Brakes by shuifanglj

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									                         BRAKING SYSTEMS

Because these are the days of carbon/carbon hydraulic six-potter ABS, it's easy to lose
sight of the fundamental principles behind how brakes work, and
therefore to make mistaken judgements in their selection or
development.

So, back to basics. The job of a brake is to convert kinetic
energy (the speeding motorcycle) into heating up the surrounding
air, nothing else. No matter how exotic the pad or disc material,
or how weird is the caliper or hub design, a certain amount of
kinetic energy will always give rise to the same amount of heat
energy, just as Sir Isaac Newton told us.

The laws of thermodynamics tell us that heat energy will always
flow from a hot place to a colder one, never the other way around
(you may laugh; it's surprising the number of engine designers
who haven't worked that one out yet). If other factors don't
change, more heat energy will be transferred as the temperature
difference increases. Therefore, if you want to dissipate the
kinetic energy of the bike into the surrounding air by heating it
up, then your braking system must get hot. All other things being
equal, the hotter the brakes, the more energy you can transfer.

This is where the problems start. Most engineering materials
change their characteristics as they get hotter; all of them
change their size, and some their shape.

More basics. A physical law which is taught to all schoolkids is that the friction
between two sliding surfaces depends only on the contact pressure and the
coefficient of friction, mu, and that mu doesn’t alter with the area of contact or the contact
pressure

This theorem is very important because it shows you that you
should never believe what you are told by those in authority; it
is utter and complete bollocks. Apart from a very few weird
situations, more area means more friction and that's an end to it.

You don't necessarily have to use friction to convert your
kinetic energy into heat as a way of slowing down, another way of
doing it would be to have an electrical generator on the wheel
which would create electricity and heat up a few bars of an
electric fire. This is not as daft as it sounds; Bosch make an
electrical brake which works like an eddy-current dynamometer,
which was very popular on buses until not so long ago. A good way
of making a really sophisticated dyno for next to no dosh was to
buy one of these from a breaker's yard and modify it suitably -
it would be good for 200bhp or more.

I have done some design work on an electric delivery vehicle which
has regenerative brakes; that is, there are leccy motors in the
wheel hubs to drive it along, but when the driver hits the brakes
these motors are used as generators to feed the kinetic energy of
the vehicle back into the battery. That way, the kinetic energy
is converted into electrical energy, and then into chemical
energy in the battery. But it's not a very efficient process and
we end up dissipating a fair bit of heat as well.

Having established the principles, we can now forget wacky
systems like generators, eddy currents, and parachutes, and
accept that in practical designs if we want to create heat we are
talking about friction between sliding surfaces. But how best to
make these surfaces?

Drum brakes are really a development of the stage-coach system
where the driver had a wooden lever to press down on the wheel
rim. I suppose this was OK when all you had to worry about was a
mere 4 horse-power.

The next big development over the wooden lever was the idea of
having a steel strap part of the way around the rim, normally
lined with leather, with a lever to pull it tight. I nearly typed
"steel band" there instead of "steel strap" but I couldn't get
the image of black guys drumming on dustbin lids out of my head.
The steel strap worked a treat as it could act evenly over a much
greater area than a wooden block. More area, more braking force.

The drum brake is basically the same thing, with the strap (in
the form a drum) rotating instead of being fixed, and the "wheel"
(the brake shoe) being pushed up against it. The great thing
about this system is that if you arrange your levers cleverly,
they will tend to pull themselves on with more force than you
apply. This "self-servo" action is why twin leading shoe drum
brakes were such a great advance over the SLS (single leading
shoe) type, because a trailing shoe tends to pull itself off the
drum but a leading one will push itself on. (Fig 1).

The drum brake is a truly superb bit of kit, and it has many
applications in engineering, but if you've ridden a CG125
recently you'll know that motorcycles aren't one of them. One job
which is particularly suited to drums is that of a parking brake,
because of their low-speed self-locking action. This is why
some cars have both drums and discs on the back wheels:
the discs are fine for high-speed work, but crap in a static
application.

