bump.
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References:
Adams, H. Chassis Engineering, HP Books,
1993
Daniels, J. Car Suspension at Work, MRP,
1998
Gillespie, T. Fundamentals of Vehicle
Dynamics, SAE, 1992
Van Walkenburgh, P. Race Car Engineering and
Mechanics, HP Books, 1992
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