Brake System Design and Optimization: Brake Fluid
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Brake Fluid
08.58
| 00:00 | - We've already covered how brake fluid is responsible for supplying hydraulic pressure throughout the whole braking system. |
| 00:07 | After being pressurised by the master cylinder, it's then responsible for supplying that pressure to the brake calliper pistons. |
| 00:15 | One of the main properties of the brake fluid that we care about is its compressibility, that is, we want to minimise any volume change of the fluid when pressure is applied. |
| 00:25 | The lower the compressibility of the brake fluid, the less compliance we'll have, leading to a more direct action between the driver's pedal input and the brake calliper force. |
| 00:36 | To some extent the chemistry of different brake fluids does affect its compressibility. |
| 00:41 | But compressibility becomes the biggest problem when the brake fluid is heated above its boiling point. |
| 00:48 | When this occurs, small air bubbles start to form inside the fluid. |
| 00:53 | Air is much more compressible than the brake fluid and therefore results in a lot of compliance. |
| 01:00 | To the driver, this will result in more pedal travel being required to build a given amount of brake pressure. |
| 01:06 | This will often be described as a soft, spongy or long brake pedal. |
| 01:11 | It's not a nice sensation and quickly destroys the driver's confidence in the brakes. |
| 01:15 | In extreme cases, this can result in the pedal reaching its maximum mechanical travel before sufficient brake pressure is reached with obvious dangerous consequences. |
| 01:26 | The source of the heat that causes the brake fluids to boil comes from the thermal energy absorbed by the brake callipers during braking events. |
| 01:35 | While the brake discs may operate at 800°C or more, the brake callipers operate at much lower temperatures. |
| 01:43 | With that said, this still makes controlling calliper temperatures and choosing a suitable brake fluid critical to avoid boiling. |
| 01:51 | One of the factors that has a large impact on the boiling temperature of the brake fluid is the amount of moisture it has in it. |
| 01:59 | Most brake fluids are hygroscopic, meaning that they readily absorb moisture and we want to keep the moisture content of our brake fluid as low as possible as it makes it more susceptible to boiling. |
| 02:11 | The more moisture present in the fluid, the lower the boiling point. |
| 02:16 | This is the reason that when you read the label on brake fluid, you'll see that there's a dry and wet boiling point temperature listed. |
| 02:22 | You'll notice that the wet boiling point is significantly lower than that of the dry. |
| 02:28 | This is also why when we open a new bottle of brake fluid, we'll see that there is an air tight seal that must be broken before we can use it. |
| 02:36 | By sealing the brake fluid from atmosphere, it prevents moisture from the humidity in the air being absorbed. |
| 02:43 | This is also why using a brake fluid from an opened bottle is not recommended. |
| 02:48 | Officially at least, the idea is to throw away unused brake fluid once the bottle has been opened. |
| 02:56 | When it comes to classifying the different types of brake fluid, you'll likely be familiar with the term DOT ratings. |
| 03:02 | DOT or dot stands for department of transport and in relation to brake fluid, it lays out the official standards for approved fluids. |
| 03:11 | The full standards include a long list of criteria but the main things we're concerned with for motorsport use is choosing a fluid that won't boil for our application. |
| 03:23 | DOT 3 is the lowest common standard and for a fluid to meet DOT 3, the dry boiling point must be above 205°C and the wet boiling point of above 140°C. |
| 03:38 | To meet DOT 4, the dry boiling point must be above 230°C with a wet boiling point of 155°C. |
| 03:48 | Finally, to meet DOT 5, it must have a dry boiling point of 260°C and a wet boiling point of 180°C. |
| 03:59 | It's important to understand that just because a given brake fluid meets a given DOT rating, it doesn't mean it'll necessarily behave identically to another product that has the same rating. |
| 04:10 | All it means is that the fluid meets or exceeds the minimum standards laid out by that particular DOT rating. |
| 04:18 | It would be easy to think that choosing a brake fluid with the highest possible DOT rating for your racecar would be the best idea. |
