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Understanding brakes

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Introduction

 

First things first, it is important to understand what exactly the brakes do, they use a basic law of physics that states that ‘you cannot destroy energy’, so a vehicle, and therefore the brake drum or brake disc when
moving possesses energy.  This is called ‘Kinetic Energy’ (the energy of motion / movement), and
this energy cannot be destroyed, so with the aid of the braking system and another force of nature called friction, we can convert kinetic energy into heat energy and dissipate it to the air.
The motion of the disc pad, or brake shoe on drum brakes, across their working surfaces produces a prodigious amount of heat, 300 to 500 degrees Celsius.
Excessive heat can be counter-productive and the ‘correct’ operating temperature can be a relative fine line.
One of the main determining factors of the effectiveness of a brake is its ability to dissipate the heat generated by the friction quickly to the airflow around the brake assembly.
This can be achieved in a number of ways, fins on a brake drum, to increase the surface area, or in the case of discs, ventilating ducts within the disc itself.
The wheel design will also impact on the heat transfer to the passing airstream. And fitting incorrect wheels can seriously affect the airflow around the brakes and hence the interest of some insurance companies
with regard to fitting non-standard wheels. Brakes and what do they do!

Drum brakes
The drum brake system is desperately ineffective, with the friction area being contained within the drum, making it hard to dissipate any heat produced to the surrounding air.
The other problem with drums is that, by creating the heat by friction, the temperature of the drum increases, the steel or cast drum itself expands away from the shoe, so if this increase in heat is not controlled, more and
more input from the brake pedal is required with diminishing results, causing what is known as ‘Brake Fade’.
A twin leading shoe assembly on the front axle was about the best evolution for the drum brake as both shoes gained a self-application effect with the rotating drum pulling the leading edge of the shoe into
itself.
Now I know this will be controversial, but I can't help myself, you should file a chamfer on the leading edge of a brake shoe, because if you do, you will defeat two of the most important designs of the twin leading shoe
concept.  One you remove the ability of the built-in self application, and two, you effectively stop the shoe from cleaning itself and the drum, in fact you are making a perfect ramp and wedge area for all the brake debris to collect, build up and hold part of the shoe off the drum.  This can also make the brakes screech when applied.


Disc Brakes
If we cast our minds back to those long drawn-out Physics lessons at school about leverage we will remember that, the longer the lever used and the closer to the object you place the pivot point the easier it is to
move & the same thing applies for the disc brake.  The bigger the disc diameter is, the further away the calliper can be mounted and this effectively increases what is known as the torque arm length, so for any given disc diameter a two piston calliper is less effective than a four piston and conversely a six piston being even better.
There are of course limitations - what can be physically fitted inside the desired wheel is obvious, but the main limitation is the brakes ability to overcome the tyres grip with the road and vice versa.  Too much braking and we lock the wheels, reducing the tyres grip with the road and vastly reducing retardation, not enough braking and we are producing excessive amounts of heat so the disc and pad become ineffective. (Brake ‘Fade’), again greatly reducing retardation.  On that point, the disc has one advantage over the drum, as it heats up it
expands into the pad and not away from it.  Just a point at this stage, a servo is purely a device to reduce the pedal pressure required to operate the brakes, so if you find yourself standing on the pedal, pulling on the steering wheel and gritting your teeth together to stop the car a servo might be the order of the day, but it will
not increase the effectiveness of the brakes.
Ventilated discs have pretty much become the norm on modern cars, they have the ability for more effective heat dissipation over a solid disc and can be fitted into a smaller area, couple this with our multi piston
calliper you start to realise how such small discs can be squeezed into tiny little city cars, also, the reason most sport and big SUVs have such big long callipers, covering most of the disc down one side, is because
they have multiple pistons in them to reduce the disc size by pushing the torque arm effect to its maximum.
Slotted and drilled discs are very much misunderstood by many as it is generally assumed they are there to assist in cooling the disc, they are not, the only reason for slotting or drilling a disc is to wipe the pad
surface.  In operation the heat creates debris and gases between the disc and pad surface, reducing their effectiveness, so the slots and/or holes clear this away.
A case in point, if you read the article about my Mini (see featured vehicles) you will see I have elected to run the car on 10 inch wheels opposed to the 12’s, and this meant the disc size had to drop from 8.4 inch to 7.5. so to
compensate I replaced the 2 piston calliper with a 4 piston increasing the torque arm and giving me a more efficient and hence better stopping power than the original 8.4 with 2 pistons.
The other gain made was the callipers are of aircraft grade aluminium so the ability to dissipate heat was massively increased over the iron ones.
We have only really scratched the surface, there are the hydraulics, master and wheel cylinder size, front rear balance, pedal ratio.  Lets face it the motor industry spent millions developing these systems and we
have limited space.  I hope this has been of some interest to you all, if so, and you would like
more I will ramble through another subject for you.  
I have taught Marine Engineering to B tech Second Diploma and can examine it to that level.
I have been teaching Engineering for 30 plus years with the Cadet Forces (Navy)
If I don't know, I probably know where to find out or someone who does.

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