Ladder Bar Set-up
his own Project Pop as an example, Steven Goodridge follows
his ‘Front End Low Down ‘feature by looking at
different methods of rear axle location and suspension.
many ways it’s a lot easier to work out the rear suspension
than the front, as there is much less happening (we haven’t
yet seen a rod with four-wheel steering). Although at first
glance it may seem there’s very little to go wrong,
there are still some basic principles that we must incorporate
into the design if we are to gain the maximum benefit from
our labours. We’ll deal with the theory as we come to
it, but let’s first look at the various options open
LEAF SPRING OR COIL-OVER?
A beam (or ‘live’) axle can be hung very simply
using leaf springs, or a type of four-bar linkage can be designed
to suit. The decision as to which is best for your own particular
application will depend on simple criteria such as looks,
space, money and skills. On this basis the leaf spring set-up
may be discarded on the grounds of looks and space, but it
is by far the cheapest and this might be your most important
Major manufacturers have now abandoned leaf springs for a
variety of reasons; space being one crucial element and efficiency
another. Simply said the coil spring works better, lacking
the inherent friction found in the leaf spring design. Besides
the leaf spring itself, space will also have to be found for
the damper and its mounting.
For rodders familiar with current state of the art technology
in the Street rod field, the only real choice will be between
which of the various four-bar set-ups to choose. Unless you
anticipate a lot of drag strip use the old style ladder bar
will be a non-starter, especially in the case of a low-slung
Ford Pop where space is at a premium. Although in a traditional
hiboy such a set-up using either a transverse leaf spring
or coil-overs and a Panhard rod might be considered essential
for styling reasons. Again it comes down to personal criteria.
The four-link suspension as we know it today is basically
a development of the ladder bar system. It evolved as space
for the full sized ladder bar disappeared due to the downsizing
of the Pro Stock racers in vogue at the time.
The four-bar set-up can be described as a truncated ladder
bar, although it is not a totally correct analogy as there
are geometry changes with a four-bar that just don’t
occur with the ladder bar. This is due to the rigid triangulated
nature of the bars themselves together with the single pivot
point on which the whole rear suspension rotates.
This fixed point is referred to as the instant centre. This
instant centre, along with the rear
tyres where they touch the ground, must be imagined as two
points on a line (see diagram). This line represents a line
of force through which the car’s power (as measured
at the tyres’ contact patch) is translated into forward
motion and therefore acceleration. The theory becomes much
more complicated when this principle is applied to the four-bar
system, because as the axle rises and falls so also will the
instant centre which is no longer fixed. On the average street
rod this will have little impact on the way you build the
car, but it could be crucial if building a drag racer as the
location of the instant centre has an important effect on
traction. The theory for this needs an article on its own
as it’s beyond the scope of this general introduction
to the subject.
Technically superior to the Panhard rod, but
more difficult to construct and find space for. As the axle
moves up and down the two locating rods attached to the central
pivoting fixture ensure that the axle moves without sideways
Simpler and quite adequate for most uses, the
Panhard rod will locate the rear axle when using a non-triangulated
The rear locating links can be placed so that
they are parallel with each other and the ground, and although
this works very well it does require the addition of a Panhard
rod to stop the axle moving from side to side. A four-bar
with triangulated upper or lower arms eliminates the sideways
movement of the axle without the addition of a Panhard rod.
This could be an important factor when looking at the space
available under a small car like a Popular or Y Type. As the
Panhard rod would normally be located behind the axle this
may limit the size of the petrol tank, if that design is chosen.
While the upper and lower bars can be placed parallel to each
other and the ground, it is a good idea to angle the lower
bars up at their front locating point. This incorporates a
geometry known as anti-squat. As the name suggests, the use
of such geometry tends to prevent or reduce the habit of the
back end to squat or sit on its haunches under hard acceleration.
This could be crucial if the tyres are already close to the
fenders or wheel wells. The inclusion of anti-squat geometry
also has the advantage of working to really plant the tyres
on the tarmac for good traction.
When calculating and planning the four-bar set-up up try and
get the arms as long as is practical or possible. With a triangulated
four-bar set-up the upper arms will be shorter than the lower
-reference to a Mklll, IV or V Cortina, Viva or Victor will
show the general arrangement. You can then incorporate similar
dimensions in your own design. At each end a urethane bushing
from Wisbech Engineering is a much more sensible proposition
for the road than a spherical bearing or rod end where the
metal to metal contact, although positive and precise for
race track use, transfers much more noise and vibration to
the occupants than a urethane bushing. This would of course
be even more noticeable in a closed car. Besides, they cost
more and require regular maintenance.
One end of the bar should
be adjustable, to optimise position and alignment. The tube
used for the bars themselves should be at least ¾”
x lOswg cold drawn seamless for a small car like a Pop or
Y Type, and 1” x lOswg cold drawn seamless for a larger
vehicle like a Model B or similar-sized American car. Whatever
size is chosen the tube itself can either be tapped directly
for the adjustable end, i.e. ¾” UNF, or ends
can be machined from solid, suitably machined, drilled and
tapped for the appropriate thread and welded into the ends
of the tube. Be sure to get good penetration!
