DISC BRAKES FOR CYCLES
An Analysis:
GORDON H. JENNINGS
RECENTLY, we concluded a series of tests on a machine that is, in one sense at least, the motorcycle of tomorrow. The motorcycle was a Japanese Pointer, made by Shin Meiwa and imported by Pointer Sales of America in Parma, Ohio, and the feature that firmly linked it to the future was its brakes: disc-type units developed for Shin Meiwa by Airheart Products, of Van Nuys, California. (Cycle WOrld, January '62). We approached this entire matter with a great deal of skepticism, but experience and time spent in contemplation of the project convinced us - as it will eventually convince everyone - that here is a design trend that should not be ignored.
Disc brakes have been around for a long time: to the best of our knowledge, the first example appeared at just about the turn of the century. However, not muchheadway was made with them until the landing speeds of heavy military aircraft began to overwhelm the conventional drum-type brakes that were being used. As developed for aircraft, the disc brake took the form of a motorcycle clutch, with multiple discs of bare steel and discs covered with some friction material. In this form, the disc brake proved to be just the thing — for aircraft. The vast rubbing area of these brakes gave them a “oneshot” heat capacity that could hardly have been bettered, and an airplane needs just one strong braking action following the landing touch-down.
After the disc brake proved its worth in aircraft, the automotive men became interested. With many of the aircraft units available on the “surplus” market, experimenting was wide-spread. Great things were claimed, initially, but as is often the case, further experience indicated that this ointment was not without its fly. In its aircraft-oriented form, the disc brake may have been great at absorbing heat, but it was rather poor at geting rid of it. The long periods of “rest” between braking applications that are almost inevitable in aircraft are not normally available where cars, particularly racing cars, are concerned. Consequently, the disc brake that would stop a 10-ton airplane from 100 mph in a single mighty effort was reduced to a smoldering cinder by the less intense, but more extended, task of slowing a fast-lapping racing car.
Goodyear, a company that is best known for tires, saved the disc brake insofar as the automobile was concerned. They developed the spot-type disc brake, in which a single disc is gripped by a pair, or several, small friction pads concentrated at some point around the disc. This arrangement leaves most of the mechanism exposed to the air, and cooling is rapid. Fade is unlikely: in the first place, heat is lost from the disc so
quickly that it stays relatively cool and secondly, there is no drum or large brake shoe to heat-warp out of contact. Generally speaking, failures with disc brakes will occur only when the mechanism begins to melt. The only fade, as such, that can conceivably occur is a boiling of the brake fluid. Boiling would form bubbles of steam in the brake lines and these would, like air, render the hydraulic system partially or completely inoperative. Experience indicates, however, that this boiling is extremely rare and the possibility can, in most instances, be ignored.
The reader will note that we have assumed that the actuation of these disc brakes will be through some type of hydraulic system. Naturally, it is possible to arrange for the actuation of these brakes through a purely mechanical hookup; several examples of mechanical actuation are being used successfully in the karting field. However, as our interest is directed toward motorcycles, we should confine the discussion to hydraulics. Disc brakes are entirely lacking in self-servo effect, and if high brake-pedal/lever pressure requirements are to be avoided, the high mechanical-advantage ratios possible with the hydraulic system make that system a “must.”
Basically, there are two distinct varieties of spot-type disc brakes: automobiles usually have the opposed-
piston type, in which pistons located on opposite sides of the disc force friction pads into contact. The other broad grouping is that of single-piston disc brakes, which have either the disc or the caliper (the part that holds the pistons and friction pads) in a floating mount. With the floating disc, the piston moves a friction pad until it contacts the disc, which is then pushed over until contact is made with the fixed pad on the opposite side. The floating caliper type gives much the same action, but the piston-actuated friction pad moves until contact is made with the disc, and then the entire caliper slides over — pulling the fixed pad on the “anvil” side of the caliper into contact with the other side of the disc.
Something may be said for each of these basic types. Where plenty of space is available, it is probably better to use the single-piston type, as it is often lighter and usually cheaper than the opposed-piston variety. Cars seldom have any room inside their wheels and when they have disc brakes, those brakes will be of the opposed-piston type. On the other hand, motorcycle requirements would seem to indicate that the single-piston type might be best — and that is the type used on the Pointer.
At the heart (no pun intended) of the Airheart/ Pointer brake mechanism is the patented Airheart actuator. This device is not just a piston that pushes the friction pad out and into contact with the disc; it is also a very clever adjuster that maintains a constant 1 /64in. clearance unless the brake is being applied. This is a feature that is unique: most actuator pistons have some kind of pin that drags past a friction collar to provide an automatic adjustment, of a sort, for wear. And further, most of them also have a spring-load to back the pad away from the drum a fraction of an inch when the brake pedal is released. Unfortunately, these devices only operated “in”, and due to a swelling of the friction pads under heat, and distortion of the calipers under load, their built-in clearance is easily lost — after which the pads drag until they have worn away enough to provide clearance. Rapid pad wear is, of course, the inevitable result.
In the Airheart actuator, there are really two pistons: a large one, pushing against the pad, and a much smaller one that simply extends out of the back of the caliper. When the brake is applied, and the line pressure rises, the hydraulic fluid pushes against both pistons and a small spring between them is compressed until a lock-ring reaches its stop — the total motion being just 1/64-in., the running clearance we mentioned before. After this point is reached, the main piston overpowers the smaller one and drags it along, moving out of its cylinder to push the pad into contact with the disc. The small piston has a grip ring that keeps it from sliding freely, and after the main piston drags it out toward the disc, the grip ring holds it in position. Then, when the brake pedal is released and the fluid pressure falls, the spring caught between the main piston and the adjuster expands once more — pulling the main piston and the pad back 1/64-in. away from the disc. It makes no difference if the pad swells, or the caliper distorts, the clearance will be there. Distortion and swelling only force the adjuster piston back through the grip-ring and when the pressure begins to fall, the mechanism makes adjustments at the grip ring until the last moment, and then the adjuster piston is held firm while the spring pulls the main piston and pad away from, the disc. The excellence of Airheart’s actuator unit may be judged by the fact that they are now used on virtually every Indianapolis racing car, and the U.S. government often requires that the manufacturers of calipers and discs use Airheart actuators. All things considered, they seem to be the world’s best.
