The earliest evidence we have of the existence of the wheel dates back almost six thousand years. Presumably, it wouldn’t have been too long thereafter when the necessity of impeding the downhill progress of primitive carts became painfully apparent. The first brake might have been some sort of chock or anchor, or perhaps a sprag attached to the chassis that could be stuck into the ground.

When the bicycle appeared a couple of centuries ago, the only way to slow it down was to wedge your boot between the front fork and the wheel. But that made deceleration an exercise in twisted legs and precarious balance, so in 1838 Kirkpatrick Macmillan, a Scottish blacksmith, invented the spoon brake — a lever that pressed a block of wood against the “tyre” (actually an iron band). You’ve probably seen wagons with the same sort of arrangement.

It was natural that early motor vehicles would resort to the spoon brake, although some used a variation in which the block contacted one of the transmission pulleys. Regardless, the whole idea is woefully inadequate. Not only was friction limited to what could be produced with a relatively small surface, but the action involved resulted in heavy side-loading of wheel or pulley bearings, and there was no real possibility of stopping evenly.

Next came the external contracting brake, essentially a steel strap or cable encircling the hub, with or without friction blocks between them. This increased drag area, and even had a rudimentary servo action as the turning of the hub helped tighten the strap. This idea was also applied to driveshafts.
Still here

Shortly after the turn of the century, a huge improvement in stopping power appeared: the internal expanding drum brake. Attributed to Louis Renault (although you may not think much of their cars, the French produced a great many of the early advances in automotive engineering), the principle is still in wide use. Most of today’s cars have rear drums, and even Renault’s flat cam arrangement for spreading the shoes can be found on modern motorcycles.

Originally, drums were steel stampings. They flexed, amplified squeals, and were generally not too great. Cast iron drums showed up shortly, and in 1919 Hispano-Suiza introduced finned aluminum with iron liners (and you thought that was a new idea when Buick started using it in ’59).

We should mention that early cars had rear brakes only. It was believed that applying drag to the front wheels would cause swerving and instability, just the opposite of what we know now (jam on the parking brake of a modern car and you’ll see what we mean — the rear end tends to go away). Also, stopping power was greatly limited because of weight transfer. But the idea of braking the wheels that also steered daunted designers for many years, so hard stops resulted in lots of spin outs and tire skidding until four-wheel brakes caught on in the ’20s.
Muscle amplification

All drum brakes have some self-energizing action as rotation tends to wedge the leading shoe against the drum. But, especially with the elephantine conveyances of the teens, more help was needed. In 1920, a true servo design showed up. This had a leading shoe linked to a trailing shoe with no anchor between so that the rotational action of the first made the second apply forcefully. In 1922 the idea of a star wheel adjuster in the link was adopted.

Other means of increasing the power of mechanical brakes were also tried. After all, it was quite a bit to ask of a puny human leg to haul down two or three tons of iron from the considerable speeds of which cars were already capable. Bugatti used a novel approach: The actuation cable was routed in such a way that the twisting of the axle on decel helped pull the cam lever. Two other premium makes, Rolls-Royce and Hispano-Suiza, employed rotational power from the transmission. As the brake pedal was depressed, a clutch was engaged that tried to pull the brake actuating rods with it.

The idea of using engine vacuum to supplement muscle power was developed from about 1920 onwards to what most cars have now. Hydraulic boost, with pressure taken either from the power steering or an electric pump, is a relatively recent addition. It has the advantages of being more compact and giving numerous fully augmented stops from psi stored in an accumulator should the pump fail.
Pinchers

Drums did a pretty good job, but there was room for improvement. Heat dissipation was the biggest problem — if you wanted to keep something warm, you could hardly do better than to place it in a cast iron pot (sort of like a Dutch oven). And, as styling dictated sleeker sheet metal and lower bodies, air flow over drums was reduced even as potential speeds increased. Water was another problem. You could lose stopping ability altogether after fording a flooded street.

Enter the disc brake, an idea that’s been around since the 1890’s, believe it or not. One of the earliest versions was used on the front wheels of an electric car designed by Elmer Ambrose Sperry in ’98, wherein an electromagnet forced a pad against the rotor.

The first design we know of that remotely resembles what we have today appeared on the ’49 Crosley. After the style of aircraft brakes, the disc was clamped between two round pads. In the mid-’50s, both the English and the French started installing disc brakes as standard equipment on several makes, but they didn’t appear on American cars again until a decade later. At first, all domestic versions were of the four-piston, rigid-caliper type, but by the late ’60s the single piston variety started to show up. While the pads may not have lasted as long, there was only one quarter the potential for fluid leaks, and the calipers were lighter, simpler, and less expensive.

If you’re middle-aged, you remember the furor over the introduction of discs in this country. It was said the independent shop or service station simply wasn’t going to be able to handle these high-tech halters, so all repair work was going to have to be done by dealerships or specialists. Predictions abounded about the demise of the independent garage, at least as far as brake business was concerned. Well, the American technician is a highly adaptable breed, and was barely fazed by this supposedly insurmountable upheaval. We don’t know about you, but we’d rather service discs than drums any day. It’s faster, easier, and there’s less hardware and no stubborn return springs to deal with.
Hot stuff

Of course, the evolution of friction materials has been a crucial part of brake history. As we said, spoon brakes usually had wooden blocks, but they were sometimes supplemented with a leather lining. Band-type brakes were either metal-to-metal, or used wood or leather, too. The earliest drum stoppers had iron shoes against steel, then some strange things were used, such as the walrus hide linings of the English Wolseley.

