HOW TYRE MILEAC IS WASTED
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No. II—Sundry Mechanical Faults
TYRE life -depends, to a great extent, on the mechanical condition of the vehicle. In the course of many thousands of miles
, certain parts lose their original setting and added stresses are imposed on the tyres. The most common (and certainly the most drastic) form of adverse mechanical influence on tyre life is front-wheel misalignment. This has already been fully covered in No. 5 of this series.
The present article deals with those causes of wear which are less important than misalignment, but which, nevertheless, are responsible for considerable mileage waste.
First, we will consider braking. In article No. 7," Speed and Braking," we saw that the brakes, even if properly adjusted, were one of the chief factors controlling tyre life. I showed how the rate of wear during a brief braking period of, say, 30 yds., might easily be far greater than in 30 miles of brake-free running.
When Brake Is Deadly Influence
This, in itself, is serious enough, but if, due to incorrect adjustment, the rate of -wear be accelerated, the brake becomes a deadly influence which will grind rubber off the tread at an unbelievable rate.
a In order to understand the problem fully, it is necessary to know exactly what happens when the brake is applied. The tyre,is the only point of contact between the vehicle and the road, and is responsible for the transmission of the forces of propulsion and retardation. It can fulfil these duties properly only so long as
it retains its grip of the road surface.
If, in response to either force, the tyre spins 'or skids, then it has lost its grip and is not propelling or retarding the vehicle in the same ratio as the force which is being applied to it. Road adhesion, then, is absolutely necessary for the full application of the power of engine or brake The tyre must be geared" to the road.
Braking Forces Are Concentrated
This, of course, is a practical impossibility, and there is, inevitably, a certain amount of slippage, which varies according to the factors of tread pattern, road surface, speed, weight, eta. The point which I really want to emphasize, however, is that all the stresses of braking are concentrated in that small area of each tyre tread which is in contact with the ground, bearing in mind, particularly, that the road surface is hard and abrasive, whereas the tyre tread is—just rubber. It must also be borne in mind that this small are& of rubber is already forced into heavy contact with the ground by reason of the weight which it is supporting, i.e., the vehicle and its load.
When the brake is applied it works in the opposite direction to that of wheel rotation. Momentum carries the vehicle onwards, but the brake is trying to hold it back. During this period of deceleration the tyre is "twisted '.' between the opposing forces, and it is then that its force of " grip " of the road surface is exerted to its uttermost. The Op, of course, is frictional, and that is why the rate of wear is so rapid when the brake is applied.
Bearing all this in mind, it is easy to see that unbalanced braking is bound to make matters far worse for the tyre. If the stresses of braking and road adhesion be equally imposed on all tyres, then the stress per tyre is normal.
But if, as a result of incorrect linkage (for example), one brake be a little in advance of the others, the stress off that particular tyre is considerably increased. ' Momentarily, by reason of its advanced adjustment, it is called upon to retard the vehicle unaided, and even when the other brakes come into somewhat ta rdy action the one tyre is still doing most of the work.
This form of unbalanced braking is serious, because most of the work falls on one tyre. It is far more serious for one brake to he too early in coming into use than too late. On a four-wheeled vehicle, if one brake be early, the tyre is made to do the. work of four.. But if one brake be late, the the remaining three are made to do the work of four. This is not quite so bad, but it is bad enough. In either case the tyre is the ultimate sufferer.
It isnecessary, then, to make periodical checks of brake efficiency, in order to ensure that each unit is doing no more nor less than its proper share. Particularly is it necessary to check the relationship between front and rear brakes. A slight lag will do considerable damage.
Similarly, a brake which sticks on, or binds, will cause rapid tread wear. Its effect is just the same as in the case of unequal brake application. Once again, the vehicle is trying to carry the tyre onwards, whilst the brake is trying to hold it back. The " twist" is exerted and the tyre-toroad friction is increased.
Symptoms That Are Usually Peesent
The following are the symptoms which usually accompany these faults :—(1) One or more of the tyres wears down more 'rapidly than, the others; (2) brake efficiency is not what it should be; (3) there is often a tendency to skid in wet weather; (4) acceleration after braking is poor (binding brake).
Confirmation can usually be obtained by feeling the brake drums immediately after a period of normal braking. Inequalities are usually indicated by variations in tempera ture. In all cases where one tyre wears more rapidly than the others a check of alignment and braking should be made. Apart from relative braking Inequalities, as between the different wheels, it is quite usual to find that an individual brake will exert varying pressure for each revolution of the wheel. The commonest cause of this trouble is brake-drum ovality. Other causes are scored drums, caked oil, loose facings, etc. Although the pressure on the brake pedal may remain constant, the amount of braking force exerted will increase and wane with each rotation of the wheel—like the rise and fall of a cam, almost.
