Basic Theory on Brake Operation

[Cpt. Ash]

The Basics of Brakes put simply – BRAKES MAKE HEAT!!

To accelerate your car, you burn fuel and the heat energy accelerates
the mass of the vehicle. Brakes reverse that process. By applying your
brakes, you are converting kinetic energy into heat, pure and simple, a
straight thermodynamic conversion. The amount of heat to be absorbed (and therefore dissipated) by the brakes is a function of the
velocity of the car, the amount it weighs and the rate of deceleration.
Acceleration/Deceleration are mathematically “square functions”,
meaning to stop twice as hard takes four times as much effort,
generates four times as much heat and wears out the brakes four times
as quickly. Stopping four times as hard requires 16 times and so on.
It therefore follows that a big heavy vehicle stopping quickly from high
speed requires large heat absorbing capability and the ability to
dissipate that heat to the atmosphere.

Brake Pads and Disc Rotors make the heat that stops
the car

Brake Pads are manufactured from fibres, metals and binders chosen
for their friction properties. Essentially, all heat the pads generate has to
be passed into the disc rotor. The pad itself has to be able to handle
the extreme heat generated but it is not expected to do much towards
dissipating the heat, as its all done by the disc rotor.
A larger pad surface area is required for reducing the specific heat
load; a very small pad would become extremely hot, even if running
against a large rotor. It therefore follows that the key to stopping a
heavy car is good ability to absorb and then dissipate heat before
being called upon to do it all again. If the system is not sufficiently
cooled, fade will occur. The type and quality of the cast iron, the
design of the internal cooling vents and the surface friction
characteristics of the rotor all have an influence. Calipers make the forces to push the pads onto the disc. Hydraulic pressure from the master cylinder is converted to force on the pistons and act on the pads to apply friction to the rotors. Replacing a single piston caliper with a four small piston with the same
size pads will have only a small effect. However when fitting a larger
rotor, you can get the best from them by having a larger pad; but
applying a greater force via a single piston can cause the pad to bend
in the middle. Therefore the recent trend to twin piston/four piston
calipers which allow greater force and more even loads on the pad.


Master Cylinders Make Pressure

To apply a hydraulic fluid pressure to the brake calipers, we use a
master cylinder, in essence small pistons connected to your brake
pedal, which generates fluid “pressure” according to the force applied
to it and the area of the piston by the formula.
Pressure = Force
Area
Therefore a smaller piston generates a greater pressure. However there
is a lot of elasticity in a brake system and so master cylinder piston
diameters are sized according to pedal travel constraints as much as
pressure requirements.
A small piston may give nice high pressure and thus reduced efforts,
but at the expense of excessive pedal travel. Master cylinder bore sizes
are always a balancing game between fluid pressures and pedal
travel. Everything has to be balanced including front to rear.

Pedal Ratios and Brake Pedal Leverage

Brake Pedal leverage ratios affect pedal effort, the greater ratio giving
the greater force on the master cylinder, but once again the trade off
is increased pedal travel. Ratios range from 5-1 up to 7-1. Physical size
as well as ratio does come into it. The free play in every system means
that the push rod must move a small distance before the brakes begin
to apply, therefore a reasonable length from push rod to pivot must be
used. A very short pedal length with a good ratio of 5-1 will still move a
long way before taking up the slack. In the end, you have to balance
getting the pedal position comfortable, the ratios and sizes correct,
and still fit the assembly to the car. Car companies rarely make
mistakes so if not sure, see how they do it.


Brake Systems for Modified cars

Whenever fitting larger brakes, you have to balance master cylinder,
caliper and wheel cylinder hydraulic sizes. Small cars usually have small-bore master cylinders (Eg. 13/16-inch and 7/8-inch etc.) and
matching small bore calipers and wheel cylinders. Larger cars are
usually 1-inch to 1.1/8-inch. Fitting big calipers and rotors to your small
car will necessitate fitting a matched bigger bore master and maybe
booster, otherwise the car will actually stop with less pedal effort but
will have excessive pedal travel. Once again, a good rule of thumb is
to see what the car companies do and use matched sets, master,
calipers etc. all off the same model or at least of the same bore sizes.

Mixing and Matching Parts

Modified cars are often concoctions of different sources of parts, with
one brand of master cylinder with a different brand of booster and so
on. Sometimes they appear to bolt straight together but looks can be
deceiving. You must ensure that the output push rod at the booster is correctly set to the master cylinder so that there is no free play, but also that the
seals within the master are fully returned past their ports so that the
brakes will release. The push rod may need lengthening or shortening
to suit. As little as 1mm will make a big difference to initial pedal take
up travel. The booster input shaft should not be preloaded but should
have minimum free play too. On disc brakes, you can check that the
master cylinder is not over adjusted by opening a brake bleeder. It
should drip under gravity if all the ports are unobstructed. If not, the
brakes will drag, get hot and lock on.

Brake Boosters

Brake Vacuum Boosters act by using atmospheric air to assist the force
on the master cylinder. When not in use, both sides of the diaphragm
are under vacuum from the engine. When applied, the central valve
closes and the valve closest to the firewall opens allowing a shot of air
in to push on the diaphragm.
Generally, and particularly on heavy cars, disc brakes will require
boosting or pedal pressures will be too high. Lighter cars or older types
with high friction asbestos type pads were used but with still high pedal
efforts.
Drum brakes are often self-energising, that is the rotation of the drum
acts to pull the shoes on harder. Most modern cars with rear drums are
boosted to all four wheels, however if using a single VH40 remote
booster, you can compensate for not having a rear booster by fitting
larger rear wheel cylinder sizes (Eg. go from 7/8 to 1.1/8 bore for higher
forces on the rear shoes), and so on.
Conversely, if your car suffers from rear brake lock up, you can consider
smaller bore wheel cylinders. When mixing large rear/small front tyres,
the same applies. If space constraints mean remote boosters are
needed and you have four wheel disc brakes, then two remote
boosters will be needed with the accompanying cost and piping.


