|
Brakes
How they work |
6
Dec 2000 |
This
time we’re going to look at just how brakes do what they do, and ways to
improve them.
Without
a doubt the most important point to get sorted at the outset is just what makes
brakes do their thing - FRICTION. The sole purpose of the brake set-up on any
vehicle is to convert kinetic energy into thermal energy. In English that’s
motion into heat - friction. The motion of the disc/drum across the surface of
the pad/shoe produces a prodigious amount of heat. The contact of these components
and the heat generated creates friction - making continued motion very
difficult. Try gently applying hand pressure to a drill chuck when it’s slowing
down. Apart from speeding the slowing down process, you’ll notice your hand
getting considerably warmer - friction.
Excessive heat can be counter-productive. The correct operating
temperatures are a relatively fine line so this also needs consideration.
The
amount of friction developed is dependent on a collection of components, but
ultimately occurs at the disc/drum end of things, so the rest of the system and
components should be built up around that. As drum brakes are desperately
ineffective, I am going to concentrate on discs - the most common type of
conversion. Anyone converting from a decent disc-brake set-up back to drums for
any reason other than originality needs their head examining.
Swept
area
This
is the total surface area of the disc that the pad operates on, so generally
the bigger diameter the disc, the bigger the pad area can be, and the more
friction can be generated. Bit like the bigger a box full of something is, the
harder it is to push it along. There are limits though - the wheel size and
type will dictate what will fit. Concentrating on our Mini situation our
alternatives are the 7.5” disc from the S/1275GT (ten inch wheel), 8.4” disc
from late 1275GT/all Minis from circa 1985 on with 12” wheels, and the much
applied Metro 8.4” disc (in both solid and vented form). See 'Brakes -
Fundamental considerations for further information with a table illustrating
all the necessary data. The greater the disc diameter, the longer the torque
arm effect too…
Torque
arm
Back in school during one of those seemingly endless physics lessons, you were taught that the longer a lever you use with a pivot point as close to the subject involved, the easier it is to move it. Same thing for brakes. The bigger diameter a disc is, the further out the caliper can be mounted, so increasing the effective torque arm length. Multi-piston calipers generally have their piston centres mounted in such a way as to move this point further out. So for any given disc diameter, a two piston caliper is less effective than a four piston caliper, a six piston one being even better and so on. There are limits of course (more later). A picture paints a thousand words - so check out the diagram for a more visual comparison
 |
Click picture for a larger image |
OK,
so far in both swept area and torque arm descriptions, I have mentioned
limitations. Apart from physical fitment, the main limitations centre around the
ability of the brakes to over come the tyre grip and vice versa. The first
tends to lock the wheels up, vastly reducing the grip on the tyre on the tarmac
- therefore also severely reducing retardation. The second develops excessive
heat, building up to a point where the pads and discs become ineffective -
otherwise known as ‘fade’. Again retardation is greatly reduced.
Therefore
trying to cram an 8.4” disc into a ten-inch wheel set up isn’t generally a good
idea. 7.5” discs on a thirteen-inch wheel car are going to be disadvantaged
too. Fitment of a servo isn’t the answer, as this tends to magnify one or other
of the problems. The servo is purely an instrument to reduce the pedal pressure
necessary to operate the brakes. But I’m jumping the gun on ‘other brake
components’ here.
As
with all modifications applied to up-rate/improve your Mini, careful thought is
needed. All the other components in the system need consideration, and whether
they are functioning properly. I do not intend to cover full servicing of the
braking system here - suffice to say that maximum performance will only be
attained from correctly serviced and properly functioning parts.
Driver
input
A
very necessary part of the system this as it is he/she that instigates the
braking events. Applying ones foot to the pedal is the start of the retardation
process. This component is very variable, and sometimes each vehicle sees a
number of different ones. No matter, the only over- riding factor here is how
much spinach each one has consumed as to the amount of input required. The
human body being the amazing piece of machinery that it is allows each to have
a totally variable input independent of the sum of the other components on the
Mini, combining thought with feel - i.e. ‘I want to slow down quicker’ so the
pedal is pressed harder. That sort of thing.