Varying Stresses On Tyre Tread
Obviously, as the braking force rise's and falls, the friction between the tyre and the road rises and falls in exactly the same proportion. Bear in mind that the relative position of tyre and brake drum does not change, because they are bolted together as a unit. Naturally, then, the rise.and fall of the braking force always fall on the same parts of the tyre. While the brake is applied, the different parts of the tyre tread are subjected to varying stresses in direct ratio to the amount of braking force which their relative part of the brake drum is capable of applying. This causes uneven wear in a small area of tread (Fig. 1).
Alternatively, it may take the form 'of a gradual "fade put" of pattern over an area of, say, 25 per cent. of the total tread (Pig. 2). Often, when the cause is brake-drum ovality, \rapid wear occurs at two points on
the tyre treat. Usually, these are diametrically opposite each other. Another cause of rapid wear is the fierce brake—the " grabbing " brake, if you like. Fierce braking is regarded by some people as an indication of braking efficiency; Its value, however, is deceptive. • A braking system which takes hold gently, and reaches its maximum'proe gressively, can be even m9re efficient than one which, comes into action violently so soon as the pedal is , depressed. Naturally, it is far bettef for the tyre to take up the job of braking gently, rather than to be suddenly " snatched " into action.
Watch Brakes Most Carefully
This -matter of braking cannot be too closely watched. Remember that everything that the brake does is effected via the tyre. Consequently, everything that the brake does wrong can be measured in terms of tyre wear. You.. want to keep your rubber on the tread—not grind it off on the tarmacadam.
So much for the brake. It lia,s taken up considerable space in this discussion of mechanical faults affecting ,tyre life because, after alignment, it is the most serious. Let us turn to some of the others.
Most of these defects are associated with front wheels. The general assembly of front axles is much more complicated than that of rear axles, because the steering is included. The. extra movements necessary and the additional parts involved (i.e., king-e' pins, bushes, etc.), are all points of potential wear and loss of adjustment.
• As usual, the tyre is called upon to bear the extra burden resulting from mechanical deterioration of the vehicle. .
Wear Caused by Excessive Play
A familiar form is seen in Fig. 3, showing scoopy, irregular wear due to excessive play in front-axle parts. I have noticed that this trouble is far more prevalent on motor coaches fitted with low-pressure tyres than on goods-carrying vehicles having highpressure equipment. Do not ask me why, beIcause I cannot tell you. I mention it because it is a point which coach operators should guard against.
It may, possibly, be due to the fact that a low-pressure tyre exerts a greater amount of road grip than a high-pressure tyre and, consequeptly, • imposes heavier stresses on the steer
ing parts. These stresses enay, in turn, cause more rapid wear on these parts and the resultant play would affect tyre life.
In the article on misalignment mention was made of tir settings c4 camber and castor. These two settings are closely related to that of toe-ireand each is interdesiendent on
the other twO. Very small variations from the tl-ue adjustment will give rise to rapid and irregular tread wear.
Look at the tyre in Fig. 4. The camber of the -front wheels has been excessive and only the outside shoulder of the tread has been in contact with the road. Consequently, on one side the rubber is worn right away to" the canvas, whilst the other side is almost untouched. " Spotty " wear i,s another result of excessive camber.
A mechanical defect which is often allowato go. unchecked is the escape of oil from the back axle on to the tyre walls (Fig. 5). Oil, as I have said before, is most destructive of rubber. If the condition be allosied to continue it will almost certainly lead to failure. It is not sufficient merely to check the leak. The caked oil should be scraped off the tyre and theremainder washed away with petrol. The petrol should then be washed off by vigorous hosing with water.
Damage from Faulty Springs
Much damage can be caused by broken or weakened springs. In their mildest form, they may result in a " pitchlr*" of the load towards the weak point, which is a form of load transference causing overloading of the tyre. More serious is the effect. when the load, being too great for the spring, forcks the mudguard down on to the tyre; Fig. 6 is an almost unique example of this. The tread was literally ripped off in . chunks.
Still another example of the damage caused by a broken spring is shown in Fig. 7. The broken leaf slipped out of position and its sharp edge came into contact with the tyre. wall. As the wheel revolved a deep cut was scored into both rubber and canvas; and thistyre. (brand new) was ruined in just over a week.
There are many other causes -of tyre wear attributable to mechanical defects, but most of them are too rarely met to justify special mention. A minor. impact, for instance, may cause slight distortion of a chassis part which will throw an added load on the tyre.
Recounting a Personal Case
Such a case occurred to me only a short time ago. A bus hit the off-side rear corner of my car, giving it a glancing blow. So far as the eye could see, the only damage was the denting of the panel. A short time afterwards, however, I noticed that one rear tyre was much more worn than the other, despite the fact that they had been fitted together. Closer examination of the tread revealed the unmistakable signs of misalignment. The feather edge was there, together with the scuffed appearance of the rubber. It seems that the collision Bud slightly displaced the rear axle.'
In order to make this article as comprehensive as possible, I would like to recall, in closing, two mechanical defects which have already been fully discussed in a previous article—bead failures 'due to faulty rims, and' bead bursts due to heat conducted from ill-ventilated brake. drums. Both of these are covered in No. .9 of this series, entitled "Bead Failures."
• L.V.B.