Ferrari 599XX lighting the brakes up [1]
Braking Bias Consistency
By Trav Mays
Today we will begin a series that uses data acquisition to look at an area where we can all improve, braking. The series will look at braking points, the transition from the throttle to the brake, brake speed or aggression, the smoothness of the braking release, braking consistency to name a few. But Before we are able to assess a driver's braking ability, we first need to have a look at the braking system, we need to check it is working correctly.
Rear Brake Line Pressure vs Front Brake Line Pressure
An extremely simple way to check the braking system, assuming you are monitoring front and rear braking pressures, is to create a scatter plot with the rear brake pressures on the Y-axis and the front brake pressures on the X-axis. Given that fluid is incompressible, theoretically we should see a straight line with no deviation. But as nothing is perfect, some scattering is expected. Below is an example of the type of scattering you’d like to see, consistent rear brake pressures for a given front brake pressure. At 20 bar front line pressure we have a variance of about 3.5 bar in the rear brake line pressure, pretty good. The driver will be very confident that they know fairly precisely what front and rear brake pressures they will get for a given level of brake pedal travel.
Rear Brake line Pressure (Y-Axis) vs Front Brake line Pressure (X-Axis)
Compare that to the scatter plot below. Clearly the below graph has far more scattering throughout the whole pressure range. At 20 bar front line pressure, this car has a variance in rear brake line pressure of 7 bars, double the above example. This means that when the driver presses the stop pedal, whilst the front line pressure is 20 bars, the rear could have a pressure of anywhere between 15 and 22 bars. This variance will result in a different bias setting every time they press their foot. At 20 bar front line pressure the bias could be anywhere between 57.1% and 47.6%, it could be either front or rear brake biassed, I can’t imagine this is a very nice car to drive.
Rear Brake line Pressure (Y-Axis) vs Front Brake line Pressure (X-Axis)
Click here to see the brake bias maths channel equation
Brake Bias is a simple maths channel to create, but one that is extremely helpful in analysing the braking system and evaluating the braking balance of the car.
Brake Bias vs Front Brake Line Pressure
The next step in checking the braking system is to create a front brake bias (Y-axis) vs front brake pressure (X-axis) scatter plot, below is an example using the same data from the first graph. The first thing you probably noticed is the large concentration on the left of the graph, this is mostly from slight increases in pressure in either the front or rear brake lines whenever the driver is not touching the brakes, going down a straight for example. We could remove these values by gating the channel (only showing values above a certain value), in this example 4bar would have been good, but I don’t think it’s necessary and doing that could potentially hide information.
Front brake bias vs front brake line pressures
The next thing we can see is the tight concentration of values across all pressures. The front brake bias shifts rearward slightly as we increase the pressure, moving from around 60% down to 55% at the very tip of the plot. As you have probably guessed, this is an example of a good scatterplot, the consistent brake bias values allow the driver to jump on the brakes and fairly accurately predict what will occur. Claude Rouelle, from Optimum G, suggests that we should be aiming for a distribution that has 5% variation above 13.8 bar (200psi) [2]. We don’t have that here, we have 7.5% variation at 14 bars, but we do reach it at 17 bar, so not perfect, but close, there is still room for improvement in the braking system.
If we again compare the above scatter plot with one created using the second graphs data, we can see that this scatter plot is far less concentrated. This brake system has a variation of 12.5% at 14 bar and doesn’t reach 5% variation until 27 bars of pressure, just shy of the total pressure experienced, 31 bars. We can also see that the car is rear brake biassed at low pressures, graduallying moving to front wheel biassed as the pressure is increased.
Front brake bias vs front brake line pressures
Causes
Both of these cars are purpose built race cars with dual brake master cylinders, they should, in theory, have a perfectly linear rear pressure vs front pressure graph like the first graph. So what’s going on? The cause could be any number of reasons, but the most likely is play within the pedal box.
Click here to learn about brake proportioning
Brake bias on a road car is governed by a brake proportioning valve that essentially controls the amount of pressure the rear brakes receive. Weight transfer means the rear axle has less load on it under braking, less load means less grip (see our series on the grip here and our post on longitudinal grip here), less grip means the rear axle will be able to lock up easier for a given pressure. The brake proportioning valve stops this by allowing the two pressure lines to increase together to a point, after which the front brakes receives more. The rear line pressure vs front line pressure graph becomes a two step graph, similar to what you see below.