The Pointer’s disc brakes look a bit unusual, due to the fact that they use an annular disc, with the calipers in floating mounts on end-sections of the axles, reaching from the inside, out, to grip the discs. This in no way impairs, or alters, their basic characteristics, which are like those of any other disc brake. The actuation for both front and rear brakes is from a foot-brake lever, and there is an emergency mechanical-actuator on the front caliper that is controlled by a handlebar-mounted grip. Braking force is transmitted from the foot-lever through a single master cylinder, with a piston diameter of 13/16". Each of the brake cylinders is 1 Vi " in diameter and with a line pressure of 1000 p.s.i., about 1750 pounds of pressure is forcing the pads against the disc. The maximum dynamic torque capacity of the brake, at the 1000 p.s.i. line pressure, is 4440 inchpounds. With the wheel and tire used, this can be translated into approximately 400 lbs. of direct braking thrust at the tire contact point. If the braking load was divided evenly between the front and rear wheels, that would mean a total braking effort of 800 lbs., but this is not quite the entire story.
Inertia, acting through the center of gravity for both machine and rider, tends to carry the entire mass forward while the brakes, acting at ground-level, try to stop it. Usually, the center of gravity will be 18 or 20 inches above the ground, and inertia applied here, reacting against the retarding force at ground-level, produces a torque that tries to make the entire motorcycle tumble forward. It does not actually come to that, however, for the force required will always exceed the frictional grip the tires have on the pavement, and the tires will skid before the motorcycle overturns. Even so, the force is there, and it is too strong to be ignored. The effect it has, in point of fact, is to force the front tire against the ground while'trying to lift the rear tire. This loading and unloading of tires accounts for the fact that it is easy to make the rear tire skid; much more force is required to slide the one in front.
Because the inertia-torque produced weight transfer goes up with load, we find that while braking very lightly the braking effort is divided very evenly between the wheels. At higher rates of retardado^, naturally, the braking effort is not the least bit even. On the average machine, running on dry pavement, nearly twice as much actual braking effort woüid be needed at the front to make both wheels lock together. Thus, the braking effort must be divided up according to the overall stopping rate, and only a very experienced rider ever comes really close to achieving atheoretically perfect stop. That is one reason why the speed/distance braking test commonly given motorcycles is of such dubious value. The results depend too much on the skill of a human operator, who may be having a bad day and who is, at best, a variable quality that cannot be adequately compensated for in evaluating performance.
Further complicating the question of braking effort distribution is the lack of uniformity in the frictional characteristics of the road surfaces over which we must travel. If the surface is oily and wet, the wheels will both skid when the braking effort is nearly equal, while — as we have just said — a good surface may set the required braking force nearly twice as high for the front wheel. The experienced human operator will be able to handle part of the chore, but in most instances, the rider will elect, on a “bad” surface, to ignore the front brake entirely and “feel” for the skidding point of the rear wheel — safe enough, but not a very efficient way to stop. Indeed, it seems highly improbable that even the best of riders gets the best from his machine’s brakes. Particularly, he must approach braking effort very gingerly, for a front-wheel skid can send him sprawling quick as a flash.
Airheart was well aware of these factors when they developed the braking system for the Pointer. They experimented for ä while with dual master cylinders — one for each of the wheels, but the production version will have the single master cylinder. It was decided that, on a bike as small as the Pointer, the added cylinder was not necessary. Consequently, as the brake discs, pads and actuating pistons are the same size, the braking effort is divided evenly. Theoretically, of course, this arrangement is somewhat lacking: the rear wheel will begin to skid long before the front. Flowever, theory notwithstanding, the results are very good. The rear wheel does lock, but not until the Pointer is slowing at a very impressive rate.
To a rider long experienced in hand-and-foot braking, the single foot control on the Pointer seems a trifle strange. Strange or not, though, it set us thinking. One of the serious obstacles facing the beginner is the necessity for learning to work both hands and feet like a Chinese acrobat. It may not seem like much after one gets the knack, but this need for complete reorientation is responsible for many people not learning to ride at all. Hence, we think that the single-control feature of the Pointer is a devilishly gopd idea.
Carried further, the idea could be of benefit to us all. If someone can devise an automatic ratio changer to adjust the distribution of braking effort to meet the overall braking load, the hand brake will be needed only as an “extra.” Suppose for example, that the brake pedal fulcrum was on a spring-loaded slide, and that the amount of pressure on the pedal compressed the slidespring and a linkage adjusted the braking effort distribution in proportion to the amount that the fulcrum slide-spring was compressed. By finding the right'spring tension, we could actually achieve “perfect” braking.
In practice, of course, it would be prudent to set the ratio-changer so that the rear wheel would always lock before the front wheel, to give the rider a warning that he had reached the limit. It may all sound rather outlandish, but it seems to us that such a scheme would be worthwhile on racing (especially road racing) bikes right now — whether they have drum or disc brakes. Such an arrangement on production-type motorcycles would have to wait until the cost of the mechanism was reduced to an acceptable level.
In any case, the disc brake is coming, generally, and it is going to be available very soon on the Pointer. Words do not really do the system justice — you will have to try it yourselves. •