The
only direct contact the driver has with the braking system is the pedal. The
pedal has a lever ratio all of its own, being calculated by the difference in
length of the pedal to the pivot pin, and from there to the master cylinder
cotter pin. Increasing the length between the pedal and pivot pin will give
more pedal travel, but reduce the effort needed to apply the brakes. Reducing
it has the opposite effect. Therefore an increase in ratio gives a softer pedal
with more ‘feel’, reducing it the opposite. Changing this particular component
is a bit involved though, and is generally unnecessary as it suits most
combinations of the other components.
Hydraulics
The
master cylinder translates the pedal movement into fluid movement. The bore
size dictates how much fluid is moved for any given pedal pressure, and is
directly related to the main brake component - the caliper. Bigger diameter or
multiple pistons will need more fluid displacement to make them work properly.
This generally means a larger bore master cylinder is needed for any given
application. However, as this component can be very costly to replace, most
folk stick with the standard one. It will work OK, but means that on the multi-piston
calipers there is more pedal travel than normal. The good news is that a
smaller bore means higher line pressure for any given pedal pressure - less
driver input for same braking effort, and better ‘feel’.
'Feel',
incidentally, is the sensation of what the brakes are up to, and is an
important part of our thought process as to how much pressure we need to apply
to the pedal in any given situation.
For
information sake, the Mini master cylinder has a 0.70” bore. The old Cooper 997
had a bore of 0.750” largely because of the pathetic size of the discs - a
great deal of pressure was needed to slow the car down but would be ace on a
four-piston caliper set-up. The old
For
race Minis, going to a 0.875” bore master cylinder is best. There isn’t one
that looks and fits just like the Mini one, it necessitates going to a master
cylinder with separate fluid reservoir and making a suitable push-rod. No
problem though as Girling and AP make a cylinder that will fit the stud
pattern, a standard Mini push-rod can be easily modified to suit, and the
reservoir is easily mounted to the bulk head. And is surprisingly cheap - less
than a new Mini split-system one in fact!
When
trying to get the best master cylinder bore size for application, you need to
remember that the amount of hydraulic pressure produced at the pedal is
INVERSELY proportional to the master cylinder bore. So if you are locking the
brakes up too easily, you need to INCREASE the bore size. Consequently if you
are standing on the pedal, pulling of the steering wheel and gritting your
teeth together to lock the brakes, a smaller bore is the order of the day.
Disc
Mods
Improving
the performance of the disc itself has seen three types of modification -
venting, slotting and drilling.
Vented
discs have become pretty much a standard item on modern cars as a more
efficient disc temperature wise can be fitted into a smaller area. This
contradicts the 'bigger is better' principle, but modern technology has seen
improvements in pad materials, so small cars that are fairly heavy can have
good brakes without going to huge wheels to fit them in. The Metro for
instance. A smaller vented disc does have slight advantages over a bigger solid
disc in the effects of inertia stakes.
Slotting
discs has been pretty much misunderstood by many. It is generally believed that
the slots are there to improve cooling. They are not. They are there to wipe
the pad surface. In operation, the heat creates debris and gases between the
disc and pad surfaces - reducing their effectiveness. The slots clear this
away. To be totally effective though they need regular cleaning as the debris
fills the slots up. Now, it has become fashionable to have loads and loads of
slots in discs. AP Racing recommend only four slots in a disc as small as the
Mini. Bear in mind that friction area is needed to make the brakes effective -
lots of slots markedly reduce the surface area of the disc and thus the
available friction area...
Drilling
discs is open to the same misunderstanding that slotting is. The same actual
reasons apply, except that holes are more effective over time as they are more
or less self-cleaning. The only major draw back (apart from going mental on the
number of holes - friction area reduction again) is that in discs with
insufficient mass - too small in diameter or too thin - they tend to crack and
fall apart. I know motorbikes don’t have
these problems, but they are a totally different kettle of fish! The 8.4” disc
can be drilled - but needs to be done by experts, not at home in the shed -
it's your life (and possible other's) that you're fooling around with.
Useful
part numbers:
|
GBD101 |
7.5" S/1275GT disc, high-quality
English-made in GR260 Grade iron to BS1452 standard (each) |
|
C-21A1265 |
As above, but 4-grooved for disc/pad
cleaning |
|
C-GBD496 |
8.4" Vented disc, grooved and
slotted |
|
C-21A2612 |
8.4" Mini-type disc, grooved and
slotted |