Description of the model

The purpose was to implement if possible a rack-and-pinon steering mechanism with the usual Ackermann trapezium as geometry.

The difficulty is to set axles at an angle, since the orientation of Lego axles is fixed.

I built the model first with real parts, in at least four iterations.  The first two used modified parts:  a ball joint link at 90º and two quarter bushings, neither of which exist as standard Lego parts.  As this was no good for publication to readers who might wish to built their own instances, I used the second two iterations to come up with a model that can be built.  The test here is that it must be possible to make it in LDD (Lego Digital Designer).

Please do not call this model a MOC:  it has no other merit than to show that the trapezium geometry of the Ackerman steering is possible.

I spent no attention to ruggedness and certainly none to good looks, so it is futile to criticise it.

Those who wish can download the lxf file and in “Building Guide Mode” see how it was done.

A few comments

There are difficulties in attempting to build some realistic functions in Lego, unless special parts are available.

It is also very difficult to mix bricks with knobs and beams (with no knobs), to change direction (because of the anisotropy) or to position parts “outside the grid”.  In addition, sometimes one needs to “grip” an axle but there is no “gripping hole” at that point, or conversely to let an axle turn in a position that presents only a gripping hole.  Finally, the bearing bricks (with knobs) come in lengths that are an even number of units whereas the beams come in odd numbers.

The net result is that very often a lot of ingenuity has to be applied before even a simple principle can be demonstrated.  Lego parts were primarily designed for static structures.  The possibilities of the Technics parts are indeed amazing considering the rigid constraints in which they have to be applied.

Just a short list of the problems encountered in this fairly simple model:

• the front is built with beams and therefore has a width that is an odd number of units
• the angled liftarms used to get the Ackerman trapezium lead to a length of the connecting rod that is necessarily a transcendental number, but fortunately it is within the limits accepted by LDD.
• the rear is more easily built with plates and bearing bricks, and so has a width that is an even number of units, leading to an awkward construction to expand by half a unit on the left and the right, done by inserting a half-width beam for which no friction pegs exist forcing the use of axles ans bushings to hold things together.
• the rack must link to the trapezium, which is best done by a ball-joint link, but the simplest way to do it is with one of the ball pegs vertical and the other horizontal, a configuration for which no link rod exists; I used an awkward axle-in-a bearing-plate system.
• linking the pinion to the steering wheel is always a problem; I chose a quick-and-dirty way out.
• the rod linking the rack to the trapezium needs axles to be gripped in holes that do not, forcing the use of extra pieces and pegs.
• because the trapezium needs to be built with angled liftarms, the assembly holding the front wheel axles becomes very high.

I'm convinced that it can be done better, smaller and more robust, but just this simple thing cost me more than a day to finish.  If anyone finds a nicer solution I will be very glad to hear about it!