Brake System Design and Optimization: Discs, Callipers and Pads
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Discs, Callipers and Pads
13.17
| 00:00 | - In the last module we took a deep dive into understanding the longitudinal load transfer and associated tyre forces generated in a maximum power braking event which allows us to calculate how much brake torque we're aiming for at the front and rear tyres. |
| 00:17 | Now it's time to use those calculations for the next step. |
| 00:20 | We want to work our way from the tyres back towards the pedal by choosing components appropriate for our application and that will achieve our brake torque requirements for a suitable pedal effort. |
| 00:33 | With all of the components we discuss here, if you need a refresher on the fundamentals of each, be sure to revisit the corresponding module in the components section of this course. |
| 00:44 | On top of the requirements we've already outlined, there are some additional ones. |
| 00:49 | One consideration is that our brake package, more specifically the callipers and discs need to actually fit within our wheels, both radially to clear the barrel of the wheel and laterally to clear the spokes. |
| 01:03 | But with enough clearance to allow for some movement with heat and the flexing of components. |
| 01:09 | It's almost always best to maximise the disc size on the front axle as this results in more brake torque and more thermal capacity, even though this can be at the expense of unsprung and rotating mass. |
| 01:23 | Brake calliper suppliers will specify what diameter and thickness disc is suitable for each calliper and should also be able to provide a drawing or a template that can be used to check the measurements against our wheels to ensure there won't be any clearance issues. |
| 01:39 | Let's say that for our example we're using 18" wheels which have an internal diameter of 416mm and plenty of clearance to the spokes. |
| 01:48 | Rather than searching through every supplier's offerings, in this example to keep things simple, we'll look at a range of callipers and discs from Endless, a reputable Japanese manufacturer. |
| 02:00 | Their Racing MONO6 calliper, as the name suggests, is a 6 piston forged monoblock calliper. |
| 02:07 | Although not Endless' top tier offering, this is a high end motorsport style calliper that's more than sufficient for our application. |
| 02:16 | Featuring staggered pistons, a durable anodised finish and the ability to run anti knock back springs which are small springs behind the piston to prevent the common issue of pad knock back. |
| 02:28 | Something we'll be discussing in more detail later in the course. |
| 02:32 | One thing to note is that these callipers don't have a removable bridge in the interest of weight and stiffness. |
| 02:39 | This won't be a major issue for our application as we shouldn't need to change brake pads quickly during an event as might be the case with endurance racing. |
| 02:48 | It's also worth noting that this is a radial mount calliper so at this point we need to check that it can be mounted to the vehicle and if needed we can design an adaptor bracket and top hat that'll work. |
| 03:01 | As we can see here on the specs, these callipers are designed to work with a 355 to 390 mm diameter disc of between 32 and 36mm thickness using a 25mm pad. |
| 03:16 | By looking at the drawing provided by the supplier, we can see that the distance from the outer diameter of the disc to the outer edge of the brake calliper is 79.4 - 63.5 so that's 15.9 or 16mm on the safer side. |
| 03:34 | If we find the internal radius of our wheel by dividing the 416mm diameter by two to get 208mm and then subtract 16mm and another 5mm for safe clearance, we get a disc radius of 187 and therefore a diamter of 374mm. |
| 03:56 | That means that the Endless 370mm x 34mm floating disc will safely fit under our wheels with the MONO6 callipers. |
| 04:07 | Depending on your vehicle, there may be off the shelf calliper brackets available or if not then you can design these specifically for your application. |
| 04:16 | If you don't know how to do this, I highly recommend checking out our 3D Modelling and CAD for Motorsport course linked underneath this module. |
| 04:25 | This is a perfect example of a job that you'll no longer need to outsource and can absolutely do yourself once you learn how. |
| 04:34 | Lastly we need to consider the disc cap offset and bracket geometry together, ensuring that they allow enough clearance to the wheel spokes. |
| 04:44 | In most cases, the disc cap mounts to the hub surface and the wheel mounting surface will sit flush with the front of the disc cap. |
| 04:51 | The basic process involves first understanding the clearance from the inside of the spokes to this mounting surface then positioning our calliper with at least a few millimetres of clearance to the spokes. |
| 05:04 | After this we can find the disc cap offset required to centre the disc in the calliper. |
| 05:10 | Disc caps are often available in a range of offsets from the supplier or can be machined to suit the application. |
| 05:18 | Finally with an understanding of where the calliper mounting holes are on the upright relative to the mounting surface, we can design a bracket to mount our calliper in the desired position radially and laterally. |
| 05:32 | Before moving onto the rear brakes, let's look at the brake pads as we need to make sure our callipers allow for a suitable pad compound. |
| 05:40 | Calliper suppliers will generally state the suitable pad part numbers or outlines and give the dimensions of the pads so it's crucial to check with them before purchasing any pads. |
| 05:53 | Brake pad suppliers will have offerings for factory vehicles but also aftermarket callipers so some research is required here to cross reference the parts and find suitable combinations of calliper, pad shapes and compounds. |
| 06:08 | Sticking with one manufacturer for all the parts keeps things a bit more simple so let's look at the pad compound offerings from Endless. |
| 06:17 | Here we can see that there's a range of options recommended for sprint racing. |
