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Professional Motorsport Data Analysis: Brake Locking

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Brake Locking


00:00 - When braking doesn't quite go to plan, one of the possible outcomes is a locked wheel.
00:04 At best, this will slow down our lap and at worst, it'll destroy a tyre or lead to a loss of control.
00:13 As we discussed in the previous module, as a tyre exceeds its peak slip ratio, more and more of the tread begins to slip against the road surface.
00:22 The most surface slip occurs, the less longitudinal force is created.
00:26 As we discussed earlier, in braking the point where the entire surface has transitioned to slipping is when the wheel can be considered locked.
00:36 And once the wheel is locked, the longitudinal force that it's contributing towards slowing the car, is far less than when it's at peak slip ratio.
00:45 The most spectacular type of locking, front locking, often results in puffs or streams of tyre smoke.
00:52 Front locking almost always occurs on the inside front tyre on corner entry and will ususally lead to understeer in that corner from failing to slow the car enough on entry.
01:02 The primary reason for this is that the inside front tyre has less vertical load on it because of the weight transfer that has taken place on corner entry.
01:13 Bumps in the road and surface changes like painted curbs and transitions in the asphalt all compound this to increase the risk of inside front locking.
01:22 Modulation of the brake pressure with steering input is an important tool to control front lockup.
01:27 Corners that are particularly susceptible to front lock may require more braking in a straight line to reduce the amount of time the inside front tyre is spent braking with reduced vertical load.
01:40 Which will occur on the inside front tyre at turn in.
01:43 Setup has a large part to play in controlling front locking as well.
01:46 Uneven front corner weights will automatically give a bias to make front lock more of an issue in one turn direction.
01:54 For a similar reason, stiff front anti roll bars will tend to unload the inside front tyre as compression of the outside tends to take the inside with it which reduces the effectiveness of having an independent front suspension.
02:08 The stiffer the springs are, the more susceptible the front axle will be to lock, particularly on rougher surfaces.
02:15 Excessive levels of front damping will also make the front axle lesson compliant and give a lower lock margin.
02:22 Brake bias, which we'll discuss in its own module next, also clearly contributes as excessive front brake bias will make it easier to lock a front wheel.
02:31 Locking on the rear axle is also a problem, although the tyre damage is usually less of an issue, as the rear axle is naturally more lightly loaded than the front during braking.
02:41 Rather than resulting in understeer as with locking on the front axls, rear locking will generally lead to instability on corner entry which is a more dangerous condition for the driver to deal with.
02:52 Many of the same setup considerations we discussed that make the front axle more prone to locking, also apply to the rear.
02:59 Aspects like corner weights spring and anti roll bar stiffness and brake bias.
03:04 In a rear wheel driver car, there's another factor that will lead to rear lock and that's the gear downshifts.
03:11 As we discussed in the braking techniques section, smooth application of braking forces is important to maintain traction.
03:18 Any shock loading to the rear tyres from downshifts that aren't as smooth as possible goes against this.
03:25 To prevent shock loading of the rear tyres during downshifts, synchronisation between the clutch, gear change and throttle blip is required.
03:33 While all this is happening, brake pressure needs to be kept as smooth as possible.
03:38 Even if the synchronisation of the downshifts is good, large variations in braking pressure due to rocking your foot onto the throttle, need to be avoided as this in itself tends to make locking and instability more of an issue.
03:52 To allow for this, the setup and ergonomics of the pedal is important.
03:56 Both when it comes to the spacing between them and the relative height of the brake and throttle pedal while the brake is heavily applied.
04:03 Trying to keep the downshifts to later in the stop can help as the later the downshifts are made, the less acceleration the engine will need to do on each downshift, lessening the shock.
04:15 Obviously there's a balance to be found between not shifting down too early and not leaving shifts so late that they occur while you're busy turning into the corner and adding lateral load to the rear of the car.
04:26 There are a few different ways you can visualise locking on either end of the car.
04:31 The first is simply by overlaying the wheel speeds.
04:34 You can see in this example where we have each of the front wheel speeds plotted, it's clear to see we have some front locking occurring on corner entry.
04:43 It's easy to see the locking in this case but when you have more than one set of data overlaid on top of each other, as shown here you can see that it quickly gets difficult to understand.
04:54 For this reason, when overlaying data, I prefer to calculate the amount of lock and show it in a separate channel and only plot a single speed for the entire vehicle to make overlays between different data sets clearer.
05:06 To calculate the locking, all we're interested in is the difference between wheel speeds while we're braking.
05:12 For the purpose of this example, let's look at just the front axle.
05:17 Although there are a number of different ways you can quantify the locking, in this case, we'll keep it simple.
05:23 Looking at this channel for the front left locking, we're using a choose statement with two conditions before we calculate the locking.
05:30 The first is that we need to be braking, using the same "is braking" condition we made earlier in the course.
05:36 The second is that the front right wheel speed, minus the front left must be greater than 10 km/h.
05:42 If both of these are true, we can calculate the magnitude of the locking.
05:46 The calculation is just a subtraction of the front left wheel speed from the front right.
05:51 Which for the conditions we've used, will always produce a positive value when looking occurs.
05:57 Adding this channel and the accompanying one for the front right locking into a time/distance plot, we can see that this is cleaner to look at than when using just the raw wheel speeds, particularly when we add overlays.
06:09 We can also use other display types depending on our preferences.
06:13 We could look at locking using a status light or an XY plot of locking magnitude vs vehicle speed.
06:21 So far, the locking math we've discussed has been for analysing the locking in your logged data inside your analysis project.
06:27 If your logger has the ability to calculate live math, you have the option of displaying some live locking feedback to the driver in real time.
06:36 In a long stop, the locking often creeps in in stages which makes it possible to give the driver some warning so they can make some correction or at least use some caution in their braking.
06:48 One of the ways to do this is to make use of a series of lights.
06:51 Normally, in 2 to 3 stages that indicate the severity of the locking.
06:56 You can use similar math logic to what we used for the locking math previously but instead of making the output continuous, you'll want to output a range of values that match the intensity of the locking.
07:09 So for example, if the magnitude of the locking is between these values, output a value of 1, or if the magnitude of the locking is between these values, output a value of 2.
07:20 You can then use this logic to trigger the different stages of warning lights.
07:24 The point of the lights is to give a series of warnings before full lock occurs.
07:29 You can decide on which thresholds make sense for your case based on reviewing your logged data and tune it from there.

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