The credit for first taking the scientific approach to friction materials goes to an Englishman, Herbert Frood, and Ferodo, the company he started early in this century. Using a water wheel-powered friction test machine set up in a shed, Frood experimented with numerous materials (even cotton!) and bonding agents, sort of like Edison looking for a suitable light bulb filament. In 1908, he hit upon resin-impregnated woven asbestos reinforced with brass wire, and the era of safe stopping began.

But asbestos isn’t ideal. Fade is a major problem, and in recent years we’ve learned to worry about the health effects of breathing the stuff. Sintered iron linings were used in heavy-duty applications to handle the heat, then we got semi-mets, which had some teething problems such as excessive rotor wear and squealing. New formulas are vastly improved, and now we’re seeing non-asbestos, non-metallic materials that seem to do everything right.
Fluid force

So much for the parts that actually stand the friction. How about the system that transmits the “whoa” signal from foot to brake? The hand lever gave way to the pedal early on, but mechanical apply set-ups — cams, cables, and levers — remained for quite a while. But no matter how clever the design, they were almost impossible to equalize perfectly, and required constant adjustment, so the idea of using hydraulics to do the job intrigued designers from about 1897 onward. It took many years to develop reasonably dependable systems, however, and the first domestic car with fluid pressure-actuated brakes was the ’21 Dusenberg. Chrysler followed in ’24.

Mechanical brakes had serious drawbacks, but they’d always stop the car, something that couldn’t necessarily be said for early hydraulic systems. Henry Ford was so adamant on this point that the vehicles that bore his name didn’t get juice brakes until 1939, long after almost everybody else had decided the pros outweighed the cons.

In 1967, it was federally mandated that all cars sold in the U.S. have two separate hydraulic circuits. But that wasn’t when they first appeared. Cadillac, for example, introduced a dual system in ’62.
Fine tuning

The typical front disc/rear drum arrangement brought with it the need for additional control of the apply pressure. Besides proportioning (which had been used before on four wheel drum cars to mitigate the rear wheels’ tendency to lock up on hard stops, but was even more important for the disc/drum combo because discs have no self-energizing or servo action), a metering function was necessary to keep the fronts from doing all the work. Since there’s virtually no clearance between the pads and the rotor, disc brakes start dragging the instant there’s any pressure in the system, whereas there’s space to be taken up before drums begin to apply.

Proportioning was brought to a higher degree of development with the load-sensing proportioning valve, which limits pressure to the rear brakes according to the distance between the chassis and axle. The first one we ever saw was on an early ’70s Fiat, but it can be found under late model pickups and utility vehicles.
Keeping control

Anti-lock braking system is the hot brake topic today. The idea is far from new — patent applications were made for mechanical versions in the mid-20’s, and electronic systems were offered for a while in the early 70’s, but neither was dependable or affordable enough to be accepted. With the fantastic advances in technology that have occurred in the last decade, however, ABS has become a realistic proposition.

How’s it work? Basically, the system monitors the speed of each wheel, and if one slows down more than its brethren during brake application, hydraulic pressure to that wheel or circuit is released, then reapplied up to 15 times per second, which prevents lockup. Not only does this greatly increase control during hard decel (a locked wheel can’t be steered) and reduce stopping distances, it can also save an expensive set of tires from destruction in a panic stop.

Ford gets the prize for being the first company to embrace RABS (Rear Anti-lock Braking System — at less than $100 per vehicle for about 80% of the benefit of a four-wheel system, it was a safety bargain). In ’87, it appeared on F-series pickups, Broncos, and Bronco II’s as standard equipment. Chevy followed with a similar system on its redesigned ’88 C-series, and calls it RWAL (Rear Wheel Anti-Lock).
From now on

The future? It’s safe to say we’ll be seeing drum brakes at the rear for a long time. After all, they have much less work to do than the fronts, less still with FWD, and they make parking brake design easy. It’s our opinion that rear discs are more a hyped selling point than a real advantage — you don’t want a lot of stopping power there except maybe on a race car (you know what happened when GM put duo-servo brakes on the rear of the A-Car: recalls and law suits).

As far as the friction-producing components, most of the hydraulic system, and boost are concerned, you can expect the same gradual evolution you’ve become accustomed to. Which is not to say there’ll be nothing to keep up with. The number of different brake designs out there is amazing, and it’ll continue to grow. As Jim Diamond of Lee and Gibson, a manufacturer of brake hardware, told MS, “When Lee Bigler started this company in the late ’40s, his first program was in springs. There were eight part numbers then that covered most of the cars on the road. Today, there are 1,400.”

But the only real challenge for auto repair technicians will be the proliferation of ABS, which will appear even on the most inexpensive cars. They’ll have to learn new diagnostic procedures and how to interpret trouble codes. Repairing these systems will never amount to a very large percentage of brake work, however, because they’re quite durable and dependable. But everybody involved in auto service should remember to advise their customers whose cars are so equipped that changing the fluid at least every two years is cheap insurance against expensive problems.

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Author: natalierrramirez

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