HOW TYRE MILEA
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#1 IS WASTED No. 9
Bead Damage
BEAD failures are often very costly for four reasons. They are as follow ;HI) Frequently they occur long before the tread is worn out, thus causing cons:derable mileage loss. (2) They are mostly unrepairable. (3) Even when the tyre i8 worn out it is unfit for retreading
and is a narional loss. (4) If the failure be in the nathre of a burst, the tube also is ruined. There are many causes, most of
which are avoidable. Here are a few : —The use of incorrect rims, use of faulty rims, incorrect fitting of flanges, heat (usually transmitted from brake drums), overloading, underinflation, the fitting of wellbase tyres with the aid of levers; this applies to the smaller sizes only.
incorrect Rims Lead to Trouble
Let us deal with each in turn. The use of incorrect rims often leads to 'bead failure, because of the distortion which it entails. The relationship between a tyre and its rim is Much the same as that between a foot and its boot. If a boot be too large, too small, or ill-fitting, the foot will suffer in some way or ether. If the rim be too large, too small, or ill-fitting, the tyre will suffer likewise. '
So far as undersize or oversize rims are concerned, we are . brought back to our old theory of "working shape. "A tyre is designed to run in a certain shape. Its beads should be at a predetermined distance apart, con-. trolled by the distance between the rim flanges. If the beads be compressed on to a narrower rim, or stretched on to a wider rim, the -intended shape is altered and trouble will follow.
A Practice Having Harmful Results
An instance is provided in the frequent (but incorrect) practice of fitting 32 by 6 heavy-duty tyres on 32 by 6 truck-type rims. The, heavyduty tyre is designed to run on a rim of 20 ins, diameter and 4.33 ins. width. The truck-type rim is 20 ins, in diameter but only 3.75 ins. wide. Consequently, when the larger tyre is fitted to the smaller rim the beads are compressed .58 in. The working stresses of a tyre are so great that it is important that no " foreign " stresses be added. The correct rim is designed to fit, the tyre almost as snugly as a bearing fits a connecting rod. It may look all right on an incorrect rim, but it cannot fit properly in the bead area.
In addition, the compression of the beads causes a distortion of the remainder of the carcase which imposes stresses on both tread and side walls outside the normal stresses that the tyre is designed to bear.
The fitting of the 32. by 6 heavyduty tyre on a 3.75-in. rim (which I quote particularly because •it .is the most common instance of the use of incorrect rims) causes a particular form of bead trouble. Most 3.75-in, rims are of the two-piece type employing a single-spring flange which secures the tyre in position. The ends of this flange are usually separated by a gap of about in. When the heavy-duty tyre is compresSed on to a truck-type rim these ends gouge their way into the beads, and several plies of cord may be cut through. (See Fig. I.) This, of course, does not constitute tyre failure until it reaches an advanced stage, but it is a harmful influence which must be avoided. •
The use of faulty rims leads to similar trouble. If a flange be buckled, or the rim be distorted in any way, the snug fit between tyre
and rim is impossible. A "high spot" is created, and severe abrasion follows. The example shown in Fig. 2 is a case of complete bead failure on an almost new 36 by 8 tyre, due entirely to the use of a damaged wheel. The rim had split at the junction with the fixed flange (a fairly common trouble). The flange was gradually forced farther away from the rim because of the internal tyre pressure, and
the split widened. The picture gives some idea of the damage which followed. Stilldealing with the re Stilldealing with the re
Causes of bead failure in the order previously mentioned, we come now to the of
incorrect fitting the detachable flange. Obvi ously, if a flange be not bedded down evenly all round it becomes slightly eccentric to the periphery of the rim. Thus it will cut into the tyre bead at the point of greatest ecceio tricity. This trouble is more commonly found with the endless type of flange, which is fitted on much the same principle as a tyre on a Well-base rim. Faulty fitting leads to the type of ' damage shown in Fig. 3.
If difficulty be experienced in getting the rim properly bedded down, the tyre should be inflated to a pressure of about 10 lb. per sq. in. and the ring tapped into place with a heavy hammer. In no case should the pressure exceed 15 lb. before the ring is properly fitted.
The root causes of this trouble are the use of distorted flanges or the accumulation of dirt and rust in the flange bed. The cure is obvious.
The fourth cause of bead failure, heat, is not so easily dealt with. I know of a firm who burst, on an average, one 36 by 8 tyre ev'ery fortnight, mostly after negligible mileage._ and due entirely to heat. The fault, in this case, lies with the vehicle designer. The brake drums almost fill the " dish " of the wheels. Consequently, there is inadequate ventilation for the dispersal of the heat generated in braking. The heat from the brake arums is conducted to the rims, and from the rims to the tyre beads. The effect of heat on rubber and cotton is well known. The result is shown in Fig. 4.
This type of failure is costly, because it is often premature (i.e., before the tyre is worn out), is absolutely unrepairable, and almost invariably includes the complete allure of the tube.
There is not much that can be done to offset this fault iii design, except to educe braking to a minimum. , Prolonged braking, as in the case of steep descents on 'long hills, builds up terrific heat. On vehicles where the dispersal of heat from brake drums is inadequate it should be made a rule that long descents be negotiated in a lower gear, to avoid the use of the brakes.
The stresses imposed on all parts of the tyre by overloading and underinfiation have already been dealt with under their respective headings, and I do not propose to go into the subjects again. Both these factors give rise to side stresses on the tyres, more particularly on corners and steep cambers.
The tyre is anchored to the rim by its bead, but the rest of the carcase is wrenched from side to side by the roll of the vehicle. Thus, the .unyielding bead is subjected to considerable stress which may easily result in failure. Other effects of overloading and underinflation leading to bead failure are described in articles Nos. 2 and 4 of this series.
Use of Levers Causes Bead Injury •
Finally, we come to the injury caused to the beads of a well-base tyre when fitted with the aid of levers. Well-base tyres are used on only the smaller sizes of delivery van and are, in fact, cat-. tyres. The correct method -of fitting depends on the pressing into the well of one side of the tyre, in order that the other side may be pushed easily over the edge of the rim. This can be done with the feet or, better still, with a rubber hammer. Levers should never be used for fitting, but are necessary when undertaking the work of removal.
When levers are used the edge of the rim is used as a fulcrum', and the fitter can exert considerable force without being. aware of it. This, in its mildest form, injures the rubber covering.of the bead toe (Fig. 6), but, in more severe cases, may even break the wire.
°nee the wire of a well-base tyre is broken it is absolutely scrap, because it will not stay on the rim.. So soon as it is put.into use it blows off.
How to Avoid Bead Failures
To sum up, bead failures can be eliminated by : (1) the use of the correct size of rim—information on this point may be obtained from our Tyre Advisory Department; (2) the avoidance of rims that are in any way damaged or faulty; (3) careful attention to the correct fitting of flanges; (4) avoidance of heat due to prolonged braking periods; (5) avoidance of stresses due to overloading and underinflation ; (6) never using levers when fitting a well-base tyre.
If you be in doubt of the cause of any particular bead trouble study the accompanying illustrations. They are -typical examples covering most forms of failure, and will probably provide the answer to your problem.
• L.V.B.