Allow me to jump in here to expand upon two interrelated (yet substantially different) points that are often missed in a brake balance discussion:
1. Mechanical Balance: While it is important to get front/rear brake balance dialed in for a specific vehicle, there is no "optimum" mechanical balance -- only trade-offs. What I mean by that is the theoretically ideal F/R balance for a stock vehicle on stock springs and dampers is no longer ideal for the same vehicle that has been lowered and rides on stiffer springs/dampers. This is due to the reduction in forward weight transfer under braking on the lowered car. Since less weight is transferred forward in this case, the rear axle can contribute more before rear wheel lockup becomes a concern. This is why race cars have two separate master cylinders and a balance bar that can be adjusted by the driver. As the fuel load decreases, the car's weight balance shifts and the driver adjusts for those changes.
So in order to get a street/track brake system ready to be sold to the public (as opposed to a pure racing system that is fined tuned by engineers at each individual track for a chosen suspension setup and a driver's particular style of driving), a mechanical balance is chosen that is often slightly more aggressive than the OE system. But mechanical balance can't be shifted excessively rearward or it would be unsafe for some drivers with some vehicle configurations.
By the way, "balanced brake systems" are nothing new. In fact, AP Racing has been doing this for nearly 100 years (originally under the Automotive Products plc parent). But as important as mechanical balance is, it is only the starting point -- not the complete end goal.
2. Thermal Balance: This is the art and science of getting each axle to contribute to the best of its ability while maintaining a predetermined temperature range. Although many of us prefer a linear response from our brake pads (although some don't!), the pad/rotor temperature has a lot to do with the way the pad responds. If we are using the same pad compound on both front and rear, we would want the temperature of both the front and rear rotor to remain in the same range. Identical F/R temps are nearly impossible to achieve for a variety of reasons, but the goal is to stay in the sweet spot at all times -- on the street or on the track.
To achieve optimal thermal balance, often special thinner rear rotors need to be made. If they are too thick and/or heavy, they will run substantially cooler than the front and won't contribute to overall brake balance the same in all conditions. That is, the system may be well-balanced when cold, but could start to become out of balance as temperatures increase. This is not ideal at all as the front pads might be in the middle of their sweet spot and the rear pads could be running too cold and not doing the job as well as they need to be doing it.
When considering a 4-wheel brake upgrade (which I recommend serious 370Z owners do), check on the thickness of the rear rotor. If it is 28mm or 32mm instead of 24-26mm, it was probably originally designed as a front rotor that was later adapted to rear applications. It is often found paired with a revised front caliper, now on the rear with smaller pistons. Yes, it can be set up for a theoretically correct mechanical balance, but it will run a lot cooler than the front due to a rotor and caliper that are over-sized for the job at hand. Also, the rear brake system will be a lot heavier than it needs to be -- another unfortunate trade-off.
One example: Thermal balance concerns are the primary reason that development of the carbon-ceramic system on the R35 GT-R led to a front cooling system only. After brake temperatures were monitored during a variety of street and track conditions, it was discovered that the front brakes needed cooling, but the rears were fine. In fact, adding cooling to the rear brakes would have hurt overall brake performance as they would almost always be running too cold.
Are rotor size, number of pistons and piston diameter important? Certainly! But just as important are the thermal properties of a brake system. This is where years of experience at the top level of motorsports pays off. Sure, the products may end up being a little more expensive (although sometimes not by very much!), it is always worth it to go with a system that is derived from decades of experience with extreme duty applications.
The magazine article I referenced above gives a good indication of what a brake system is capable of if it is balanced -- both mechanically and thermally.
Chris Bernal
STILLEN / AP Racing
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