Brake System Design and Optimization: Brake System Compliance
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Brake System Compliance
04.38
| 00:00 | - The term compliance refers to the deflection of a component or system and so in the context of braking, compliance is something we want to avoid and that's what we'll be discussing in this module. |
| 00:12 | The compliance experienced by a driver in the braking system is felt through the brake pedal. |
| 00:18 | This results in additional pedal travel over and above what's required to operate the braking system normally. |
| 00:26 | This is often described as a soft brake pedal by the driver. |
| 00:30 | The extra travel and soft feel of the brake pedal can make it more difficult to get a consistent feel in the brakes for the driver as well as being both unnerving and confidence sapping. |
| 00:41 | If there's one thing that'll make a driver second guess how aggressively they can approach a corner, it's inconsistent brake performance. |
| 00:49 | There are different common sources of compliance that will result in a soft pedal feel. |
| 00:55 | One of the most common is flex in the pedal box assembly. |
| 00:59 | Sometimes this can be from the parts of the pedal box itself, other things it can be from where the pedal box is mounted like the firewall and dashboard support assembly. |
| 01:09 | In normal road driving, this isn't something that would cause issues but when high pedal pressure is used, as it is on a racetrack, this can often show up. |
| 01:18 | One fix for this is to add some way of stiffening or bracing either the pedal box itself or the master cylinder from inside the engine bay. |
| 01:28 | Another common culprit is the expansion of the brake hoses that allow for flexible hydraulic connection between the calipers and the chassis. |
| 01:37 | In road cars, these are commonly made from a rubber based hose. |
| 01:42 | While these are cheap to produce and perfectly adequate for road use, the high hydraulic pressures used in motorsport tend to make them expand and flex excessively. |
| 01:52 | The solution for this is to replace the original brake hoses with stainless steel braided hose. |
| 01:59 | This hose is made specifically for brake lines, being both chemically suitable and having much higher stiffness which makes them expand far less when subject to high internal pressures. |
| 02:11 | It may be surprising but many brake calipers exhibit significant amounts of compliance. |
| 02:17 | This is where the shape of the caliper changes as the hydraulic pressure increases. |
| 02:22 | Many OE road going calipers will do this and it can be seen for yourself by having someone apply a lot of force to the brake pedal while you watch the caliper flex. |
| 02:33 | In the aftermarket, you'll often see this in calipers that are made in multiple pieces which is done to reduce manufacturing costs. |
| 02:43 | Generally the most pieces the caliper is made in, the lower the overall stiffness will be. |
| 02:48 | This is one of the main reasons many high end calipers these days are of monoblock design. |
| 02:55 | This means the entire caliper is made from one solid piece, being made from a single piece can increase the stiffness substantially although it does come with a financial penalty. |
| 03:07 | This is not to say that any multi piece caliper isn't worth using. |
| 03:11 | There are many extremely high quality multi piece brake calipers out there in the market with more than enough inherent stiffness. |
| 03:19 | The design and manufacturing processes have just as much to do with the stiffness of the final product. |
| 03:26 | Another potential source of compliance is within the brake fluid itself. |
| 03:30 | For all practical purposes, when the brake fluid is in good condition, it's effectively incompressible. |
| 03:36 | Meaning the volume doesn't change when subject to pressure. |
| 03:40 | If the brake fluid is heated beyond its safe limits, then gas bubbles can form within the fluid. |
| 03:46 | These gas bubbles suspended within the fluid are compressible and will cause a soft brake pedal. |
| 03:53 | We'll discuss this phenomenon and how to fix it in more detail later in the course. |
| 03:59 | It's important to point out that each of the components we've discussed will always have some amount of compliance in practice. |
| 04:06 | This is unavoidable. |
| 04:08 | It's just about minimising it as much as is practical. |
| 04:11 | In summary, compliance in the brake system will be experienced as a soft pedal with excessive travel. |
| 04:18 | This can make it difficult to be accurate with the brakes and will reduce the driver's confidence. |
| 04:23 | There are a number of components that can contribute towards poor compliance but pedal boxes, brake hoses, calipers and overheated brake fluid are the most common culprits. |
