RE: What is a disc brake? PH Explains
Discussion
ads_green said:
I'll have a go...
Usual disclaimers - not an automotive engineer and E&OE...
The way brake pad material interacts with the disc is complicated and is worthy of a scientific branch all of it's own similar to that of tire dynamics.
However it is useful to look at the mechanics as three phases:
When pads and discs are cold the only way a brake can generate friction is by good 'ole fashioned surface to surface contact. Microscopic imperfections of both the pad and disc interlock as pressure is applied and generate a useful amount of friction and heat. As there are now two metallic based surfaces in direct contact you can also get squealing noises. This is especially apparent with "Sporty" braking systems containing more elaborate compounds (usually have something ceramic).
Whilst slowing the car down this isn't most effective or efficient use of the braking system. Things get more interesting once conventional surface friction generates some meaningful heat into the system.
With the brakes now hot, the material in the pads starts to become more "elastic"... to the point where small amounts of the pad material will jump to the disc surface. This reaction also generates heat which in turn can dramatically improve braking performance. The compounds in the pads can also stick to each other far easier than other materials further increasing the maximum friction available.
At this stage, the brakes will feel alive as the layer where the pad material is transferring creates a surface that responds well to pedal input whilst providing solid feedback to the driver.
One advantage of this process is that when the braking force is removed, pad material remains on the disc surface ready for the next brake application. With pad material pre-loaded on to the discs it takes considerably less time to return to operating conditions even if the brakes have cooled somewhat.
However, if the brakes are cold and then used overly aggressive, the direct surface contact can scrub this layer off and the whole process needs to start again.
As the braking process is achieved through more of a chemical reaction than anything else, it is very sensitive to the operating temperatures encountered. Continual high speed application of the brakes can overwhelm the system's ability to expel the heat until a threshold point is reached causing the reaction to perform badly. Aside from the common and obvious side effect of boiling the brake fluid, high heat levels stop the pad material transferring properly and brake down the bonds to the point where the material no longer provides a good, consistent layer between disc and pad.
So how much force can the brakes actually apply? It's a common misconception to say that "my brakes can lock the wheels at any speed so they are up to the job".
Pretty much any modern disc brake system should be able to grab and hold a wheel from turning - that is relatively easy. However if the disc isn't turning then the brakes are contribution a big fat zero to slowing the car. All the slowing down is achieved by the locked tires sliding over the road surface.
It varies between tire types, manufacturers, compounds but most road tires give peak traction when they slide a small amount (between 5%-15%) over the road. So for braking, the ideal is for the brakes to keep the wheel speed slightly slower than the vehicles' road speed. Operating at this edge of the envelope is what puts a large amount of heat into the brake system and can overwhelm undersized setups.
The pad material process discussed above works well based on two factors - one already covered is heat however another is pressure. It is possible to have the brake system press so hard on the pads that they effectively push through the material transfer layer and return to contact friction. Too little pressure doesn't get sufficient uniform contact between the pads and disc. As with most things there's a Goldilocks zone that's just right.
Brake rotors are essentially a simple lever - the bigger the disc, the further from the centre the pads operate then more force can be applied to the axle for a given amount of force. So being simplistic, if a disc is scaled up by 10% in diameter then the same amount of braking force applied to the disc by the pads will give about 10% more braking effort to the wheel. Similarly, the extra size of the disc can also mean the same braking force can be obtained with less pad pressure. By carefully manipulating the disc size the required level of braking can be achieved with the optimum pad pressure and size.
This is a problem for fast road cars however. As a mechanical system this careful consideration of component sizings only work for a specified and quite narrow range. If the car is on track it'll work flawlessly but when trapped in stop start commuter traffic it'll never go anywhere near it's optimum zone.
In these cases it is actually very easy to cause damage to the discs by under using them which would require replacement or re-skimming the rotor surface. It feels contrary to common sense that not using the brakes hard causes more wear and damage than using them in a spirited manner.
Consider a reasonably heavy, fast saloon car with a large brake package - VXR8 or RS4 for example.
The brakes fitted are monstrous in size - 365mm on the VXR8 and similar to what you would find on a Lamborghini.
From the above, this allows a large amount of braking force to be applied with a firm, consistent pressure on the pads... But what if you don't need threshold braking emergency stops? Then the sheer lever size of the rotor only requires a tiny amount of pressure between pad and disc to get normal day-to-day braking effort.
At a microscopic level, the disc is full of peaks and valleys and insufficient pad pressure results in only the peaks being in contact with the pad. The peaks starts to heat up and enters the pad material transfer process but the valleys do not. The peaks collect more and more pad material making them even more proud of the disc. The situation gets worse and worse until the effect can be felt through the brake pedal as vibration under braking with the assumption that the disc has warped.
For cars with big impressive brakes, a good rule of thumb is that if you don't need to slow down more than what you can achieve by downshifting then you don't need to use the brakes. If you do need to use them then don't feather or baby them.
Much better than the original article Usual disclaimers - not an automotive engineer and E&OE...
