Brake System Design and Optimization: Brake Bias
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Brake Bias
10.42
| 00:00 | - Brake bias is a term you'll hear thrown around a lot in any discussion of braking and motorsport and as we'll see in this module, there are some nuances we need to understand to get a full picture of why it's important. |
| 00:14 | The concept of brake bias describes the amount of braking force applied at each end of the car. |
| 00:20 | Generally that means we're referring to the ratio of braking force between the front and rear axles. |
| 00:27 | In broad terms, we're trying to match the ratio of the braking force on each axle to the vertical load applied to the tyres at each axle. |
| 00:36 | The idea being that this will give us the highest possible braking force by making use of all the available tyre grip. |
| 00:44 | As we learned in the longitudinal tyre forces section, the amount of braking force a tyre can produce is closely linked to the vertical load applied to it. |
| 00:54 | Increasing the vertical load increases the amount of braking force it can supply. |
| 00:59 | There are two main factors in determining the amount of vertical load applied to each tyre when we brake. |
| 01:06 | One is the static weight distribution and the other is the longitudinal load transfer. |
| 01:11 | The static weight distribution is determined by how much the main components of the car weigh and where they're positioned. |
| 01:19 | The engine and transmission, driver, fuel tank, suspension and the chassis itself are some examples. |
| 01:26 | Taking the case of a front engined rear wheel drive car, you might have 55% of the total mass being supported by the front axle. |
| 01:36 | Alternatively, if you have a rear engined car, you could instead have 60% of the total mass supported by the rear axle. |
| 01:45 | Longitudinal load transfer only occurs when we brake or accelerate the car and is determined by this equation. |
| 01:52 | As we can see, the load transfer is a function of the magnitude of the longitudinal acceleration, the mass, the centre of gravity height and the wheel base. |
| 02:03 | This equation shows us that increasing the longitudinal acceleration, the mass, or the centre of gravity height, will increase the longitudinal load transfer and increasing the wheel base will decrease the longitudinal load transfer. |
| 02:18 | In the longitudinal tyre forces module, we learned that while increasing the vertical load on a tyre, we'll increase the braking force it can supply because the coefficient of friction between the tyre and the road reduces as we increase the vertical load. |
| 02:34 | The less load we transfer, the more effective our braking will be. |
| 02:39 | So one way to increase our braking performance is to reduce the amount of longitudinal load transfer. |
| 02:46 | In terms of the car itself, referring back to our previous equation, we can decrease the total mass, lower the centre of gravity or lengthen the wheel base. |
| 02:55 | With these factors considered, we can start to understand why brake bias is important. |
| 03:00 | The braking force each tyre can produce is a function of how much we decelerate and the configuration of the car. |
| 03:08 | From a simplified perspective, we want to match the braking torque supplied by the brake system to the braking capacity each tyre has. |
| 03:18 | It should be clear that all else being equal, a car with more front weight distribution will require more forward brake bias than a car with a more rearward weight distribution. |
| 03:29 | This means that a higher proportion of the braking force will happen at the front axle. |
| 03:34 | In the same way, a car with a higher centre of gravity, more total mass or a shorter wheel base, all factors that result in more longitudinal weight transfer, will need a more forward bias than a car with the opposite traits. |
| 03:49 | As we've already discussed, longitudinal acceleration during braking, otherwise known as deceleration, also affects the amount of load transfer. |
| 03:59 | Obviously the driver has some control over this with how hard they press the brake pedal. |
| 04:05 | However in many cases, the car will be lock limited, meaning there'll be a point where any more brake pressure will lead to a locked wheel which on a sealed surface at least, is bad for maximising braking force. |
| 04:20 | Some important factors that affect the lock threshold include the coefficient of friction between the tyres and track surface which also changes with track conditions as well as any aerodynamic downforce, which essentially changes the vertical load through the tyres. |
| 04:37 | In most cases, the baseline starting point for brake bias is to set it such that the front wheels lock slightly before the rears. |
| 04:45 | This ensures that the car will be on the side of stability at maximum braking because the front wheels being locked is a more stable condition than if the rear wheels were locked. |
| 04:56 | Obviously the application and competition type can have some bearing on this too. |
| 05:02 | While having the front tyres locking first might be the preference for high speed circuit racing, in something like a rally car, or low speed autocross, having the rears lock earlier may be an advantage to help the car rotate on corner entry. |
