Derby Steering

The steering linkage on the Derby Cars is, in simple terms, a conventional ball and socket system with an element of damping built into it. The damping is to control the natural gyroscopic effects of having two massive flywheels at each end of the front axle and the disturbing forces from bumps in the road transmitted through these flywheels.

The damping element comes from the spring preload on the ball joints and relies heavily on a “super force” between the ball and socket called “FRICTION”. Friction in engineering is either the desirable or highly undesirable depending on the application. For the steering joint it has to be both, desirable to provide the damping effect, and undesirable for light steering. The joints are lubricated via the “one shot” system on the Derby and at the time the cars were first built “straight” oils were the norm and would break down under extreme loads. Ideal then for a damped joint when the damping required occurred with a lateral jolt on the joint causing the oil film to disperse and the two metal elements to be in direct contact for a while and not willing to move relative to each other because the magic force “FRICTION” took over. When the jolt was over the oil film returned and normal service resumed.

It may be worth noting that areas and loads have a bearing(no pun intended) on this since it is the pressure between the two surfaces of the joint which determine what force is required to cause the oil film to disperse. In simple terms the greater the area of contact the greater the force required, so a well fitted ball and socket requires more force to disperse the oil film. Rolls- Royce recognised this and produced a modification to reduce the area of contact to match the performance of the oils available at the time.

 

When the cars left the factory there was then a known damping force on the steering system, this, coupled with the correct settings of the geometry of the suspension and correctly balanced and fitted wheels gave the Derby perfect steering characteristics making it a joy to drive on all road surfaces.

 

When the cars had been used for a while a phenomena known as “Steering Shimmy” appeared, an indication that it is time to pay attention to the steering linkage, wheel balance and general geometry of the front suspension. The geometry will be unchanged unless there has been some sort of damage to it since there are no adjustments available without resorting to force or shims. Balancing the wheels must be done and the preload on the ball joints checked, remembering to retain the permitted lateral clearance.

 

There has been much written on the causes of steering shimmy, but remember the steering was perfect when each car left the factory so, barring some sort of accident, it will be down to normal wear and maintenance - and that includes the one-shot lubrication system. Also make sure that there is no damage to the gaiters on the ball joints.

 

A cautionary warning. Modern multigrade oils, semi and fully synthetic oils are dramatically different to the straight oils available at the time the car was built and are designed to carry much higher pressures before the oil film disperses. Put any of these into the one shot system and steering shimmy becomes much more likely to occur. The preload needed on the ball joint is much higher, or the contact area must be reduced. In simple terms Rolls-Royce matched the preload to the available oil – and stuck to that oil to maintain the correct settings. Change the oil and the settings need to be changed to match it.

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