The Skidding Controversy.
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The Relative Importance of Wheel Diameter and Road Camber.
Our contemporary " The Surveyor " has, we are pleased to note, responded to our invitation that it should put, forward data in substantiation of its charge against wheel diameter as a cause of skidding. We may recall that, in our issue of the 19th February, we gave an approximate scale, showing the tendency of the smooth rubber-tired wheels of a motorbus to skid as adhesion is diminished, due either to camber or the state of the surface of the highway.We there tentatively established a scale by which it may be seen that the power of holding the road is only 10 per cent. on " greasy " asphalt of that which cbtains oii dry asphalt. We proceeded to show that there was probably no margin between requisite adhesion for driving, and maximum available adhesion, on a gradient or cross-fall steeper than 1 in 30. Our eon. temporary took the view that. more importance should be attached to wheel diameters, and it specific. ally disclaimed any influence due to the tire itself, although we believe that this vital factor was rejected before the ease had been properly considered.
Our contemporary, with whom we are nauch pleased to have a friendly discussion, made the following general comment: " Both side-slip proper and the skidding of braked wheels are influenced by the diameter of the wheels, and those of motor omnibuses are•mueli too small, while the design of the brakes themselves is defective." We dissented from the view of " The Surveyor," in so far as it declared or inferred that skidding and side-slip of motorbuses were more due to inadequate wheel and tire dimensions, than to excessive angles of cross-fall in street or road construction, and in so far as it made, to our mind, an unjustifiable attack upon motorbus brake design (issue of the 26th March, page 78 ante). We find that " The Surveyor " devotes its first leading articie to the subject of this controversy, in its issue of the 10th inst., and we now publish its whole case, which has evidently been given careful consideration, so far as this caa be nia.de to ascribe a tendency to side-slip or to skid to the clement of customary wheel diameters. Whilst we are not averse to accepting the main arguments of the Editor of " The Surveyor," they must be regarded, in practically all senses, as dependent. upon assumptions, in so far as they can be applied to any differences due to variations of wheel diameters on public-service vehicles—even though a range he granted so wide as between, say, 40 in. and 54 in., and as disclosing less important effects than (a) greasy and dirty surfaces, or (b) common variations in camber and side-fall. The big wheel will not overcome either, except Iii minor degrees.
We will concede that the points, each admirably and simply put forward, are not to be disregarded, but we cannot believe that the Editor of " The Surveyor" himself believes or Imagines that they approach in importance, severally or jointly, in these days of increasing rubber-tired motor traffic, the resultant of more-appropriate cleansing of surfaces and reduction of camber.
Four Points in Wheel Diameter.
As regards the effect of the diameter of the wheel upon the tendency to skidding, there are certain considerations which cannot he set forth except by a careful presentment of the mathematics and dynamics of the subject. There are, however, four more simple reasons why there is less tendency to skid with wheels of larger diameter. The larger the diameter of the wheel the greater is the area of road surface covered, for the same width of tyre, or, at least, the length of this area is greater, and the probability that a patch of sufficient roughness to resist side-slip occurs in the portion of road covered is correspondingly increased. Li the case of a wood pavement this would practically mean that the pro.
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bability of covering an effective piece of imbedded flint is increased. This particular advantage is not very great frit lubber tires, but it counts. Afore important is the fact that a wheel of larger diameter holds the road better, and is less liable therefore to that momentary slackening of grip which often initiates skidding. Thirdly, there can be with the larger wheel a brake-drum of greater diameter. It can, while, allowing for the necessary clearances, be larger in proportion to the diameter of the wheel, which is an advantage, reducing the tendency to locking of the wheels. But even if the radius of the drum be no greater in proportion to the diameter of the wheel, the combined lengths of the two levers, brake-drum diameter and wheel diameter, will be greater ; and therefore for the same actual retarding effect the actual strain due to putting on the brake will he greater, measured circumferentially. This means that the force exerted in the production of the necessary degree of circumferential strain at the tread of the wheel will he smaller. Therefore, the loads being the same, a smaller coefficient of friction suffices, and the wheel has less tendency to skid when the brake is put on. The same reasoning applies to those changes in the force at the tread which occur as the braked wheel rolls on.
What They May be Worth.
These changes of force are due to important changes in the amount of the load, as the vehicle sways, or as the wheel passes over humps and hollows, corresponding to changes in the degree of compression of the spring. The wheel and biake with the longer strain, measured as an angle or circumferentially, will absorb less extra coefficient of friction when the load becomes less, because the distance through which the force acts, while that condition attains, is a greatei distance. Force multiplied by distance is equal to the work done. Putting it another way, we can say that the elastic reluctance of the wheel and brake drum, and therefore their efficiency as an intermediary between the force on the brake and the force on the road, as a measure of one of the factors of efficient braking. The same considerations, it may be remarked in passing, apply to propulsion, and the wheel of larger diameter is therefore less severe upon the road. Lastly, the wheel of larger diameter and the same strength will be subject to an angular strain, sideways, of greater dimensions for the same force exerted centripetally at bends, or when the vehicle is being steered to one side, or when, having begun to side-slip, its wheel comes to a rougher or less slippery part of the surface. This reduces the chances of sideslip, because the amount of the coefficient of friction needed to prevent or check the side-slip is smaller than it would be with a wheel of less diameter. To show that the combined effect of a number of factors, such as the advantages stated above, can be very considerable, assume that the advantage in the first case is so small that it is expressed by the facto/ ten-ninths, the second by nine-eighths, the third by six-fifths, and the fourth by eight-sevenths; the resultant advantage is expressed by the quantity twelve-sevenths, an advantage of over 70 per cent, numerically, but really much greater, because in these matters the difilerence between just enough and not quite enough is very great, and may be all-important.