The Dunlop Maxaret
Whenever I see a large air-liner land, especially on a wet runway, I am intrigued by the way the pilot applies relatively small pressure to a hand or foot lever and 30 or 40 tons of aircraft are brought to rest with no apparent fuss. Since 1952 a device has been available for aircraft which has revolutionised heavy braking, especially on wet and slippery surfaces, this being a mechanism which is fitted into the normal braking system and which prevents a brake from locking the wheel should the coefficient of friction between tyre and runway be reduced, or should too much pressure be applied to a brake. As the average airliner pilot is sitting as much as 30 feet away from his wheels and brakes, it is impossible for him to have any idea of what is happening at his points of contact with the ground, apart from the directional attitude of the aircraft, and if a locked wheel is putting the aircraft into angular velocity relative to the runway, he is in real trouble. Bearing in mind that it takes less than 3 seconds for a locked wheel on something like a Viscount to burn right through the tyre, and once that has happened, the alloy wheels and suspension struts would virtually disintegrate, it can be appreciated that the mechanism that prevents brake locking is vital to the safety of large aircraft.
Designed by the Aircraft Division of the Dunlop Rubber Company, the Maxaret, which is the trade name for this anti-locking device, is now almost universal on aircraft and is being made under licence in other countries. Quite simply it is a mechanism that removes all "feel" from the pilot's hands, he is merely operating a lever which opens a valve letting hydraulic pressure go from a basic source to the wheel brakes. If tyre-to-runway conditions are perfect for adhesion then the chances are that the brakes can operate happily with full pressure applied, but if the conditions are not perfect then full pressure is liable to prove to be too much and a brake may lock the wheel, thus losing all braking efficiency between tyre and runway, and if this happens on one wheel only then the aircraft will swing. The instant that a wheel is about to lock, the Maxaret comes into play and releases the hydraulic pressure from the offending brake, by means of a by-pass valve, even though the pilot is still applying full braking pressure, and of course with no pressure going to the brake the wheel is prevented from locking. Now, the moment the wheel is running free (i.e., with no braking being applied) the Maxaret closes the by-pass valve and pressure goes once more to the brake, and if it still wants to lock, the Maxaret releases the pressure again, and so on. These cyclic variations take place at about the rate of 10 times a second, so that from an alternative of a locked wheel and zero braking effort you now have a fluctuating braking force consistent with the tyre-to-road conditions. Naturally, as soon as conditions are such that full de-acceleration can be applied to the wheel without locking then the Maxaret sits quietly doing nothing, awaiting the moment de-acceleration drops below a pre-set figure, and then it comes into play again. With one of these instruments on each wheel braking is kept at the best possible figure consistent with surface conditions and irrespective of what the pilot is doing, once he has applied the brakes.
Anyone who has driven fast must, at some time, have had to brake heavily on a wet or slippery surface and suffered locking wheels with all the consequent embarrassments. As you know, if you lock the front wheels you lose all steering ability and the car will slide straight on; if you lock the rear wheels, and have any sideways forces acting on the car at the time, the tail will come round and you will be in real trouble. In fact, at almost any time a locked wheel will get you into trouble and call for a lot of skill to counteract. Rally drivers, who learn to brake on snow, ice, mud, loose-gravel or wet surfaces, and brake hard, have taught themselves, by trial-and-error, that a pumping action on the brake pedal can give a reasonable degree of de-acceleration and still retain control. If you have been driven by someone proficient in this art you will have seen how the brakes are put on and let off in short dabs, each one being enough to just begin to lock the wheels, but not enough to lose control, and it is easy to appreciate the skill needed to do this with finesse and still slow the car down without losing control. The Rally driver is doing nothing more than turning his senses and muscles into a Maxaret unit applied to all four wheels from the source of brake pressure; which is the driver's foot.
Over the past two years part of the Dunlop Aircraft Division engineers have been applying their Maxaret knowledge of aircraft braking to that of motor car braking, and early in 1958 a report on their findings was written by Mr. F. R. Mortimer. At first they adapted some aircraft units to a Mark VII Jaguar, and later made some special units essentially applicable to road vehicles, and last November on one of the dampest, foggiest and greasiest days possible I was able to go out to a disused aerodrome with Mr. Mortimer and try out the Jaguar fitted with the latest experimental test equipment. One important factor with Maxaret-controlled braking is that the brake must be a full-power one, which means simply that a constant source of hydraulic pressure must be available, and not only when the driver applies pressure with his foot, as on a normal braking system. The Mark Vll had been modified to full pressure by having a hydraulic pump driven off the front of the crankshaft which kept a reservoir of hydraulic fluid at a figure between 1,500 p.s.i. and 1,700 p.s.i., the pump cutting in automatically below the minimum and cutting out at the maximum, this range being about equal to the pressures generated at the brakes in a normal hydraulic brake system where the driver's effort is transmitted directly to the shoes or pads of the brakes. In passing it is worth noting that the DS 19 Citroen is the only production car at present using a full-power hydraulic system, but in time all advanced designs will incorporate it. Extra powerful disc brakes were fitted to the Jaguar, to ensure that wheels could be locked from almost any speeds if required, and solenoid operated switches were added to the front pair and rear pair of brakes so that the Maxaret could be switched out of use when desired, for test and comparison purposes. The driver's brake pedal was coupled to an aircraft-type hydraulic valve in the main pressure system.
