iRacing GTP tuning series — 5, Brakes
Ok, let’s talk about brakes this week. It is an important part in terms of performance, as well as handling and driveability. Awell tuned brake settings will bring more consistent lap time over a full stint, and reduce lockup moments when it matters. Let’s get into it.
Brake settings in iRacing
As usual, let’s get the iRacing settings cleared so that we are all aligned on what we are going to discuss.
1, Cylinder size
This is the most important setting in iRacing brake tuning. Basically the smaller the cylinder size is, the more brake force it could supply. The front and rear are independent. Generally speaking, you would want either equal front and rear, or front being 1 click smaller(providing more force) than rear.
2, Pad compound
It’s a shame that iRacing has not introduced the pad changes over their endurance races. There are 3 pad compounds, from low to high. They provide different friction. But basically with tuning the cylinder size, we could ignore this and leave it in Medium.
3, Brake Bias
This is a very common thing across all race cars. The bigger the number is, the more brake force is applied to the front.
Note that this is from the master cylinder, not the actual brake force applied front to rear — meaning, if you’re using equal front to rear cylinder size, the bias(55% is the ballpark) will be way more forward than a front one click smaller set(50% or even 48% is the ballpark).
4, BMIG
This is a fancy feature in GTP(and F1). What it does is to dynamically change the brake balance across the range of brake pedal input. If remembered correctly, for example, a base 52% brake bias with a -2 BMIG would equal to 52% bias at 100% brake pedal and 50% bias at 1%(0%) brake pedal.
Note that it’s not necessary to use this for a good brake feeling, as the available grip front to rear does not follow exactly the same trend as the BMIG says. It is possible to tune the car without BMIG(keeping it at 0). And a more common application is to deliberately using negative BMIG to create more rotation while releasing the brake pedal.
5, Brake target
Another fancy feature. This can quickly offset the overall brake settings. By saying quickly, I meant that the brake bias can do the same thing, but it takes 4 clicks to move 1% whereas this brake target is 1 click for 1%.
I never used this but one possible application is the magic button: if you really want min/max on the formation lap, this brake target could quickly move the bias to the rear to evenly heat up front and rear tyres under braking. Then revert it quickly before green.
With every setting explained, let’s get into the procedure of brake tuning.
Cylinder size and max brake force
Apparently, deciding how much max brake force you will need without sacrificing performance is the first step.
To achieve this scientifically, we will need to use telemetry: we should check your practice laps, and if there is the 100% brake pedal for a long time(over 0.3s) at each heavy braking zone, that means you are actually losing time under braking as the car is not decelerating as quickly as it could.
So reduce cylinder size by 1–2 clicks and check this again. Ideally you want it still hit 100% occasionally, but not having a 100% brake pedal plateau for too long.
And on the other hand, if you never hit 100%(or even worse, not hitting 70%) for most of the time. It actually means the brake cylinder settings are too small(brake force too much) and it’s better to increase cylinder size. The reasoning behind is that, you want to utilize all your brake pedal travel, as that gives you more range for precise brake modulation.
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Some may argue that they “feel” stronger brake settings(small cylinder or higher pads) could help them stop the car quicker. But when the telemetry is pulled, it’s quite often that these guys had never hit 100% brake pedal at all, or even worse, not having threshold braking at all: when you brake at only 50% of the pedal travel, of course the more aggressive brake setting would provide shorter braking distance, given that both settings are not locking up.
And one tip from most iRacing high downforce templates: they are often slightly conservative on the brakes, meaning that you will often find the 100% brake plateau in your telemetry. I often reduce cylinder size by 1 click before actually building the setup nowadays.
Brake bias, front to rear cylinder size
The other critical process is to find the correct brake bias for your specific setup(spring rate, static ride height and even LS dampers).
We will use the lockup indicator in the car as our main tool here. As you brake, in these GTP cars, there are several led stripes showing how much each tyre is locking up.
1–2 led often means minor lockup(which is actually what you want for threshold braking).
3–5 is a medium lockup, others will see smoke coming out of your tyres. And you will possibly miss the apex, if not off track.
A full bar is a really bad lockup. You will leave tyre mark on the tarmac and people would know that there was someone running off track earlier(and possibly hit the wall and towed.)
With our reference established, the procedure is quite simple: first you should try brake hard without any trail braking. The goal is to simultaneously light up the front and rear by 1–2 leds. If the fronts are having more leds than the rear, move the bias backward. And vise versa. After several laps, you should be able to get a good brake bias for straight line braking. (And also understand whether your’re using 100% brake pedal briefly or not for the cylinder size tuning mentioned above).
This would be a great brake bias baseline, it has the best braking efficiency. However, for a race tune, we will have one more step:
Now you want to reduce(move to the rear)the brake bias slightly, to the point that when you’re threshold braking, the rear leds would light up 1–2 without any front leds light up.
The reason behind this step, is trying to avoid front lockups in the long run as front lockups are more lethal. And the other thing is, although not hitting the maximum deceleration efficiency, this way of setting it up will help a bit rotation under trail braking, which will bring better handling feel of the car.
A step further of this specific purpose, is to tweak the front to rear cylinder size.
First of all, you want to keep their max difference being 1 click, as there is limited range of brake bias settings(about 40% to 60%). With 2 clicks difference, most of the time it will lock up one end even with the bias on the extreme side: it simply can not compensate the brake force difference.
Then, to the business, occasionally, it is beneficial to use equal cylinder size. With this setup, the rear would generally provide more brake force than those front cylinder 1 click small setups. This will inherently reduce the difficulty of trail braking, the fronts will have less chance to lock up as the rear brake pressure is relatively high across the whole brake pedal range.
This way of setting the cylinder size is good for tracks that require a lot trail braking(such as the Watkins Glen coming next week).
The rain
When it rains, you would want to adjust the brakes(mainly bias and BMIG, unless it is a rain race from the beginning).
The general rule is to avoid front lockups. Thus a 1–2% reduction in bias, combined with -1 to -2 clicks of BMIG should serve the purpose pretty well.
There is a point that getting too far with it and then you start locking up the rears, which feels like a handbrake turn. This could happen, so don’t go too far with the adjustments, especially during a race with changing conditions.
It definitely feels confident to have no front lockup risks before it spins under braking.
So, that’s pretty much about how to setup the brakes. Indeed, there will be more specific tweaks based on how you drive the car and how the handling baseline is. But with these general steps, the brake performance should be close to the car’s potential rather than leaving any performance on the table.
See you next week at the Glen 6hr, good luck and have fun!
