To: NSXTECH list <NSX-TECH_at_LISTSERV.NSX.NET>
Sent: Tuesday, February 15, 2000 5:15 PM
Subject: Brake Shudder ( high speed judder) explained
Back in December I asked the question: What is the physical process taking place during brake shudder? There were several replies, mostly of the "I think it's.... but I don't know for sure" variety. My curiosity was motivated by the fact that I had experienced tremendous brake shudder when decelerating from over 135 down to 50 at the end of the front straight at Firebird Raceway and yet I never actually *lost* my brakes.
Through the List I was put in touch with an individual that is a mechanical
engineer employed by the #1 producer of brakes in Japan, and of OEM brake
pads in the States. He was here in Phoenix in January doing brake tests
General Motors and I had the opportunity to have dinner with him. We spent most of the evening discussing all the technical/engineering aspects of brakes and he debunked a lot of the "urban legends" regarding brake shudder.
I also was able to convince him to write an explanation of the phenomenon which he agreed to do under one condition: That I do not give out his contact info as "I do not want to be bombarded with rebuttals or requests for more information!" Below is his write-up. Perhaps Lud can add it to the FAQ
High Speed Braking Judder (Shudder)
Causes: Brakes are essentially devices that convert kinetic energy into
energy. Since kinetic energy increases as a square of velocity, higher speed = much higher heat. Also because of this, the heat energy involved in
a deceleration from 100 to 80 is as high as slowing from 60 to zero. The change in kinetic energy difference between 100 and 80 miles an hour is the same, no matter how quickly you slow down. Therefore, ignoring in-stop cooling, the amount of heat generated is not proportional to deceleration rate, but to initial and final braking speeds. This is why judder is a complaint for both moderate and threshold brakers if they are going fast enough.
Most common mechanism for the disturbance: A higher order (approximately 8-10 per rev.) disturbance caused by regularly spaced "hot spots" on the brake rotor. These hot spots expand (thicken) the rotor locally, causing brake torque pulsations. It is an unstable situation, which gets worse as the hot spots get hotter and expand further. When the brake rotor cools, the hot spots go away, and so does the problem.
Possible Remedies: Increase rotor thermal mass. Bigger rotors can handle
more heat before the unstable mode can begin. Rotors with thicker
(the two solid iron friction areas separated by the rotor vanes) are also helpful. Increased cooling to the brakes also helps, because it lowers the initial temperature at the start of braking and therefore also the maximum temperature.
What doesn't work: "Turning" the rotors. The most likely mechanism for the
problem is not tied to rotor run-out. If rotor runout is an issue, shudder
will also be apparent at much lower speed, and with cool brakes. Note: as
general maintenance tip, do not have your rotors turned when brake pads are replaced unless you have low speed, cool brake shudder, scored (deeply scratched) rotor friction faces, or some other specific problem. The machining equipment at speed or brake repair shops is not nearly as accurate
as that used in the original manufacture of a rotor, so you may be paying money to create a problem where there wasn't one before. Also, cross-drilling of rotors is intended primarily to vent the gases released during brake fade, therefore improving brake effectiveness in the fade condition. This is not the same as a high-speed judder condition, and therefore is not a path to solve this issue. Note that both turning rotors and cross-drilling them also reduce rotor mass, which is not directionally correct. Also note that most real race cars don't run cross-drilled rotors.
Secondary possible cause: Breakdown of the raw materials in the brake pad
very high temperatures, leading to an uneven and thick (relatively speaking:
it's still measured in microns) film deposit on the rotor. This film may or
may not be visible. The solution in this case is to find a brake pad designed for higher temperature operation. One possible avenue is a pad homologated for sale in Europe (where high speed driving is much more common) on your vehicle. European requirements for certification of aftermarket brake pads are far higher than U.S. aftermarket standards. The quality of U.S. aftermarket pads is generally very poor compared with original equipment.