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The Effect of Weight on Skidding.

21st January 1915
Page 14
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Page 14, 21st January 1915 — The Effect of Weight on Skidding.
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Which of the following most accurately describes the problem?

The fact that the present winter season is proving one of the wettest on record might of itself be sufficient excuse for reverting to this by now almost hackneyed subject. The writer, however, has further and _more cogent reasons, based upon a knowledge of what is happening daily in the course of operations on lines of communication at the Front. It is a fact that the most prominent difficulty confronting the members of the Mechanical Transport branch of the A.S.C. is that due to skidding. As may well be imagined, the continuous passage, over rain-sodden roads, of trains of .heavy motor lorries, not to mention that of horses, men, guns and all the other impedimenta of a modern army, has practically converted what may have been, at one time, passable or even good highways, into road-wide ditches. It is a pleasure to be able to record that this problem is being successfully tackled by the men on the spot. At the same time, there can be no doubt that any new light which may be thrown on the subject will be most welcome The Normal Winter Grip on Road Surface.

In the first place, it is very obvious that a skid is at nil times due to the tire's losing its normal grip of the road surface. The conditions under which this is -possible are, unfortunately, normal, at any rate during a considerable part of the year. Taking the most usual material employed for road surface, macadam, probably the only favourable surface condition would be that obtaining immediately after a good shower of rain, the road having thoroughly dried ; unfortunately • the periods when these circumstances hold are, of necessity, short in duration and of infrequent occurrence. Varying weather conditions detract, to a greater or 'less degree, from the ideal state. Under the influence of rain, if the surface is in fair repair, macadam still gives considerable holding power to rubber tires, provided the degree of wetness be constant ; if, however, subject to considerable traffic, and inclined to 'be muddy in places, care is necessary in negotiating -roads constructed of this material.

The Menace of the Tramways.

On asphalt, dry, a good grip can be obtained by -rubber tires ; unfortunately, if wet, this forms the most treacherous surface extant. Granite setts -possess, in a slightly less degree, the properties of

• asphalt. Neither of these may be treated with impunity, and careful driving is needed on either. 'Tramlines, usually found in conjunction with setts, are a continual menace to the safety of the motorist driving any kind of self-propelled vehicle. A fairly quick turn, necessitating the crossing of tramlines with the steering at full lock, usually involves one in a skid if the rails are wet. Frequently, too, this is that skid of skids—a front-wheel glide.

The Limitations of Non-skids.

Of all the various " non-skids " or devices for preventing skidding, although some of them undoubtedly achieve their object, under specific conditions, each

,certainly has important disadvantages. Taken as -types, the first and commonest is that which consists

of moulded projections on the tire surface ; this is of service only on muddy stretches, and even then only when new, exception being made here to that type of tire, with deep orifices at frequent intervals round the tread, which operates by suction ; on the other band, this latter form has a slowing effect. There is another method, which is admittedly of service only under very severe conditions of mud or snow, or both, perhaps. The writer refers to that type, similar to the Parsons chain, in which some foreign object is tied or fastened in some way, so as to cross the tire at intervals ; alternatively, in the case of twin tires, something in the nature of a chain is fastened round the groove between them.

Hard and Soft Tires. Pneumatic-Cushion Types.

As pointed out, these devices are only for use under special circumstances. Most people will 'agree with the writer, who thinks their use should be entirely prohibited on ordinary roads. The combination of soft and hard tires on the same wheels has been tried, but their success has not led to their very general adoption. It is hardly to be expected, perhaps, that this combination should have any great effect on the coefficient of friction between tire and road. The writer had the pleasure of a test run recently on a new type of tire, the principle being the reverse of the suction type referred to above, combined with a narrow tread. The device was effective.

The Necessity of "Holding the Road."

A little consideration should make it quite evident' that alterations or additions to the tire cannot have any positive or absolute effect as a non-skid. It is probable that if the pressure between tire and road could be kept at a constant value,, and if the driving force at the wheel rim could be arranged so that no rapid changes were imparted, no skidding would take place. If this can be presumed, then it becomes obvious that any permanent improvement must take place in the "holding the road" properties of chassis. Driving conditions being what they are, it is not to be expected that any amelioration of the harsh usage frequently necessary will be forthcoming.