The Maxaret units are remarkably simple when dissected, though demanding such accuracy of construction that they rank as scientific instruments. Contained in a small cylinder just under 3 in. diameter and 2 in. long is a flywheel attached to a central spindle which is coupled directly to the road wheel or brake disc, in this case to brake disc in internally toothed ring on the disc and a gear wheel on the Maxaret spindle, running the flywheel at about five times the speed of the road wheel (5,000 r.p.m. at 100 m.p.h.). In the flywheel is a cam-contoured groove in which lies a small ball-bearing on a cross-shaft, this cross-shaft being coupled to a rod within the central spindle of the unit, and at its end is a tiny poppet valve. On one side of this poppet valve is the main hydraulic pressure from the brake system, and the other side is a take-off back to the reservoir, so that if the poppet valve is opened the main pressure is exhausted back into the reservoir instead of going to the brake-operating mechanism. While the roller follower is in the neutral position of the cam-slot in the flywheel the poppet valve is kept closed, and it is retained in this position by a small coil spring which has a designed operating load that opposes the de-acceleration of the flywheel to to a predetermined figure. When the brake on the wheel under discussion is applied, the flywheel which is running at approximately five times the speed of the road wheel, has to slow down (or de-accelerate) along with the road wheel, being positively geared to it. Providing the rate of de-acceleration does not exceed the figure calculated to give maximum retardation, the spring in the Maxaret can keep the poppet valve closed. If the wheel should lock and skidding take place between tyre and road, the flywheel is obviously de-accelerated violently the cam-slot over-rides the roller and the cross-shaft operates the push-rod, collapsing the spring and opening the poppet valve which releases the hydraulic pressure through the exhaust passage back to the reservoir. The moment this happens the road wheel unlocks, the brake pressure having been taken off, and the wheel accelerates again, as does the flywheel in the Maxaret and the coil-spring closes the poppet valve again. This operation can take place at the rate of about 10 per second, and though a graph of the brake pressures on the wheel and the de-accelerations and accelerations of the wheel can be traced as a wavy line, the mean is a constant braking force of the maximum possible consistent with the available coefficient of friction between tyre and road.
One of these Maxaret units is fitted to each wheel and looks after the retardation of its own corner, irrespective of what the other three are doing. On the test Jaguar Mark VII the driver could switch them all off or the front only or the back only.
With visibility on our test day at about 60 yards, and slime and grease on the runway, as well as a layer of water sprayed aver the test area, conditions were at a pretty low level for braking, and to convince myself I first of all did some test runs in the Porsche. Braking at a marker from about 70 m.p.h. soon had me sliding helplessly down the runway with the front wheels locked hard on and having no directional stability, while another run on which I played with the brake pedal while trying to stop, showed that there was still not much chance of stopping in a hurry. I then tried going through a marked-out ess bend and stopping at the same time, and though the Porsche managed it, it was only by reason of its in-built stability and control, coupled with lots of on-off movements on the brake pedal and fast inter-related steering corrections, in other words a dicey-old manoeuvre in which I was kept jolly busy. I then tried the Mark VII Jaguar from the same speed with the Maxarets switched off and it really was impossible to retain any control at all, either in straight braking or braking and cornering. Dunlop's test driver gave a demonstration of trying to brake and corner at the same time in the Jaguar without the Maxarets and spun round in a big way. Switcing on the Maxarets I braked as hard as possible at the first marker and on this wet slippery surface experienced absolute straight-line braking of a force that is known as "the big restraining hand" where you have to brace yourself against the scuttle. I have experienced similar retardations on dry roads in sports-racing cars, but to achieve this on a slippery surface was just uncanny. On the initial press of the brake pedal there was a feeling that nothing was going to happen, but then the Maxarets took over and the car literally came to a "grinding halt." I then took it through our artificial ess bend with the brakes hard on and it was just silly, it steered as if on a dry road on a neutral throttle opening, without any braking. I did another very fast run keeping the throttle open with my right foot and braking with the left foot and the way that great gormless Mark VII saloon cornered on the very limit of adhesion was truly remarkable, for this time although the car was not losing speed the Maxarets were working overtime to keep tyre adhesion at the maximum possible for the conditions, and with maximum tyre adhesion available, one could utilise it for cornering power.
To convince myself I was not being fooled I played some more games with the Porsche, and got into most difficulties apart from putting it on its roof, so I was quite convinced that the aerodrome surface was very bad. Over lunch I was able to have a long and interesting talk with Mr. Mortimer about the application of Maxaret to road vehicles (and in particular to my Porsche !) only to diseover that cost is prohibitive at present as the whole application is still experimental. However, one can foresee the application in the near future to commercial vehicles, especially lorries with trailers, or articulated lorries, for how many times has one seen a lorry driver braking with one trailer wheel locked on solidly, there being no indication in the cab, which might be 60 feet away? In rallies, especially the snow and ice ones, they are an obvious answer, though they would need overriding controls for the skilled rally driver who wants to spin his car in the special tests by means of locking his wheels, while for racing in bad weather they are an obvious winner. No doubt, like disc brakes, Dunlop will eventually get the Maxaret unit down to a production price, especially for really fast cars like the 3.4 Jaguar or the XK150 and if Jaguars can persuade Dunlops to co-operate with them the next time they race at Le Mans, and it is raining, it will be a cake-walk. To the purist racing designer the only drawback is having to have a full-power hydraulic system, with its extra weight and complications, and the addition of 2 lb (the weight of a Maxaret unit) to the unsprung weight of each wheel, but I have no fear in saying that such disadvantages would be more than repaid in a wet race. Whether Mr Joe Soap will ever have them on his family saloon is unknown at present, but like independent suspension, automatic transmissions, hydraulic brakes, even pneumatic tyres, the Maxaret will probably be an inevitability as it is with aircraft. -- D. S. J.
Footnote: For further reading on this subject see Test Report 293 of the Dunlop Rubber Co., Aircraft. Division, Coventry; The Automobile Engineer for July, 1958, and The Autocar, June 13th. 1958.