Brake System Design and Optimization: Setting Brake Bias
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Setting Brake Bias
07.12
| 00:00 | - Brake bias is one of the key setup and tuning tools at our disposal for adjusting the balance and stability of the car under braking as well as the outright braking performance. |
| 00:11 | As we've mentioned previously, we're generally trying to match the ratio of the braking force on each axle to the braking capacity of each axle. |
| 00:20 | However, stability is just as important as stopping quickly since if the driver can't maintain control of the vehicle, then the braking performance is irrelevant. |
| 00:30 | For this reason, we tend to target locking of the front brakes first, as this is a stable condition but we'll touch more on this soon. |
| 00:39 | The foundations of the brake bias are largely set by the mechanical components, being the disc size as well as the pad compound and size which is a by product of the calliper size. |
| 00:50 | On the hydraulic side of things we're mostly concerned about the brake pressure in the front and the rear circuits and how this is translated to the clamping force of the callipers. |
| 01:01 | Naturally this is determined by the size of the master cylinders and the calliper pistons. |
| 01:07 | But also the bias bar and how it distributes the force into each master cylinder. |
| 01:13 | In this module we're going to assume that all the components in the system hydraulic or mechanical have been sized correctly to achieve a foundational bias appropriate for our vehicle. |
| 01:23 | From here, we can use the bias bar as a tuning tool, using its relatively easy tunability to make small adjustments when needed to optimise the braking balance for variable conditions. |
| 01:35 | Rather than a bandaid for incorrect mechanical bias. |
| 01:39 | So what does this all mean in practice when the driver is out on track putting the system to use? As we mentioned earlier in this module, we typically aim for the front tyres to lock first as this is a stable condition but the adjustments are there to suit different conditions, car setups and driving styles. |
| 01:57 | Like when there are multiple drivers in endurance racing for example. |
| 02:01 | Having the bias towards the front also means that the front brakes are being used more and will therefore get hotter as well as increasing the possibility of understeer while trail braking. |
| 02:12 | Conversely, too much rear bias will make the car more unpredictable and difficult to drive, decreasing the driver's confidence, though it can be used to help promote rotation in some instances like auto cross. |
| 02:26 | Naturally the temperature of the rear brakes will also rise with a more rearward bias and there will be a possibility of oversteer on corner entry while trail braking. |
| 02:36 | Either of these conditions aren't optimal and will lead to longer stopping distances but like I've mentioned, braking performance is irrelevant if the driver can't control the car. |
| 02:47 | Before going out on track, it's worth doing a quick check with the car in the air to make sure the bias is in the ballpark and nothing seems obviously wrong. |
| 02:56 | If we don't have access to a vehicle lift, the first step is going to be jacking up the car and supporting it safely on axle stands. |
| 03:04 | Have an assistant sit in the car and have them press down on the brake pedal slowly while you rotate one of the front wheels. |
| 03:11 | As soon as you can't rotate a front wheel anymore, have the driver hold that exact pressure on the pedal and move to the back wheel. |
| 03:18 | You should be able to turn it but only just. |
| 03:21 | This technique doesn't take into account any vertical loading on the tyre or grip available and it can take a while to get a feel for what's good but it can at least ensure we're in the right ballpark safely. |
| 03:34 | From here it's time to get out on track and make adjustments. |
| 03:38 | As important as our calculations and planning are for setting a good starting point, at the end of the day, there's just no substitute for real testing. |
| 03:46 | The driver needs to find a safe and representative area, start with the bias adjusted forward and then gradually bring it backwards until there's some instability. |
| 03:56 | After this is't best to move the bias slightly forward again for some safety margin. |
| 04:00 | In some cases, it can be difficult for the driver to determine exactly which wheels are locking so it can be helpful for someone to help as a spotter or to record video for review. |
| 04:12 | Data from wheel speed sensors will really be the best most reliable method if it's in the budget though. |
| 04:18 | With all this said, in some cases, particularly shorter stints in the car like time attack or qualifying laps, we might want to target a slightly rearward bias to help the car rotate on corner entry. |
| 04:31 | This wouldn't usually be the case for a full race stint as it accelerates the rear tyre degradation and places more demands on the driver's concentration. |
| 04:40 | While this course focuses on motorsport applications as this is where getting the absolute optimal braking performance is most critical, it is still possible to use the same ideas when changing the bias on a vehicle with an OE style pedal box without any type of bias adjustment. |
| 04:57 | It's just less convenient as the components need to be changed. |
| 05:01 | Outside of making complete changes to the master cylinders, an adjustable proportioning valve is still a good solution to approximate bias corrections with various input pressures and conditions. |
| 05:13 | So we use proportioning valves to reduce the pressure to the rear brakes relative to the input pressure above a certain threshold. |
| 05:22 | Adjustable proportioning valves are available and the process of setting them up is much the same. |
| 05:28 | Starting with as much front bias as possible and working our way back until we get some signs of too much rear bias, then moving slightly forward again for some safety margin. |
| 05:38 | Finally, let's quickly discuss data analysis which as we've learned about in the previous section of the course is the best method of getting real factual information on the brake bias during on track testing. |
| 05:52 | By monitoring the front and rear brake pressures, we're able to build a brake bias math channel for logging. |
| 05:59 | This will allow us to see how the brake bias changes over a lap and various other bias related aspects. |
| 06:05 | For example, if we're also logging wheel speed sensors, we can understand how the driver's adjustments to the brake bias affect the lock up events. |
| 06:15 | Brake bias and other brake performance subjects are covered in more detail in the HPA professional data analysis course so if you want more information on these topics, I'd recommend checking that out. |
| 06:27 | That rounds out our discussion on tuning brake bias so let's summarise the main points. |
| 06:32 | With all the mechanical and hydraulic components specced appropriately, the driver can make adjustments using the bias bar to fine tune the bias for different conditions and a proportioning valve can be used to help improve the bias over a range of input pressures. |
| 06:49 | Finally, it's onto testing on track, starting with a safe overly forward bias and slowly working rearward to find an optimal point for the conditions, vehicle setup and driver preference. |
| 07:01 | We can also use data logging of brake pressure to get some hard numbers and give us a better understanding of the effects of our adjustments. |
