1960’s Double Adjustable Automotive Struts

1960’s double adjustable automotive struts

The majority of these struts appear to have been manufactured by Armstrong of Hull/Beverly/York. They were of the twin tube design, the gas pressurised damper appearing on the scene a little later. The twin tube internals do not lend themselves to having separate bump and rebound damping external adjusters fitted to the strut body. This is especially true of rear Chapman struts for Lotus Elan, Elite, 15,16, 17 etc, where the damper body is pressed into a light alloy casting.

If we consider how damping can be adjusted we can perhaps throw light onto the difficulties of external adjusters for both bump and rebound. To adjust damping the pressure built up above and below the piston attached to the damper rod must have a means of control. It is relatively complicated to do this via internal means, and more difficult to provide external means of adjustment. The simplest means is to have internally adjustable elements within the dampers, Early 60's Armstrong and Ford struts used a range of cartridge components that offered damping adjustment, these being fitted into the foot valves at the base of the struts to alter bump (compression) damping, and into the base of hollowed out damper rods to alter rebound (extension) damping. Such struts had separate rebound and bump damping adjustment, but they appear to have been few and far between, possibly associated with works supported vehicles, and the damping was adjusted in the workshop - great if you have the budget and facilities. They are also a pig to set up, requiring much bench and associated track testing, but once set up perform very well as long as there is sympathy for the technology, materials and thinking of the period.

To provide external damping adjustment is rather more difficult as not only do you have to intercept the damping forces, but external elements get in the way. For example to adjust compression damping you need to have control over the compression forces below the piston. This is accomplished by altering how freely the piston and foot valve pass oil, more freely reduces damping, less freely increases damping. However, in order to provide low rod speed damping control, some bleed of pressure has to be allowed past the piston and foot valve at all times. Piston bleed affects rebound damping, and foot valve bleed only affects bump damping, so low speed bump damping affects both bump and rebound damping. The control of the bulk bump damping forces involves adjusting how much oil can flow past the foot valve above that flowing through the low speed galleries. This flow is controlled by either an internal cartridge as above, or via a mechanism that includes a shaft that rotates a disk or barrel with slots/holes of varying flow properties. The shafts have to pass through the body of the damper construction via a seal that can withstand high fluid pressure, and this makes seal housings bulky. This bulk is either installed inside the damper body where it takes up valuable space, or externally. Given the location at the bottom of the damper/strut, such adjusters cannot be fitted where the a the strut has a stub axle with a ball joint or similar fitted ti the base (Mk1/2 Cortina, etc), or is pressed into a casting (Elan, Elite, 15, 16, 17, etc). We have seen foot valve adjustment provided via prongs on the base of the piston which requires the damper to be fully compressed after removal of a mounting and any bump stop fitted over the rod, and via a shaft running down a central drilling in the rod. I have not seen either of these means of foot valve adjustment as early as the period you are researching, but this does not mean it did not exist.

The external adjustment of rebound damping is similar in that some means of altering the compression above the piston is required. this can be accomplished via a hollow rod and a shaft that rotates a disk on the piston to allow more or less oil to bleed through the piston. This means of adjustment simultaneously alters the compression damping so does not result in true separation