TOWARDS GREAT AFETY ON HILLS.
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The Shortcomings of Present-day Braking Circumstances in which Front-wheel Brake id Some Suggestions for Brake Improvement. tomatic Sprags Make for increased Safety.
THE importance of providing adequate la-Liking power on any and every vehicle using the roads cannot be over-emphasized, because, apart from the danger to the occupants of an under-braked lorry or coach, other road users are imperilled by such a
vehicle.
It is quite impossible to consider all the aspects of this big question in the scope of a single article, so that it has been thought advisable to confine this consideration of the subject to the problem of providing greater safety on hills. Other questions, such as safety on greasy roads and stopping power on the .level, have been fully dealt with recently in The Commercial Motor, likewise the problem of suitable brakes for the front wheels.
Considering safety on hills, the subject naturally falls into two parts, viz., safety of descent and safety of ascent. As regards the former, the principal factor to consider is the alteration in weight distribution between one axle and the other, which occurs when a downward gradient is being negotiated. It is clear that the weight must, to some extent,be shifted forward, which means that, when a vehicle is on a gradient, with the, front wheels at a lower level than the rear wheels, the adhesion available for brakes acting on the back axle will be below the normal.
The difference in adhesion depends upon the gradient and the wheelbase and other characteristics of the vehicle concerned. However, broadly -speaking, if, 'in a motor mach standing on level ground, the rear axle weight is 70 per cent, of the whole, then, on a fairly severe gradient, this weight may easily be reduced to a value of N per cent. If rear wheel brakes are fitted, it follows that the adhesion available is diminished in the proportion of 70 to .60, involving a reduction of braking effect amounting to about 14 per cent. In itself, this might not be considered very serious, but it must be remembered that the retardation produced by the brakes themselves-has a very similar effect on the axle weights. The average vehicle is braked by applying shoes to revolving drums, and it is easy to see thati the pull. exerted -by the drums upon the shots is precisely equal to the braking torque exerted on the wheels.
This pull on the shoes is transferred through the anchorage, the axle and the springs to the frame of the vehicle, and can only be counteracted by an increase in the weight thrown on the front axle and a reduction of die back axle weight. If the brakes • are really effective and are applied to their full .extent, as much as 10 per cent, of the total weight may be transferred from the rear to the front. B32
No*, in descending a steep gradient one may be certain that the brakes will be fully applied, so that the net result of the slope and the retardation will be very considerable. For this reason, a vehicle with rear-wheel brakes, which may he quite satisfactory on the level, may be almost unsafe to handle on a steep gradient. It will easily be seen that the reduction in'the weight acting • on, the rear wheels means that the brakes will be able to lock these wheels fairly easily.
A driver descending a steep hill, who finds that his rear wheels are locked and are sliding down out of control, can only remedy matters by releasing the brakes temporarily and then re-applying them. This cannot be considered a very safe solution, particularly on a hill which may abound in hair-pin bends. Furthermore, it must be remembered that in order to be popular a motor coach must run on a route embracing the best scenery of the district, and it will usually be found that where the scenery is up to the mark the roads are usually badly worn by the traffic. If the surface is loose or greasy, the ease with which the back wheels can be locked is greatly increased, so that there is quite a number of considerations which lead to the conclusion that, however satisfactory rear-wheel-braking systems may be under average conditions, front-wheel brakes are highly advisable for vehicles used in hilly districts.
It must also be remembered that, even if one fits a transmission brake, a rear-wheet servo mechanism or an air-brake attachment on the engine, the retarding effect is, in each ease, finally applied to the teat wheels only, so that the criticisms already advanced are still applicable. However, such mechanisms are of value in the case of a really heavy vehicle, because they reduce the labour required on the part of the driver.
On a long down-grade the hand lever and its"ratchet can, of course, be made use of, but, in order to safeguard against undue heating of the linings, it is usually necessary to ring the changes on the brakes, which means that the pedal must be applied during part of the descent.