| 06:22 | The ME20 compound is stated as the standard pad for sprint and medium distance racing with good initial bite and modulation without being too aggressive on discs. |
| 06:33 | The average coefficient of friction across the temperature range is 0.35 - 0.4. |
| 06:39 | Some experimentation with pads is usually necessarily to find the correct combination of front and rear that are ideal for our application. |
| 06:49 | But this should provide a good starting point. |
| 06:52 | It's usually best to run the same pad on the front and rear if our setup allows for it. |
| 06:57 | Often when we see mix and match pads, it's to compensate for some other issue but since we're designing from scratch, we should be able to get everything correct or at least in the ballpark as a starting point. |
| 07:09 | With our callipers, discs and pads chosen for the front axle, we can now calculate the front circuit pressure require to achieve our target brake torque of 1673 newton metres for each front wheel. |
| 07:25 | First we need to calculate the effective radius of the disc. |
| 07:28 | This is essentially the radius that the friction for of the pad is centred on. |
| 07:33 | A 370mm diameter disc has an internal diameter of 260mm but uses a pad with a 53mm radial dimension so the effective radius is 370 divided by 2, minus 53, giving us 158.5 mm. |
| 07:54 | To achieve 1673 newton metres brake torque with a brake compound coefficient of friction of 0.4, we would require approximately 26,388 newtons of calliper clamping force which is actually 13,194 newtons on each pad. |
| 08:15 | Next we need to know the total calliper piston area on just one side of the calliper of course. |
| 08:21 | Since the area of a circle is equal to pi times the radius squared with the 27, 32 and 38mm diameter pistons, we get a total piston area of 2510mm² . |
| 08:36 | Dividing the force in newtons by the piston area in metres squared, gives us the required pressure of about 5257 kPa or 762 psi. |
| 08:49 | At this stage it's important to compare this to the safe limits of the callipers and the potential range of master cylinders we plan on using. |
| 08:58 | If we're above these limits then we either need to increase the pad friction, the disc diameter or the calliper piston area to reduce the pressure requirement. |
| 09:09 | We'll move onto the rear for now and then come back to consider all the parts together. |
| 09:14 | The brake torque requirement and thermal capacity is significantly less here and if we achieve the bias primary through the sizing of the mechanical parts, then the hydraulic system can be more balanced, helping to prevent large angles of the pedal box bias bar. |
| 09:31 | Sticking with the same family of callipers from Endless, we can use the MONO4rs which are intended for use on the rear of the vehicle. |
| 09:40 | These are 4 piston callipers designed for use with a 280mm - 355mm diameter disc between 28 and 32mm thick and will be more than sufficient for the rear axle without the extra expense and weight of the 6 piston variants. |
| 09:59 | Which would also likely result in a large offset of our hydraulic system. |
| 10:05 | An effective way of determining the size of the rear discs relative to the front is to consider the thermal mass. |
| 10:12 | Obviously if the thermal capacity of the front and the rear was the same then the front brakes would run at a much higher temperature as they're doing much more work and as we know from our discussion on brake pad compounds, the friction varies and generally increases with temperature so this would lead to more front bias than desired. |
| 10:32 | If we're running the same compound front and rear, we want the brake discs and pads to run at the same temperature. |
| 10:39 | The mass of each front disc is about 8kg, without the hat and since 74% of the work is done by the front brakes, the mass of each rear disc should be just under 3kg. |
| 10:51 | A 280x30mm floating disc option from Endless puts us around the 4kg mark. |
| 10:58 | It's beneficial to have slightly more thermal mass in the rear disc than these theoretical calculations tell us, since it's a lot more practical to use cooling ducts to control the temperature of the front axle compared to the rear. |
| 11:12 | It's also at the lower limit of what's suitable for the MONO4r callipers and would obviously fit under our 18" wheels with no problems. |
| 11:21 | Although we've considered our brake disc sizing in the interest of thermal capacity, none of what we've done really addresses the thermal capacity requirements meaning getting an understanding of how much heat will be generated and how much cooling will be required. |
| 11:37 | The cooling system really needs to be designed after we gain some understanding of the heat management requirements from testing. |
| 11:46 | Completing the same calculation, the 27 and 32mm piston diameters give us a total piston area of 1377mm² which along with the disc's effective radius of 116.5mm when using a 47mm radius pad of the same compound, shows we require a pressure of 4574kPa or 663 psi to achieve our target brake torque of 587 newton metres. |
| 12:20 | We'll finish this module here and carry on with our design process in the next one to keep things digestible. |
| 12:26 | To recap what we've covered in this module, with our braking requirements already defined, we generally want to start by maximising the disc size that will safely fit under our front wheels. |
| 12:37 | At the same time we need to consider the calliper to ensure the package clears the wheel barrels and spokes. |
| 12:44 | We also need to choose a calliper that gives us a range of options for pad compounds that are suitable for our application. |
| 12:52 | We can then size our rear brake package relative to the front to achieve the bias and thermal requirements of the system. |
| 13:00 | This way, as we move forward, our hydraulic system is well balanced to avoid issues with the bias bar. |
| 13:07 | From there, we calculate the hydraulic pressure required for our calliper, disc and pad to generate our target brake torque. |
