Lots of Info Here
It's been a while since I've posted, it seems other daily tasks here keep me away from the front lines...
Let me first start by saying "I'm not a brake engineer, but I play one at work every day." That being said, I do interact each day w/ Steve Ruiz, our Engineering Manager, and a well respected expert in the brake industry (racing and OE)
Here are some basic things regarding a lot of what is being stated in this thread.
Balance is critically important to optimal brake performance, that is to say, the amount of brake torque being applied to the front wheels needs to be optimally proportional to the amount of brake torque in the rear. Each vehicle has an optimal front-to-rear brake torque balance, which can change with variables such as suspension, tires, aero down force etc. I'll not so much address the non-brake variables; just presume all is equal except the brake components for this discourse. If the balance is not optimal, either the front or rear tires will not be doing their fair share of stopping the vehicle. Too much front brake and you'll see the tree through your windshield before you slide into it; too much rear brake and the tree will be in your rear view mirror when you hit it... If you lock the fronts, you have a chance to let off and recover. If you lock the rears, you will likely have the rear end step out making recovery less likely. This is why we (and OE brake engineers) always have a bias toward the front.
Notice I used the term "brake torque", not “brake force”, “clamping load” or similar term. Brake torque is arrived at by calculating the effect of several variables: System Pressure, Piston Area, Co-Efficient of Friction (Cf) of the Pads, and Effective Radius of the Clamping Force. System Pressure is the output pressure from the master cylinder, which varies with pedal effort. Piston Area is the area of the caliper pistons; larger pistons means a given system pressure will exert more clamping load on the pads/rotor. Cf is how much bite a particular pad has. Different pads have vastly different Cf's and can vary with temperature and condition of the bed-in of the rotors and pads. Effective Radius is the radius of the centerline of the pistons on the rotor; for simplicity sake, call it rotor diameter and presume the centerline of the pistons is basically the same distance from the outer edge on different set-ups. Change any of these 4 variables and you change the brake torque being applied. Only the system pressure is a constant front-to-rear (presuming a single bore, stock master cylinder); Piston Diameter, Effective Radius and Cf can all be changed on just one end of the vehicle. Here is a link to an article on the Technical Page of our website with more details on the formulas:
Formulas for Vehicle Braking Dynamics
Let's presume the Cf of the pads is a constant, and look at caliper piston and rotor sizing. If you use a caliper with a larger total piston area, you will increase the brake torque. One of the scary things being talked about is using front calipers in the rear. Front calipers typically have larger piston areas then rear calipers, so putting ANY front caliper in the rear will dramatically increase rear brake torque. An ABS equipped vehicle may manage it (not very effectively) and keep you straight, but lose an ABS sensor coming out of a corner and the next brake zone will get very exciting... A larger caliper in the rear won’t only be working at say 20% of its capacity, it will be working more like 50% OVER capacity for what the rear tires need. The system pressure is the same; the caliper is gripping the rear with the same force as it would the front, which is too much for the rear.
So what about upgrading all the calipers to larger piston sizes in equal proportion? If you use the same master cylinder, you will increase pedal stroke to consume the extra piston volume and when system pressure is applied, you will have more brake torque at all corners. “More brake torque!”, sounds great, but what does it really mean? It means the pedal is more sensitive once you use up the extended pedal travel and the system will be more prone to lock-up (or ABS intervention) making modulation of the brakes less manageable. You could do the exact same thing by changing the master cylinder and keeping the original calipers. The limiting factor is really the Cf of the tires. If you can lock them up with the stock calipers, you will only be able to lock them up with less effort with the larger calipers, you won't stop any shorter. It should also be noted that the number of pistons has nothing to do with brake torque either. An eight piston caliper with smaller pistons will feel the same to the driver and perform the same as a 4 piston with an equal total piston area (less larger diameter pistons). The only advantage the 8 piston may have is a larger pad area meaning more heat capacity.
