Dunlop's Maxaret was the first anti-lock braking system (ABS) to be widely used. Introduced in the early 1950s, Maxaret was rapidly taken up in the aviation world, after testing found a 30% reduction in stopping distances, and the elimination of tyre bursts or flat spots due to skids. Experimental fittings on cars and motorcycles demonstrated mixed performance, and ABS systems would not appear on mainstream, non-sporting cars until the 1970s when electronic controls matured.
The Maxaret system had four main parts, all of which weighed only 4.7 pounds and fit within the small confines of a main landing gear wheel. The system was entirely mechanical and worked by measuring the relative speed of two spinning disks. The first, the "drum", was surrounded by a rubber disk and arranged within the wheel's internal periphery, so that the rubber maintained strong mechanical contact with the wheel. The second, a flywheel, was attached to the drum with a one-way clutch. Normally, with the wheel turning, the wheel would spin the drum, which would spin the flywheel, so that all of the moving parts were spinning at the same speed. [1]
When a skid developed, the wheel would stop, stopping the drum along with it. The flywheel, driven by the one-way clutch, continued to spin. If the relative angle between the drum and flywheel reached 60 degrees, the drum would be driven forward to press on a valve. This released brake fluid into a reservoir, lowering hydraulic pressure, and releasing the brakes. As soon as the drum started spinning again and reached the (slowing) speed of the flywheel, the valve was released and the brakes re-applied. The system could cycle about ten times a second and could hold the brakes off for up to four seconds in total before the reservoir was full. [1]
Aircraft have a much lower ratio of tyre contact patch to vehicle weight than automobiles and operate at much higher speeds. For these reasons, it is much easier to enter a skid in an aircraft through the over-application of brakes, and threshold braking is essentially impossible as the skid develops so rapidly. This makes landings in marginal conditions very difficult and leads to many common weather conditions precluding flying. Slippery conditions from heavy rain, or even light snow or ice, will close a field.
In early testing on the Avro Canada CF-100, the Maxaret allowed landings to be safely made on runways covered in ice. [2] Since the operational requirements of most aircraft are defined by the best take-off or landing distances under all weather conditions, Maxaret allowed aircraft to operate at 15% higher all-up weights. [2]
Another benefit was initially unexpected. Braking effect is greatly reduced at high speeds; the coefficient of friction between a tyre and concrete is about 0.7 to 1.0 at 30 miles per hour (48 km/h), but decreases dramatically to 0.3 to 0.5 at 120 miles per hour (190 km/h). [1] This means that it is much easier to skid when first landing, a fact that led pilots to hold off on the brakes until the aircraft was firmly down, and then slowly increase pressure to avoid skids. With Maxaret, they simply applied full braking as soon as they touched down, knowing that the system would prevent skids. As a result, braking distances even in perfect conditions were greatly improved on the order of 30%. [1] A later modification allowed the pilot to push on the brakes before landing, with the valve actually applying the brakes only after the wheel had spun up at least once.
When skidding occurs, the tyres can be rubbed flat, or even burst. Aircraft tyres have much shorter lifetimes than cars for these reasons. Since Maxaret reduced the skidding, spreading it out over the entire surface of the tyre, the tyre lifetime was improved. One early tester summed up the system thus:
The runway was very wet on the first landing, and the aircraft's all up weight was at least 12 per cent above the maximum landing weight. The brakes were held on at approximately 1,200 lb/sq in pressure from a speed of 80-85 knots, until the aircraft came to rest. The braking distance was estimated at 1,200 yards. The tyres were completely unmarked. Landing previously in an identical machine without Maxarets, and at approximately the same all up weight, great difficulty was experienced in stopping the aircraft in an estimated distance of 1,600 yards, with the braking parachute streamed at approximately 70 knots. On this occasion two tyres were burst, and the remaining six were damaged beyond repair. [2]
Maxaret, developed by Dunlop in the UK, quickly found uses on most UK military aircraft, such as the Handley Page Victor, BAC TSR.2, [3] and English Electric Lightning. Civil aircraft included airliners such as the Hawker Siddeley Trident. [4] Many companies followed suit, both in military and civilian models. One interesting variation was used on the Fokker F-27 aircraft, which did not have a hydraulic system, and instead used a high-pressure pneumatic system to actuate the brakes, including the Maxaret anti-skid system.
