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by P. A. C. Brockington, mimechE
Roll stiffness could spell danger
A LEADING OIL company technician has stated publicly that the roll-over tendencies Df long attics carrying a high load should be the subject of intensive research. And the Deed to increase the roll stiffness of semirailer axle or bogie suspension as a means of improving stability has been highlighted hy a number of authorities since the rollover hazard, associated with long artics, was first publicized some nine months ago.
An improvement in roll stillness can be obtained in the case of a beam axle by autrigging the springs which increases the spring base, roll stiffness being proportional Lc) the square of the spring base. If independent suspension is employed, the effective spring base is equal to the track, and the gain in roll stiffness may be as much as four times the stiffness obtainable (in the absence of an anti-roll bar) with a conventional beam-axle layout. Moreover, with some types of independent suspension, notably the swing-axle type, the roll centre is raised above the normal location and this also reduces roll.
When a vehicle rolls, the overturning couple is a product of the lateral force acting through the centre of gravity and the height of the centre of gravity. Moreover, a secondary roll couple augments the roll-over tendency and this force can be considerable if roll is excessive. As the centre of gravity moves towards the outer wheel as the result of roll a secondary couple is created, which is the product of the weight of the sprung mass and the lateral distance moved by the centre of gravity.
A high-roll resistance would, therefore, appear to be a favourable factor in terms of stability, but in practice the flexibility of the frame or superstructure represents a variable that can influence the value of roll resistance. The frame of a conventional platform semi-trailer is torsionally flexible, while a typical frameless tanker may be regarded as a rigid structure comparable to that of a modern car. A vehicle with a flexible frame that is twisted by centrifugal force on a curve may have very different handling characteristics to a similar vehicle with a torsionally-stiff structure.
Roll-over tendencies When considering roll-over tendencies, it is necessary to differentiate between the attitude of a vehicle on a corner after entering the curve from a straight path and its attitude on the second curve of an S-bend or the equivalent. In the first case any increase in the severity of the curve results in a progressive increase in the lateral forces acting on the vehicle produced by centrifugal force acting through the centre of gravity, and the roll frequency of a semi-trailer has little effect on stability unless the secondary roll couple is excessive.
If, however, the vehicle changes direction transitionally as it moves from one curve to another, stability may to a considerable extent depend on roll frequency. Given that the frequency matches the time occupied by the vehicle when travelling from the first maximum-roll point to the point at which the direction of maximum roll is reversed on the opposite lock, roll-back increases the total force tending to roll the vehicle over. In this case, high roll resistance of the suspension should be advantageous as it increases the roll frequency rate, a low roll frequency being associated with a high roll amplitude. Good damping and the use of a variable rate suspension are also advantageous.
Because the frame of a conventional articulated vehicle is torsionally flexible, it may be regarded as a part of the suspension system. At one time this was also true of private cars and when cars were first built as frameless rigid structures, new handling problems were created. It is appropriate to cite particular problems that resulted in the early days from fitting suspensions having a high resistance to roll.
In an article by Donald Bastow published in the January 26 1944 issue of Motor, a photograph is published in which the inner back wheel of a car (with a relatively high centre of gravity) driven to its limit on a curve, has lifted some 12 in. while both front wheels are firmly on the ground. The author observes that the lift-off of the inner rear wheel proved that the roll stiffness at the back was higher than the stiffness at the front, and it has since been acknowledged that this is unfavourable to stability as shown by cars fitted with an anti-roll bar at the rear only. The greater the roll stiffness the higher is the load transfer from the inner to the outer wheel on a curve. And the cornering power of the tyres is reduced by high-load transfer.
Overturning couples In an article in Motor written 23 years later (February 18 1967 issue) Charles Bulmer, the Editor (then Technical Editor) deals at length with overturning couples and allied aspects of car handling. Charles points out that "chassis tuning (of car chassis) is largely concerned with roll, lateral weight transfer and the distribution of this transfer between front and rear wheels—all things which are affected by anti-roll bars, spring stiffnesses and roll centre heights". And he adds that "these topics seem to cause endless confusion".
Later in the article the author emphasizes that "the fundamental proposition to bear in mind is that total weight transfer for the whole car has nothing to do with suspension variables—it depends on the height of the centre of gravity and the width of the track. Since the overturning couple is fixed it follows that anything you can do to increase the weight transfer at one end of the car will automatically reduce it at the other".
In line with this concept it would appear that increasing the weight transfer at the rear end of a semi-trailer having a rigid structure by improving the roll stillness of the axle or bogie suspension reduces the weight transfer at the front end, that is on the axle of the tractive unit. But this is not necessarily a valid assumption in the case of a torsionally-flexible chassis frame of the conventional type, which allows the roll force to twist the frame and a part of the force to be transferred to the front. Despite the fact that excessive roll produces a secondary overturning couple, this can be a favourable factor. If a large proportion of load transfer is at the rear, the roll-over tendency could be increased.