So why upgrade? Heat capacity is the most compelling reason. You need to take all the kinetic energy of a 3000# object moving at 150mph and convert it to heat through a set of pads with less area than your computer keyboard (think about that next time you complain about pad life…) The pads transfer the heat to the rotors, which serve as the main means to shed off the heat. Note I'm being somewhat simplistic in terms of how the heat is dissipated, suffice to say, the pads and rotors work together to manage the heat and the calipers absorb some heat as well. Larger rotors manage the heat better. Better airflow through the rotors also dissipates the heat better. Larger pads do their share as well. On a given deceleration event, from a given speed, with a given vehicle mass, a specific amount of heat will be generated. That amount of heat will be virtually the same no matter what the brake system components. The key is how much heat can the system absorb or shed before the next deceleration. A lower temperature at the start of the brake zone means the total kinetic energy controlled by the system will result in a lower system temperature at the end of the braking zone. It all adds up and all needs to be shed off; hence larger rotors, better air flow, nice brake ducts etc.
Notice that nowhere in the brake torque formula is pad area taken into account. A larger pad has no effect on brake torque, so a larger pad does NOT provide any better braking, except in terms of heat management and pad wear. In theory, a brake pad the diameter of a quarter, with the same Cf and effective radius of a larger pad will give the same brake torque output. Of course the pad will immediately overheat and the Cf will fall to zero, but at the moment of brake application, it will perform the same as a larger pad. Here is as good a place as any to say that balancing a brake system with different pads is a band-aid at best for a poorly designed system. Add the complications of the pads heating at different rates and ending up at different temperatures front and rear and you can really start chasing the balance. A well balanced system should allow some choice of pads with a manageable effect on brake performance. If you must run a more aggressive pad on one end or the other to keep the system in balance, something is not right.
Increasing the rotor diameter will not only give more heat capacity, it will also increase brake torque as you move the caliper further away from the centerline of the axle. That means in order to maintain balance, a larger rotor would need smaller caliper pistons. Just another variable we take into consideration when designing a brake kit.
The one way around caliper piston sizing not being optimized is a dual master cylinder set-up. With dual masters, you can change the system pressure front to rear using different master cylinder diameters and also by changing the linkage ratio from the pedal to the individual master cylinder push rods. This is typically driver adjustable, and a great way to fine tune the brakes for varying track and vehicle conditions (tire wear, fuel load, etc) Installing a dual master cylinder is a major undertaking, and typically reserved for dedicated race cars. Most Viper owners stay w/ a stock master cylinder, making caliper piston sizing most critical.
As this is being written on and off over the course of the day, I think it may be a bit dis-jointed. Let me try and re-cap what I'm trying to get across:
A balanced brake upgrade is critical to optimal brake performance. Get the balance wrong and you will increase your stopping distance. Get it really wrong and you can make the platform completely unstable. ABS will mask a lot of sins, but a misbalanced system will still have a longer stopping distance, even w/ ABS (we've proven this in testing numerous times.) Heat capacity is what you are really looking to improve, and increasing brake torque has nothing to do with heat control. Larger rotors, higher heat capacity pad compounds, larger pad area all help with heat control. You need to consider pedal feel as well, and with a stock master cylinder, you are limited in ways to vary that. Larger caliper pistons will make for longer pedal travel and a more sensitive pedal effort. This may not always be a negative if you can adjust the pedal height and like a softer pedal feel, however most oversized calipers result in a loss of effective pedal modulation, especially under threshold braking.
This is not magic, but it is science. Not hard science, except in the amount of time and effort to sort it out. We base our business and reputation on designing, producing and testing our systems so when a customer bolts it on their car, it provides the best balance and heat capacity possible. We get you really close, and individual vehicle variables won't get you too far out of line. This is not to say there aren't other good solutions, but make sure the Laws of Physics aren't being asked to take a day off in order for everything to work. It is not as simple as taking what works well on one car and presuming it will work well on another. You would not take the shocks and springs off a Porsche, cut off the ends and weld on adapters to make it fit a Viper and think the suspension would be better... Why would anyone think that would be true with the brakes? Yet there are systems being sold out there that do exactly that; be it Porsche, Ferrari, or even a Viper with different suspension, master cylinder, ABS etc.
All I'm saying is do the math and/or rely on trusted sources and their first hand experiences. And give yourself a little room to do some testing before you use the system in an aggressive mode.
Here is a link to the Technical Page of our website:
StopTech Technical Page
I hope this helps, not confuses...
Please feel free to e-mail me directly with any questions as I may not be back online for a day or 2.
Matt Weiss
StopTech
mweiss@stoptech.com
