Brake System Design and Optimization: Mitigation Plan
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Mitigation Plan
06.52
| 00:00 | - In this module, we'll continue to use the calculator to develop a mitigation plan to address the issues with the City's braking system and work towards a more balanced and adjustable setup. |
| 00:12 | The relatively similar sizing of the front and rear callipers and discs, seems like a bit of a red flag for a front wheel drive vehicle like this that has a very forward weight bias. |
| 00:23 | The larger rear master cylinder sizing has been used as a bandaid for the sizing of the mechanical components and the result is an unbalanced hydraulic system that could result in bias migration from severe angles of the bias bar. |
| 00:37 | The extreme difference in the compound of the front and rear pads is also less than ideal, however this isn't too concerning and can still allow us to achieve a balanced and adjustable system. |
| 00:49 | Regardless, the previous setup suffered from too rearward bias which can only be accounted for by maximising the bias bar adjustment, essentially rendering it useless for varying conditions. |
| 01:01 | Since the last outings, the build has progressed somewhat with the addition of a fuel surge tank, power steering with a remote power steering pump and a rear wing. |
| 01:10 | So it's likely the details in the vehicle data section of the calculator need to be updated. |
| 01:16 | The wing is an aluminium piece and the wide body itself, which isn't fully installed yet is a carbon fibre kit from a VW Golf TCR racecar. |
| 01:26 | A front splitter is in the plans to round out the aero package, although this hasn't been fitted yet. |
| 01:32 | While we have no real testing or data to base the aero performance on for this specific car, we do have data from other very similar setups. |
| 01:41 | With a conservative approach, we can assume the front and rear downforces would be around 20 and 80kg respectively at the top speed of between 180 and 200km/h. |
| 01:52 | These are significant for such a light car so we'll factor them into our calculations, adding our 20kg front and 80kg rear aero loads. |
| 02:02 | We also reweighed the car, finding an increase from the original 835 to 885kg but a slightly better weight distribution at 65% front. |
| 02:14 | The CoG height however remained very close to the original value of 450mm. |
| 02:21 | Updating our calculator, we can now see that the optimum bias line on our plot moves downwards to show we're now targeting a more rearward bias. |
| 02:30 | This is because even though we've transferred about 10kg more load from the rear wheels to the front wheels, we now have a better static weight distribution and significant rear aero downforce. |
| 02:42 | So the load on the rear tyres has almost doubled while the load on the front tyres has only increased slightly. |
| 02:49 | Since the previous setup had such little load on the rears, it's quite easy to make significant improvements. |
| 02:56 | With these changes to the mass and downforce of the car, our target bias has now shifted so much that the current brake system setup should be able to provide results we're after with only minor changes. |
| 03:09 | And also with the bias bar in the central positon, allowing for adjustability in either direction. |
| 03:15 | Naturally if we were starting from scratch, we'd ideally want to be using a front calliper and disc that's larger relative to the rear as this would allow us to use a much more balanced hydraulic system. |
| 03:27 | However, since the time and money has already been spent on the current system, we'll try to work with the callipers and rotors we already have. |
| 03:35 | There's still a few different paths we could take and the bigger question here is do we use a proportioning valve or not? If not, the only change we'll make is increasing the size of the rear master cylinder further. |
| 03:47 | In which case we'll use a 1" size as this is the next size up from the 7/8" from our supplier. |
| 03:54 | This change would theoretically give us stable front bias up to about 1.4G assuming we get sufficient heat into the rear tyres and good pedal travel and effort requirements. |
| 04:05 | The downside here is that we're increasing the imbalance of our hydraulic system so the difference in master cylinder travel will be increased by 1mm to about 7mm which is just at the upper end of what we'd usually consider acceptable and may result in some bias migration as the bias bar tilts on brake application. |
| 04:28 | On the other hand, if we're using our proportioning valve, for example the Tilton lever type, we could have it set to wide open for maximum reduction to the rear brakes in dry conditions where the activation pressure or knee point is 175psi. |
| 04:44 | This would allow us to use the same 5/8" master cylinder on the rear circuit as the front in which case the difference in master cylinder travel would almost be eliminated. |
| 04:55 | As well as the adjustability of the proportioning valve to move the bias rearward in lower grip conditions. |
| 05:03 | The issue here is that the pedal travel is on the long side of what's acceptable. |
| 05:07 | Since the pedal effort isn't too high, we could lower the pedal ratio to offset this a small amount but not much. |
| 05:15 | This is concerning as proportioning valves typically require a small amount of extra fluid to be displaced, adding to the pedal travel even more. |
| 05:23 | The other thing to consider is that proportioning valves offer more advantages for vehicles with more longitudinal load transfer, from extra weight, a higher CoG, a shorter wheel base or higher deceleration rates. |
| 05:38 | The City doesn't transfer much load in the grand scheme of things, as although the wheel base is relatively short, the weight and CoG are fairly low. |
| 05:48 | We can see the advantage of using a proportioning valve by overlaying the plot is relatively small. |
| 05:54 | For these reasons, we'll avoid introducing a proportioning valve as our first step but keep it in mind as a potential future change that's relatively simple and only requires a different rear master cylinder. |
| 06:06 | To recap, due to the extra components being added to the vehicle, the weight, weight distribution and aero downforce were changed. |
| 06:14 | In this case, helping to improve the vehicle's braking ability by minimising the amount of longitudinal load transfer. |
| 06:21 | This should allow us to retain most of the original setup and only change the rear master cylinder from 7/8" up to 1" diameter. |
| 06:31 | This will allow us to use the full range of bias bar adjustment at the risk of introducing bias migration which is just something that we need to be aware of. |
| 06:40 | From here, we'll head out to the track and test the setup as is and then make these changes testing again and using our data to validate the results. |
