Brake System Design and Optimization: Setting up a Brake Bias Bar
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Setting up a Brake Bias Bar
06.14
| 00:00 | - In the brake system components section of the course, we discuss the brake bias bars and how they can be used to adjust the brake bias which is essentially a mechanism that transfers the force from the brake pedal to each master cylinder when we have a dual master cylinder arrangement in a motorsport pedal box. |
| 00:19 | In this module, we'll be diving into more detail in regards to how to set up a bias bar correctly as this is critical to its function as well as being commonly overlooked or misunderstood. |
| 00:31 | Naturally the bias bar is only so long so there's only a window of adjustment here which is why the rest of our braking system needs to be accurate for our targeted bias. |
| 00:43 | Ideally our bias bar would be completely centred under the pedal force equal to a heavy stop. |
| 00:50 | Allowing for adjustability in each direction. |
| 00:53 | To be clear, by completely centred, I mean that the bearing for the pedal will be centred between the clevises for the master cylinders. |
| 01:01 | With all this in mind, let's take a look at how we actually go about setting up a bias bar in this central location. |
| 01:08 | With the master cylinder rods disconnected from their clevises, the first thing to do is set the position of the centre bearing to be centred between the clevises by rotating the bias bar. |
| 01:20 | We want to make sure the spacing of the clevises matches the spacing of the master cylinders in the pedal box. |
| 01:27 | Next we thread the master cylinder push rods into each clevis, starting with the length equal to determine the pedal position. |
| 01:35 | Once the is is correct, if one rod is threaded out then the other needs to be threaded in to maintain the pedal position. |
| 01:42 | What we really want is for the bias bar to be perpendicular to the master cylinder centrelines when the pedal is pressed fully at the force equal to a heavy stop. |
| 01:52 | This, along with the bias bar being centred means 50% of the pedal force is distributed into each master cylinder under maximum brake pedal application. |
| 02:04 | Depending on how the bias is being achieved, it's common for a larger hydraulic leverage ratio to be used on the front brakes. |
| 02:12 | Meaning the front cylinder will often require more travel and a higher feed rate to displace more fluid than the rear. |
| 02:20 | So this would mean that the front push rod needs to be out further than the rear to make sure the bias bar is at the desired perpendicular position when the pedal is depressed. |
| 02:31 | Of course this can only be set when the brakes have been bled and we want to try to minimise this difference when designing the system. |
| 02:39 | But about a quarter inch or 6mm of difference in push rod length is reasonable. |
| 02:45 | If the rods were equal in length with the pedal relaxed, then the feed rate of the rear cylinder is too high relative to the front, resulting in the rear brakes engaging before the front. |
| 02:57 | However, having the push rod lengths adjusted correctly, the bias bar is level under compression and therefore the front and rear brakes should engage at roughly the same time. |
| 03:09 | Of course, this will result in the bias bar being angled when the pedal is relaxed so we need to make sure that neither master cylinder is pre loaded. |
| 03:20 | They must be completely free of any pressure when the pedal is relaxed. |
| 03:24 | With the bias bar set up correctly we can now tighten the jam nuts on the push rods to secure them in place. |
| 03:31 | For a more accurate method of getting the brake bias bar set up, we can use data to find the brake pressure generated for our target brake force and bias. |
| 03:41 | It's possible to set up a digital dash to show when the pressure is achieved. |
| 03:46 | So we can then have an assistant sit in the driver's seat and apply this pressure while we check the bias bar is in its desired position. |
| 03:55 | Setting up this data acquisition is well outside the scope of this course so if you're interested in learning more, make sure to check out our data analysis fundamentals course. |
| 04:06 | So with our bias bar now set up and ready to go, we can use its adjustment to fine tune the brake bias. |
| 04:13 | Depending on the pitch of the threaded bias bar and other geometry, each full turn of bias is generally equal to about a 4% change in force distribution. |
| 04:24 | This is something we'll be covering in the next module. |
| 04:27 | Before we wrap up, another topic I want to touch on here is maintenance. |
| 04:32 | Hopefully it's obvious as to why maintenance is so important for all the components in the braking system and the bias bar is no exception. |
| 04:40 | It's imperative to follow the recommendations of the manufacturer, both for wear and general conditions as well as the general setup and requirements around factors such as clearance. |
| 04:52 | On the topic of clearance, there are a few key areas we want to watch out for, one being when the bias bar has too much pitch, it can contact the housing around the central pedal bearing. |
| 05:03 | Another if the bias has been adjusted too far in one direction in which case one of the clevises would also contact the housing for the central bearing. |
| 05:13 | We want to avoid any unintentional contact like this as it can damage the components or prevent the brake system from functioning correctly. |
| 05:22 | So let's summarise this module before moving on. |
| 05:25 | After our core brake system components are sized for a suitable brake bias, the next step is to set up the bias bar. |
| 05:34 | Ideally we want our bias bar to be centred, meaning the central bearing for the brake pedal is halfway between the clevises for each master cylinder. |
| 05:43 | This means 50% of the pedal force is distributed into each brake circuit, giving us a wider range of adjustment in each direction and keeping the angles of the bias bar and master cylinder push rods to a minimum. |
| 05:57 | With the bias bar centred, we can then connect the master cylinder push rods to each clevis. |
| 06:03 | Generally the front push rod will be set further out than the rear to ensure the front and rear brakes engage at roughly the same time. |
