How Stuff Works
Fuzz explains kingpins
’The beam axle and kingpin combination brought amazing advances to vehicular stability’
Back in the days when roadgoing vehicles had an old nag up front directing the steered wheels, steering was a crude construction of a beam axle with a wheel at either end and a pivot at its centre.
Attached to the axle were two shafts and between them stood our equine chum, connected to them via a collar and straps or chains. The driver of the vehicle sat right behind the, ahem, exhaust and guided the animal via reins and tapping whip.
This centre pivot, in effect, made the cart something akin to a three-wheeled vehicle and this, in combination with its high centre of gravity, gave it a tendency to topple.
Removing the sentient animal and replacing it with a lump of iron meant that the source of power would no longer respond to a gentle tap on its hind quarters and so a decent method of direction control was required.
The beam axle remained at the heart of the assembly, but now the wheel at each end had its own individual pivot and central to this was the kingpin. This turned the beam axle from a one-piece affair into three, with the outer sections to which the wheels were attached now being designated as stub axles.
Each stub axle was required to head in roughly the same direction as the other and so some form of solid link had to be made between them, this being the track rod. In turn, the track rod would have been ineffectual if connected to the centre of the pivot and so each stub-axle was fitted with a steering arm, rather like a tiller on a boat.
To prevent tyre scrub on cornering, the steering arms’ geometry was set to a line from the centre of each kingpin, through the centre of each steering arm ball pin to a point, nominally at the centre of the rear axle. With the steered wheels turned directionally, this gave all four wheels a common centre point.
Not only did kingpins allow this to be achieved, but they could also be set in the beam axle to allow camber and caster angle, to suit vehicular application.
Looking at the vehicle headon, in simple form, the kingpins could be set to 90 degrees from a horizontal plane. However, to allow for greater tyre contact during spirited cornering, tapering this plane so that the top of the kingpin was set inwards to the centre line of the front axle, with the bottom correspondingly kicked outwards, gave negative camber. This is a crude explanation, as a kingpin’s inclination may be set differently to the camber of the wheel itself, to give in effect the lighter steering advantages of a positive camber.
Similarly, looking from the side of the vehicle, inclining the top of the kingpin’s centre line forward, with the bottom correspondingly trailing behind, gave ‘positive caster’, rather like the wheels on a shopping trolley, giving the vehicle greater straightahead steering stability.
Thus, the humble beam axle and kingpin combination brought amazing advances to vehicular stability and controllability, right up into the days of independent suspension, and beyond.