In country districts where steep hills abound, a heavy vehicle entails very considerable labour in operating the brake pedal, and it is here that servo systems, engine brakes and other similar methods are of Particular value. . There is no doubt that accidents have occurred simply through driver's fatigue resulting in his inability to apply full pressure an the brake pedal, or in the foot inadvertently slipping off the pedal for a sufficient period to allow the vehicle to attain a dangerous speed. In this connection it is worth noting the increasing popularity of the Westinghouse system of power operation, in which the driver has merely to operate the
handle of a valve. . ..• •
• Engine brakes have already been mentioned, a good example of this system ,being found on the Saurer. Then, again, therei are hydraulic systems such as the Lockheed, which are widely Used in America. In this system contracting brake bands are operated by small pistons to which oil pressure is applied when the pedal is depressed.
Servo mechanisms have not attained great popularity, possibly because of the widespread idea that a servo mechanism, whilst effective when tha vehicle is moving forwards, is not efficient under the reverse conditions. It is, however, quite possible to design a servo mechanism which will be equally effective in both directions, and to prove this contention we may draw attention to two systems which have recently been patented. The first of these is due to the bunbeam Motor Co., and is shown in one of the illustrations we
reproduce. Here there are..two concentric drums fitted .with expanding shoes, and only the inner shoes are directly operated by the pedal. When these make contact with the drum they tend to drag round, anki this motion is transmitted to a carrier plate which is permitted to move through a small angle on the axle. Teeth cut on this plate mesh with a sector and communicate the movement to the cam operating the large shoes.
It is clear that this brake will be equally effective in either direction of rotation, and in an interesting modification of the design, the dragging effect of the rear shoes is made to operate a pair of frontwheel brakes. This is a simple system which should be very effective .
Another method, recently patented by Mr. H. E. Merritt, MSc,, also employs a movable,earrier for the brake shoes, but is dissimilar from the Sunbeam system in all other respects. It will be seen that th0 shoes are operated through a pair of pivoted links, and that the angle through which the carrier. is dragged round is limited by a fi.xed stop butting against one or other of the shoes. Mr. Merritt has coined the term " servo-reflex " for this system,
because the servo effect produced on the second shoe by the first is returned through the medium of the links, with the final result that the shoes take equal shares in the work, and the braking effect is amplified very considerably.
Mention of equally effective braking in both directions of motion brings us to the question of safety
when a gradient is ascended. In all orthodox vehicles the gear lever has to be moved through a .neutral position when changing .down on a hill, and there is, therefore, always a danger that the driver will not succeed in meshing the lower gear, and will then have to rely entirely upon the brakes to prevent the vehicle from running backwards.
It is, of course, clear that there will be adequate adhesion available on the rear wheels in a case of this kind, but the point is that if the driver should lose his head for a moment and neglect to apply the brakes immediately, the vehicle may attain -a dangerous speed in a backward direction.
Of course, there is the 'possibility of running the rear wheels into the bank, if, indeed, such a hank borders the road, but even if this be done, there is a great danger of the vehicle overturning. So many accidents have occurred in this way that no excuses should be required for emphasizing the need for some automatic spragging device, to which attention has so often been directed by this journal.
It should also be remembered that an efficient auto "natio sprag would make it possible for the driver to stop -at will•at any point in the ascent of a steep hill, and then restart without the complex series of physical actions required for the simultaneous operation, of the brake lever, gear lever, accelerator pedal and clutch.
First of all, it is clear that a trailing pole will not meet-the case, because, apart from other drawbacks, it cannot be left in continuous operation, and •lf dropped after the vehicle has started to move backwards it is usually ineffective.
It appears that an automatic spragging brake is really required, which should be continuously in action, but not subject to wear, foolproof, and able to stop the vehicle without doing any damage. Various mechanisms haved5een designed, from time to time, with thee ends in view, most of which embodied a form of over-running _pawl _gear which, in our opinion, puts theni out of court. Such a gear in continuous operation is bound to be noisy, and is apt to give way just when it is most required.
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• For aese reasons we should like to call attention to the description of a frictional sprag which appeared in our associated journal, The Motor, some months ago, and which is depicted. in one of the illustrations reproduced. Briefly, the mechanism consists of a cone running upon a quick-threaded screw cut in the shaft projecting at the tail end of the gearbox. The angle of the screw is such that
is rotating anti-clockwise---as will be the case when the vehicle is running forwards—the tendency is to hold the cone out of engagement. So soon as the direotion of motion • is reversed, however, • the inertia of the cone will tend to make it cease to revolve, and will therefore cause it to tun forward along the' shaft; When it comes into contact with the surrounding cone member the resistance to rotation is greatly increas.ed, with the result that the parts are pressed firmly into . contact and produce a very
efficienti braking effect.
