Choosing brake pads for daily and track

[JRitt]

Originally Posted by CSG Mike

Actually, sort of yes and sort of no. Multiple factors here.

- Cooling capacity (surface area and airflow) is increased with most BBKs. Most BBKs have more surface area via increased rotor thickness and more vanes.

- Thermal capacity (rotor mass) is decreased with most BBKs. The brake rotor is a heat sink that absorbs the kinetic energy which is turned into heat under braking. The rotor absorbs the heat from the pads, and then releases it into the air. If you have less rotor mass, the same amount of energy (heat) is still going into the rotor, so you actually run the rotors *hotter*

Almost every BBK for this platform increases cooling, but decreases thermal capacity. You don't see increased thermal capacity until you get to to 325mm 1 piece rotor BBKs, or 355mm 2 piece rotor BBKs.

Any BBK that advertises weight reduction, is a decrease in thermal capacity. You have to find a balance between cooling and thermal that is right for you.

Mike,
You're not capturing the whole picture with this assessment of discs.

It's not just about the total mass of the disc (the number you see when you drop it on the scale). The most important mass is that in the immediate vicinity of the friction area. That also includes the vanes since they do an excellent job of transferring the heat to the air, which is absolutely critical. The mass of the bell and hub flange are of little help with respect to heat dissipation, and on a one piece disc much of the disc mass is in that area (dead weight). The more relevant question is how much mass is in the swept area, which includes the wall thickness, number of vanes, radial depth, etc.

Cooling ability is critical to assessing the disc's overall thermal capacity. If you have a system that can transfer the heat to the air quicker/more efficiently, you can have less effective mass (that between the friction faces) and achieve no higher of a peak temperature on the track. That means a well-designed smaller disc is not necessarily running hotter than a larger disc with a less efficient design.

Finally, a proper floating disc can go to a higher temperature than a one-piece disc without having some real issues with staying flat or being over-stressed, since the outer ring is free to expand as needed as it increases in temperature. A good racing system (discs assembly, pad, and caliper) can operate consistently at higher temperatures and still perform admirably, because it is designed to do so.

With an efficient design, a given BBK that provides a weight reduction can most definitely absorb and shed far more heat than brake systems with larger discs and less-efficient designs (fewer vanes, thinner disc walls on the friction faces, mass concentrated in the hat/flanges). Our AP Sprint and Endurance Kits have proven that over, and over, and over again on tracks all over the world (as have our BBK's on other platforms such as the M3, Corvette, etc.). Bigger is not better. Efficient is better!

[CSG Mike]

Originally Posted by JRitt

Mike,
You're not capturing the whole picture with this assessment of discs.

It's not just about the total mass of the disc (the number you see when you drop it on the scale). The most important mass is that in the immediate vicinity of the friction area. That also includes the vanes since they do an excellent job of transferring the heat to the air, which is absolutely critical. The mass of the bell and hub flange are of little help with respect to heat dissipation, and on a one piece disc much of the disc mass is in that area (dead weight). The more relevant question is how much mass is in the swept area, which includes the wall thickness, number of vanes, radial depth, etc.

Cooling ability is critical to assessing the disc's overall thermal capacity. If you have a system that can transfer the heat to the air quicker/more efficiently, you can have less effective mass (that between the friction faces) and achieve no higher of a peak temperature on the track. That means a well-designed smaller disc is not necessarily running hotter than a larger disc with a less efficient design.

Finally, a proper floating disc can go to a higher temperature than a one-piece disc without having some real issues with staying flat or being over-stressed, since the outer ring is free to expand as needed as it increases in temperature. A good racing system (discs assembly, pad, and caliper) can operate consistently at higher temperatures and still perform admirably, because it is designed to do so.

With an efficient design, a given BBK that provides a weight reduction can most definitely absorb and shed far more heat than brake systems with larger discs and less-efficient designs (fewer vanes, thinner disc walls on the friction faces, mass concentrated in the hat/flanges). Our AP Sprint and Endurance Kits have proven that over, and over, and over again on tracks all over the world (as have our BBK's on other platforms such as the M3, Corvette, etc.). Bigger is not better. Efficient is better!

All about finding the right equilibrium for your needs. Years of observation with this platform have shown a consistent formation of ID cracks with the lighter kits, which is a large indicator of insufficient thermal capacity. A 2 piece system allows for this, whereas a 1 piece system has cast metal there, reinforcing the structure, and preventing it from happening.