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Factors A ecting ACCURATE CONTROL

5th September 1947
Page 33
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Page 33, 5th September 1947 — Factors A ecting ACCURATE CONTROL
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Which of the following most accurately describes the problem?

By J. PICKLES, A. M.I. M ech. E.

pREVENTION of roll is one of the most important considerations in vehicle design. Roll is the angular displacement of the vehicle centre line under the influence of a side force, as distinct from side sway, which is the linear displacement under the same influence.

For analysis purposes, the mass of the vehicle may be -considered as concentrated at the centre of gravity of the sprung body. This rnass will be found to revolve about Tor-Tie point on the vertical centre line and is known as the roll centre.

The vertical position of this point cannot be readily determined geometrically with any degree of accuracy on a vehicle fitted with laminated springs, but an approximate position would be at about the height of the spring eyes.

Centrifugal Force Thus, we may, in theory, replace the body by a lever, the length of which is equal to the distance between the e.g. and the roll centre, which we May call "L," and consider centrifugal force acting on the mass concentrated at the e.g. (Fig. 1). Roll will be resisted by the springs which are positioned on each side of the centre line and at a distance " b " from it.

Centrifugal force will act upon lever "L," and the product of force and length will give the rolling moment. Similarly, the spring force, multiplied by their spread, gives the resisting moment.

With a given rolling moment it will be obvious that, for a given spring rate (lb. per in. deflection), the farther the springs are spread, the more they will resist roll. From Fig. 2 it Will be seen that the replacement of the spring by a weight will cause an increase in resistance directly proportional to the distance As the resistance is provided by a' spring and not a weight, we may see from Fig. 3 that the greater the distance b," the more will the miring he compressed, and the -greater will be the resistance which is imposed.

By spreading the springs, roll resistance is thus increased as a result of the greater leverage, and because of the larger spring deflection. In engineering terms, the roll resistance varies as the square of the spring spreads. As an example, a chassis which has a spring spread twice that of another will have four times the roll resistance, other things being equal. • The rolling moment is directly proportional to the length of the hypothetical lever "L "—zero length giving zero roll—and every effort should be made to reduce this to a minimum. The height-of the e.g. is determined, to a large extent, by the 'body design, but this is invariably such that the weight is carried as low as possible. In the case of the roll centre, too, the use of laminated springs limits the height, Overslung springs are better than the underslung type, but although they may readily be used at the front, it is not easy to make this improvement at the rear without increasing the height of the e.g. Lifting one at the expense of the other is, of course, worse than useless.

Should independent suspension be used, there is much more scope for variation in the height of the ,roll centre, and although most designs are inferior in this respect, they have compensating advantages.

Parallel-action Suspension

• In the case of the true parallel. action type, the roll centre falls at ground level, so that the rolling moment is at the maximum. The spring, spread is, however, equal to the wheel track, so that, remembering that resistance increases with the square of the spread, it will be seen that, on balance, it is greatly superior to the orthodox design (Fig. 4).

Suspensions of the Porsche, Dubonnet and most of the wishbonetype systems have this characteristic of road-level roll centre. By raising the inner end of the lower wishbone it is possible to increase the height of the roll centre, and at least one design, in which the upper link is shorter than the lower, employs this principle.

The exact centre may be found by producing the centre lines of the two wishbones until they intersect a line drawn through the wheel roadcontact point, and this intersection will cut the vertical centre line at the roll centre. By this method it is possible to reduce the dimension "L" by 3-4 ins, at the expense of some 0.25 in. of wheel side movement.

Swing axle suspensions are rather despised in this country, partly as the result of an erroneous idea that heavy tyre wear is incurred. In this design the spring spread is again equal to the wheel track.

It must be emphasized that, in the case of independent suspension systems, the term "spring spread" is not sti ictly correct, in that the distance apart of the springs is of no importance, as they usually act through a system of levers. What is implied by this term is the spread of thtextreme pivots of the springloaded levers, and the spring strength is, of course, also to be considered to be at this point

Swing Axles In the case of the swing axle, the wheel is carried on a lever pivoted at or near the centre of the frame. With a centre pivot the roll centre is contiguous (Fig. 5), but where these pivots are spread a little on each side of the centre point, lines produced through the wheel road contact points, and through the swing pivots, intersect the vertical centre line at the roll centre (Fig. 6).

Because of the high resisting moment and low rolling moment, the troll resistance is abnormally high and has caused great difficulty in handling when used in conjunction with a suspension of low resistance.

To return to the orthodox suspension, at the rear the springs are invariably spread wide, so that only a running clearance is left between them and the tyres. The resisting moment is thus fairly high, although the low spring height results in a high rolling moment.

At the front it is necessary to mount the springs fairly closely together to maintain clearance for the wheels when on lock. Although the rolling moment at the front is low, it does not offset the disadvantage of low rolling resistance. An average vehicle would have a spring spread about half that of the wheel track, so that, in theory, its resisting moment is about a quarter that of the swing axle

Roll and Spring Twisting

In mitigation of this fault, the roll can occur only with the accompaniment of spring twisting. With 5 degrees of roll the spring eyes will be twisted by this amount, leaving the axle anchorage at the angle of the axle.