As mass-produced moronwagons are cost engineered down               to
fractions of a penny, addition of drums where a car
already had discs shows just how useful they are, but not for
bikes.
The liability of all drums is that as they get hotter, they
expand and distort. Their expansion means that you have to push
more on the brake shoes to keep up, and the distortion means that
not so much shoe is in contact with the drum, so the braking
force decreases. These two processes add up to classic fade; the
temperature goes up, the drum expands and distorts, the braking
force falls off, the rider falls off.

You can design your drums to try and limit these two problems of
expansion and distortion, of course. Putting deep fins on them
works well by making them more rigid, as well as dissipating heat
better. Huge "bacon-slicer" fins were sold to the cafe racer
crowd as aftermarket add-ons, and big air scoops were fitted to
the hubs to try to get a cooling draught around the brake shoes.
But the limitation, though smaller, was still there. If you were
really clever, you would take this liability and use it as an
asset. Enter the disc brake.

The great thing about disc brakes is that they are discs. No,
really. You see, when a complex shape like a brake drum heats up,
it distorts; when a simple, flat shape like a disc heats up, it
usually just gets a bit bigger without distorting. That means
that your friction surfaces (the brake pads) will still make good
contact with it and you won't get fade providing the materials
don't change their properties too much with the rising
temperature.

It's actually even better than that. If you can keep the brake
pads in the same position, then as the disc heats up and gets
bigger it will automatically increase its stopping power for you
- the great liability of friction brakes, their expansion with
temperature, is thus turned into an asset. Isn't that just
marvelous? A little thing like that might seem trivial to you,
but as a design engineer I get quite a thrill just thinking about
it.

The size and shape of discs and pads is one of those vexing
hassles that can give you real grief because of the variety of
problems which they can throw at you. It's all about a balance of
energy transfer really. The harder you brake, and if you do it
through a wider speed range, the more heat you put into the
brakes. As their temperature rises, they can dissipate more heat
into the surrounding air. There is a certain amount of lag in
this process, so it's not at all simple.

Imagine the level of water in a bathtub being the equivalent of
the heat energy in a set of brakes. The higher the water level,
the faster you could get rid of the water down the plughole.
Every now and then, you might well want to turn one of the taps
full on, or just have it trickling, for varying amounts of time.
Having water splash over the side is the equivalent of fade - too
much energy in the system for it to handle.

If there's no water at all in the bath, you can't wash the soap
off your hands so you can't turn the tap on, and this is the
equivalent of having brakes that won't work when cold. You could
always guarantee to have water in the tub by making it really
big, but then it's so heavy that it might damage the floor - and
the direct comparison to motorcycle brakes is pretty obvious.

The balance of energy flowing into and out of the system carries
designs to extremes depending on their application. A Tornado
military assault aircraft has triple disc brakes on each main
wheel, with carbon pads in between, stacked together rather like
a big clutch. As there is no area of disc exposed to the air to
dissipate the heat energy, they get well hot. But the military
don't really care as the aircraft only has to land once or twice
a day and the brakes have hours and hours to cool down. This is a
classic example of fast in, slow out, for heat energy.

Another extreme was the old Broadspeed Jaguar saloon racing cars.
Known for extreme lardiness, their brakes had to get rid of
energy just as fast as it was fed in, to keep performance
constant over an endurance race. Therefore a large area of
exposed disc was required to get the heat out, so (as is standard
racing car practice) vented discs were used. These are hollow,
with the pads gripping on the outside, and air being fed through
the inside. A duct is used to feed cool air to the centre of the
disc, and its rotation sucks the air through to the rim like a
centrifuge. This wasn't enough for the Jags, they had to feed
water jets in there as well to stop everything melting.