| 05:18 | The brake bias is primarily set by the components we use. |
| 05:22 | The bore sizes of the master cylinders and caliper pistons, how the force is proportioned to the master cylinder from the brake pedal, the relative friction of the front and rear brake pads and the size of the front and rear brake discs respectively. |
| 05:37 | The brake friction and disc size can be thought of as the mechanical brake bias. |
| 05:42 | Whereas the ratio of hydraulic pressure applied to the front and rear circuits respectively is termed the hydraulic brake bias. |
| 05:50 | With the chosen components fitted to the car, the hydraulic brake bias is what we use to fine tune the brake bias of the car on the track. |
| 05:59 | This may be done for varying track types and conditions throughout a race weekend or even throughout a lap for each corner individually in extreme cases. |
| 06:09 | In most cases, when people are discussing brake bias settings and changes, they're talking about the hydraulic brake bias as this is the most convenient thing to change. |
| 06:19 | Calculating the brake bias is simple, first we take the brake torque of the front brakes, divided by the front and rear added together and then multiply the whole thing by 100. |
| 06:31 | This gives us the brake bias as a percentage applied to the front axle which is the convention. |
| 06:37 | We'll be covering the methods and mecahnisms by which the brake bias is set and adjusted in more detail later in the course so let's not worry about that side of things for now. |
| 06:49 | One situation where a driver would usually make a brake bias adjustment would be when track conditions change from dry to wet. |
| 06:57 | Intuitively you might think that for wet conditions, you would adjust the brake bias towards the front axle to increase stability on a lower grip track. |
| 07:07 | The opposite is actually true. |
| 07:10 | This comes from the fact that when the overall track grip is lowered, the car will have less deceleration available. |
| 07:17 | Referring back to the load transfer equation from before, this means we'll be transferring less load from the rear axle to the front. |
| 07:26 | This means that if we didn't adjust our brake bias, we'd increase the chances of locking a front wheel when we're at the grip limit. |
| 07:33 | This is because the rear tyres have more of a part to play in braking the car on a lower grip surface because the vertical load remains on them during a lower braking G stop. |
| 07:45 | By not adjusting the brake bias, we'd have too much hydraulic pressure acting on the front axle relative to the rear and we'll end up with the front locking at the braking limit as a result. |
| 07:56 | This example also brings up an important point, the optimum brake bias is a function of the deceleration rate because of load transfer. |
| 08:06 | So if we had no load transfer, the target brake bias would be equal to our static weight distribution. |
| 08:12 | As we increase the amount of load transfer from the rear tyres to the front tyres during braking, the main reason being due to the additional deceleration from pressing the brake pedal harder, our optimum brake bias changes. |
| 08:28 | So the harder we brake, the further forward our optimum brake bias is. |
| 08:33 | Something we can see on this plot where the optimum bias increases linearly with the deceleration. |
| 08:40 | Anything to the left of the optimal line would be too much front brake bias and anything to the right would be too much rear brake bias. |
| 08:48 | This presents and issue where for a simple system, the brake bias can only be optimised at a single value of braking G force. |
| 08:57 | In motorsport case, the baseline bias is targeted at the max G stop. |
| 09:03 | A conventional brake bias bar system isn't able to change the braking bias during a stop due to friction. |
| 09:10 | Even if the driver was capable of tweaking it during a braking event. |
| 09:14 | So we can see that our actual brake bias line on the plot is a horizontal line that intersects the optimal line at 1 braking G force. |
| 09:23 | To help get around this issue, devices called proportioning vales exist which we'll get onto in a later module. |
| 09:31 | Other reasons we might want to move our brake bias would be varying fuel loads, tyre degradation and different brake friction characteristics as the brake temperatures vary. |
| 09:41 | Depending on the fuel tank location, the height and longitudinal location of the centre of gravity may vary considerably as a race stint goes on. |
| 09:51 | As tyres wear throughout a stint, the front or rear may degrade quicker in some cases. |
| 09:57 | This will affect the maximum available braking force at each axle. |
| 10:02 | As we'll discuss in more detail later in the course, the operating temperature of the brakes has a big effect on the available brake torque. |
| 10:10 | Changing the brake bias to account for varying front and rear friction characteristics is an important tool. |
| 10:17 | In summary, brake bias refers to the relative braking of the front and rear axles. |
| 10:23 | This can be described in mechanical or hydraulic terms with hydraulic being considerably easier to adjust. |
| 10:31 | The brake bias must be set and adjusted based on the different car characteristics, tyres and track conditions. |
