Technofile - the rod end bearing
Among the wreckage of a German fighter plane was found the future for racing car suspensions. Keith Howard reveals the origins of the rose joint.
Compliance. If you had to encapsulate the essential difference between race car and road car suspensions in a single word, that would be it: compliance, the reciprocal of stiffness. Road car suspension systems require carefully applied compliance to achieve acceptable ride quality and provide isolation from the noise and vibration generated at the tyre contact patch. So they make liberal use of elastomeric suspension bushes, meticulously positioned and dimensioned in modem cars so as to cause minimal compromise of wheel control. In a racing context, by contrast, refinement is barely an issue and accurate wheel control paramount, so rubber bushes are anathema.
Look closely at any modem race car, particularly from the senior formulae, and you’ll find it littered with a component which has become truly ubiquitous because of this: the rod end bearing. It is used to pivot suspension arms, at either end of anti-roll bar drop links, in gear linkages, on pedals: anywhere, in fact, where a low friction, very low compliance, zero play and, above all, adjustable pivot is needed.
Reel back 40 years, however, and the rod end bearing – although already widely used in aircraft – all but disappears from view. Right into the late 1950s/early 1960s even Formula One cars more commonly used (horror!) rubber bushes in their suspensions, albeit stiff ones. It was the desire to eliminate this parasitic suspension compliance, particularly with the development of wider, grippier tyres, and to incorporate increased suspension adjustability that brought the rod end into the picture and how.
In essence the rod end is delightfully simple, cornprising two, or at most three, principal components: a pierced ball with flattened poles which provides the articulation; a banjo-shaped housing into which the ball is secured during manufacture; and, in the most sophisticated types, a thin, usually PTFE-based, self-lubricating liner which is interposed between ball and housing to ensure low friction. What attracts the race car designer to the rod end is that it is,able to sustain high radial loads and, thanks to the male or female screw thread incorporated into its shank, is inherently adjustable, making it an ideal pivot in suspensions and elsewhere.
The origin of the rod end bearing has an edge of intrigue about it. Reel back another two decades to the first year of World War II and the desperate days of summer 1940. One of the first Messerschmitts to be shot down – presumably a Bf109E, although I haven’t been able to confirm that – crashes into a field, where its wreckage is pored over by Air Ministry experts. They are surprised to find a component in the flight control system that nobody in Britain has ever seen before: the rod end bearing.
It’s such a simple, elegant, effective device that Rose Brothers, later Rose Bearings Ltd, is asked to copy it for British aircraft to use. In due course, the same happens in the US with the HG Heim company. So intimately do these two manufacturers become associated with the rod end that to this day it is still commonly referred to as the rose joint or heim joint on either side of the Atlantic, the loss of capital letters a sign of how the names have become generics. Both companies still exist, albeit now as part of larger groups, and both still manufacture rod ends.
The first race car to employ a rod end bearing may well have been the Vanwall. Rose Bearings has a record of supplying a special order to Vandervell in 1957 (and to Rover a year earlier – but for what?) and photos of the front suspension of the 1958 car clearly show a rod end at the outboard extremity of its long radius arm. Later Rose made bearings to order for Cooper (1958) and Lotus (1960) among others, but the list is inevitably incomplete: as well as there being other sources of rod ends, Rose may have supplied off-the-shelf items of which there’s no record. In the US rod ends were certainly in use by 1959: the Watson-Offy Indy car of that year had them at both ends of its rear radius arms.
It’s all somewhat academic anyway because the rise of the rod end was gradual, as a perusal of early ’60s Formula One cars shows. By 1961/2 when the Lotus 25 was designed, for instance, rod ends were more widely deployed: Chapman used them at the outboard end of the lower front wishbone and rear radius arm, at both ends of the upper and lower rear lateral arms and at either extremity of the rear anti-roll bar drop links. Five years later, in the Lotus 49, the rod end was truly entrenched it was used virtually everywhere it could be.
Prior to the rod end’s adoption the Metalastik bush had held sway: two coaxial metal cylinders, the smaller to carry the mounting bolt, the larger to press-fit into the wishbone eye, separated by a thin layer of rubber. An odd choice, you might think, given that today a solid metal bush would be preferred, as it is in road cars adapted for track use. But rubber served an essential purpose in those days of gas-welded wishbones and suspension pick-up brackets: it provided enough ‘give’ to accommodate manufacturing tolerances.
The articulating ball of the rod end was likewise forgiving of dimensional inaccuracies, and without the cost to wheel control introduced by rubber. Also it facilitated, for the first time, the wide-ranging adjustability of suspension geometry we now take for granted although whether the teams and drivers had much idea how to juggle all the new variables at their command in those early years is another matter. Today the rod end continues to perform essentially the same functions, largely unchanged, although some F1 constructors are now incorporating spherical bearings – basically rod ends without the shank – directly into their carbon fibre composite push rods and suspension arms.
The next time you tuck into a Cadbury’s Roses chocolate would be an apposite time to reflect on how the simple rod end bearing transformed racing practice. Because, believe it or not, the name of the chocolates that grow on you derives from the fact that the clever machine originally developed to wrap them was designed by Rose Brothers the very same whose name would later become synonymous with the rod end.
Our thanks to R G Bearings for providing the NMB E-series stainless steel rod end used in the photograph. ‘Rose joint’ is a registered trademark of Rose Bearings Ltd.