Magazine for the auto service professional
Issue link: http://asp.epubxp.com/i/780462
34 | ASP February 2017 Technical caliper must be mounted so that the smaller piston end is closer to the entrance or "attack" of the rotor, with the larger piston end toward the exit path of the rotor. e smaller piston end creates the beginning of the clamping force and the larger piston end provides slightly greater clamping force, the combina- tion of which helps to compensate for pad taper wear. A multi-piston caliper that features differ- ent-diameter pistons must be mounted so that, as noted earlier, the rotor travel path hits the smaller pistons end first. is means that the calipers must be dedicated for the right or left side of the axle. is staggered piston diameter design also helps to alleviate any potential for pad resin gas "pressure ramping" effect. is pressure begins just after the point of attack as the pad meets the rotor, and continues to progressively build along the pad until it can escape at the exit end of the pad. Depending on the brake pad compound, as the pads compress onto the rotor, heat builds and resins in the pads react and gases are released to the pad surface, building a sudden "pressure ramp," which forces the pads away from the rotor, which pushes the pistons back into their bores. A staggered piston setup uses the larger diameter piston(s) at the exit portion of the pad to counteract this gas-push by applying greater force behind the pad. If the rotor disc design includes a series of slots or holes, this provides a faster escape path for these gas buildups, which function in unison with staggered piston calipers. Performance brake fluid It's time to pay attention to brake fluid, especially from a performance standpoint. All too often, a brake system is filled, bled and then forgotten. Age, humidity, operating temperature and potential air ingestion can begin to take its toll. ere are two basic types of brake fluid: poly glycol and silicone. Glycol fluid (DOT 3, DOT 4 and DOT 5.1) is "hygroscopic." at means that the fluid is capable of attracting and holding airborne moisture. NOTE: Do not confuse DOT 5.1 with DOT 5 fluid. DOT 5 fluid is silicone, while DOT 5.1 is simply a high performance DOT 4 type fluid with a slightly lighter viscosity and generally a higher wet boiling point. As water begins to be absorbed into the fluid, this has two effects: it begins to lower the wet boiling point, which diminishes the fluid's ability to obtain solid and dependable braking performance. Water absorption also creates the potential for holding water inside the system, which can contribute to corro- sion of steel brake lines and can lead to wheel cylinder and caliper piston sticking, especially for vehicles that are stored for extended periods. Because of the propensity for glycol fluid to absorb moisture, it's important to change the fluid on a regular basis. Brake fluid does not last forever... it needs to be changed to maintain proper braking performance. Silicone brake fluid (DOT 5) is not hygro- scopic, so it does not absorb moisture from the air. Silicone fluid is also not harmful to painted surfaces, as compared to glycol fluid. at's why some owners prefer silicone (to prevent paint damage in the case of a leak or spill) and because many people assume that, since silicone does not attract and absorb In an effort to eliminate pad squeal, vibration- damping shim layers are featured on pad backing plates. Shown here is an example of Raybestos' Element3 pad that utilizes multiple shims of steel with vulcanized rubber in-between and a thermo-set adhesive, the combination of which serves to dampen vibrational harmonics. Photo courtesy of Raybestos