Why is brake pad backing plate material so important?
The backing plate is a critical component in brake pads
Car makers in the U.S. must meet critical safety standards for all brake components. But those safety standards DON’T apply to the brake pads you buy at auto parts stores. These “aftermarket” brake pads can be made to fit any vehicle without matching the car maker’s original specifications. When poor quality steel is used in the brake pad backing plate, your safety is at risk.
In depth studies conducted by the Global Brake Safety Council (GBSC) show that inferior brake pad backing plate material is a leading cause of early brake pad failure.
What does a brake pad backing plate do?
A brake pad backing plate must remain flat and stable as the brake caliper piston pushes against the center of the backing plate. That means the backing plate should not flex at the edges as it’s being forced against the rotor.
In addition to maintaining flat during application, a brake pad backing plate must maintain dimensional stability so the abutment areas resist wear and slide smoothly in the anti-rattle clips.
Finally, the brake pad backing plate must resist rust to prevent the friction material from separating from the back plate. Friction separation can cause noise and early failure, both of which represent a safety issue.
What is black steel?
When steel is hot rolled at the mill, the surface develops mill scale. Mill scale is actually iron oxide that can affect the bond between the friction material and the backing plate. If the iron oxide isn’t removed properly, the backing plate can develop “rust jacking,” where the rust formation builds and actually pushes the friction material away from the backing plate.
Once stamped, dubious brake pad manufacturers often remove mill scale by shot blasting. Then they paint the “cleaned” backing plates. Unfortunately, shot blasting can deform and compromise the backing plate’s critical dimensions. Once compromised, those altered dimensions can affect fit, function and even safety when installed on the vehicle.
Salt spray tests conducted by GBSC on these black steel backing plates show that after 96 hours of contact with an industry standard salt spray (ASTM B117) test all of the painted black steel backing plates failed the test. To pass, the steel plates must show no more than 5% red rust. Worse yet, all of the samples failed before the test passed the 20-hour mark. Not only did the backing plates show rust, but most showed out of spec dimensions in critical areas. Some of these inexpensive backing plates were out of tolerance right out of the box!
“Of the thirty two (32) disc brake pads measured prior to testing, many of the pads from sets 7 and 8 had
critical dimensions already out of the OE specified tolerance. Evidently, these samples were
manufactured out of the operational limits of the OEM even before they were subjected to salt spray.
The frequency of the out of tolerance parts indicates poor dimensional capabilities in the stamping
process. It is also possible the manufacturer does not know what the actual dimensional tolerances are
for this particular vehicle application and has designed their tooling based on incomplete reverse
Sets 1, 2, 7, and 8 each had flatness exceeding the maximum tolerance on several pads. This could
partially be for the same reasons as above, especially considering how far out of tolerance these disc
brake shoes were. The OES set and set 4 each had one pad slightly over the maximum flatness
specification. It is reasonable to conclude that the same residual stresses that pulled in on the abutment dimensions may also have disrupted the flatness on all of the pads during the tumbling operation.”
All the tested brake pads showed in increase in critical dimensions after the salt spray test. Average growth was 0.10-mm. The largest increase was 0.25-mm, with some samples showing an increase of as much as 0.50-mm.
Rust jacking is a safety issue
As rust forms, it builds height and that height lifts the friction material off the backing plate causing cracking and delamination, which decreases braking performance.
Abutment width increases based on quality grades
Economy grade brake pads exceeded the upper tolerance limit
Mid-grade: 3 of 4 samples exceeded the upper tolerance limit
Next, the GBSC tests examined metallographic cross sections using Scanning Electron Microscopy, X‐Ray Spectrometry, and Microstructural Analysis. The report showed:
Iron oxides (mill scale) were found embedded in subsurface locations of the black steel samples and in the bond layer of suspected black steel disc brake pads.
The only effective way to combat backing plate rust
GBSC tests proved that backing plates made from SAE 1010 hot rolled steel with mill scale removed by acid cleaning (commonly called pickling) followed by oiling resist rust better than black steel and hold dimensional stability better. Further, GBSC test show that additional zinc plating further protects the steel from rust.
Shot blasting should not be considered an acceptable method of removing all mill scale from disc brake
Shot blasting the disc brake shoe after it has been stamped to the required tolerances may deform and
compromise critical dimensions of the disc brake shoe causing fit, function, and safety issues in the brake
caliper assembly.Posted on by Rick Muscoplat
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