Metal Construction

Other Projects (9)

Home

Steering of Vehicles

With the construction of a radio controlled vehicle it is also necessary to deal with a suitable steering.

First of all let be pointed out, that the steering of a "real" vehicle is a very complex mechanism which in a model surely is not justified.
In the following hence only the (static) geometrical correlations are assumed , which can be applied to a model and which normally are easy to realize.

Swivel Axis Steering

As the construction type Pivoted Bogie Steering this simple and oldest kind of a steering e. g. is used at horse-drawn vehicles and handcarts.

The benefits of this steering are the large realisable steering angle up to 90° and the feature, that the steered wheels automatically run on their proper geometrical curve radius.
The disadvantage beside the demanding required space is the increasing tipping gradient with increasing steering angle, because in this case the platform more and more is reduced from a rectangle to a triangle.

As the construction type Ball Race Steering the swivel axis steering today virtually is used at all truck trailors.

Swivel Axis Steering
Source: "Lenkung von Dreirädern"
von W. Stiffel

At a model trailor perhaps is to notice the ease of movement, which possibly requires the installation of a roller bearing.

Buckling Steering

This kind of steering is reserved to only a few special vehicles like small size tractors and building site vehicles; mentioned e. g. is a wheel loader or the VOLVO BM-Dumper.

VOLVO BMA 25 with Buckling Steering
(6 Zyl., 6724 ccm, 244 PS)

Benefits of this steering are the low required space during shunting and the feature, that the steering mechanism is not so heavy polluted like the mechanism of an axle-pivot steering.
According to the swivel axis steering the steered wheels also automatically run on their proper geometrical curve radius.

Single Wheel Steering

As the construction type Fork Steering the single wheel steering today almost solely is used at two-wheelers and three-wheelers.

But also at road rollers the fork steering became accepted, at which the fork can be mounted along or transverse to the longitudinal axle of the vehicle.

Fork Steering at a Road Roller

Another kind of single wheel steering is the Axle-Pivot Steering.
Today this is the mostly used steering an shall be considered within a separate chapter.

Axle-Pivot Steering

Exepting the above mentioned vehicles today nearly all passenger cars, lorries, busses and other commercial vehicles are provided with an axle-pivot steering.
The essential benefits of this steering beside the low required space are the unchanged stableness of the vehicle during steering as well as the constructive alternatives and the resulting different types of construction.

So as lorries and other commercial vehicles as a rule a stiff front axle (Rigid Axle) is used while all passenger cars - and increasing also busses - throughout are provided with an Independent Wheel Suspension.

Principle of an Axle-Pivot Steering

At this more pretentious kind of a steering the wheels are placed on separate steerable stub axles which are provided each with a track rod arm.
The track rod arms are aligned approx. parallel to the longitudinal axis of the vehicle and are connected together by means of a track rod; in this way both wheels are steered simultaneous.

Important is the alignment of the track rod arms.
Are they e. g. exact parallel to the longitudinal axis of the vehicle, then axle, track rod arms and track rod form a rectangle (see figure); with this both wheels are steered with the same angle.
This steering works indeed and is used in very simple models, but it doesn´t comply exact with the geometrical reality.

At a steered vehicle all wheels only then can roll with the minimal resistance, if they run non-slipping on their exact circular arc.
That´s given, if the extended stub axles are intersecting in one center.

By graphics one can show that the condition is complied if the inner wheel is more steered than the outer wheel.

Geometry of the Axle-Pivot Steering

This circumstance for the first time the in Munich living carriage of car builder Georg Lankensperger found out. In his publication of the year 1816 with the so called Steering Trapezoid he also provided a way for practical realization.
To get a patent right also in UK he assigned the in London living art dealer Rudolph Ackermann with the patening. On this account the exact steering angles are called Ackermann-Angles resp. this kind of steering is called Ackermann-Steering or A-Steering.

The theoretical dependence cannot be realized by simple means, but only an approximation.
One possibility is the already mentioned steering trapezoid. The track rod arms of the stub axles hereby are not aligned exact parallel to the longitudinal axis of the vehicle but a little bit to the inner side (to the center line of the vehicle) or also to the outer side (in dependence of the position of the track rod.)
Now axle, track rod arms and track rod don´t form a rectangle but a trapezoid.

