FODENS TACKLE FADE
Page 52
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in secondary braking
E intention of the requirements in the amended Construction and Use Regulations is to improve the efficiency of the brakes and also to safeguard against failure by providing an emergency system which will still give reasonable efficiency should the service brake fail. The brake efficiency can readily be improved by using larger brake and air chambers; but this introduces another major problem, as a brake which meets the requirements for a laden vehicle can be dangerous when the vehicle is empty. When the power provided is too great, resulting in locked wheels with consequential loss of directional control. This can be overcome in theory by fitting load-sensing valves between the axles and frame, these proportioning the braking load according to the weight carried. Another device which is being developed is one which reduces the braking load as the wheel stops rotating. This has the advantage that it takes into account road conditions and prevents dangerous situations from arising under icy and wet conditions.
However, neither of these systems is fully developed and when they do become available they will bring with them their own problems of reliability and servicing. Everything possible must be done to make vehicles safer because unless such mechanisms are extremely reliable a condition could arise where more accidents were caused by failure of these devices than if they were not fitted, the driver not being aware of faults until an emergency arose—for under normal braking the effects of them are not noticeable.
As an interim step Fodens have modified the foot valve of the service brake, as it has been found that, if normal pedal pressure is limited to two-thirds of the maximum, this meets all average usage for a laden vehicle and still gives good control of the braking when empty. To obtain the last third of the braking the driver has to apply considerably more pressure on the pedal. This is an automatic reflex reaction in an emergency and only then is the full braking power obtained.
A secondary advantage of this arrangement is that the driver does not brake more severely than necessary in normal conditions, and this reduces wear and tear of brakes and tyres, while also giving a more progressive and even braking so that following traffic has more time to react.
Another, feature of this system is that it is arranged for the main air reservoir to be charged first and only then does air feed into the auxiliary tank. This is done c32 because numerous accidents have been caused by drivers ignoring the air tank warning devices when moving off. If the main and auxiliary tanks are arranged in parallel, as is often done, both build up at the same rate so that the time taken to build up the air pressure fully for the footbrake is twice as long. This increases the time during which the driver has inadequate braking.
There are some who advocate that the emergency system should be a duplication of the service brake, operated by the same pedal. In theory this is fine; the driver would then have full braking should there be a failure in the main air system. Should this occur, hoWever, leaving the braking system apparently as good as ever, he would tend to ignore the warning devices and then he would be back to the condition where he had no emergency system. For this reason I believe that the emergency system should be operated by a different control, so that there can be no doubt in the driver's mind that the . failure of some vital braking part has occurred.
In order to keep the same number of controls, Fodens have combined the operation of the emergency brake with the handbrake lever, which is normally used for parking purposes, and is the control to which the driver would naturally turn in an emergency. When the driver takes hold of the hand lever and pulls, the first part of the motion gives controlled braking on triple diaphragms on the front axle and at the same time on the trailer axle or axles via the auxiliary circuit. Further movement of the handbrake lever then begins to apply the transmission handbrake. This process of applying the brake on the extremities of the vehicle first is deliberate and designed to give controlled braking and minimize jacknifing. When the hand lever is released, the air pressure is also exhausted so that the vehicle is parked on the transmission brake. This is necessary to ensure that a " runaway" is prevented by the vehicle being parked on the air cylinders and then the pressure reducing.
Furthermore, it is known that under adverse road conditions a transmission brake gives greater lateral stability than other systems. The usual pawl and ratchet is provided on the handbrake lever, but the lever has to be pulled on firmly before the first ratchet teeth are engaged. This is done to prevent the driver from descending gradients on the handbrake and is intended to prevent overheating and reduction of the efficiency of the transmission brake, this being provided to give a good emergency brake which will be maintained at its maximum efficiency. The transmission brake alone is designed to give an efficiency of 20 per cent and in conjunction with the triple diaphragms on the front and trailer axles gives a combined efficiency of 45 per cent.
We believe that the transmission handbrake still gives certain very desirable features which are necessary if the overall braking system, and not just the air pressure system, is to be made more reliable. Many braking failures are caused by fade and, unfortunately, with air pressure actuation the driver is not made readily aware of this fade characteristic. Unlike mechanical or hydraulic linkages, the travel of the brake pedal gives no indication of the brake shoe travel, so that it is not until the brake is in an advanced stage of fade that the driver becomes aware of the problem. Unfortunately, as the brakes are progressively improved in design to combat fade the driver's natural tendency is to use the brakes much more severely, so that no overall improvement is made. Also, the use of air braking systems has reduced the fatigue effect on the driver and, whilst this is highly desirable, it does not make him aware of the amount of effort being put into the Wakes. For these reasons, we feel that it is important to have an entirely separate emergency system and, providing that care is taken to design properly for its application, a transmission unit can be satisfactory in this secondary role of emergency brake.
It should be appreciated that very great loads can be developed on the rear axle gear teeth by the transmission brake, but this can be taken care of if a wormgear axle is used, as this design is capable of withstanding very severe loads for long periods without damage to the worm teeth. The other advantage of using an overslung gear in conjunction with a transmission brake is that the brake is approximately 9 in. higher than the wheel brakes, whilst the drum, being located on the rear end of the worm-gear casing, is protected from water entry. This has been found to give an effective emergency system when travelling through flood water with the wheel .brakes wet and ineffective.