Front and Rear brake line pressure graph - Data is made up by me
Obviously we are not talking about road cars, we are talking about race cars, so what governs it for them? The race car system uses two master cylinders with a linkage (balance bar) between the two. The relative distance between the two master cylinder pushrods and the input (brake pedal) pushrod controls the amount of pressure each line gets. If we look at the image below, we can see a dual master cylinder system from the viewpoint of looking directly down. This system has an even distribution of pressure to both the front and the rear disks, we have 250 lbs being supplied to the system from the drivers foot, with both the front and rear brake callipers receiving half of it. If the input rod was to be moved in either direction along the horizontal, for example closer to the front master cylinder pushrod, reducing the distance to 1.2” and increasing the distance to the rear master cylinder’s pushrod to 1.8”. The front master cylinder would receive a higher proportion of the inputted pressure, 150 lbs in this example, whilst the rear master cylinder would receive just 100 lbs. The adjustment is done simply by either screwing in or out the balance bar, first undo the locking nut on the right and adjust away. Most race cars also allow for bias adjustment within the cockpit, this allows the driver to alter the bias as needed for things such as fuel being used, track condition changes, tyre wear etc.
Dual master cylinder set up with balance bar and input shaft [3]
Adjusting the balance bar isn’t the only thing that is adjustable within a braking system, we can also alter the master cylinder pushrod lengths. Increasing the length of one will increase the amount of pressure supplied to that master cylinder and therefore brake bias. The brake bias can also be changed by altering the size of the master cylinders, callipers, etc, but we will save that discussion for another post.
I don’t know the exact cause of the poor braking consistency for the second example, but if I had to guess, I would think it is probably excessive play between the balancing beam and front master cylinder’s push rod (check out the drop down above for a little explainer about these components). This would explain why at low pressures the car is rear brake biassed, whilst at higher pressures, the increase in foot travel takes up the excess slop, which moves the brake bias to a safer and more desirable front brake bias. It would also explain the high level of variance in the front vs rear brake line pressure graph, but this could also be explained by play somewhere else in the pedal box, so a good examination is needed.
Pedal box play isn’t the only thing that can cause a high level of variation, failure in any of the braking parts could. Typically if you have a higher levels of variance at higher pressures you have a component failing somewhere, it could be brake lines expanding, brake pads or callipers deforming, the master cylinder not working correctly etc. At lower pressures it is more likely play within the pedal box or something sticking, master cylinder or brake callipers pistons not sliding correctly. The higher pressures are enough to unstick the pistons. As with all things, start with the easiest and quickest thing to check, leaks, pedal box play, brake lines etc.
Driving with inconsistent balance
To have a look at the impact inconsistent braking balance can have on a driver, let's have a look at how the driver of the second graph navigated the slowest corner on their fastest lap. As the driver begins to apply the brakes for the corner, the car has a front wheel bias of 42.47%, the rear brake line pressure is increasing faster than the front. The driver continues to increase the brake pressure until the right rear wheel has a slight lock up (a sign there could be too much rear brake bias), they immediately release the brakes slightly and the wheel is no longer locking. They keep the pressure constant for about 6m, at this point the car’s braking bias flips to the front brakes. As the driver starts to decrease the brake pressure the front wheel bias continues to build, reaching a maximum of 53.3%, the brake bias has changed 11% in this one braking zone. The driver continues to release the brakes and at the same time they are increasing steering angle. They have almost completely come off the brakes but needed to add another correction and reapplied the brakes. This brake correction came just before peak steering angle, right when you would least want it to. To make matters worse, the brakes decided now was the best time to shift their bias again, the car is now rear brake biassed again. Lucky for this driver they didn’t need to apply the brakes too hard, as they were in the danger zone, high steering angle, rear brake biassed, and a little bit of brake is a recipe for a spin (see another example of this issue in my post here).
As you can see, inconsistent braking bias makes the car a handful to drive, reducing driver confidence and with it, lap times. But most importantly it increases the risk of drivers getting injured, something none of us want. This simple check can help you find and eliminate any issues you might have in your braking system, helping to decrease both lap times and the likelihood of accidents.
This brings us to the end of this post, but isn’t the end of our braking series, we will be back shortly with another instalment. If you can't wait, check out our other articles on this site below and above. And don’t forget to sign up so you don’t miss out on any of our upcoming posts. If you have any comments, suggestions, or noticed I made a mistake, write it down in the comments or get in contact via email or using the contact page, I’d love to hear it. Thanks for reading and I hope you have a great day.
By Trav Mays
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