IN DEFENCE ="4 DESIGNERS
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By A. W. Haigh
A.M.I.Mech.E.. A.M.I.A.E.
A N examination of the mechanical
causes of tyre wear which can be
directly attributed to chassis designers will throw light on the controversy that has ,bean the subject of recent articles and will support the views of " L.V.B." Modern commercial vehicles can not, in the writer's opinion, be designed to give greater tyre economy.
Assume, first of all, that the chassis to be investigated is of orthodox fourwheel design, with beam front axle, semi-elliptic springing all round, fourwheel brakes and engine in the normal front position and has a weight distribution of approximately twice as much load on the rear axle as on the front, when laden.
This last point is important, because a weight distribution in this ratio ensures equal tyre loading, with twin rear wheels, under normal running conditions. Owing to the camber of the road, however, the load on the near side is always slightly in excess of that on the off side (Fig.
In designing its Q-type bus, the A.E.C. concern even remedied this detect to some extent by mounting the engine and transmission on the off side of the vehicle, but it would be illogical to suggest that all chassis should be designed on the " Q " principle Merely to eliminate this slight difference. Many other features of far greater importance have first claim on the designe:.
Weight-tranference Affects Both Axles When a vehicle accelerates, weight is transferred from the front to the rear wheels; likewise, when the brakes are applied or even when the chassis decelerates naturally, .werght is transferred to the front wheels. For every acceleration there must be a corresponding deceleration, so it is permissible to say that weight transference, which is proportional to acceleration or deceleration, is equal on both axles, and, as unit pressure on all tyres is the same (because of the predetermined weight distribution) wear from these causes must take place at the same rate on all six tyres.
Traction effects, however, are borne by only the rear tyres, and it may be thought that by altering the weight distribution, by moving the load forward so that more is carried by the front axle, wear due to driving the rear wheels can be compensated for. This could be accomplished only at the expense of braking efficiency and vehicle stability.
On a vehicle with 331 per cent. and 661 per cent, weight distribution the most advantageous braking ratio has been found to be 60 per cent, at the front and 40 per cent, at the rear. For tneoretically correct braking this ratio is directly proportional to the relative loads on the two axles under maximum retardation conditions. Any alteration in static weight distribution would naturally result in a different dynamic distribution, and, as dynamic distribution is the governing factor in the layout of brakes, the braking ratio, with an increase in front-axle weight, would be 60 pet cent, plus on the front wheels and 40 per cent minus on the rear wheels, which means more braking on the front wheels than has been found satisfactory. This excess braking is a definite cause of decreased front-end control.
The fully laden commercial vehicle, whether goods or passenger, reacts more readily to centrifugal force than any other type of automobile, because of its high centre of gravity. In consequence body roll is a feature which must be guarded against as the intermittent pressure on the tyres resulting from the leaning of the vehicle first to one side and then to the other, may cause undue wear, apart from its effect on the control of the vehicle, the • comfort of the passengers or the movement of the load.
Stiffer springs would relieve roll to some extent but would also mean a higher periodicity which reac ts adversely on passengers, besides applying less-absorbed road shocks to the chassis frame. The latter item may • not seem of great importance, but a sharp blow—such as would be given by a wheel striking an obstruction at speed—transmitted to the frame via the stiffened spring, has considerably more distorting effect than the same blow more absorbed by softer springs.
Heavier shock absorbers would have the same effect on the frame as the stiff springs, whilst stabilizers cause each wheel to rise an equal amount when the vehicle is moving straight ahead and one wheel strikes an obstacle. Fig. 2 illystrates this point, the near-side wheel has come into contact with an obstruction in the road and risen the amount shown, the offside wheel, because of the stabilizer mechanism, is forced to rise an equal amount with the result that both sides of the frame receive an upward thrust.
Stabilisers Alternative to Harder Springs Although stabilizers do cure roll, the effect on tyre wear of a vehicle' dropping from even a small height, when the wheels have increased their speeds, due to lack of resistance to the engine, during the short time they have been in the air, is obvious. Stabilizers then should definitely not be considered and even the adoption of stiffer springs or 'harder shock absorbers would probably prove more harmful than Otherwise.
The effect of frame design on tyre wear on vehicles with orthodox suspension is nil. Whether a frame is constructed with channel or box crossmembers (both commonly used for goods yehicles), which allow it to twist and to help the springs in coping with the unevenness of bad roads, or whether it incorporates tubular cross-members (used extensively for double-deck bus chassis), which give it torsional rigidity, and provided., of course, that 'the suspension is correctly designed,
the road-holding qualities of the tyres and their wear are absolutely unaffected.
The use of independent suspension on commercial .vehicles is only an experimental venture, 'consequently the very adverse effect of this type of springing on tyre wear need not be,discussed. Orthodox axles, both front and rear, give the most economical tyre wear possible. The wheels are not affected by body roll, as is the case with independent suspension, they always run at the same angle to the road, unless the axle be tilted by one wheel striking an obstacle, and even the tilt caused in this. event is so stnall and of such short duration that no adverse wear on the tyres can result.
