Brake System Design and Optimization: Brake Friction
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Brake Friction
05.12
| 00:00 | - A critical property of a conventional braking system is friction. |
| 00:03 | This is simply the resulting force we get when two bodies that are moving with respect to each other are forced together. |
| 00:11 | If we take a block and push it along a flat surface, there'll be some resistance to the sliding of the block on that surface. |
| 00:20 | The amount of force we need to apply to the side of the block to make it slide is determined by the coefficient of friction between the block and the surface it sits on and the mass of the block. |
| 00:32 | Don't worry, we'll be explaining what this coefficient of friction is and why it matters in a few minutes. |
| 00:38 | First though I want to quickly note the difference between static and kinetic friction. |
| 00:43 | Static friction is the resistance force when there is no relative motion between the surfaces. |
| 00:50 | if the block on the surface is stationary, and we gradually applied a force to the side of it. |
| 00:55 | The reacting friction force would increase, matching the applied force up until the threshold of motion. |
| 01:02 | This threshold is determined by the coefficient of static friction. |
| 01:07 | Kinetic friction on the other hand is the resistance force experienced when the bodies are already moving relative to each other. |
| 01:14 | Naturally since the brakes are generally applied in a vehicle while it's moving and therefore there is a relative motion between the pads and the discs, kinetic friction is our primary focus for the disc and pad interface. |
| 01:29 | The tyre and road contact is the other important frictional interface that we're interested in. |
| 01:34 | And this concerns static friction. |
| 01:37 | As although the tyres are rolling over the surface, there is no relative motion at the contact patch. |
| 01:43 | At least in most cases where the wheel is not locked or breaking traction. |
| 01:48 | The key point here is that the coefficient of static friction is almost always greater than the coefficient of kinetic friction, all other things equal. |
| 01:57 | Meaning that the force required to move the objects relative to each other in the first place is more than the force required to keep them moving. |
| 02:06 | For a simple system like a block on a flat surface, the coefficient of friction between these two bodies is governed by the material of each of them and their surface finish. |
| 02:16 | We can vary the coefficient of friction by using a different combination of materials and by making the surface finish rougher or smoother. |
| 02:25 | The symbol that's generally used to represent the coefficient of friction is the greek symbol mu. |
| 02:32 | The definition of the coefficient of friction is the ratio of the friction force between two surfaces in contact and the normal force between the surfaces which is the force pressing them together. |
| 02:44 | Essentially we're describing how much the friction force between the surfaces varies with the normal force. |
| 02:51 | If two objects have a coefficient of friction of one, this means that the friction force is equal to the force pressing them together. |
| 02:59 | A coefficient of friction less than one means we need less force to overcome the friction force than the force pressing them together and a coefficient of friction greater than one means we require more force. |
| 03:12 | The friction force we're most interested in for braking systems is that between the brake pads and the discs. |
| 03:19 | Looking at a disc from the side on when we squeeze the pads against the disc, we get a friction force building up. |
| 03:26 | This force depends on how hard we squeeze the pads, being the normal force as well as the coefficient of friction between the pads and the disc. |
| 03:35 | Because this friction force is acting at a certain distance from the centre of rotation of the disc, this results in a torque about the centre of the disc. |
| 03:45 | The torque is equal to the friction force multiplied by the distance between the force and the centre of the disc and therefore a larger diameter disc, greater clamping force or higher coefficient of friction will result in an increase in brake torque. |
| 04:00 | With brakes, there are two other important parameters that affect the coefficient of friction between the pads and the discs. |
| 04:08 | The materials and the temperature. |
| 04:10 | The materials apply to what the discs in the pad are made of and there's a wide range to choose from. |
| 04:17 | The temperature the brakes are running at also has a bearing on the coefficient of friction and a lot of this comes down to the specification of the pad material. |
| 04:25 | The temperature is constantly changing throughout each stop as well as we absorb more braking energy, the temperature of both the pads and the discs rises. |
| 04:36 | We'll discuss the disc and pad materials as well as the temperature relationship in more detail later in the course. |
| 04:42 | In summary, the coefficient of kinetic friction comes from the ratio of the friction force between two objects with relative motion, divided by the force holding the objects together. |
| 04:54 | The friction between the pads and the discs is one of the most critical parameters to understand in a braking system. |
| 05:02 | The coefficient of friction is largely controlled by the disc and pad materials as well as the operating temperatures of both. |
