Brake System Design and Optimization: Implementation, Testing & Conclusion
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Implementation, Testing & Conclusion
07.41
| 00:00 | - With our mitigation plan in mind, it's time to head out to the track for some real world testing to validate our theoretical calculations. |
| 00:08 | We do this by analysing the current setup and any changes we make using datalogging and of course driver feedback. |
| 00:16 | Before testing, the hydraulic system was freshly bled and the bias bar was checked on the hoist using the method of rotating the wheels by hand to ensure we were already in the ballpark. |
| 00:28 | 100 tread wear Nankang AR-1 semi slicks have been fitted for the test day rather than a full slick as we had some concerns about not getting enough heat into the rear tyres and at least a semi slick will offer some grip from cold. |
| 00:42 | We'll keep this in mind moving forward as it does have an impact on the results. |
| 00:47 | We also applied AP Racing brake temperature paint to all the brake discs. |
| 00:52 | The green paint changes to white at 430°C, the orange changes to white at 560 and the red at 610. |
| 01:02 | As always, good practice dictates that the brakes should gradually be brought up to temperature before hard application to avoid thermal shock to the discs and other components. |
| 01:12 | We also want to get the tyres up to temperature to minimise the chances of locking the wheels and to reproduce true racing conditions as closely as possible. |
| 01:22 | With the bias bar set in a safe and stable overly front position, we head out to begin testing. |
| 01:28 | On a test day like ours with the track to ourselves we were able to make use of the empty track by using the main straight to brake in a straight line with plenty of run off and little consequence. |
| 01:40 | Naturally on a busy track, care needs to be taken to not get in anyone's way and risk collision. |
| 01:47 | The first step is to ramp up the braking from a relatively low application pressure and initial speeds to high speed full power stops. |
| 01:57 | With the bias bar gradually wound rearward until the driver feels instability or the rear wheels beginning to lock. |
| 02:04 | This can be validate by a spotter or in our case by looking at the logged data from the wheel speed sensors. |
| 02:10 | From here, we adjust the bias bar forward again to prevent the instability and then testing the change around different areas of the track while building to a solid race pace. |
| 02:20 | Following this method, we confirmed that using our current setup, using the 7/8" rear master cylinder would still lock the rear wheels under full power braking even with the bias bar wound all the way to the front to maximise front bias. |
| 02:36 | However the driver reported that it was still very predictable and easy to control. |
| 02:41 | Looking at the data, we can identify these lock up events, alongside the brake pressure and deceleration data to see that a rear pressure of around 350 to 400 psi or 2400 to 2750 kPa, causes the rear brakes to lock when braking at over 1.1G. |
| 03:02 | Comparing this to our calculator, we can see there's some correlation. |
| 03:07 | The rear brake pressure required to achieve 1.1G of deceleration is around 2650 kPa or 385 psi. |
| 03:17 | And the bias line on the plot crosses the optimal line and into the overly rear bias area at about 1.15G. |
| 03:25 | We can also see from the data that the City was able to achieve a maximum longitudinal deceleration of up to about 1.3G on multiple occasions which is impressive on semi slicks at this stage of development. |
| 03:39 | On the temperature side of things, our concerns around rear tyre heat were clear. |
| 03:43 | We just couldn't get enough temperature into the rear tyres which is most likely a result of too much rubber on the rear and therefore just too much thermal mass. |
| 03:53 | After 5 laps of hard driving we used the probe to measure the tyre temperature in the rear of around 30°C. |
| 04:01 | For reference, the optimum temperature range for the Nankang AR-1 is 71 to 104°C and although a semi slick like this will still provide decent grip when cold, the grip increase with hot tyres is significant to say the least. |
| 04:17 | We measured the front tyres at the lower end of this range at around 70°C. |
| 04:23 | Staying on the topic of temperature, after some hard laps, the green paint on the front discs had turned white but the orange did not, indicating a maximum temperature somewhere between 430 and 560°C which is well within the optimal range for the Hawk DTC60 front pad compound. |
| 04:43 | The rears again were on the cooler side with no changes to the disc paint, meaning the temperature never got to 430°C and in fact would never after the full days of testing. |
| 04:56 | The Wilwood purple pad compound is effective at less than half of this and the rear brakes were clearly working effectively, causing the rears to lock so this isn't a concern. |
| 05:06 | Since the driver reported that the car is predictable and stable on the brakes, it's fair to assume that the pad compounds are suitable for the time being. |
| 05:14 | The max speed of brake applications was around 170km/h, 20-30 lower than that we would achieve at our most commonly driven track. |
| 05:24 | Regardless, each lock up occurred at much lower speeds where the rear downforce would have been reduced significantly. |
| 05:31 | In this situation, it would usually be up to the driver to use less brake pressure when less aero grip is available. |
| 05:39 | But it should be noted that the driver was purposefully being aggressive to provoke lock ups for testing purposes. |
| 05:46 | On top of this, by achieving slightly higher deceleration forces than expected, the longitudinal load transfer is increased, shifting load off the rear tyres and further reducing the rear grip. |
| 05:57 | With all of this considered, it's clear the root cause of the issue is simply not enough rear grip which is more of a result of the tyre temperature than brake setup. |
| 06:07 | Regardless, since we had a 1" master cylinder on hand, we changed it out to replace the 7/8" rear master cylinder at the risk of increasing the bias bar tilt and bias migration. |
| 06:19 | On this note, by looking at this plot of front brake pressure vs brake bias, we see that the bias remains relatively stable as the pressure changes during a stop, showing that bias migration wasn't an issue with the old setup and therefore is unlikely to present issues with the updated setup. |
| 06:37 | Back out on track and following the same method of starting with an overly forward bias, the bias bar could be adjusted to about halfway between the central position and maximum forward position before showing signs of instability from rear locking. |
| 06:52 | The driver also reported they could apply much more pedal force without locking the rears and the pedal feel and travel were still well suited to the vehicle and allowed for good modulation and heel and toe downshifts. |
| 07:05 | This is a clear improvement from the previous setup that now allows for some adjustment in either direction, although to be clear it's still not the optimal solution. |
| 07:15 | Moving forward, the plan is to change to a much narrower rear tyre in an attempt to get the rubber into its optimum temperature range. |
| 07:23 | This may seem counter intuitive since decreasing the rear contact patch could likely result in less rear grip, making the locking situation worse. |
| 07:32 | However, generally speaking, with a motorsport tyre, heat has a much larger impact than the contact patch. |