The torsional resistance of the springs may put up the rolling resistance by 25-50 per cent., even in the case of an orthodox spring, and with the aid of special anti-roll clips even higher figures can be achieved. Resistance from this source :las, however, the disadvantage of increasing the resistance to one-wheel bumps and, therefore, the suspension sulfers.to a small extent.

One of the primary reasons for the adoption of independent suspensio:1 is the desire to improve handling characteristics. A widespread demand

by car owners has caused such equipment to be adopted by many manufacturers, although the need for such a refinement does not exist in small cars. Large coaches have, however, all the qualities which go towards making independent wheel suspension a necessity.

It will probably have been noticed by drivers that when travelling at speed, and passing over a bump in the road, the steering kicks. In many cases this kick does not cause the steering wheel to be turned in a

direction such as would result if the affected road wheel were forced rearwards, as would be the case at low speed. In fact, the road wheel will turn towards the bump (Fig. 7). This phenomenon comes about as the result of gyroscopic force.

To analyse the cause of this curious effect, we must consider the motions possible with a steered road wheel. There is, first of all, rotation of the wheel on its bearings, so that the stub axle is, in effect, an axis of rotation, which we may describe as the primary.

Action on "Bumps" When one wheel of a normally axled vehicle passes over a projection on the road surface, both wheels are tilted, and rotation may be said to occur about a horizontal axis. To reduce the argument to essentials, the vehicle may be considered as moving in a straight line, so that this secondary axis is at right angles to the first, and both are in the same horizontal plane.

The coincidence of the two planes of rotation tends to cause rotation about a third axis, the mechanics of which are rather complicated and outside the scope of this article. Suffice it to say that this does, in fact, occur, the third or precession axis being at right angles to both primary and secondary axes and, therefore, vertical.

It is, therefore, more or less coincident with the king-pin axis about which the wheel may revolve, except that it is restrained by the driver. His force is multiplied by the leverage of the wheel, by the reduction ratio of the steering box and augmented by the friction of the linkage and steering gear.

Gyroscopic Force

In most cases when gyroscopic force is applied to the wheel, the energy is lost in overcoming friction, but on a few occasions its magnitude may be such as to be felt at the steering wheel, calling for rapid correction. At high speeds there is the danger that these forces may cause the vehicle to be uncontrollable.

The magnitude of the gyroscopic force is dependent on the rotational speed of the road wheel, its weight, and on the velocity of rotation about the secondary axis. In the matter of weight, coaches are, of course, far worse than private vehicles, and the modem trend towards soft suspension permits of more rapid wheelrising speed.

Gyroscopic force, it must be remembered, is a result of wheel tilt; bodily lifting of the wheel (motion in a translatory sense) causes no M effect. In consequence, suspension designers have sought to eliminate gyroscopic force by arranging the road wheel to move in a plane parallel with the centre line of the vehicle, so that if the body remains undisturbed wheel lift can occur without tilting.

Steering Kick-back The complete suppression of gyroscopic force cannot, however, be considered ideal. When travelling slowly, it has already been agreed that the orthodox-axled vehicle is subjected to a kick-back on the steering because the wheel is forced rearwards as the result of passing over a bump. To put this more accurately, resistance to the motion of the wheel is imposed while the motion of the vehicle is continued.

Gyroscopic force, as has been stated, causes a reversal of this force, with similar ill-effect on control. With the usual use of compromise, the designer of a first-class suspension system attempts so to reduce the gyroscopic disturbance that the rearward push is most nearly balanced.

With the object of reducing change in track when wheel rise or fall occurs, it is customary, with suspension of the wishbone type, to arrange for the wheels to tilt slightly. In many instances tilt reaches an angle almost equal to that of the orthodox suspension, although the total gyroscopic force is only half that of the latter type as both wheels must tilt when one passes over a bump. With independent suspension one wheel only is affected.

Wheel § that are unbalanced can have a bad effect on the steering, and many high-grade vehicles are provided with means for correction—a feature which could well be employed on coaches The effect of an unbalanced mass, when freely suspended, is to revolve about its centre of gravity, so that if it be mounted on an axis which is not coincident with this point, there must inevitably be a tendency to revolve the mounting axis bodily about the centre ol gravity Tyre " Tramp " In the case of the front wheels of a coach, up-and-down motion of a wheel from this cause can occur only by deflecting and relieving the load on the tyre, which, in extreme cases-, may lead to " tramp " or " patter " Similarly, the horizontal component arising from this unbalance may cause the wheel to rock backwards and forwards about the kingpin. As the centre of the wheel road-contact area is some distance behind the pivot axis (the trail dimension), the angular rock about the king-pin tends to cause the axle beam to be oscillated bodily beneath the frame to an extent which depends largely on spring, and partly on frame flexibility.

Inferior independent Suspension In a properly designed independent suspension system, rigid linking of the wheels to a frame which is sufficiently strong to resist the imposed loading materially reduces road vibrations. In the past, however, insufficient strength has caused trouble with independent suspensions of inferior design With orthodox suspension systems, careful design can ensure ample rigidity. Shackle and spring bushes of adequate proportions are essential, and improvement has been effected, in certain instances, by eliminating the shackle and employing a sliding arrangement for the spring end

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Organisations: Gyroscopic Force

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