by Geoff Kremer
FOUR BARS
(An alternative form of drinking!)
Sorry about the pun! So you have a solid or live axle and you need to fit it to your pride and joy. The four bar set up is perhaps the most common way to do this. It also seems to be the easiest to get wrong.
The basic concept of the four bar suspension lends itself to a variety of installations but the basics as below must be followed.
Many manufacturers use the four bar set up for the location of the rear axle for instance. The Ford Cortina uses a modified version with smaller, steeply inclined top bars, this idea serves two functions; first to locate the axle in all planes and second to save money. Theoretically it should not work and indeed it would not except for the
large rubber bushes dotted all over the place, basically it’s a bodge!
Rule number one; All four bars must be of equal length.
Rule number two; The pair of bars each side must run parallel with each other.
The reasons for this is as follows. I you look at the top diagram you will see the bars form a parallelogram and as the suspension moves this relationship is maintained. If you alter either the length of or the mounting point of a bar this relationship will be broken and what will happen is the caster angle will vary with the suspension movement leading to instability, especially driving in a straight line at speed. Also it will lead to binding and added stresses when one side of the axle is deflected more than the other.
The Panhard rod or sway bar does only one thing, keeps the axle in line with the chassis but it works in conjunction with the four bars so it has to designed to work with them not against them.
In the final diagram, I show here the Thames rear axle design. It’s a variation on the simple four bar theme.
