[GarySheehan]

Originally Posted by Dean

I am going to concede the bigger disks are better for all the reasons you listed. I think I have been biased in favor of width since my wider/flat/directional vaned but same diameter kit on my Stealth was so much superior as compared to the not as wide/cross drilled/directional vaned but larger diameter kit that I put on my A4. I realize they are completely different systems, and it is likely that one is just a better fit for the vehicle than the other, but it has warped my impressions.

My question to you on that would be all other things held constant; would you prefer 10% more diameter, or width? OK, maybe this isn't easy...

You describe the Stoptech front only upgrade system for the WRX as shifting 10% bias to the rear. With a bigger diameter rotor, and I assume a higher coefficient of friction pad than stock supplied with the kit, the only way I can see this could be done is with significantly less piston area than stock. Do you have the stock vs. Stoptech piston area and piston circle diameter? I just don't recall ever seeing an aftermarket brake kit company selling a kit with less piston area than stock.

Dean, I am glad you are willing to concede. Acknowledgment of denial is the first step in recovery!

To answer your first question, I would always take 10% diameter over width if I could fit it under my wheel. While a wider rotor will provide additional cooling, a larger diameter rotor generates less heat to begin with. Less heat generated is less heat to shed.

You said you don’t recall ever seeing an aftermarket brake kit company selling a kit with less piston area than stock. That’s because you’ve been looking at kits that are not sized properly to the application, like the kit that went on your A4. If it had larger front pistons and larger diameter front rotors, it moved even more braking bias to the front of an already front biased brake system. The end result would be a system that locks the front brakes WAY too early as well as have a longer brake pedal and lighter pedal pressure. Nasty.

The stock Subaru WRX calipers have pistons that are 42.5mm in diameter per my calipers. The Stoptech ST-40 calipers have a leading edge piston of 36mm and a trailing edge piston of 40mm. Here is an aftermarket brake kit company selling a kit that is properly designed for the application with larger front rotors and smaller front pistons.

Originally Posted by Dean

Now on to the theory stuff.

I don't know your background, but for the most part, I come from a geeky science/physics/engineering background. Often in that world, they take things to extremes such as my Infinite CF pin and hole example. I realize it isn't how brakes normally work, but it serves a purpose.

My college degree is in electrical engineering. I have had plenty of physics courses thrown in as well. I will not discuss theory with you on how brakes work. I will tell you how brakes work. There will not be any need to discuss an infinite CF pin and hole example. It does not apply. If you have questions on my explanation, feel free to ask. But don’t bring theoretical extremes into the discussion.

Originally Posted by Dean

You said "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."

My example was to address precisely the transients you mention.

STOP. There are no transients in the coefficient of friction between the brake pads and the rotors other than the direct input of line pressure from the driver. The transient between dynamic friction to static friction between the brake pads and rotor is IRRELEVENT because the tire has been “locked up” for a considerable and measurable amount of time before the wheel actually stops rotating. I will explain.

The tire stops the car. A tire has a limited amount of adhesion while rolling. As you add brake pressure and use the tire to slow the car, you approach the limit of the tires adhesion. As you continue to add brake pressure, the tire will actually start to rotate slower than road speed. Just as this starts to happen, maximum tire adhesion has been reached. As you further add more brake pressure, the tire continues to slow in relation to road speed. As the delta between actual road speed and the rotational speed of the tire increases, the tire loses adhesion as it transitions to sliding friction. It will get to a point where the speed difference between the road and the tire becomes so great that the friction between the tire and the road is the same as if the wheel were actually stopped, even though the tire is still rolling. For all intents and purposes, the tire is locked because minimum friction is being generated between the road and the tire.