Fortunately, the arrival of carbon brakes meant that bikes, being
light enough that they didn't need to shift really vast amounts
of energy, never got as far as vented discs. There is no real magic with carbon, it's just a
material that is very light and can operate at huge temperatures without changing its
properties much. This moves you on to a different part of the
heat/temperature map for less weight and pad area. The
disadvantage is that they only work well when stinking hot.

It is very disconcerting using them for the first few times, you
stamp on the pedal (I think of pedals because I've only used
carbon brakes in cars, never on a bike) and nothing happens. Just
as panic really sets in, the temperature goes shooting up and the
frictional force with it, so you have to lift off to avoid
locking up. This is very arse-about-face compared to normal
systems and takes some getting used to. That's why the bike
racers run shrouds on them, to keep the temperature closer to the
working range.
As demands on road bike brakes get more extreme with higher
speeds and better tyre grip, so the accent falls on fade-
avoidance with the penalty of poor low temperature performance,
like carbon systems for little boys.

What doesn't help with road bikes is that they are designed for
pose value as well as maximum efficiency. Thus, though the Yamaha
TZ250 racers of a few years back had a superb wopping big single disc
front brake setup, with a sensible caliper, the equivalent
Kawasaki road bike (the KR1) just had to have twin front discs
for marketing reasons. They were therefore small diameter spindly
horrible affairs drilled full of holes which scored up and wore
out in no time. The calipers and pads were also tiny little naff
things like out of Xmas crackers and the proddy racers were
getting them to last for about one meeting.

The reason why discs sometimes warp after hard use is connected
with the extreme temperature differences which they see. Not only
does the whole disc have to suffer wide variations, but in use
each part of the disc will be heated by the pad, then cooled by
the airstream, for every revolution of the wheel. This rapid
thermo-cycling builds up all sorts of internal stresses and the
disc will sometimes cry enough after a while. Cracks appear for
the same reason.

Disc brake design has developed logically to date. It is better
to have a disc diameter as large as possble, because the caliper
then exerts more leverage over the wheels for the same force. Of
course, you still have to fit a caliper around it, and if it's
too big then the caliper will foul the wheel rim. An old racing
car with drum brakes had a gearbox in each wheel hub, to run the
drums at a higher speed than the wheels, because of this problem.

When the disc is as big as it can get, you can increase the
braking force by making the pads bigger. But then, the bit of pad
at the outer edge of the disc is so much more effective than the
bit closest to the middle, that it's better to extend the pad
around the disc. For this you need four hydraulic pistons instead
of two, or you can even run to six. The limitation here is that
as you extend the pads, you end up with no area of fresh air for
the disc to cool itself in, just like on the Tornado.

It is important that as much heat energy as possible goes      into
the disc; if it gets through the pad and into your caliper    it
will distort your hydraulic pistons, boil your fluid, and     all
sorts. So calipers sometimes have ceramic plates behind the     pads
as insulators.

It is commonly believed that calipers are mounted behind fork
legs because if you put them in front they cause more dive under
braking. This is bollocks; the whole front wheel unsprung system
can be contained in what a mechanical systems engineer would call
a "control volume" and the position of the caliper in relation to
the forks makes no difference.

There are three reasons why the calipers go behind the forks; (a)
the polar moment of the bike is lessened by moving the mass of
the calipers closer to the centre of mass, and the moment of the
forks around the steering head is reduced (making tank-slappers
less likely), (b) more air can get to the part of the disc in
front of the fork leg to cool it, and (c) it is less likely that
stones or other debris will get caught in the works.

Rear brake calipers normally sit under the spindle because torque
arms holding them can be made lighter if they act in tension
rather than compression. On motocross machines, the caliper lives
on top of the swinging arm because minimum weight is less
important than keeping the caliper out of the way of rocks, crud,
roots, and other parts of nature's great vista of wonderment.

The saving grace of MX bikes is that since they have so little
grip, they don't need very effective brakes anyway. Indeed, the
same notion applies to bikes in general - with a small contact
patch and a rounded tread profile, and a small all-up weight to
stop, it's a matter of making the most of what you've got.
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