Other aircraft fitted with Maxaret were the Avro Vulcan, Vickers Viscount, Vickers Valiant, Folland Gnat, de Havilland Comet 2c, de Havilland Sea Vixen, and later aircraft, such as the Vickers VC10, Hawker Siddeley 125, Hawker Siddeley HS 748 and derived British Aerospace ATP, and BAC One-Eleven.
By 1966 an electronic version of Maxaret had been developed, called Maxaret Mark X.[ citation needed ]
There were numerous applications of the Maxaret to various vehicles, including experimental fits to a Royal Enfield Super Meteor and some production use on semi-trailers.
Most noticeable to the general public was its use on the Jensen FF, the British sportscar that introduced ABS, all-wheel drive and a traction control system. [5] Sports Illustrated called it the "safest car in the world" in a 1965 article. [6] In this case the system had an undesirable side-effect; the relief valve fed directly into the master pump, and caused the brake pedal to drive back towards the driver when it actuated.
An anti-lock braking system (ABS) is a safety anti-skid braking system used on aircraft and on land vehicles, such as cars, motorcycles, trucks, and buses. ABS operates by preventing the wheels from locking up during braking, thereby maintaining tractive contact with the road surface and allowing the driver to maintain more control over the vehicle.
A tire or tyre is a ring-shaped component that surrounds a wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide traction on the surface over which the wheel travels. Most tires, such as those for automobiles and bicycles, are pneumatically inflated structures, which also provide a flexible cushion that absorbs shock as the tire rolls over rough features on the surface. Tires provide a footprint, called a contact patch, that is designed to match the weight of the vehicle with the bearing strength of the surface that it rolls over by providing a bearing pressure that will not deform the surface excessively.
A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction.
A traction control system (TCS), is typically a secondary function of the electronic stability control (ESC) on production motor vehicles, designed to prevent loss of traction of the driven road wheels. TCS is activated when throttle input and engine power and torque transfer are mismatched to the road surface conditions.
Aquaplaning or hydroplaning by the tires of a road vehicle, aircraft or other wheeled vehicle occurs when a layer of water builds between the wheels of the vehicle and the road surface, leading to a loss of traction that prevents the vehicle from responding to control inputs. If it occurs to all wheels simultaneously, the vehicle becomes, in effect, an uncontrolled sled. Aquaplaning is a different phenomenon from when water on the surface of the roadway merely acts as a lubricant. Traction is diminished on wet pavement even when aquaplaning is not occurring.
Electronic brakeforce distribution or electronic brakeforce limitation (EBL) is an automobile brake technology that automatically varies the amount of force applied to each of a vehicle's wheels, based on road conditions, speed, loading, etc, thus providing intelligent control of both brake balance and overall brake force. Always coupled with anti-lock braking systems (ABS), EBD can apply more or less braking pressure to each wheel in order to maximize stopping power whilst maintaining vehicular control. Typically, the front end carries more weight and EBD distributes less braking pressure to the rear brakes so the rear brakes do not lock up and cause a skid. In some systems, EBD distributes more braking pressure at the rear brakes during initial brake application before the effects of weight transfer become apparent.
Vehicle braking system fade, or brake fade, is the reduction in stopping power that can occur after repeated or sustained application of the brakes, especially in high load or high speed conditions. Brake fade can be a factor in any vehicle that utilizes a friction braking system including automobiles, trucks, motorcycles, airplanes, and bicycles.
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Cadence braking or stutter braking is a driving technique that involves pumping the brake pedal and is used to allow a car to both steer and brake on a slippery surface. It is used to effect an emergency stop where traction is limited to reduce the effect of skidding from road wheels locking up under braking. This can be a particular problem when different tires have different traction, such as on patchy ice for example. Its use in an emergency requires a presence of mind that the situation itself might preclude. Cadence braking is supposed to maximize the time for the driver to steer around the obstacle ahead, as it allows the driver to steer while slowing. It needs to be learned and practiced. For most drivers of modern cars, it has been entirely superseded by ABS, however it is still a valuable skill for drivers of non-ABS equipped vehicles such as classic cars.
A hydraulic brake is an arrangement of braking mechanism which uses brake fluid, typically containing glycol ethers or diethylene glycol, to transfer pressure from the controlling mechanism to the braking mechanism.
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