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TIRE BEAD FAILURE DUE TO HEATING
Aluminum's heat dissipation property is excellent, keeping the wheels at a lower average temperature than steel wheels.
The purpose of this instruction is to provide general observations on bead failure from heating, and to emphasize that failure to observe criteria for safe vehicle driving may lead to bead failure from heating regardless of the type of wheel in service (radial, disk, steel or aluminum).
The text below is reproduced in full from the Technical Bulletin published by the ALABA - Latin American Tire and Rim Association, to which Alcoa belongs.
For additional assistance contact your Alcoa representative or call Alcoa Rodas de Alumínio Ltda.- After-Sales Department - Phone 011 3611 3086.
1. INTRODUCTION
Accidents, with or without blowouts, often occur as a result of brakes overheating.
Very high temperatures on the brake pads (above 250º C) lower the efficiency of the system by gradually reducing friction between the brake pads and drums. In addition, the heat spreads, damaging the pieces of the system, including the tires, inner tubes, valve stem (tube tires), and valve core.
Depending on the levels of temperature transmitted to the wheels and the exposure time, there may be damage ranging from cracking near the bead, melting of the tube, breaks on the rubber of the beads during dismounting, to explosion of the tire (temperatures above 140ºC).
Hence, brake overheating causes serious risks of accidents and damage to components, in particular to tires and their accessories.
2. TIRE BEADS
The beads are responsible for attaching the tire to the vehicle through the wheel, and hence they are part of the most critical region of the tire, where the efforts of acceleration and braking transmitted from the vehicle to the ground are concentrated.
Overheating in this area (temperature above 80º) causes irreversible damage to the rubber that sustains the anchoring of the carcass cords to the rim of the tire.
Excessive temperature causes the rubber to lose its physical properties, allowing the internal pressure of the tire to expand the carcass, through the abrupt or continuous unrolling of the bands enwrapping the rim of the tire. *
THE CONSEQUENCES:
- In more critical cases, rupture of the bead (see examples at the end);
- Emergence of circumferential cracking at the beads;
- Bubbling (wrongly treated by bakeliting) which makes the bead rubber more likely to break when tire is mounted or dismounted;
- Deteriorations in the inner tubes and protectors rubbing against each other, or, depending on the degree of heating, melting of inner tube, causing rapid loss of tire pressure;
- With tubeless tires, damage to the bead, causing loss of pressure, as this area must assure the seal of the wheel-tire combination.
- Likewise in tubeless tires, the rubber valve stems deteriorate, causing loss of air pressure.
3. INFLUENCE OF TEMPERATURE ON LIFE OF TIRES
To enjoy maximum potential tire durability, the temperature of the beads (measured at their base) must be kept near 80ºC.
Above this temperature the beads begin to degrade, gradually reducing the life of the tire.
In the graph below, the potential life of the tire is a function of maximum temperature reached on the beads, continually, with all other factors excluded:
4. MAIN CAUSES OF TIRE OVERHEATING
Various factors contribute, together or by themselves, to tire overheating. We can divide them into two groups: operating and maintenance.
4.1 Operating
- urban traffic in large cities requires more frequent use of the brake, often associated with an aggressive driving style, prompted by stress;
- Mountainous areas also require more intensive use of brakes when the rules of good driving are not observed, e.g., using the motor brake properly, going downhill in the right gear, etc.;
- transport with excessive load considerably increases heat on the brakes during braking;
- excessive speed or speed not suited to traffic or road conditions also forces use of the brakes, causing greater heat dissipation onto them (kinetic energy, it should be kept in mind, is proportional to the square of speed; Ec= 1/2Mr2. For example, if speed doubles, kinetic energy quadruples);
- For cab and semi trailer and/or "Julieta" rigs, the incorrect and abusive use of manual brakes (through hand brake) strains the mechanical brake system, concentrating excessive heat on the brakes themselves, leading to very serious consequences;
In some European systems this system has now been abolished and in others its action has undergone the limitation of brake pressure, in order to avoid the risks from its improper use.
By way of example, the risk of one of these rigs "jackknifing" after excessive use of the SR brakes (through the hand brake) increases significantly, because the overheated semi-trailer brakes lose a good portion of their efficiency. Under these conditions, a light touch on the brake pedal, which acts on the whole rig, will affect only the cab, since the trailer brakes are not very efficient. At that point it will take very little for the semi-trailer to "overtake" the cabin.