Rule of Thumb to build a Steering Trapezoid

The calculation of an optimal steering trapezoid is difficult, and probably this is the reason, that up to now it was not possible to find usable calculation instructions in the internet.

However on can find the "rule of thumb" that the extended track rod arms at straight ahead position of the wheels shall intersect in the center of the rear axle.

This rule of course can be applied, but this doesn´t result into an optimal approximation inevitably. So e. g. also the space between axle and track rod has an effect, and this the rule doesn´t consider.
Furthermore one can find, that with the renunciation of large steering angles (e. g. > 30°) the small angles can be approximated so much the better.
Aside from this generally valid properties the construction of a steering for rigid axles is subjected to other conditions, as the construction of a steering for single wheel suspension.

Steering with Steering Trapezoid and One-piece Track Rod

(applicable on models with rigid axle)

This steering can be constructed according to the classical trapezoid steering (see above) with an one-piece trackrod.
Disadvantageous hereby is, that the track rod of a spring mounted axle can move in 3 planes, and this complicates the drive of the steering mechanism. At metal models often a crank arm is used, which allows movement of the spring suspension too.
A steering angle deviation between the wheels during springing can not occure, because the complete steering mechanism follows the motion of the spring suspension.

The exact resp. desired dimensioning of the steering trapezoid depends on the size of the model; as a first approximation the mentioned rule of thumb is valid.
If the steering trapezoid shall be optimised to other aspects, one can find a calculative method in the chapter
Calculation of a steering with steering trapezoid and one-piece track rod
(only in German).

Steering with separated Track Rod

(applicable on models with single wheel suspension)

At this the wheels are mounted on separate transverse control arms and move on a circular path around the suspension of the transverse control arms during springing. The resulting space changes between the track rod arms with one-piece track rods lead to a steering angle deviation, which can´t be accepted at real vehicles.

Thus the track rod is separated in 3 parts at which the central segment is mounted parallel to the front axle and only can move transversal. In practice most this is a rack driven by a steering gear (Rack Steering).
The outer segments connect the central part with the track rod arms. Thus the wheels are decoupled, and with a proper length adjustment of transverse control arms and outer track rod segments the steering angle deviation during springing almost can be compensated.

Geometry of a Steering with
separated Track Rod

Because of the additional eligible length of the track rod segments the steering trapezoid can´t be determined by the rule of thumb. Under consideration of a minimal steering angle deviation the construction is to design for each individual case. A calculative method one can find in the chapter
Calculation of a steering with separated track rod
(only in German).

Steering with Steering Quadrangle

This construction is only mentioned for the sake of completeness and surely not justified in a metal model.

Schematic of a Steering with
Steering Quadrangle

Independant from an one-piece or a separated track rod in all cases this constructions supply only an approximation to the ideal geometry.

For an improvement of the approximation the steering trapezoid can be substituted with 2 steering quadrangles. With this one get 2 additional parts for adjustment which allow nearly exact geometry up to steering angles of 30°.

For the design of a steering quadrangle exist several alternatives. But even the version with the best approximation is hardly to realise in passenger cars because of the required space.

Steering of more Axles

Sometimes more than 2 wheels shall be steered.
Also such a construction can be shown by graphics and with the picture as below one can see, that the wheels behind the front wheels have to steer less than the front wheels.

Mostly the wheels before the (stiff) drive axle are steered.
But also interesting are constructions with additional steered wheels behind a drive axle (trailing axle).

Several steered Wheels

Steered Wheels of a Trailing Axle

For the practical realization of this constructions mostly several alternatives exist, but in any case all wheels are to connect together anywise.
At this often the component´s clearances sum up in this way that some wheels possibly steer quite undefined.

For reduction of the clearance also several alternatives exist:
In the simplest case the bolts of the joinings can be "thickened" by means of a short piece of insulation tube or hose. Thus the bolts are more tight in the wholes of the perforated strips; surely the tube will wear out in the course of time.

Another version is to fasten a fish plate onto the end of a perforated strip, namely with the slotted whole onto the last but one of the strip.
By displacing the fish plate the last whole of the perforated strip can be made any smaller.

The best solution surely is the use of spherical head joints and screwed rods which allow a free of clearance connection in all directions.

Home