Posted Aug. 12, 2015
This article originally appeared in the Fall 1976 issue of Inside Line, the voice of the Association of American Motorcycle Road Racers. AAMRR was the north-east U.S. equivalent of the American Federation of Motorcyclists in the 1970s. Permission was granted to reprint the article and it appeared in the July 1977 issue of the Lap Times, the AFM newsletter. The AAMRR isn’t around anymore, so I’m assuming their permission is still valid. In a few spots I’ve interjected text within square brackets […], and left some stuff out for the sake of brevity.
BRAKES – They’re Not Getting Worse, You’re Getting Better
by Kevin Cameron
Brakes, like any other equipment, must be matched to the job and to the user. The brakes that you think are great in your first novice race may very well be dangerously inadequate when your skill has improved and you are going in deeper and pulling the lever harder.
Back in 1964, users of the first TD1-B Yamahas were amazed at the power and predictability of their [drum] brakes. By the end of the season everybody was looking for something else. Next year there was a new lining material, and that was great. After that, people went looking for other linings than the Japanese linoleum, and the faster runners were showing up with Ferodo, Friendo, and other better linings. This could only go so far, though, because people were pouring the heat to the brakes faster than even the drum could stand. By 1968, the last year of the small, two-shoe drum, the iron liner of the drum itself would crack in one hard race, and have to be thrown away or re-lined.
For 1969 Yamaha released their now-famous four leading shoe 250mm drum brake, which was right off the successful factory 250 GP bikes. When we saw this for the first time, with its two-inch spokes and massive finning, we thought it was the millennium for sure, factory equipment at last.
The honeymoon lasted as long as it took to find you could stop even harder and faster than before, and people began to put on air scoops and to apply Ferodo racing linings. Since it was believed that using the famous “Green Stuff,” a notoriously grippy lining, on all the shoes would make the brake too fierce, people crept up to the problem by using less sensitive material on the leading edges of the shoes. Again. however, riding technique wasn’t far behind brake technology, and Green Stuff began to appear from one end of the shoe to the other, and riders were happy to suffer “on-off switch” type brake performance in return for getting stopped quickly.
Because of its large lining area and mechanical actuation, a drum brake requires a lot of pressure to stop quickly. The only way this could be achieved was by the leading shoe principle, in which the rider’s hand presses the shoe against the drum, and the rotation of the drum tends to wedge the shoe even more tightly against itself. It is a self-servo device. The higher the friction coefficient of the lining, the stronger the action.
A point was reached where brake performance was good, but control feel was poor. Even a slight difference in lever pressure could command a large change in brake torque, because for every pound of hand force there were several pounds provided by the brake itself. This would not be so bad by itself were it not for the friction of the cable, cam pivots, and the cams themselves, which conspired to make it hard to communicate a small change in rider control. This required fine judgment on the part of the rider, and a lot of commitment to a certain line into the corner, because brake torque could not be accurately modulated. You rushed up to your braking point and turned on the retarder. Abrupt changes in plans were not in the program.
[A Smith-Kanrim front brake, representing the pinnacle of drum brakes. Four leading shoe, adjustable cam leverage, full coverage cooling fins, air scoops to collect cooling air and hot air exit ports. Photo from victorylibrary.com.]
Here entered the disc brake. It has no self-servo action, so for every change in rider control force, you get a proportional change in brake torque. Its smooth hydraulic force multiplication greatly reduced control friction, so that small changes in control force could be accurately reproduced at the wheel. While thermal expansion of a drum brake moves the friction surfaces apart, there is no such efficiency-reducing heat effect with a disc brake. The hottest surfaces of a drum brake are inside, and heat generated there must be dissipated to the air stream outside. In a disc brake, the hot parts are all exposed to the airstream.
The limits of braking were extended, therefore, again. It took a while for riders to develop the new techniques that were possible, but the early disc brakes were indeed a pleasant surprise for the above reasons. The more competent riders discovered they could go in deeper still, using the accurate control-ability of the disc brake to approach the traction limit more closely. Heat was poured into brakes at higher than ever levels. There were problems. The disks that had been a marvel in March were junk in July. This sent people scurrying for solutions,. There were drilled discs, thinned discs, alloy discs, dual discs, different calipers, and stacks of mystery friction materials. There still are. How can any sense be made of it?
[The RD400F front brake has a single piston caliper; the piston presses the outside brake pad into the disc and the caliper pivots slightly on a pin to even the pressure on the static inside brake pad. You can see a stack of washers just above the bleed nipple showing the location of the pivot pin. As a result the pads wear at an angle — a worn pad will have almost no pad material left on one side but plenty of pad on the opposite side.]
First there is the question of leverage. Everyone knows there are different size master cylinders. What do they do? The relationship is a simple one. The ratio of the area of the master cylinder piston to that of the caliper piston is the leverage ratio. For example, a small bore master cylinder has an I.D. of 14mm and an area of 1.54 square centimeters. The Yamaha caliper has a bore of 50mm and an area of 19.6 square centimeters. The ratio is 19.6 to 1.54, or 12.7 to one [i.e., one pound at the lever produces 12.7 pounds at the caliper]. If we substitute a large bore master cylinder of 16mm I.D. and an area of 2.01 square centimeters, we will have a reduced leverage ratio of 19.6-2.01, or 9.7 to one. Increasing the master cylinder bore reduces the leverage, which means we must pull harder on the lever to get the same pressure at the caliper. A similar change will take place if we vary the bore of the caliper piston….
Of course it all gets complicated because some calipers have one piston and some have two pistons and there are many different sizes of both master cylinder and caliper bores. The simplest way to review the facts is to specify brake performance in terms of how many pounds of control force (lever squeeze) are required to stop a machine at one “G” deceleration. [See Table 1.] A small figure in lbs per G means a lot of stop for not very much pull on the lever (sensitive), and a large number means the opposite.