As regards holding the road, several factors have to be considered as contributing, in a more or less degree, to this quality. One of these, the question of weight distribution, is apparently, as a rule, either entirely disregarded, or subject to considerable diversity of opinion by designers, if one may judge from the differences which exist, as between one make and another, in wheelbase dimensions for vehicles of the same load capacity. The problem, no doubt, is a difficult one, and complicated to some considerable extent by its inter-relation with others. For example, to view the matter from an unusual standpoint, in the opinion of the writer the ideal weight distribution for a chain-driven vehicle, having a comparatively light dead axle, must be different to that adopted as the best for a Jive-axle machine, with a rather heavy unsprung weight on the rear rubbers. The general practice seems to be to make the wheelbases too long. Probably the best way to examine the problem is by means of concrete examples. It should be explained that the writer's theory is that the two axles should have an equal tendency to cling to the ground. In his opinion, if this desirable object be achieved, neither axle will tend to skid to a greater or less degree than the other, and a sort of equilibrium will be set up, the whole grip of four wheels being thus, under ordinary circumstances, available to resist the skidding tendency. It can be attained, other things being equal, by arranging that the total weights on the front and rear wheels are in proportion to the unsprung weights of the front and rear axles respectively.

Resistance to Skidding.

The above, of course, is only one aspect of the question; • the writer's excuse for devoting so much space to it is his belief in its novelty. The more usual method of considering the effect of weight distribution is as illustrated by Figs. 5 and 6. The radius of gyration of the loaded lorry is found, supposing it to gyrate about the centre of the front axle. This quantity is calculated for each of the chain-driven vehicles, and a figure showing the skidding tendency is arrived at ; a figure showing the relative resistance to skidding afforded by the rear tires gripping the road is also given. It may be observed that the skidding propensity is then found to be 2-i per cent, greater in the long wheelbase lorry, and the resistance increases by practically a like amount Bad Effect of Differential.

The effect of the differential is on occasion for, and at other times against, skidding. Bearing in mind the broad principle laid down above, that to avoid a skid it is necessary to make changes in the power transmitted between wheel and road gradually, it can be seen that, from this point of view, a differential is a good servant only when actually rounding a curve, its action then aiding in obtaining the required effect. On the other hand, on a straight run, if one wheel begins to slip, it is given every opportunity to continue to do so.

The Four Diagrams of a Two-ton Lorry.

The examples taken, which are illustrated by four diagrams herewith, are of a two-ton lorry. Nos. (1) and (2) show a chain-driven vehicle with wheelbase of 10 ft. and 12 ft. respectively, Nos. (3) and (4) illustrate a live-axle machine, of similar dimensions. The total weight of the loaded lorry, less axles, is taken as acting through the centre of gravity, roughly 6 ft_ 8 ins, behind the centre line of the front axle, and the actual axle weights are given_ The drawings being self-explanatory, it is only necessary to point out that the shorter wheelbase gives the desired result in each case ; the alternative puts a larger load, in proportion, on the front axle, with a tendency for it to act as a centre upon which the vehicle may, and

does, swivel. The reverse effect, of -course, is obtained by making the base short.

Will the Front-wheel Drive Cure Skidding ?

Whether the front-wheel drive will prove a cure for the evil or not is, for the writer, a moot point. In all probability, on vehicles driven in this manner, the tendency will be to put excessive loads on to the front axle, to enable it to grip satisfactorily for hill climbing. Unfortunately this will render the tail rather. more liable to wag than it is with the more usual construction at present.

Improved Weight Distribution Wanted.

To sum up, possible improvements may be looked. for in the direction of improved weight distribution, possibly on the lines suggested ; it seems that this side of the question has up to the present received scant consideration. There may be more careful attention given to springing and to suspension generally.

Better arrangeme‘nts for brake compensating, and more accessible lubricating arrangements for the mechanism, also brake linings of a material which will engage smoothly, seem small points, but in this connection they loom large.Detail modifications in transmission, with the object of eliminating shock, or sudden changes in the effort curve, will only come gradually ; efficient clutches, with gradual take-up, and the provision of a shock absorber in the main drive are further trifles which go to make perfection. In this connection, the standard type of gear-changing mechanism has much to answer for, owing to the occasional jerks when changing, which are aI times quite unavoidable.

A Troublesome Subject.

There may be opinions herein expressed that are liable to criticism ; certainly differences have before now arisen with regard to some of the points that are touched upon. If this article have the effect of bringing to bear on a troublesome subject an increased amount of Consideration, the writer's object will, to some extent, have been achieved.


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