Wow, I wish I was involved in this thread a little earlier. I'm going to try and address several points on this page of the thread. I fear it's going to be a little lengthy...
Dean,
The Stoptech kit is only a front upgrade system. The pistons are custom sized to the rotor AND the WRX platform to move brake bias 10% more towards the rear over stock. Stoptech hardly throws bigger parts at anything (unlike many of it's competitors). They design, engineer and extensively test every application before it becomes available for sale. Stoptech uses the same caliper (ST-40) for most of it's applications. However, they machine unique piston sizes for each of the applications. Stoptech has 14 different piston sizes they can utilize to generate optimum brake bias, pedal travel and pad bite characteristics. Not taking piston size into consideration in your analysis ignores one of the more critical factors in a braking system.
BAN SUVS,
Reducing front brake torque is an excellent thing, not a bad thing for the WRX. The WRX comes with too much front brake bias, as do all production cars. Moving some of it rearward helps improve front/rear brake bias, which will reduce stopping distances. With the stock front calipers and rotors, I was able to consistently lock the front brakes with Hoosier R3S03 race tires as well as with front downforce via a splitter. The WRX does not need more front brake torque! There is no such thing as adding more stopping power, unless the original system cannot lock the brakes. What you can do is increase brake torque with less pedal effort. However, when considering a brake system, you do not want a system that generates very high brake torque with very low pedal effort. Systems like that are very hard to modulate at the limit. A well designed system will take some effort to lock the brakes. As long as the effort is not fatiguing, the higher effort provides the driver a wider modulation range, making it easier to use the brakes to keep the tires on the edge of adhesion.
You also mentioned that you have yet to lock the brakes at the track. One of two things is happening. You are using ABS, which prevents the wheels from locking, or you are not braking hard enough. Unless of course you have faded the brake pads and/or fluid, which would make it impossible to lock the wheels after a few laps. But even with R compound tires, you should have no issues with locking the front wheels on the track while the brake pads and fluid are still within their operating temperature range.
Dean,
The Wilwood calipers are known to be some of the most flexible calipers in the aftermarket. See this chart here:
http://www.stoptech.com/technical/ca...ctionchart.htm
Company W is Wilwood. The higher the caliper flex, the worse the pedal feel, because the effort you are putting into the pedal is being wasted by flexing the caliper. Very similar to flexible rubber brake lines. Since this is widely known, I feel confident that the flexing described in this thread above is a normal occurance in a Wilwood caliper, not a hobbled together caliper made of Wilwood components.
You also mentioned that getting into ABS is not necessarily fast, and is often fairly slow. This is entirely dependant on the ABS braking system and you will find that in the higher classes of sports cars and performance cars, the ABS systems are excellent and hard to outperform. They react extremely fast and keep the tire rotating at a speed that provides optimal adhesion.
Your soapbox conclusions are mostly inaccurate. The trailing piston is larger to counteract the debris field and slight outgassing that starts developing at the leading edge of the pad and continues along the length of the pad. More force is applied to the rear of the pad to counteract this and provide even pad wear.
The melting point of iron is 2,800F. Brake rotors will glow at 1,200F. Brake rotor iron does not become maleable until it reaches the 1,700F - 1,800F degree range, however, it will undergo a loss of tensile strength at 1,600F, but will not deform. There is a phenominon known as judder associated with straight vaned rotors, where the leading edge of the pad will set up a pressure wave between vanes and will cause slight pedal kickback, but this is cured by using a curved vane rotor, which also improves cooling efficiency.
The majority of OEMs and many aftermarket BBK vendors will use rotor iron with a tensile strength of 18,000psi. This is also known as dampened iron and is used to reduce rotor noise and decrease NVH issues. This is a softer iron. Stoptech, as well as the top racing brake rotor manufacturers use iron with a tensile strength of 40,000psi, which dramatically increases it's strength all the way up to the melting point, in which case both rotors turn into puddles.
Saying that piston size differences in the caliper is designed to eliminate distortion of the rotor is completely inaccurate.
You used NASCAR as proof that larger rotors are not required. This is an unfortunate example, since on many race tracks that NASCAR runs on, they never even use the brakes. The answer to your question is yes, many weekend warriors will dramatically exceed the demands put on the brakes over NASCAR drivers. Other tracks in NASCAR, like Bristol, put a huge toll on the brakes. The cars use a much wider rotor on those tracks, but do not use a bigger diameter rotor because of NASCAR mandated wheel diameter and rotor diameter. Using NASCAR as a technological example is horrible because their rules have locked them into very old technology. The formula for NASCAR is not to produce the fastest, most technological racing in the world. It is designed to produce the closest, most cost effective racing possible.