To make some sense of these numbers here are some facts. National riders on 750s seem best pleased with control forces of 25-30 pounds, while top 250cc riders can just get by at 35 pounds. It seems that the complaining starts around the 35 pound mark. At the other end of the scale, complaints about excessive sensitivity begin at 20 pounds per G and below.
[Conclusion: the preferred setup will produce one G of deceleration with a lever squeeze between 25-35 pounds.]
This is far from the whole story. That trusty old Yamaha RD350 setup, the single 10.5” disc and 14mm master cylinder, gives a control force of 28 pounds per G, right in what seems to be the desirable range. Yet lots of racers are having trouble with it. What gives? … This small disc simply doesn’t have enough surface area to get rid of its heat, so its temperature goes up high enough to cook the friction material, reducing its friction coefficient. This is Fade. [In fairness, the RD350 front brake was meant for a street bike, not a road racer.]
I have therefore included some other figures. I have computed a table of brake power loading, in square centimeters per horsepower.
[In this table, the larger the number the faster a skilled rider will be able to stop.]
Again these figures don’t tell the whole story, because on a slow, heavy braking course such as Loudon, a 750 can’t use its horsepower to go much faster than a 250, so perhaps weight would be a better indicator of brake loading.
The Table 3 figures tell how many square centimeters of heat dissipating surface are provided by the front brake for each pound of machine-rider weight. [Again, the larger the number, the faster a skilled rider will be able to stop.]
For courses where machines can get close to their maximum speed, brake power loading [table 2] is an appropriate measure of brake adequacy, while on twisty courses, where 250cc and 750cc lap times are very close, weight loading [table 3] is the better indicator…
This is an answer to why 250’s with RD350 disc brakes are having trouble getting stopped. While their control sensitivity, at 28 pounds per G, seems to be in the middle of the desired range, both its power and weight loadings are below standard. This disc brake is just getting too hot to work properly.
[This STILL doesn’t tell the whole story.] The discs must be able to store a large amount of heat for a short time. During a single heavy braking event, such as for turn one at Daytona, a vehicle may decelerate from 175 mph to 80 mph. The kinetic energy difference between these two speeds is converted to heat and dumped into the discs. Because braking occupies only a few seconds at most, the amount of heat dissipated to the airstream is very small compared with the rate at which heat is being added by braking. The discs must therefore store the heat and their temperature rises. The rise depends inversely on the amount of mass in the discs (the greater the mass the smaller the temperature rise) and inversely upon what is called the specific heat of the disc material. “Specific heat” is the amount of energy in calories required to raise one gram of material by one degree Centigrade, and it varies considerably from one material to another. [All other things being equal, the higher the specific heat the better the material will absorb heat.] The specific heats of some materials of interest are given below.
Beryllium 0.4 – 0.5
Iron, Steel 0.1
[From this perspective] our ideal machine will have discs made of water, with the poisonous metal beryllium as a distant second choice. (Beryllium has been tried in racing, and is used in aircraft). [Drilling or thinning the discs will reduce un-sprung weight, a good thing, but will also reduce mass which makes the discs heat up more.]
What about [plasma-coated] aluminum alloy discs, such as those offered by Harry Hunt, and commonly used in factory racing? [Also used on some of the early Superbikes.] Aluminum weighs just one third as much per volume as steel, so when you put on an alloy disc you are saving weight but you are also reducing your heat storing mass as well. Fortunately, the specific heat of aluminum is 2.2 times that of steel, so it nearly makes up for the difference … the one-stop temperature rise of aluminum may be higher that that of steel, but because the thermal conductivity of the alloy is so much higher than steel, the heat generated is more uniformly distributed throughout the disc. Thus the surface of the disc doesn’t become as hot, because the heat generated there by pad friction penetrates the body of the disc rapidly.
Finally, I want to present some facts and some personal observations about friction materials. You can improve or degrade the performance of a brake system by right or wrong choices of brake pads. … Generally, the very high friction materials also offer rapid wear, but there are few applications that will wear out a set of pads in a single race.
My favorite pad material for use with coated aluminum brake discs is the Ferodo 2434, which has a high friction coefficient of nearly 0.5, and has shown little wear in my experience, typically less than a millimeter per pad at Daytona, somewhat more at Loudon. On steel discs, with their much higher surface temperature, wear will be accelerated considerably.
Another high-friction favorite is Mintex 64, standard in some Lockheed calipers. This is a soft, rapid-wearing material which Gary Nixon used at Laguna Seca, with little more than half the pad left after the first 100 kilometer section. They were willing to sacrifice the time needed to change the pads during the pause between the two 100-km heats to get the high grip. Mintex has a harder, longer wearing material, #108, which had been a [world champion] Phil Read choice, probably on steel discs.
Postscript: After reading this in 1977 I got the impression that in the early days of disc brakes, setup was a matter of juggling a large number of factors – pad material, disc material, master cylinder and caliper bore sizes, disc mass versus weight, and some magic stuff as well. Few people got it right the first time. In the late 1970s I have to admit I knew very little. The Dale Newton Ducati Superbikes I raced in 1977 and 1978 stopped well but did not have good feel. It always felt like I was close to locking up the front wheel, although I never did – my crashes were almost all because of too much throttle at the wrong time. My 1979 Ducati Darmah street bike’s brakes had better feedback than the race bikes.
When I rode the ex-Butler&Smith BMW in 1979 I experienced a race bike that stopped really fast and with great feedback. I could do “stoppies” with that bike without feeling like I was going to lose the front. Brakes on today’s bikes, especially the sport bikes, are miles ahead of what we had to suffer with in the decades of the 1970s.