Traditional Suspension

By Kev Rooney

There are many front axle location mediums that have been used on beam axles vehicles through the years. Each is pertinent to the technology available at the time and, as such, mismatching components from different eras can create problems. Some result in poor handling while others can create dangerous and potentially lethal problems. If we work chronologically through the applications the inherent problems and solutions should become obvious.

Fords original design consisted of an unboxed U channel section with the I beam front being located by a triangulated front wishbone, with a ball central pivot. There is a famous publicity shot of the times with a 34 Ford with a ramp, about 18 inches tall, under one front wheel with the other 3 planted firmly on the ground. This demonstrated the incredible flexibility of both the front axle and of the chassis. It also explains the large body shut gaps on vehicles of this era necessary to allow this sort of movement. The springing was very stiff and setting it up too soft would have resulted in a very twitchy unstable ride. To see this type of chassis/suspension in motion you only have to look at an unladen flatbed lorry travelling along the road. You can see that the position of the cab and bed bear little resemblance to one another. This becomes less obvious when the bed is heavily laden as the springing comes into action. The flexible chassis is needed, as if it were more rigid, the heavy duty springing would make the lorry jump around all over the show.

So having established that the original Ford design was wonderful at coping with the diverse conditions encountered on roads of that period, along comes Joe Hotrodder. He decides to shoehorn in a larger engine that conflicts with the original centre pivot. The answer is then to split the wishbones and move them out to the chassis rails. Now, whilst not as ideal as the original design, providing the new mounts are well made, the flexibility of the chassis and the I beams ability to flex along its length, means it will work.

The next stage for Joe is to realise that the flexible chassis does not aid handling, and so he starts to strengthen and box his chassis. Now that this slop is taken out there is a far greater load placed on the chassis mounts for the split wishbones resulting in some failing. Someone then came up with the idea of using hairpins. Now this may have been “reasoned” engineering or it may have been for aesthetic reasons. Whatever the cause there was then enough twist in the hairpins to reduce load on the chassis mounts. The boxing of the chassis also meant that the front springing became too harsh. At this stage leaves would need to be removed or a custom made spring fitted otherwise the hard springing would cause the car to bounce about unpredictably. So once again, although not ideal, we have a useable system.

The next major stumbling block comes in the form of the tube axle. As this will not flex/twist as per the beam axle, he is again placing huge loads on the chassis mounts. Unfortunately it also promotes appalling handling characteristics as the beam has no “give” as its impossible to twist tube by the rotational forces acting upon it . This then causes all suspension/cornering loads to be placed on the tyres making the car skip and dart around.

It appears that the midget and track racers were the first to notice this problem and they overcame it by going to a 4 bar location system. This acts as a parallelogram, allowing the tube axle to move freely again. To enable any beam axled car to operate well it must be set up “free”, i.e. no bind in the system anywhere. When an original type beam is used, this is also accomplished by having bolt through spring shackle mounts that allow the slight movement that is required to keep the spring from becoming misaligned and creating bind.

When beams are modified by welding a batwing fitting to them the bind returns , so “give“ must be placed back in the system for the 4 bar or hairpin to operate anyway near correctly again. Urethanes will not allow this to happen so either rose joints or rubber bushes must be incorporated.

So all this info is then used to create a suspension set up that will work on your car. It’s a question of using the correct location method, with the appropriate bushing medium in relation to the construction method of your chassis.

A very informative site can be found here http://www.eatonsprings.com/Spring%20Tech%20101.pdf

Tech Section