When buying the necessary components for your four-bars it
is a good idea to consider stainless sleeves for the bushes
themselves so they don’t rust themselves into position.
Personally, I wouldn’t use stainless bolts anywhere
on the suspension as they don’t possess the ultimate
strength of a high tensile bolt. Some may argue that the loads
imposed on a suspension fitting are not so great that a stainless
bolt would fail, which may well be the case. However, I don’t
intend to find out and will always fit an unplated high tensile
bolt. Unplated, because chroming a bolt, or other component
for that matter, leads to hydrogen embrittlement, which is
an unavoidable result of the plating process. This weakens
the component and should not be considered for such crucial
areas as the suspension and steering. Yes, the bolt will rust
because we’re talking twelve months a year drivers here!
If it is decided to fit a parallel four-bar
set-up without any triangulation (that is when viewed from
above, either the upper or lower bars will form a truncated
triangle) some form of sideways location will be necessary.
A Panhard rod or Watts linkage is the best way to control
this, the latter being technically superior to the former
as with a Panhard rod the axle will move up and down in a
slight arc determined by the length of the rod itself. So
it follows that the rod should be as long as possible. This
rod can also be mounted using urethane bushes. One end of
the rod is mounted to the axle and the other to the chassis.
In the rest position it should be parallel to the ground.
The Watts linkage is a little more complicated but eliminates
the slight movement induced by the previous design. In the
Watts linkage a pivot point is welded to the centre of the
differential case. On to this pivot a linkage is mounted which
connects the two location bars. As the central linkage is
free to rotate around its pivot, any sideways movement induced
by the locating rods is taken up by the linkage. This ensures
that the axle always rises and falls vertically, and could
be very useful if clearance between the tyre and fenderwell
is tight. Reference to the drawings should make the operation
of these devices clearer.
If space is limited a triangulated four-bar is a much more
common solution to the problem and is the one chosen for my
Popular. The only disadvantage of this design being that whatever
arrangement is chosen for the layout of the urethane bushes,
some degree of bind will be introduced if much suspension
movement is envisaged.
THEORY INTO PRACTICE
The first step is to block the chassis up to the correct ride
height, then block up the axle in a position determined by
the radius of the tyre being used. With these components so
arranged the position of the radius rods can be plotted. The
top bar drops slightly towards the front and, if triangulated,
also splayed out from a position close to the differential
in the direction of the chassis rails. Reference to the drawings
again should make this clear. The lower links rise at the
front, and this arrangement should produce a degree of anti-
squat geometry. Care should be taken to ensure that a full
range of envisaged suspension movement is possible without
the radius rods binding or hitting something! When checking
the movement of the rear axle be sure to include the propshaft
in this, as the floor and chassis crossmembers can both present
problems, especially in a low car. In the case of my Pop the
coil-overs are to be fitted in front of the axle so additional
care was taken to see that sufficient clearance was available.
Coil-over shock assemblies can either be racing- style Spax
or Koni units fitted with urethane bushes from a specialist
like Wisbech Engineering (which have the advantage of being
rebuildable), or normal stock passenger units from the back
of a Jaguar or, dare I say it, from the front of a Reliant.
Picked up at a breaker’s they needn’t cost more
than a leaf spring.
Suitable coil springs are available in an infinite variety
of lengths and strengths, and again available from Wisbech
Engineering or firms like Demon Tweeks. Most such springs
will come powder-coated for a tough finish that won’t
flake off. Powder coating avoids the problems of hydrogen
embrittlement mentioned earlier.
I decided to fit the coil-overs in front of the axle to leave
maximum space behind for the petrol tank. The tank is in the
correct place for such things, under the car leaving the boot
free for luggage . . . now that’s a novelty Such considerations
won’t be important on American cars but, on a smaller
vehicle like a Pop or Y Type, it can make a lot of difference
to the versatility of the finished machine.
Brackets will have to be made to complete the installation.
On a car of the size we are dealing with, depending on length
or whether it’s to be boxed, 3/16” or ¼”
will be quite strong enough.
For the urethane bushes, adjustable ends, coilovers and springs
I’ve always found Wisbech Engineering to be reliable
(Tel. 01553 828038).
clearly shows the difference in bar length. Ultimate length
is dictated by available space. (All pics viewed from below)
|The pick-up points for
the four-bar linkage can be seen on the crossmembers, which
will later be used to mount the floor and belly pan.
|By using the triangulated
four-bar design and mounting the coil-avers in front of the
axle, maximum space is left for the petrol tank
The shorter upper bars are triangulated to eliminate sideways
movement. This does away with the need for a Panhard rod (chassis
viewed from below)
four-bar dating back to the Thirties and popularlsed by Pete
& Jake in more recent times. Set-up needs a Panhard rod
with suitable crossmembers. This can make a cheap alternative
to the coil-over spring set-up.