Captain Caveman (great name BTW...I forgot about that guy!
),
As pointed out by others above, it's not just about piston size. To calculate brake torque you have to include the radius at which the clamping force acts upon the disc (known as effective radius). Increasing the effective radius via a larger disc increases the amount of brake torque on a given axle. You also have to take into account brake pad coefficient of friction, which I'll address below. Developing a superior system requires a proper blend of these factors.
You first look at a chassis and consider how it will be used - weight distribution, power, tire choice, aero, etc. You then choose a disc spec for proper thermal capacity and cooling. You consider what pads will be required and used based on the intended operating conditions. Then you dial it in with piston sizing (assuming you can't change master cylinders, which is a luxury we don't have on road cars).
There are several considerations related to piston sizes. The pedal force at the traction limits needs to be within a range where the driver can fine tune the brake torque with ease. If the pedal is too hard to push, one cannot provide precise control when one is straining. If the required force is too light, then a small change in pedal force creates a big change in torque output, which is not good either. The driver needs to be able to adjust the torque output in fine increments. One does not say “I need more brake, I think that I will push the pedal 3 mm farther”. One decides to push a little harder on the pedal, does so, and it moves however far it moves, which can vary.
When we were developing our e9x M3 systems back in 2014-2015, we worked closely with several race teams (
like this one) to collect driver input, and we tried several different configurations of our system. We ultimately arrived at a system that not only performs extremely well, but one that feels amazing under foot. AP Racing also continuously strives to reduce caliper flex via stiffness, and the latest Radi-CAL calipers are by far the leader in that regard. They contribute towards making the system more responsive, consistent in feel, and easier to control.
There is one more tuning factor which impacts brake torque, which is the friction material. The vast majority of our customers are running relatively high mu racing pads, so we designed our system to perform optimally with that type of pad installed. With racing pads our system has fantastic sensitivity (pedal force requirement) and excellent modulation. Not every driver is the same, but the way our system is set up has made most of them extremely happy.
In summary, brake systems aren't just about numbers. Yes, it's crucial that you are in the proper range with the numbers. You can't randomly grab brake bits off another chassis and slap them on the car (which we regrettably see people do all the time
). If you go that route, the car will not only perform poorly, it will feel like sh*t. The numbers are incredibly important, but there are numerous other factors required for creating a system with the proper sensitivity, balance, and modulation.
These are the factors the driver notices and appreciates while pushing the car at the limit.
Here's a video we made that goes into a little more detail on how we approach our designs. I'm the dashing chap on the right.
As an FYI, I was the sales manager at StopTech from 2002-2007, and one of the core people who helped build the company alongside the original owner (Bob Lee). That was long before StopTech was bought and sold several times, went bankrupt, etc. While I was there we developed their system for the e90 M3, the ST-60 caliper, and hundreds of others. I'm intimately familiar with their products, processes, cost structure, suppliers, etc. When we started our AP Racing brake kit program at Essex, we systematically went down a checklist I created, and strove to make our AP Racing systems superior in every possible regard.