To enable the driver to reverse when required, a simple fork could b•e incorporated which would hold the Cones out of engagement so long as the reverse gear was engaged. Furthermore, in the .suggested mechanism, a torque arm is embodied, anchored' to the frame, which would prevent any stresses from being transmitted to the gearbox.
There is always a danger in the use of a positive sprag of any kind, as one does not often realize that there is danger of running backwards until it is too late to' apply a sprag -which will abruptly step a vehicle. In the preceding pages -we show an auxiliary, brake which has been designed solely for the purpose under consideration, and not for retarding in a forward direction. To. save room and complication, it could be easily fitted as an outside brake to a drum already provided for an expanding transmission brake. The fulcrum pins of this brake are mounted on a plate (13), which is provided with a segmental slot, as shown at A, and is allowed a certain amount of rocking motion around its centre. When the drum is revolving in the direction of the arrow, as indicated in the left-hand illustration at
the foot of the centre pages, which shows the vehicle running backwards, the brake will grip with great force, as the tighter it grips the greater the compression of its shoes. When its drum reverses, as when forward motion of the vehicle is resumed,as shown in the right-hand illustration, the compression of the shoes automatically releases and frees the braking effect.
Such a brake, made with V-shaped surfaces to set up the necessary friction, would grip with much greater force than with ordinary flat faces. This construction would. he quite permissible with a type of brake which is only used occasionally, but would be hardly suitable were it in constant use, as wear would cause rapid deterioration.
It would, of course,-be necessary to fit a separate lever or pedal to work this brake, which should be powerful enough to stop the vehicle alone without • the other brakes. This can easily be arranged, as the brake, once in contact, will not release easily, as its retarding effect increases its grip. In designing such a brake, care must be taken that the lever or pedal actuating it has freedom to move through a longer arc than an ordinary brake lever, as, once the shoes grip the drum, the rocking plate (B) travels with the drum until the pin in the 'slot (A) comes to the position shown in the left-hand illustration referred to. This action has the effect of altering the position of the pull-rod and bringing it from the position shown in dotted lines to that shown in full lines. In cases where no brake is provided on the transmission, brakes of this type could be fitted at the rear of the worm.
One of the special advantages of such a construction is that it would allow forward motion of the vehicle to be resumed at any time without releasing the brake, as it would release itself automatically to a sufficient extent while climbing a hill. There can be little doubt that the true solution of the brake problem lies in the design of a brake which will fulfil all requirements in any direction at all times. When a brake fails to act it is usually due, in the case of a foot brake, to the pedal coming in contact with the footboards before the driver can exert his full strength, or, in the case of a hand brake, through the lever reaching the end of the segment in which it works before the driver can exert sufficient strength to stop the vehicle. It may be said that the• leverage of the brake mechanism can be so designed as to give sufficient power to the expander dams to produce the necessary retarding force. This, however, is not the case, as sufficient leverage cannot be arranged unless a very !Ong' movement of the hand lever is provided.
If a brake of ordinary construction be examined, it will he found that with a convenient movement of the hand lever, only a minute rotary movement of the expander cam is produced. An illustration on page 525 shows a design of brake in which a second or third movement of the hand lever is made possible. Under ordinary conditions, it can be used just as an ordinary brake but, should a further movementbe necessary, all that the driver has to do is to disengage the lever from one of the dogs on the face of the ratchet wheel and take a second or third pull, the path of the lever being along the sides of -the parallelogram indicated by a dotted line. The ratchet is merely provided to hold the brake on while the further pulls are taken.
In conclusion, we would like to emphasize the fact that no matter how efficient the brakes may he, their ultimate retarding effect depends upon the , adhesion available between the tyres and the road ; consequently, the suspension system deserves serious attention, because, if it is inefficient, the axles are permitted to bounce on rough surfaces and all adhesion is temporarily lost. While a wheel is in the air, the brakes will cause it to cease to revolve, so that its strikes the road in a locked condition, and a very uneven braking effect results.