Throughout this entire process, the coefficient of friction between the pads and the rotors HAS NOT CHANGED. There is no instantaneous transient of dynamic friction to static friction between the pads and the rotors. It is a constant dynamic friction between the pads and rotor based on increasing brake pressure and decreasing tire/road friction. When a wheel has finally locked (i.e. – stopped rotating) it is very late in the game and traction between the road and the tire has been long gone for quite some time. The difference in the coefficients of friction between a wheel rotating at 5mph in dynamic friction versus a wheel that has stopped rotating in static friction is absolutely negligible when the road speed of the vehicle is 100mph. The part that you have to understand and accept is that the driver has control of the rotational speed of the tire from full roadspeed to full lock, regardless of the speed of the vehicle. The reason it is practically impossible to control that is because the friction between the tire and road rolls off VERY fast as the tire begins to rotate slower than road speed. It has NOTHING to do with the transition of the friction characteristics of dynamic friction vs. static friction of the pads to rotors because a static state has not been reached.

In summary, the friction curve between the tire and the road will drop dramatically as the tire slows below road speed and becomes flat from a given rotational speed all the way to a fully locked wheel sliding along the pavement. The friction curve between the pads and the rotor stays very consistent throughout the speed range of the rotating tire. There isn’t a transition in the coefficient of friction between the pads and the rotor that would CAUSE a tire to lock, other than driver input.

Originally Posted by Dean

My pin/hole example is the ultimate example of a bad brake system as it is nothing but a single infinite transient. It instantaneously applies an infinite brake torque through an infinite coefficient of friction as the pin stops the rotor. At that instant, the torque curve is a straight vertical line from 0 to infinity as the "brake" is applied. Some non zero time later through the flexible lever that is the tire, the tires torque curve follows curving initially as the tire gives way until it goes into a skid as the sidewal etc. runs out of elasticity.

No, the worst braking system in the world works the same way because the tire/road interface rules all. Your infinite transient does not apply. The tire will always reach it’s minimum traction long before the tire actually stops rotating. Continuing to add brake pressure will speed up when the wheel actually locks.

Originally Posted by Dean

I realize it is extreme, and not remotely real world, but IMHO it shows how a brake system that has transients, or is "grabby" for whatever reason can force a tire lockup earlier than necessary.

It’s not extreme. It doesn’t apply. You are not understanding the friction curve between the tire and the road. In addition, we are not talking about stomping on the brake pedal as hard as humanly possible as an effective braking method. Neither of these scenarios are applicable to describing brake systems.

Originally Posted by Dean

This is where I got into the ABS making up for a bad brake system and where Scott came up with the idea of the ultimate ABS system that would run my pin/hole brake system in which the tire would end up absorbing close to 100% of the energy of stopping. Again, a ridiculous example, but educational none the less as we see that the energy must still go somewhere and how an excelent ABS system can improve a less than optimal brake system.

NO! The ultimate ABS system would keep the tire rotating just slower than roadspeed. The on-off nature of the ABS system would cycle the brakes between road speed and a tiny bit too slow for maximum traction. The average would be a tire rotating just below roadspeed in the maximum adhesion window of the tire.

Your example using the pinhole brake system would alternate between near maximum adhesion with the tire at road speed to absolute minimum adhesion with a locked wheel. That average adhesion would be DRASTICALLY lower than the average ABS system in production today. Today’s ABS systems do not sense lock-up. They sense a speed delta in the tire to itself and to the other tires, but the tire is always rotating.

Are you seeing why the pin and hole analogy doesn't work?

Originally Posted by Dean

I think we both agree that a huge amount of money has gone into pad formulation to minimize spikes, enhance release, provide smooth coefficient of friction curves, etc. But you can still by something close to a cow turd at your local Kragen and put it into your stock caliper with a single small pot and small pads that closely resembles a C clamp. Even though you can probably lock up a tire and/or get into ABS with this turd and C clamp brake system, I think we would both agree a better system can stop your car faster.

Yes, but a cow turd will still stop your car with the same principals. Brakes do not exhibit a dramatic increase in friction between the pads and rotors as rotor speed decreases. Your car comes to a smooth stop with the application of constant brake pedal pressure. It does not have a sudden jerk of deceleration as it approaches zero speed. Do not confuse the rebound of the stored energy in the springs once the car reaches a stop as a sign of increased braking force.

