Brake System Design and Optimization: Position Measurement
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Position Measurement
06.42
| 00:00 | - The previous two measurement topics covering temperature and pressure are the most common types of measurements associated with brakes. |
| 00:08 | Position measurement on the other hand, isn't quite as common and is usually limited to the professional level motorsport. |
| 00:16 | It is still a topic worth covering though. |
| 00:19 | When we talk about position measurement in braking systems, we're normally referring to the measurement of master cylinder displacement. |
| 00:27 | Something that's commonly used in dual master cylinder setups found in motorsport pedal boxes. |
| 00:33 | In modern applications, this is most commonly done by the use of contactless magnetic position sensors. |
| 00:40 | The function and integration of which are not the focus of this module, we're more interested in what can be learned from the data. |
| 00:47 | By measuring the displacement of each master cylinder, alongside the brake pressure, we can understand the stiffness of the brake pedal and the compliance in both the front and the rear brake circuits. |
| 00:59 | We can use this data to better understand the brake system and diagnose any problems. |
| 01:04 | It's normal for there to be some level of compliance in every brake system. |
| 01:08 | If there was none at all, after the pads came in contact with the disc, the brake pedal would be rock solid and wouldn't have any displacement regardless of how hard the driver pushed on it. |
| 01:20 | To be clear though, it would still build pressure and the brakes would still operate. |
| 01:25 | In reality, every part has some compliance and these all add up to give us the total compliance of the system. |
| 01:33 | The brake callipers is one of the most common sources of compliance where the body can deform and expand when hydraulic pressure is increased. |
| 01:42 | This means the brake pads too because even though they may feel quite rigid in your hand, some types of pads can compress significantly enough to be able to feel the difference in the brake pedal when subject to high forces. |
| 01:55 | As we've previously discussed, brake fluid can also be a common source of compliance, depending on its condition. |
| 02:03 | Even when the fluid is perfect, some types of fluid can have noticeably poorer compliance properties. |
| 02:10 | Brake lines and hoses also expand to some extent. |
| 02:13 | We already covered the upgrading of rubber based hoses to a braided hose and carrying our this modification can make a big difference in the pedal feel to the driver. |
| 02:22 | In addition to the components deflecting due to hydraulic pressure, items like brake calliper piston seals can also have a big effect. |
| 02:32 | Due to the design of the seals, when hydraulic pressure is removed after a braking event, these seals can make the pistons retract a small amount. |
| 02:41 | This is intentional, with the purpose of very slightly retracting the brake pads away from the discs. |
| 02:47 | The reason for this is to prevent the brake pads from sitting unnecessarily on the discs which makes for extra drag force, resulting in higher fuel consumption, lower acceleration and a significant effect on the top speed. |
| 03:01 | All these effects add up to giving us more compliance in the system meaning we'll end up having to displace a certain amount of additional fluid in the master cylinders as we build to the target brake pressure. |
| 03:14 | Changing the master cylinder and brake calliper bore sizes also affect this to some degree. |
| 03:20 | By measuring the amount of master cylinder displacement, we can start to understand how much compliance we have in the system and by measuring the displacement of each cylinder in a dual setup, we can start to isolate issues and changes in the system. |
| 03:35 | A common way to visualise this is by plotting the master cylinder displacement vs hydraulic pressure. |
| 03:42 | In a system that's working ideally, this relationship would be a straight line, making a perfectly linear relationship between the displacement and pressure. |
| 03:52 | In a real world system there are a few deviations from the ideal behaviour, even with the system working well which is what we see here. |
| 04:00 | The first thing is the first section where the displacement is increasing with almost no pressure increase. |
| 04:06 | Here is where the slack in the system is being taken up before the pressure can be built. |
| 04:12 | This is one area where something like the effect of the calliper pistons taking up the clearance until they're firmly against the back of the pads will be seen. |
| 04:21 | The next section is relatively linear which is a close to ideal behaviour. |
| 04:27 | The slope of this line will be determined by the master cylinder to brake calliper piston area ratio. |
| 04:33 | If you change a master cylinder size for example, the slope of this line will change. |
| 04:38 | The same is true if we change the compliance of the system. |
| 04:42 | Let's say we overheat the brake fluid which causes it to boil. |
| 04:46 | This results in the fluid becoming more compressible. |
| 04:49 | If we were to compare the data from a run where the brake fluid was in good condition to after it had boiled, we'd see a reduction in the slope, indicating a drop in the stiffness of the brake pedal. |
| 05:02 | This is an example of how using a plot like this, based on our logged data can be a really useful tool for understanding changes and issues in your brake system. |
| 05:13 | If you saw this on the data, you'd be inclined to bleed the brakes as the next step. |
| 05:17 | The final feature on a real system to note is hysteresis which is essentially the lag between the input and output caused by friction and drag in the system. |
| 05:28 | This is what we see here where the pressure to displacement ratio is different on the brake application vs release. |
| 05:36 | On release, the relationship doesn't follow the same line on the way down. |
| 05:40 | This most often occurs due to the internal friction in the master cylinders as well as the joints and linkages that are part of the pedal box. |
| 05:48 | The setup and alignment of the pedal box components can have a significant effect here too. |
| 05:55 | The result is that we don't get the same relationship on the brake release that we get on application. |
| 06:01 | In extreme cases, the effect can lead to significant differences in brake bias on release vs application which can cause issues. |
| 06:09 | However this effect can also be manipulated to intentionally vary the brake bias in order to modify the vehicle balance on corner entry in some cases. |
| 06:19 | In summary, by measuring the position of the brake master cylinders, we can understand the pressure to displacement relationship in each circuit. |
| 06:28 | This helps to monitor changes in the brake system compliance which is a helpful tool in diagnosing potential problems in brake system operation and components. |