| 04:26 | However there's a little more to it than that. |
| 04:28 | While DOT 4 has a higher boiling point than DOT 3 for example, it may also be more prone to absorbing moisture, meaning it'll likely need to be replaced more often. |
| 04:39 | DOT 5 has a completely different chemistry than all the other types of brake fluid. |
| 04:43 | DOT 5 is silicon based with everything else being based on glycol ether. |
| 04:48 | DOT 5 does have a higher boiling point than DOT 4 as well as not being hygroscopic, meaning it can't absorb moisture. |
| 04:58 | The downside of DOT 5 is that due to its different makeup, it's naturally more compressible than DOT 3 or 4, this makes DOT 5 fluid unsuitable for applications that require high hydraulic pressures like motorsport. |
| 05:11 | As a result, DOT 5 brake fluid does not tend to be used in motorsport. |
| 05:16 | You may also have seen DOT 5.1 in the local shop. |
| 05:20 | This is actually distinctly different from DOT 5. |
| 05:24 | Chemically speaking, DOT 5.1 is similar to DOT 4 in that it's glycol ether based. |
| 05:29 | But it also benefits from some of the differences that raise the boiling point to the DOT 5 standard. |
| 05:35 | This gives us the benefits of the DOT 5 temperature rating without the poor compressibility of the DOT 5 fluid. |
| 05:43 | The inevitable downside of using DOT 5.1 fluid is that it's naturally a lot more expensive than the lower rated fluids. |
| 05:51 | Another important point is that DOT 3, 4 and 5.1 are compatible with each other. |
| 05:57 | Meaning you can top up the system with any of these. |
| 06:01 | DOT 5 on the other hand, doesn't mix with the other standards and can't be used interchangeably. |
| 06:07 | This is important to remember as if you had a car with DOT 5 fluid that you wanted to remove, simply flushing your system with a more suitable fluid is not a good idea because it's possible for pockets of the original brake fluid to remain in the system after the flush. |
| 06:23 | The only way to be confident in changing the brake fluid type is by stripping the brake system and ensuring that none of the previous brake fluid remains anywhere which is obviously a time intensive process. |
| 06:37 | Brake fluid should be thought of as a consumable item, whether becoming damaged by absorbing moisture or being overheated, it needs to be changed periodically. |
| 06:47 | The rate at which it's changed will depends a lot on the application and the reason for damage. |
| 06:53 | Moisture damage can occur even when the car's braking system remains completely sealed. |
| 06:58 | This is due to diffusion where moisture can pass through some materials like plastic fluid reservoirs and rubber base calliper seals. |
| 07:07 | Brake fluid is also damaged by high temperatures which can be recognised by a change in colour. |
| 07:13 | When glycol ether based fluid is new, it'll be a clearish yellow type colour. |
| 07:17 | This will change to a brown colour as the fluid breaks down from higher temperatures. |
| 07:23 | In motorsport, exposure to high temperatures is simply part of the game. |
| 07:27 | This will depend on the application, in professional circuit racing for example, which is where you'll usually see the highest temperatures, it's not unusual to change the fluid between every session. |
| 07:40 | Unless serious club racing, you should be checking your brake fluid with a small brake bleed at least after every event. |
| 07:48 | Don't worry, we'll be covering the proper process of brake bleeding in a later module. |
| 07:53 | A car that's not raced and occasionally driven will still likely benefit from a brake bleed annually, depending on the ambient environment and components involved. |
| 08:03 | If you notice a difference in pedal quality after a brake bleed, this can also be used as a guide for how often replacing the fluid is needed for your application. |
| 08:14 | In summary, brake fluid is used to transfer hydraulic pressure throughout the braking system. |
| 08:20 | The compressibility of the fluid is the primary concern for motorsport. |
| 08:24 | Brake fluid becomes compressible when it boils and boiling resistance is usually influenced by moisture absorption. |
| 08:33 | DOT 3, 4 and 5.1 are compatible with each other but DOT 5 is not which means that if you're swapping to or from DOT 5, you'll need to do a complete flush and clean of the system to ensure none of the previous fluid remains. |
| 08:48 | Brake fluid should be treated as a consumable and replaced as often as necessary for your application. |