There are no points in the design of rear axles which can effect tyre wear, but front axles do embrace three causes of wear, two very slight and the other variable.
First comes the camber angle, that is, the angle of the wheel to the vertical. This is necessary for three reasons—to prevent the axle from having • that "knock-kneed " look, to assist the steering by reducing centre distance without excessive king-pin angle, and to keep the load-reaction line of the tyre close in to the king pin, thus reducing the bending moment in the stub axle.
This lean of the tyre, whilst rarely, if ever, exceeding 2 degrees, causes a slight abrasion on the tyre. When the vehicle moves in a straight path, the natural direction for the inclined wheel to take is 'ouhvards away from the centre line of the chassis, in a circle about a point determined by the intersection of the extended axis of the stub axle with the ground.
A condition of true rolling on this circle is that the radius of the tyre on the side nearest the centre of rotation (Fig. 3) must be less than that of the side farther away from the centre. The tyre meets this condition by . virtue of the camber angle, but it is constrained, by the steering gear, to travel in a straight line, which, for true rolling. demands that the two radii mentioned. be equal. As these radii are different, each side of the tyre travels a proportionately different distance per revolution of the wheel so that slight tyre sctub is set up.
Secondly, there is centre distance, which is more or less a function of the camber angle (Fig. 4). The distance between the intersection points With the 'ground of the centre line of the tyre and of the king-pin axis produced is the centre distance and has a great effect An steering design.
For'the -lightest steering the ideal distance is nil, but a centre distance of no magnitude tends to encourage wheel wobble whilst also causing slight tyre wear. When the wheels are turned by the steering gear they pivot about the centre point. This centre point is determined by the camber and kingpin angles, but as soon as the wheels move out of the straight-ahead position the camber angle alters and consequently the centre distance is no longer nil Relation Between Lock and Centre Distance The larger the angle of lock the larger the centre distance becomes, so that when a wheel is turned on full lock the point of contact of the tyre with the ground has described a peculiar curve which provides for neither true rolling of the wheel around the -king • pin nor true sliding. Slight tyre abrasion is the result which, in fact, is taken up ' by the elasticity of the rubber, As commercial chassis are not designed with absolute centre-point steering, the off-set of the wheel ensures true rolling of the tyre around the king pin.
The third point is what is usually called steering geometry. It is well known that the lock angles of near and off front wheels are different and that there is a definite relationship between the angles, which, if adhered to throughout all locks from straight ahead to full, ensures that both wheels roll on their correct paths. This relationship is governed by the inclination of the track-rod arms to each other. There is only one angle of inclination which gives correct steering, and if this is not specified by the designer the wheels, so soon as they are turned out of the straight, become out of track. The degree of error is governed by the discrepancy between the correct and actual angles of inclination, with resultant tyre wear which almost every operator has experienced with out-oftrack front wheels.
With regard to wheel design there is little to say. Sharp edges likely to cause damage are avoided, and flanges are rrilEe adequately high and of a slope which precludes any abrasion between them and the tyre walls. The choice between narrowand wide-base rims is governed by tyre design.
A narrow-base rim encourages laterat deflection of the tyres in that the wall height is increased. This `comparative sideways instability (I say comparative because the difference between the stability of a tyre on a wide rim and one on a narrow rim is very small) can be cured to some extent by stronger tyre walls, but added strength means more material and in these days of strict economy every ounce of rubber saved counts.
Tyres on narrow-base rims have the slight advantage of increased resilience, but this factor should be of no importance if the suspension is adequately designed and, in any case, the advantage is cancelled by the loss of stability: Therefore, if wide rims are used, maximum tyre life should be obtained.
Brake-drum Heat Conveyed to Tyre Modern brakes are designed to stop. a vehicle in a minimum distance without skidding, under normal read conditions. .During braking considerable heat is generated by the friction of the brake-shoe facings on the drams, part of which heat is transferred to the tyre. ,Excessive heat has a very adverse effect on rubber and the I.A.E. conducted recently a series of experiments on rear axles to determine the amounts of heat generated in different parts of tyres due to braking: Temperatures recorded on wheel rims under service conditions were found to be lowest when ventilated wheels 'and brake drums with external fins running parallel with the axis, were used. together with some means for increasing the air flow between the rims and drums. Where no provision was made positively tck, disturb the air, cooling being left solely to the ventilated wheels, conditions were unsatisfactory, but the use of Circumferential ribs on the drums did improve cooling to an extent slightly less than when axial fins were employed.
It would seem, then, that the main contribution which designers can make to increased tyre life is the provision of a fanning arrangement on rear wheels (the temperatures of front wheels are unaffected by any special devices for cooling them), circumferential ribs on the brake drums already being a feature of design.
Whether the design of tyres can be changed to give longer life is a matter for tyre designers, and we can rest assured that if an improvement i4 possible it will be made. The main point remains, however, that the life of a tyre is not dependent to any appreciable extent on chassis design, but on the care and attention which operators must give it.