Let's set some context, what is brake bias?
Brake bias is how a car splits its stopping power between the front and rear wheels.
There are two ways that you can mess up splitting the cars stopping power.
- Too Much Front Bias (e.g., 70% Front / 30% Rear)
The front tires do all the heavy lifting.The car becomes very stable and safe in a straight line.It will refuse to turn easily into a corner (understeer). Hard braking will lock up the front wheels.
- Too Much Rear Bias (e.g., 50% Front / 50% Rear)
The rear tires are doing the heavy lifting. The car becomes very unstable, and the back wants to come around to the front (oversteer). Hard braking will lock the rear wheels, similar to pulling the handbrake.
From the factory, BMW wants its cars to understeer as it's easier for drivers to intuitively control. This is not desirable on track. As such, many builders/racers look to shift their bias rearward. This promotes corner entry oversteer, which is faster, and allows for slightly shorter braking distances, also faster. Go to far however, and the results can be disastrous if you're not expecting the rears to lock up.
Here is a summary of what causes a need for bias to move in one direction or the other.
Mechanical Changes That Shift Optimal Bias Forward (and vice versa)- Grippier tires
- Increasing COG
- Shortening the wheelbase
- Moving weight forward in the car (i.e. battery relocation)
- Increasing front downforce relative to rear downforce
Now here's a summary of how we can control brake bias on one axle.
Mechanical Changes That Increase Bias on an Axle (and vice versa)- Higher friction brake pads
- Larger brake rotor diameter
- Larger caliper piston area
- Master Cylinder proportioning valve changes (won't be covered here)
On the BMW 1 series cars, both the 128i and 135i use stepped-bore tandem master cylinders. This means both the front and rear circuits always see the same brake pressure. The brake pedal acts on the first (larger) piston, which transfers a fixed amount of brake pressure (i.e. 80 bar) to the front brake circuit and 80 bar to the secondary (smaller) diameter rear circuit. I've attached a visual that made this clear to me. As such, changing between these master cylinders will not adjust brake bias.
This research brings me to my reason for going down this rabbit hole. After switching to my current braking setup, I've experienced a tremendous amount of understeer and it's obvious why after looking at it. I introduced a massive increase in front brake piston area and rotor size, with only a small piston area increase in the rear plus additional pad friction to counteract it. These changes moved my bias 1.2% more forward than BMW shipped from the factory, so my rear tires are underworked on track. Using a brake force calculator, I've identified complimenting the F3x front brakes with the F3x 44mm rear brakes is a way to shift my bias significantly rearward. I can achieve this while running a lower friction pad, which gives me the opportunity to move bias even more rearward if desired. It's almost like BMW designed these two to work together from the start. This ultimately arrives at a 62% front bias, which is 2.5% less than factory and 3.7% less than my current setup. This is right in line with the M235i race car which runs on slicks, so i expect to be in a safe range.
I considered the 135i rear setup as well, however it didn't provide enough flexibility to shift my forward bias lower than 63.8%. 335i brakes would've been a good option as well however Hawk doesn't make DTC60/70 pads for them.
The main downside to this setup is the weight. In total I've added ~13lbs in unsprung weight to the front and rear axles, which comes with significant performance losses.
I'm using the adapters from VYS garage to mount the calipers, DTC60 pads, rotors from an e60 that allow me to keep the handbrake, and junkyard calipers. All in cost will be ~$1000.