Originally Posted by Dean

I agree a good ABS system can stop a car faster than most humans, but it remains to be seen if the WRX system falls into the category of "sports cars and performance cars" you describe. I was also referring to overall vehicle balance at the time when I said getting into ABS was not necessarily fast. IMHO, standing the car on it's nose every time you hit the brakes is not always the best way to get around the track.

The WRX does not fall into this category. It’s ABS system is not the best.

Standing a car on it’s nose every time you hit the brakes IS the best, fastest way to get around the track. Regardless of being at maximum braking with ABS or maximum braking on your own, you should always be braking at your maximum in the braking zones (except while trailbraking). ABS will actually compensate for an unbalanced system by allowing all four wheels to do the maximum amount of work possible.

Originally Posted by Dean

I do not have first hand experience with this, and weld was probably a bad word, but I have heard tell of where probably the binding agent in the compound in the pad became fused to the rotor and required significant force to be dislodged.

You can transfer pad material from the pad to the rotor. It’s called pad deposition and it occurs when you apply a very hot pad to a very hot rotor when the rotor is stopped. But this is a result of a stopped wheel, it does not cause a locked wheel by welding itself to the rotor when the wheel is rotating.

Originally Posted by Dean

It is really hard not to get defensive about some of this, so bear with me...

To the best of my knowledge, everything is compressible to some extent, rotors, pads or diamonds. Substances do not have to be molten to do so. Unless I am mistaken, the modulus of compression for steel is 1/160x10^9 N/m^2. Yes, this is really small, but it is not zero. Brake pads also have a non zero modulus of compression which undoubtedly varies based on compound. All of these materials are also subject to thermal expansion, but that is a whole other topic.

This compression, along with the deformation due to flexing between the vanes is probably the reason for the Jutter you describe, and some of the transients you see in some brakes.

Every friction example I can think of has the potential for more heat at the leading edge then on the remaining friction surface. That is usually due to some material compression and/or sloughing off of one or both of the materials as the leading edge molecules crash into the oncoming molecules of the surface it is in contact with.

There are a ton of things that go into the design of a caliper and the rest of a brake system. I would guess that reducing leading edge pressure and therefore temperatures are contributors to this design decision in addition to the debris issues you describe.

Okay, so don't get defensive of your theory. Just accept the facts I am about to give you. I have explained that differential piston sizes are there to compensate for the debris field that forms along the pad surface. Additional force on the trailing piston is required to force the pad through the debris field and make the entire surface of the pad usable. This is not a theory of mine. It is what I have learned directly from brake manufacturers, brake designers and brake engineers. If you don't agree with what I have stated above, stop theorizing and start talking to the people that design and build brakes.

Originally Posted by Dean

I think we are splitting hairs trying to differentiate between a softer and lighter pedal. Perhaps I was using a specific braking term incorrectly, but we appear to have meant the same thing, pedal pressure, not the distance the pedal travels.

I was not splitting hairs. There is terminology in the braking world that are used to describe the characteristics of a brake system. A soft pedal and a light pedal are two different things. Regardless of what you implied, it’s important to clarify this for the other readers of the thread, because what you described was not what you were experiencing.

Originally Posted by Dean

And when I basically said bigger everything improves braking performance, I was referring to a single brake, not the entire car where bias and other issues come into play.

But you can’t speak about a car’s four brake system in those kind of generalities. They don’t work that way.

Originally Posted by Dean

To be honest, I had not considered the possible feel issue with increase in piston area. On reading another article, it appears drivers are better at modulating to a point with a firmer pressure (non light) pedal. Would you agree with this?

The driver’s ability to use the brake system effectively is critical to the operation of the brakes. This thread is discussing improving brake performance and interface to the driver is very important. Yes, a pedal that requires higher effort is easier to modulate, as long as it doesn’t require so much pressure as to be fatiquing. Think about it, with your hand on a scale, is it easier to control a range from zero to an ounce or from zero to 10 pounds? It’s much easier to select any point between zero and 10 pound than between zero and one ounce. However, if you changed it to control between zero and 100 pounds with your hand, it isn’t feasible.

I’m ready to answer questions you may have on any of the above.

Gary
Sheehan Motor Racing
www.teamSMR.com