Editing Disc brake (section)

===Run-out===
[[Run-out]] is measured using a dial indicator on a fixed rigid base, with the tip perpendicular to the brake disc's face. It is typically measured about {{convert|1/2|in|mm|1|abbr=on}} from the outside diameter of the disc. The disc is spun. The difference between the minimum and maximum value on the dial is called lateral run-out. Typical hub/disc assembly run-out specifications for passenger vehicles are around {{convert|0.002|in|mm|sigfig=3|abbr=on|lk=out}}. Runout can be caused either by deformation of the disc itself or by runout in the underlying wheel hub face or by contamination between the disc surface and the underlying hub mounting surface. Determining the root cause of the indicator displacement (lateral runout) requires the disassembly of the disc from the hub. Disc face runout due to hub face runout or contamination will typically have a period of 1 minimum and 1 maximum per revolution of the brake disc.

Discs can be machined to eliminate thickness variation and lateral run-out. Machining can be done in situ (on-car) or off-car (bench lathe). Both methods will eliminate the thickness variation. Machining on-car with the proper equipment can also eliminate lateral [[run-out]] due to hub-face non-perpendicularity.

Incorrect fitting can distort (warp) discs. The disc's retaining bolts (or the wheel/lug nuts, if the disc is sandwiched in place by the wheel) must be tightened progressively and evenly. The use of air tools to fasten lug nuts can be bad practice unless a torque wrench is used for final tightening. The vehicle manual will indicate the proper pattern for tightening as well as a torque rating for the bolts. Lug nuts should never be tightened in a circle. Some vehicles are sensitive to the force the bolts apply and tightening should be done with a [[torque wrench]].

Often uneven pad transfer is confused for disc warping.<ref>{{cite web |url=http://www.stoptech.com/technical-support/technical-white-papers/-warped-brake-disc-and-other-myths |first=Carroll |last=Smith |title=Warped- Brake Disc and Other Myths |website=Stoptech.com |access-date=18 January 2014 |archive-date=11 January 2014 |archive-url=https://web.archive.org/web/20140111042612/http://www.stoptech.com/technical-support/technical-white-papers/-warped-brake-disc-and-other-myths |url-status=dead }}</ref> The majority of brake discs diagnosed as "warped" are the result of uneven transfer of pad material. Uneven pad transfer can lead to thickness variation of the disc. When the thicker section of the disc passes between the pads, the pads will move apart and the brake pedal will raise slightly; this is pedal pulsation. The thickness variation can be felt by the driver when it is approximately {{convert|0.17|mm|in|4|abbr=on}} or greater (on automobile discs).

Thickness variation has many causes, but three primary mechanisms contribute to the propagation of disc thickness variations. The first is the improper selection of brake pads. Pads that are effective at low temperatures, such as when braking for the first time in cold weather, often are made of materials that decompose unevenly at higher temperatures. This uneven decomposition results in the uneven deposition of material onto the brake disc. Another cause of uneven material transfer is the improper break-in of a pad/disc combination. For proper break-in, the disc surface should be refreshed (either by machining the contact surface or by replacing the disc) every time the pads are changed. Once this is done, the brakes are heavily applied multiple times in succession. This creates a smooth, even interface between the pad and the disc. When this is not done properly the brake pads will see an uneven distribution of stress and heat, resulting in an uneven, seemingly random, deposition of pad material. The third primary mechanism of uneven pad material transfer is "pad imprinting." This occurs when the brake pads are heated to the point that the material begins to break down and transfer to the disc. In a properly broken-in brake system (with properly selected pads), this transfer is natural and is a major contributor to the braking force generated by the brake pads. However, if the vehicle comes to a stop and the driver continues to apply the brakes, as is customary in cars with an [[automatic transmission]], the pads will deposit a layer of material in the shape of the brake pad. This small thickness variation can begin the cycle of uneven pad transfer.{{Cn|date=August 2024}}

Once the disc has some level of variation in thickness, uneven pad deposition can accelerate, sometimes resulting in changes to the crystal structure of the metal that composes the disc. As the brakes are applied, the pads slide over the varying disc surface. As the pads pass by the thicker section of the disc, they are forced outwards. The foot of the driver applied to the brake pedal naturally resists this change, and thus more force is applied to the pads. The result is that the thicker sections see higher levels of stress. This causes uneven heating of the surface of the disc, which causes two major issues. As the brake disc heats unevenly it also expands unevenly. The thicker sections of the disc expand more than the thinner sections due to seeing more heat, and thus the difference in thickness is magnified. Also, the uneven distribution of heat results in the further uneven transfer of pad material. The result is that the thicker-hotter sections receive even more pad material than the thinner-cooler sections, contributing to a further increase in the variation in the disc's thickness. In extreme situations, this uneven heating can cause the crystal structure of the disc material to change. When the hotter sections of the discs reach extremely high temperatures ({{convert|1200|-|1300|°F|°C|disp=or}} <!-- {{convert|1200|-|1300|°F|°C|abbr=off|disp=or}} -->), the metal can undergo a [[Phase transition|phase transformation]] and the carbon which is dissolved in the steel can precipitate out to form carbon-heavy carbide regions known as [[cementite]]. This [[iron carbide]] is very different from the cast iron the rest of the disc is composed of. It is extremely hard, brittle, and does not absorb heat well. After cementite is formed, the integrity of the disc is compromised. Even if the disc surface is machined, the cementite within the disc will not wear or absorb heat at the same rate as the cast iron surrounding it, causing the uneven thickness and heating characteristics of the disc to return.{{Cn|date=August 2024}}