Brake System Design and Optimization: Current System Architecture & Analysis
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Current System Architecture & Analysis
08.34
| 00:00 | - In this worked example, we're going to go through the analysis and development of a braking system on a relatively simple race car. |
| 00:07 | Identifying the issues with the current setup and working to correct and optimise the system as a whole to achieve better braking performance that's well balanced, controllable and consistent. |
| 00:19 | The vehicle in question is a Honda City which at face value, doesn't sound like anything more than a grandma's grocery getter. |
| 00:27 | But having driven this car myself, it's anything but. |
| 00:30 | This City has a 200 wheel horsepower B18C engine and gearbox from an Integra type R and weighs around 800kg dry. |
| 00:40 | It's a fully caged track car with a close ratio gear set, aggressive LSD and a myriad of other performance parts. |
| 00:48 | The braking system is of course what's of interest to us, it's been completely changed from the factory, sliding front calliper and rear drum brake setup and now consists of fix style front and rear callipers with larger discs. |
| 01:02 | Most importantly, the factory pedal box has been replaced with a motorsport style pedal box with dual master cylinders and a bias bar. |
| 01:09 | The setup does not include a proportioning valve but we've added pressure sensors to the front and rear circuits as well as wheel speed sensors to all 4 corners to help with data logging through the AiM MXP dash. |
| 01:22 | It's important to note that the City has been widened significantly and will run on relatively big semi slicks or full slicks. |
| 01:30 | The intended application is club level track days as well as the occasional sprint or endurance race. |
| 01:37 | During the few testing sessions the vehicle has seen, the drivers have reported signs of an overly rearward bias with the rear wheels locking on hard brake applications. |
| 01:47 | Steps have been taken to try and make improvements which we'll discuss shortly but the setup is still very much compromised. |
| 01:55 | While the wide body, aero and other areas of the build are still a work in progress, this makes it the perfect time for potential changes. |
| 02:03 | Let's start by listing the specs of the current setup and adding them to the HPA calculator to help us understand how it all stacks up. |
| 02:11 | Honda Citys have a wheel base of 2400mm and this hasn't changed from factory. |
| 02:16 | Before the first testing session, the car was weighed using corner weight scales to find a weight of 835.5kg with a driver and half a tank of fuel, giving us a 65.8% front weight distribution. |
| 02:32 | We also measured the centre of gravity height at 450mm with the help of the HPA CoG calculator. |
| 02:40 | You can find this spreadsheet attached as a resource in the notes underneath this module. |
| 02:45 | HPA's suspension tuning and optimisation course is a great place to pick up these skills if you're looking for more information, you can also find a link to this below. |
| 02:55 | From the data we acquired during the limited running time of the City, we know we're pulling about 0.8 to 1G of longitudinal deceleration during a braking event which was with no aero and a compromised brake setup in regards to balance. |
| 03:10 | Albeit with lots of room for improvement so it's not unrealistic to target a slightly higher deceleration force between 1-1.2G. |
| 03:19 | As there was no aero downforce in the splitter or rear wing during any of the previous testing, we can set these cells to zero for now. |
| 03:27 | We do have both of these elements planned though but we'll come back to this in an upcoming module. |
| 03:33 | As we mentioned earlier, there's also no proportioning valve so we can leave this section of the calculator set to no. |
| 03:41 | The Tilton pedal box allows for some adjustments of the pedal ratio but we've set this to around 5:1 which is in the middle of the range to allow adjustment either way if required. |
| 03:51 | We'll come back to the pedal box soon to discuss the master cylinders. |
| 03:55 | And the pedal effort input doesn't really matter for analysing the current setup so we can just leave this at 50kg for now. |
| 04:02 | The car originally used 15 inch wheels so the front disc isn't maximised for the current 17s that are being used. |
| 04:10 | Up front we have 280mm outer diameter discs that are 26mm thick and vented. |
| 04:17 | The callipers are Wilwood DynaPro 4 pistons which have a pair of 35mm pistons on each side of the calliper. |
| 04:25 | In the rear, the discs aren't much smaller than the fronts with a diameter of 260mm. |
| 04:31 | But are only 10mm thick solid pieces. |
| 04:34 | The rear callipers are also Wilwood DynaPros but in this case, 2 piston variants with a 44.45mm piston on each side of the calliper Due to the large size of the piston in the rear callipers, the total piston area of the front to rear callipers are relatively close at around 1900mm² and 1550mm² respectively. |
| 04:59 | On the car's first test day, the front master cylinder was 7/8" and the rear was 5/8" with Hawk DTC 60 pads of 42mm radius and a coefficient of friction of around 0.6 being used in the front callipers. |
| 05:16 | Wilwood Polymatrix pads were being used in the rear with a coefficient of friction of around 0.55 and also 42mm radius. |
| 05:25 | With all of these values added to our calculator, we can see that our pedal travel and pedal effort are both in a reasonable range and we aren't getting any warnings. |
| 05:34 | With this set up, the driver's reported that the rear wheels would lock easily and it felt like the front brakes were hardly working. |
| 05:42 | Looking at our bias plot, we can easily see why this would be the case with a 43% front bias, the rear brakes are doing more work. |
| 05:51 | This is clearly way off what should be targeted for a such a front weight biased car and a result of the pressure in the rear circuit being a lot higher than the front circuit, while the brake callipers, rotors and pads compounds are all fairly similar. |
| 06:06 | After this, master cylinders were swapped around so now the 5/8" on the front and 7/8" on the back. |
| 06:12 | The feedback from the driver was that there was some improvement but the bias was still too far rear. |
| 06:18 | If we update the values in our calculator to reflect this, we can clearly see that this was a step in the right direction however the difference in master cylinder travel was increased, meaning a bit more bias bar tilt and more chance of bias migration. |
| 06:34 | Following this, the rear pads were then changed to Wilwood's purple compound with a lower coefficient of friction of around 0.4. |
| 06:42 | The compound is actually designed to be used with aluminium discs which are more common on the inboard brakes of dirt track sprint cars. |
| 06:50 | The drivers reported that with the bias bar wound all the way to maximise the front bias, the car felt stable on the brakes. |
| 06:57 | We can add these values into our calculator and actually see that with the bias bar centred, the bias is optimal at around 0.75G and with the bias bar in the max front bias position, the bias is in the stable front bias zone up to around 1.2G. |
| 07:15 | This is a perfect example of using different pad compounds and the bias bar adjustment to solve issues with the fundamental sizing of the mechanical components in the system. |
| 07:26 | Although the brakes are now working effectively and achieving an appropriate bias, they are still compromised and we've lost all the advantages of having an adjustable bias bar. |
| 07:36 | The pedal travel and effort are reasonable but the hydraulics are unbalanced with the front master cylinder moving through about 6mm more stroke than the rear which although is workable, isn't ideal for minimising chances of bias migration. |
| 07:51 | Having 2 different compounds isn't an issue in itself but we need to be aware of how the coefficient of friction at each end of the car and therefore the bias will change with temperature during a race, a lap or even a brake application. |
| 08:05 | With that, we'll wrap up our first look at the City's braking system. |
| 08:09 | In this module, we took down all the details of the City's braking system and entered them into our calculator for the various previous setups. |
| 08:17 | From this, we were able to highlight the areas that were less than ideal, comparing these results to the driver feedback of overly rearward bias so we can make some calculated changes to improve the system as a whole. |
