Al-Qassim University College of Engineering

Department of Mechanical Engineering

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Experiment-4

Ackermann Steering Demonstrator

Mechanics of Machinery Lab.

Introduction

Ackermann steering geometry is a geometric arrangement of linkages in the steering of a car or other

vehicle designed to solve the problem of wheels on the inside and outside of a turn needing to trace out

circles of different radii. It was invented by the Anglo-German inventor Rudolph Ackermann (1764–

1834) in 1810 for horse drawn carriages. Erasmus Darwin may have a prior claim as the inventor

dating from 1758.

A simple approximation to perfect Ackermann steering geometry may be generated by moving the

steering pivot points inward so as to lie on a line drawn between the steering kingpins and the centre of

the rear axle. The steering pivot points are joined by a rigid bar called the tie rod which can also be

part of the steering mechanism, in the form of a rack and pinion for instance. With perfect Ackermann,

at any angle of steering, the centre point of all of the circles traced by all wheels will lie at a common

point. Note that this may be difficult to arrange in practice with simple linkages, and designers are

advised to draw or analyze their steering systems over the full range of steering angles.

Modern cars do not use pure Ackermann steering, partly because it ignores important dynamic and

compliant effects, but the principle is sound for low speed maneuvers. Some race cars use reverse

Ackermann geometry to compensate for the large difference in slip angle between the inner and outer

front tires while cornering at high speed. The use of such geometry helps reduce tire temperatures

during high-speed cornering but compromises performance in low speed maneuvers.

Correct steering geometry is particularly important for human-powered vehicles, because if tyres

scrub as you turn, the energy wasted can significantly slow you down. It can also end up being

expensive in tyres! The design method often used to minimize this effect is also useful for lightweight

electric or solar vehicles - in fact, pretty much any multitrack vehicle.

There are several aspects to steering design:

First, you need to make sure the steering linkage turns the wheels at the correct angle when

you go round corners: this is Ackermann steering geometry.

Then, you may wish to minimize bump and brake steer by using what is known as centre

point steering or zero scrub radius geometry, usually achieved by kingpin inclination (side-

to-side).

Also, for stability and a 'self-centering' effect, the front wheels must employ some 'caster

effect' or 'trail'. This is usually achieved by inclining the kingpins fore-aft.

Finally, there are a few other considerations, such as the type of handlebars to be employed,

and some quick notes about detail design

Objective

- Steering error as a function of the steering angle with varying steering geometry

Apparatus

The objective of this kinematic model is to explain the special features of the Ackermann

steering mechanism. The lead angle of a steering trapezoid is determined as are the

disadvantages of incorrectly adjusted track rods. The units consists of two levers, an

intermediate steering rod, two track rods and two track rod arms. The indicators attached

to these rotate with the arms and indicate the steering angle on scales. It is possible to

adjust the pointers independent of the levers. By turning an adjustment nut it is possible to

change the length of the track rods. The complete arrangement is mounted on a base plate.

The unit can supplied either for wall mounting or as a bench top model.

Specification [1] Bench top or wall mounted experiment on the Ackermann steering mechanism

[2] King pin spacing 465mm

[3] Steering lock angle 50 , scale graduations 1

[4] Split track rod, length can be individual adjusted

[5] Steering rod made of aluminum

[6] lxwxh 630x280x50mm

Theory

When a trike or quad goes round a corner, it turns around a point along the line of its rear axle. As the

diagram shows, this means that the two front wheels will have to turn through slightly different angles

so that they are also guiding the vehicle round this point, and not 'fighting' the turn by scrubbing. As

the diagram below shows, the inside wheel turns through a greater angle than the outer.

Ackermann geometry is simply steering which achieves this, keeping each front wheel at the correct

angle, through the whole range of the steering motion.

Even with perfect Ackermann steering, there will still be some scrub, because of dynamic effects (the

trike tries to go straight on, the tyres push it round the corner, so it tends to understeer). Some builders

'tweak' the Ackermann model to take account of this, usually by arranging that the wheels remain

more close to parallel than exact Ackermann would suggest. Having said that, pure Ackermann works

pretty well - and it doesn't have to be perfect.

A steering system is designed to move the steered wheels of a vehicle into a geometrically accurate

position with respect to the centre point of the circular path traversed and thus to ensure that the

wheels roll properly. This presupposes that the lines extended from all wheel axes intersect

(Ackermann principle, Fiq. Below). To achieve this, the inside wheel must move through a greater

angle than the outside one, with this difference in angle being known as the advance angle. This effect

can be approximated by way of a so-called steering trapezium.

The steering trapezium is formed by the steering arm and track rod.A steering trapezium does not

entirely satisfy the required geometric conditions. The deviation between the steering angle of the

outside wheel and the theoretically required value is known as the steering error.

The Ackermann equations dictate the relationship between the turn angle of the inner wheel and the

outer wheel in a turn and the radius of turn of a vehicle. They are based on the improved Ackermann

geometry. The illustration below shows the general equation and the derivations of it we utilized in our

design.

The steering error is reduced by using a split track rod such as that of the Kl 160. for a wheel angle up

to 25˚ it should not be more than 0.5˚. With larger wheel angles, steering errors of up to 2˚ do not

present problems, as tight bends are usually only negotiated at low speed.

Experimental procedure

The following experiment is intended to determine the wheel angle αout as a function of αin. both track

rods are set to a length of 160 mm. the wheel base must be determined as it is not yet known. This

involves reading off the angle of the “outside wheel” for average conditions (αin =20˚) and using the

reading to calculate the wheel base (the distance between the front and rear axle) L. The swivel pin

spacing (the distance between the convergent point and front axle) of the KI 160 is B=464.6mm.

It is possible to adjust the pointers independent of the levers. By turning an adjustment nut

it is possible to change the length of the track rods. The complete arrangement is mounted

on a base plate. The unit can supplied either for wall mounting or as a bench top model.

αin =20˚ gives a reading of αout =16˚. The calculated wheel base is thus: L=628mm.

Report In the report:

- Fill the following tables

- Plot and find the relationship between (αin, αout), for both, experimentally and theoretically values

- Find the slope

- Find the maximum error in experimental and theoretical values

Readings to be taken

Table 1. Experimental data(wheel angle αout as a function of αin (L=628): values obtained by equation 2.

Table 1. Experimental data(wheel angle αout as a function of αin (l=628): values obtained by

experiment

αin set in ˚ 5 10 15 25

αout

calculated

in ˚

αin set in ˚ 5 10 15 25

αout read

off in˚

Questions for Further Discussion

- What are your suggestions to reduce the steering errors?

- In experiment, errors comes from where?

- How can you improve this experiment?

- what caster angle means?

Note: after you accomplish your report and all its items (introduction, theoretical basis and back

ground, description of test rig, measurement and results, discussion and ……..). Now it is time to write

your conclusion.

The conclusion, plain and simple, is the answer to your question. It should be clear, concise and stick to

the point. Resist the temptation to jump to conclusions.

Your conclusions should mesh with the objectives stated in the introduction and should be already

stated (although perhaps not as succinctly) in the Results and Discussion.

If you were to do your experiment again, would you get the same results?

Can there be differences? Why?

Ask yourself what happened when you tested your hypothesis.

What have you learned?

Discuss the results with your colleagues in your team and Write a final report summarizing your

question, research methods and conclusion.

Prepared by,

Instructor