Kingpins and Ackerman angle

Tuesday, 15 January 2013, 2pm

I’ve finished off the kingpin/stub axle units. Firstly I had to build a jig to drill the holes in the stub axles to house the kingpins –

At top is the stub axle, below it is the axle cradle, then the spacer I made to get the horizontal distance set precisely. By setting the horizontal and vertical distances precisely I was able to set the angle at the desired 14.196 degrees. Here’s the jig set up in the lathe –

I’ve added some nails to hold it in more firmly. It was drilled out with centre drill, drill and cutter, tricky and slow work. Here are the components of the kingpin after drilling –

The threaded high tensile studs and axle nut inserts (all steel) were screwed into untapped holes, 1mm smaller diameter than the threads. After gluing these parts together with epoxy/nanotube mix (pouring a couple of mils of it into each end of the axle to form a block around the kingpin as well), I made up a jig to set the Ackerman angle of the steering link pivot at the desired 20.18 degrees.

The Ackerman is the angle set so that the inner and outer front wheels go round corners at different angles, the axles aligned to a common centre pivot. This prevents the tyres scrubbing.

For those nerds interested in this (probably not many), I worked out a formula to calculate the angle myself. It goes something like this (angles upper case, distances lower case) –

Firstly you need to adjust for camber. Find the horizontal distance from the centre of the kingpin/stub axle intersection to the wheel axle centre (a) using

a = d cos A

Where d is the actual distance (from the centre of the kingpin/stub axle intersection to the wheel axle centre) and A is the camber. Then find the horizontal offset caused by the wheel camber (c), i.e. the sideways difference between the wheel centre-points at the axle and at the tyre contact point, using

c = b sin A

Where b is the wheel radius, then find the actual horizontal distance (from the centre of the kingpin/stub axle intersection to the wheel axle centre) (z), using

z = a – c (negative camber) or z = a + c (positive camber)

Then calculate the angle of the outside wheel at full lock (B) using

B = 1/tan [ w / (w/tan X + (t – 2z)) ]

Where X is the angle of the wheel from full lock to straight ahead, t is the tread width (distance between centres of front tyres) and w is the wheelbase (centre of front to back axles under load, with no people on board).

After those adjustments, we’re ready to calculate the Ackerman using this formula –

L sin C + L sin (B – C) = 2L sin (X/2) x cos [(X/2) + C ]

Where L is the chosen length between centres for the steering link pivot and C is the Ackerman angle. So X is constant (in my case 40 degrees) and 2L sin (X/2) is therefore also constant. Angles are substituted in for C until both sides are equal. Wasn’t that fun?

So here’s the finished pins –

And here’s how the whole shebang (passenger side) fits together –

This is what’s known as the ‘unsprung weight’- essential to keep it as low as possible to make the suspension as responsive as possible. The kingpin/stub axle weighed 180g, as opposed to 280g for the (smaller) steel item of the Mk 4.

I’ll be infusing the bottom bracket mounts next.