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Brake pad backing plate

Why is the 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.

The 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.

Brake pad backing plate flex

Brake pad backing plate flex causes internal cracking as well. This will cause friction material to break-off

brake pad backing plate

Brake pad backing plate flex causes friction material cracking and break-off

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 hot surface develops mill scale. Mill scale is actually iron oxide, and it can degrade the bond between the friction material and the backing plate. If the iron oxide isn’t removed properly, the backing plate can

backing plate corrosion

Black steel backing plate corrosion. Photo courtesy Nucap Corporation

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.

brake pad delamination

Brake pad delamination caused by rust jacking.
Photo courtesy Nuccap Corporation

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!

brake pad backing plate

Rust jacking causes delamination. Photo courtesy Nucap Corporation

 

Total brake pad separation

Total brake pad separation caused by backing plate rust

brake pad backing plate

Brake pad friction material delamination AND break-off

Global Brake Safety Council Test Results

“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 engineering.

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 MAJOR safety issue that puts you at risk

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. Worse yet, rust jacking breaks off portions of the friction material, so you’re braking on only a portion of the friction material. Watch what happens when you try to stop a vehicle when a portion of the brake friction material is missing.

Backing plate dimensions are critical to proper pad movement.

As shown in the study, many backing plates were outside of OE specs right out of the box. If

the backing plate is too tall, it can bind in the abutment areas causing uneven (tapered) pad wear or uneven inboard/outboard pad wear. If the abutment portions are too small, the pad will rattle. When you combine dimensional problems with corrosion and rust jacking, you get seized brake pads, resulting in poor braking, uneven brake pad wear and dramatically lower pad life.

brake pad backing plate

Excessive brake pad wear and decreased braking performance due to improper backing plate length which causes binding

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 ways 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 resists rust better than black steel. Plus, they hold their 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 shoes.
• 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.

See my next article on the next generation of brake pad backing plates that are stamped to exacting specifications, eliminate rust and mechanically bond the friction material to the backing plate. Nucap’s new NRS backing plate outlasts the friction material!

DISCLAIMER: In accordance with Federal Trade Commission (FTC) rules regarding endorsements, I have not received any financial remuneration from NUCAP or any other company in return for posting this press release on my site.

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat

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