Brake Hoses

ADR 7 requires that automotive brake hoses be fully fatigue tested,
manufactured in a way that they cannot be taken apart and pressure
tested to 3000 PSI (20,000 KPA). Amongst other things, stainless braided
hoses swell less under pressure, particularly when hot, giving better
pedal feel, but the types which are assembled at home and not
pressure tested are not street legal.

Brake Piping

Most automotive brake piping is 3/16-inch O/D or ¼-inch O/D plated
steel seamless tube with doubles flares or ball flares (drill point seats).
Under no circumstances make automotive brake lines with singles
flares. Copper lines are unsuitable due to fatigue hardening. All
approved automotive brake fluid connections are based on positive
compression seals, flares, flat copper sealing washers and lately,
positive o-ring quick connect seals. No new car was ever produced
with a taper thread fitting (ie. needing sealing compound or Teflon
tape). Some aftermarket calipers are sold with taper thread
connections and their acceptability with ADR’s is therefore debatable.
Check every connection for brake fluid leaks before driving the car.

Proportioning Valves

A well-designed brake system should always lock the front first. If the
rear wheels lock, the car will spin around. The purpose of rear
proportioning valves is to ensure that line pressure cannot rise to the
point of rear lock up. Up to that point, front and rear line pressures are
usually the same. A proportioning valve is not a band-aid for a badly
balanced system.

Handy Hints – Do’s and Dont’s

1. Don’t mix up calipers, left to right, bleeders always go to the top to
get the air out (some older European and English cars were built to
be bled backwards, making it hard to do at home). Modern cars
always bleed air at the top.
2. When trouble shooting a soft pedal, clamp all hoses, taking the
clamps off one at a time until you find where the softness is coming
from. If you still have a problem with all lines clamped, then the
master cylinder is the cause. If the pedal goes hard, it is fine.
Sometimes the hose itself can be swelling.
3. New brake pads running on worn/tapered discs will cause softness
until they wear to match and the springiness goes away. Machine
discs when fitting new pads.
4. It is well proven that undersized discs do not cool well and warp
quickly. Do not use when below minimum for safety and legal
reasons.
5. Check master cylinders for residual valves when converting to discs.
These valves are meant to stop air sucking past wheel cylinder cups
but will make discs drag. Early American Corvette calipers can suck
air and therefore need light pressure, special residual valves, all
modern calipers are fine with none. If not sure, do the dripping
bleeder test.
6. Never get any hydrocarbons, oil, kerosene, petrol etc. near brake
parts – brakes use special rubbers, which swell dramatically when
contacted by hydrocarbons. Wash brake parts in Methylated Spirits
or Brake Clean Fluid and lubricate with brake rubber lubricant
(silicon based), or brake fluid during assembly.
7. Fitting new seals to used master and slave cylinders is not always
successful. Bore wear, scratches and corrosion usually mean
reduced life of rebuilds. Buy a new assembly or have yours stainless
sleeved if necessary.
8. Cheap brake pads are a false economy; they wear quickly and do
not stop as well.
9. A reaction disc is a small rubber disc which lives under the head of
the booster output push rod. If this is lost, the brakes will be nearly
uncontrollable with stopping proportional to pedal position, not
pedal effort, sending you through the windscreen at the lightest
touch.
10.With all modified systems, check everything for clearance, through
all suspension travel and lock to lock, particularly brake hoses and
calipers versus cross members and sway bars etc. on lock.
11.When converting a four-wheel drum type car to front discs
(retaining original rear brakes), it is necessary to fit smaller rear wheel
cylinders to reduce rear brake effort, as the front discs are not self-
energising.
12.If not sure, seek the advice of an expert.

[Sinfiniti]

I have to be a bitch about this, brakes dont make heat, they produce friction - heat is a by product of that friction.


and the force on the piston crown, redirected at 90deg by the crankshaft is what move the car (torque not heat per say, but yes heat is a by product). if you could make the engine operate at lower temperatures you would have a more efficient engine because as the engine temp increases the load on the cooling system, oil (cooiling and lubricating system) is increased and friction increases - iot is that internal engine friction that is counter producting and why more of that by product, heat, is produced.



...end rant
oh i know you didnt compose this so im not having a go at you - i remember cringing at this when i first saw it on the PBR or something website.

[Chrispy]

And ADR 7 is ggggooooooonnnneeeee.

Had a big argument with lots of people about this recently (ADR's and brake lines).

[Cpt. Ash]

Originally Posted by Chrispy

And ADR 7 is ggggooooooonnnneeeee.

Had a big argument with lots of people about this recently (ADR's and brake lines).

good luck with insurance

[Chrispy]

The ADR that replaces it just states that the brake lines must comply with a ISO, BS, SAE etc standard. Pretty much means as long as it's one of the big companies Maltech, Goodridge etc then it's fine. It also states that the lines must be installed in a manner to prevent damage and interferance with suspension, brakes and wheels.

[Cpt. Ash]

ah ok, when did this come into affect?

[Chrispy]

2004.

[MagicMike]

Has everyone noticed that Caz is selling the adapters seperately now for the 13" rotors? $250odd

[Authozy09]

Oh and the chirping disapears when i apply either the foot brake or hand brake???

[ingethy]

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