- Not using the motor brake on downgrades or in stopping the vehicle forces the service brake to be used more intensively, causing excessive heat which could have been avoided.
- Failure to observe maintaining minimum recommended distance from the vehicle in front, which varies depending on traffic, leads to frequent use of the service brakes.
4.2 - Maintenance
It must be emphasized that all the factors cited in the previous item are already very harmful, even on well-maintained vehicles. If they have defective or unregulated parts on the suspension or the brakes, the risk of accidents increases.
Proper brake maintenance is fundamental for safety and for the life of components and the tires. Good maintenance means following the recommendations of the vehicle or parts manufacturers, and using the original parts specified by manufacturers, when they have to be replaced.
Predominance: in most truck cabs there is a valve that allows, within a usual band of service brake activity, a pressure differential between the brake circles of the cab and the trailer; that is, it allows for the semi-trailer to receive slightly higher pressure than the cab.
This pressure differential is factory regulated between 0.15 and 0.6 bar, depending on the manufacturer of the vehicle.
Some of these valves allow for regulations while others do not. Those that allow regulation must only be regulated at authorized shops, as special equipment is required.
In practice it is clear that most are regulated unintentionally by the maximum differential, on the order of 1 to 1.5 bar, which means that on normal braking (between 1.0 and 3.0 bar pressure, measured at the brake pedal exit) the circuit of the semi-trailer will have 1.0 and 1.5 bar more pressure and obviously its brakes will be under more demand and therefore hotter.
5. SOME RECOMMENDATIONS FOR BRAKE SYSTEM MAINTENANCE
5.1 Adjusting Pads
Brake pads must be adjusted so as not to rub against the brake drum while the vehicle is moving freely. Because of possible ovalization of the rims as a result of wear and the stresses to which they are subjected, this adjusting must be done with the axle raised.
With the vehicle resting on the ground, it is impossible to check whether there are also points of contact with the pads while moving.
In combined units (cab + semi-trailer), when the pads of the semi-trailer are adjusted, the pads of the cab must also be adjusted. In the market it is very common to leave the pads of the cab farther away than those of the semi-trailer so that the cab will brake less. This practice is harmful to overall safety, and leads to the problems mentioned earlier.
5.2 Relief valve or rapid discharge
When the brake pedal is released after braking, these valves serve to rapidly discharge the air in the brake chambers.
Otherwise the brakes would be applied longer than desired, causing unnecessary heat and wear. The functioning of these valves must therefore be checked periodically and whenever there is a problem of overheating.
5.3 Shoe return springs
These springs become fatigued with use, primarily when exposed to excessive heat.
They lose their tension, eventually allowing constant contact of the pads with the brake drum, causing not only unnecessary heating and wear, but also the brake to lock up (through contact of shoe with brake drum at high rotation).
5.4 Predominance
Not altering the original regulation of the pressure regulation valve going to the semi-trailer (through the "friendly hand"). In case of doubt consult the vehicle manufacturer directly or through its service network.
5.5 Retarders
The use of hydraulic or electromagnetic speed retarders, which considerably reduce the need to use the service brakes, is always recommended. They greatly lower maintenance costs, and significantly increase safety.
When used correctly this equipment slows the vehicle down, acting directly on its transmission system, making the use of the service brake unnecessary for a good portion of the trip, and significantly lowering heat generation in the brake drums.
Using such equipment translates into:
- Longer life for brake pads and drums;
- Longer tire life;
- Greater safety in transport;
- Lower maintenance costs.
The initial investment cost is recovered and paid off in a short time with the lowering of maintenance costs.
TIPS
1. Never be near an inflated tire that has overheated while it remains hot (especially when there is a smell of hot brake pads and rubber).
2. In descents from hills or mountains, the exaggerated use of service brakes due to faulty driving causes tires to overheat to the point where they can be damaged or blow out. It is suggested that the vehicle not be stopped after these sections in order to allow the rigs to be ventilated. When ventilation stops, the temperature in the tires increases during the first few minutes through heat irradiation from the drums. That is why some tires blow out while the vehicle is parked.
3. Do not abuse the semi-trailer brake; on downhill use the same gear as would be used in going uphill. Give preference to braking with the motor over service brakes, or when necessary, use both.
4. Observe load and speed limits established by transit authorities.
Some more serious examples of tires ruined by brake overheating.
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