Traditional Ladder Bar Set-up
Four Bar Set-up
Using 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.
In 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 to us.

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 requirement.
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 movement
Simpler and quite adequate for most uses, the Panhard rod will locate the rear axle when using a non-triangulated four-bar
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.

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).
This shot 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
Triangulated four-bar. The shorter upper bars are triangulated to eliminate sideways movement. This does away with the need for a Panhard rod (chassis viewed from below)
Traditional-style four-bar dating back to the Thirties and popularlsed by Pete & Jake in more recent times. Set-up needs a Panhard rod to work.
Leaf spring with suitable crossmembers. This can make a cheap alternative to the coil-over spring set-up.