That is why true road race cars, like the Daytona Prototypes, ALMS Prototypes, Indycars, Grand Am GT and GS and such use huge diameter rotors. These cars are much more technologically advanced than anything you would find in the NASCAR series and they all use big rotors. I'll get into why larger rotors are better at the end of this post.
On to the next point. When all four tires lock simultaneously after gradually increasing brake pressure without any steep transients, you can say that the brake system is perfectly balanced. You mentioned the difference between locking the brakes before reaching the maximum traction of the tires. This is wrong. Your example of an infinitely hard pin example, first of all, doesn't work they way you explain, and secondly is completely irrelavent to how brakes work.
A brake system will apply more and more force to the pads, creating greater and greater friction, slowing the wheel and tire combination more and more. There is never a case where the friction coefficient between the brake pad and the rotor will instantaneously jump to a level to suddenly lock the brakes. Brake systems do not work that way. There is never an instantaneous on-off. There is always a ramp. You cannot weld a pad to a rotor.
The state of a rolling tire versus a locked tire is dependant on the coefficient of friction between the road surface and the tire versus the braking force applied to the rotor based on pedal pressure. It is a linear progression that rolls off as the tire adhesion is exceeded. As a matter of fact, maximum braking occurs when the tire is rotating just slightly slower than road speed.
The characteristics of how quickly the coefficient of friction between the pad and the rotor increases is dependant on pad material. Some pads can come on very quickly, others have a more gentle ramp. NONE of them will lock a wheel prior overcoming the coefficient of friction between the tire and the road.
Furthermore, brake pads are not designed to transition from dynamic to static friction. They are designed to provide a stable coefficient of friction in a given temperature range and pressure range. Quite a bit of effort has gone into the "release" characteristics of a pad, but that is as you are coming off of the brakes, not adding pressure to the locking point. Given a stable braking system regardless of bias, locking of the brakes is a driver error.
You mentioned that bigger brakes with bigger pistons create a softer pedal. This is not true. Larger pistons will create a longer pedal, because you have to displace more fluid before the pads touch the rotor. Once the pads do touch the rotor, the pedal movement is based solely on the flexibility of the system. A very stiff system will not have a softer pedal with larger pistions. Finally, with a larger rotor and larger pistons, it will require less pedal effort to generate the previous brake torque. This is not a softer pedal, it is lighter pedal effort.
In your closing comments, you said that more pad area, better pad compounds, more piston area and hating to admit it, larger rotors contribute to better brake performance. As written, this is inaccurate. What you are saying is that if you increase the size of everything, your braking performance will improve. Absolutely not! What you've done is drastically increase the brake torque, which may not be a good thing for brake bias or pedal feel. You have to take everything into consideration to design an appropriate brake system for an application.
If you increase front rotor diameter, you will need to reduce piston size to prevent too much front brake bias. If you just increase the rotor size front and rear, the original size pistons may be too big, creating a brake pedal that is far too light and difficult to modulate. All of the components that make up a brake system must be carefully analyzed when making changes, and an increase in one parameter may require a decrease in another parameter.
Finally, I want to explain why a larger diameter rotor is better, even though you don't like the idea. A larger diameter rotor requires less clamping force to generate the same brake torque as a smaller rotor, since the torque is applied further away from the center of the wheel. Less clamping force means less heat is generated per square inch over the face of the rotor. Less heat is a good thing. Also, the air speed in the vanes of a larger diameter rotor is faster than a smaller rotor, since the larger rotor is passing through more air per revolution, allowing it to draw heat away from the rotor more quickly. Smaller rotors are built wider to help flow more air, but they are also built with thicker steel, because brake temps will be higher (due to higher clamping forces required to slow the tire) and the additional mass is used to strengthen the rotors to run at the higher temperatures.
One of the major benefits to a larger rotor is the ability to modulate the brakes closer to the edge of the tire's adhesion. This is because there are more rotor inches passing through the caliper. This gives you higher "resolution" with the larger rotor. It makes the brakes easier to modulate, keeping the tire consistently closer to the limit of adhesion.
To your credit, you did mention that the tire stops the car and the tire's characteristics are very important. Somehow along the way, the information provided to you led you astray and you contradicted yourself several times as well as used analogies that do not relate to braking systems.
Please don't be offended by any of my comments. You said yourself you are not a brake expert and can stand to learn a few things. I am not a brake expert either, but 11 years of roadracing experience and working closely with brake manufacturers have provided me with a solid working understanding of brake systems.
Let me know if you have any questions.
Gary
Sheehan Motor Racing
www.teamSMR.com