How do brakes, er, "brake"
Braking is all about turning kinetic energy into heat.The way brake pad material interacts with the disc is complicated and is worthy of a scientific branch all of it's own similar to that of tire dynamics.
However it is useful to look at the mechanics as three phases:
Cold
When pads and discs are cold the only way a brake can generate friction is by good 'ole fashioned surface to surface contact. Microscopic imperfections of both the pad and disc interlock as pressure is applied and generate a useful amount of friction and heat. As there are now two metallic based surfaces in direct contact you can also get squealing noises. This is especially apparent with "Sporty" braking systems containing more elaborate compounds (usually have something ceramic).
Whilst slowing the car down this isn't most effective or efficient use of the braking system. Things get more interesting once conventional surface friction generates some meaningful heat into the system.
Hot
With the brakes now hot, the material in the pads starts to become more "elastic"... to the point where small amounts of the pad material will jump to the disc surface. This reaction also generates heat which in turn can dramatically improve braking performance. The compounds in the pads can also stick to each other far easier than other materials further increasing the maximum friction available.
At this stage, the brakes will feel alive as the layer where the pad material is transferring creates a surface that responds well to pedal input whilst providing solid feedback to the driver.
One advantage of this process is that when the braking force is removed, pad material remains on the disc surface ready for the next brake application. With pad material pre-loaded on to the discs it takes considerably less time to return to operating conditions even if the brakes have cooled somewhat.
However, if the brakes are cold and then used overly aggressive, the direct surface contact can scrub this layer off and the whole process needs to start again.
Overheated
As the braking process is achieved through more of a chemical reaction than anything else, it is very sensitive to the operating temperatures encountered. Continual high speed application of the brakes can overwhelm the system's ability to expel the heat until a threshold point is reached causing the reaction to perform badly. Aside from the common and obvious side effect of boiling the brake fluid, high heat levels stop the pad material transferring properly and brake down the bonds to the point where the material no longer provides a good, consistent layer between disc and pad.
Brake sizing
With the pads interacting with the disc, the brakes use the wheels contact with the road to keep the disc turning generating heat, slowing the car.So how much force can the brakes actually apply? It's a common misconception to say that "my brakes can lock the wheels at any speed so they are up to the job".
Pretty much any modern disc brake system should be able to grab and hold a wheel from turning - that is relatively easy. However if the disc isn't turning then the brakes are contribution a big fat zero to slowing the car. All the slowing down is achieved by the locked tires sliding over the road surface.
It varies between tire types, manufacturers, compounds but most road tires give peak traction when they slide a small amount (between 5%-15%) over the road. So for braking, the ideal is for the brakes to keep the wheel speed slightly slower than the vehicles' road speed. Operating at this edge of the envelope is what puts a large amount of heat into the brake system and can overwhelm undersized setups.
The pad material process discussed above works well based on two factors - one already covered is heat however another is pressure. It is possible to have the brake system press so hard on the pads that they effectively push through the material transfer layer and return to contact friction. Too little pressure doesn't get sufficient uniform contact between the pads and disc. As with most things there's a Goldilocks zone that's just right.
Brake rotors are essentially a simple lever - the bigger the disc, the further from the centre the pads operate then more force can be applied to the axle for a given amount of force. So being simplistic, if a disc is scaled up by 10% in diameter then the same amount of braking force applied to the disc by the pads will give about 10% more braking effort to the wheel. Similarly, the extra size of the disc can also mean the same braking force can be obtained with less pad pressure. By carefully manipulating the disc size the required level of braking can be achieved with the optimum pad pressure and size.
This is a problem for fast road cars however. As a mechanical system this careful consideration of component sizings only work for a specified and quite narrow range. If the car is on track it'll work flawlessly but when trapped in stop start commuter traffic it'll never go anywhere near it's optimum zone.
In these cases it is actually very easy to cause damage to the discs by under using them which would require replacement or re-skimming the rotor surface. It feels contrary to common sense that not using the brakes hard causes more wear and damage than using them in a spirited manner.
Consider a reasonably heavy, fast saloon car with a large brake package - VXR8 or RS4 for example.
The brakes fitted are monstrous in size - 365mm on the VXR8 and similar to what you would find on a Lamborghini.
From the above, this allows a large amount of braking force to be applied with a firm, consistent pressure on the pads... But what if you don't need threshold braking emergency stops? Then the sheer lever size of the rotor only requires a tiny amount of pressure between pad and disc to get normal day-to-day braking effort.
At a microscopic level, the disc is full of peaks and valleys and insufficient pad pressure results in only the peaks being in contact with the pad. The peaks starts to heat up and enters the pad material transfer process but the valleys do not. The peaks collect more and more pad material making them even more proud of the disc. The situation gets worse and worse until the effect can be felt through the brake pedal as vibration under braking with the assumption that the disc has warped.
For cars with big impressive brakes, a good rule of thumb is that if you don't need to slow down more than what you can achieve by downshifting then you don't need to use the brakes. If you do need to use them then don't feather or baby them.
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