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The disc brake pad underlayer – the undervalued layer

January 1, 1

Most brake pads of cars and commercial vehicles have an underlayer between the friction material and steel backplate. This underlayer ensures good bonding strength with the backplate, dampens vibration and noise generated in the braking system, and gives a thermal and moisture barrier. And, depending on the raw materials used in the underlayer, it may save costs.

Despite these key functions, the brake pad underlayer is often undervalued and underappreciated. There has been relatively little work on its development, but its properties are critical to the performance and safety of the whole brake system.

The underlayer is glued to the backplate using an adhesive (see Figure), in most cases phenolic based. 

friction automotive brake pad underlayer material backplate detail Rockdelta

Underlayer constituent parts

Like the friction material in the brake pad itself, the underlayer can incorporate Lapinus mineral fibres to improve strength, thermal insulation and damping.

Mineral fibre concentrations in brake pad friction materials are typically between 0% (European low-metal formulations) and 15% (Non-Asbestos Organic brake pad formulations) by weight. In the underlayer, they can be up to 40%.

Mineral fibre concentrations in brake pads

In the friction material up to
0

by weight

In the underlayer up to
0

by weight

In OE disc brake pads the cost of the underlayer can be even higher than the friction material. This is to ensure that the combination of the friction material and the backplate give the best possible performance. In aftermarket brake pads the cost of the underlayer is normally lower than the cost of the friction material.

Even when underlayer material costs are lower, there must be no compromise on performance. One of the most important features of the underlayer is to provide sufficient shear strength so that the friction material cannot separate from the backplate. This must hold after years of operation in various conditions: low and high temperatures, different humidity, and in winter often in harsh conditions when roads are salted.

Key properties

Besides high shear strength, key underlayer properties include thermal insulation, damping and conformability – the way it conforms to the exact shape and surface of the counter material.

A passenger car brake pad friction material is generally around 12 to 15 mm thick. The underlayer is much thinner, around 2 to 3 mm. Commercial vehicle pads are much thicker, up to nearly 20 mm with an underlayer of 2 to 4 mm. The thickness always needs to be constant for consistent braking performance.

Vehicles that use regenerative braking have much lower friction material wear, and the pads can even last the lifetime of a vehicle. This means even more stringent requirements for the underlayer. Moisture should never penetrate between the backplate and the underlayer because it may cause corrosion and delamination from the backplate.

Because of the lower wear, brake pads for electric and hybrid vehicles may be made thinner to reduce weight and costs. However, thin pads may result in more noise problems because there is simply less material for damping.

Also, the heat transfer from the braking surface to the backplate will be higher. Thermal insulation then becomes even more important to avoid overheating of the backplate, brake piston and brake fluid. That may reduce brake pressure, causing a reduction in braking performance. Requirements for the underlayer are therefore even more stringent, and this may in the future result in the development of new, higher performance underlayers.

Mark Segeren Lapinus
Author
Mark Segeren

Business Manager, Lapinus