Electric vehicles (EVs) typically use regenerative braking, where the electric motor generates electricity that is stored in the battery. However, there are several brake conditions under which regenerative braking may not be an option (i.e., at low speeds, at speeds where deceleration is above a certain threshold value, and during emergency braking). Consequently, EVs are also equipped with traditional means of braking, typically disc brakes or drum brakes. As EVs carry a heavy battery, disc brakes are often chosen due to superior braking performance over drum brakes.
Efforts to limit brake noise
During braking with a disc brake, a large amount of energy is released, most of it as heat, however, a small part of it is generated as vibrations in the disc and pad. These vibrations are within the audible range and may reach levels as high as 100-120 dB in the form of brake squeals. A such level of noise can be disturbing for the driver and for the environment surrounding the car. Brake noise can also lead to high warranty costs and poor customer reviews. For these reasons, large efforts are put into the noise-damping of disc brake systems. This includes modifications to the brake caliper, discs and pads, but also a stronger focus on the application of anti-noise shims to the backplate of the brake pad.
Figure 1. Overview of a disc brake system. The brake pad consists of friction material, a backplate, and an anti-noise shim.
Steel, rubber, adhesive: Constrained Layer Damping
Anti-noise shims are typically made of a steel layer onto which rubber is attached using advanced thin film coating techniques. The rubber-coated steel plate is then attached to the brake system of the car using specially developed adhesives with noise-damping properties. The overall concept is called Constrained Layer Damping (CLD).
Cancelling cold noise: how to tackle low noise acceptance in EVs?
Figure 2: Disc temperature for a typical EV and for an ICE vehicle. The disc temperature of the EV is much lower than that of the vehicle with an ICE. Ambient conditions will also have a significant impact on the disc temperature of an EV.
Typical disc temperatures for EVs are below 150°C, meaning that the typical maximum backplate and shim temperatures are in the temperature range of 50 – 70°C.
The ambient conditions play a key role in the temperature levels of the disc, backplate, and shim. Lower temperatures are in general beneficial for the durability of the brake pad and shim. However, at lower temperatures, there is an increased risk for water and ice to remain longer on the disc, resulting in an increased risk of noise issues. Additionally, brake pads last longer on EVs due to lesser usage. This allows for a decrease in the thickness of the friction material of the brake pad, which will significantly add to the risk of increased noise levels.
As the noise acceptance level is generally lower in EVs, due to the overall lower noise levels, it is critical to find an anti-noise shim solution that is suitable for the vehicle and brake system in question.
The need for innovative solutions
Anti-noise shim materials of today already cover a broad temperature range and have a main damping centered around +20°C. As the noise acceptance level is even more critical for EVs, it requires development of additional solutions to dampen noise from -20°C to +80°C. In some cases, standard solutions can be used, and in other cases, innovative solutions are needed.
The importance of adhesives in anti-noise brake technology
There are three main types of adhesives used in anti-noise brake shim technology: silicone, phenolic, and acrylic adhesives.
Silicone adhesives: excellent compressibility – higher cost
Silicone adhesives have the advantage of high-temperature stability up to 250-300°C for prolonged periods of time. They have a low compressibility which is beneficial for the overall brake performance. One disadvantage is that the cost of silicone adhesives is higher than acrylic adhesives.
Phenolic adhesives: broad damping – energy & time consuming
When combined with rubber, phenolic adhesives provide good damping in a broad temperature range. However, phenolic adhesive requires a bonding process to the backplate that involves heat (>200°C) and pressure for longer periods of time (up to 3 min). Although an extremely good bonding between the backplate and the shim is achieved, which is very suitable for high-performance cars, the complex bonding procedure is energy-intensive and time-consuming. Phenolic adhesive is therefore gradually being phased out of the market.
Acrylic adhesives: price competitive – lower performance at high temperatures
Acrylic adhesives are price competitive, their damping properties can easily be modified, and the adhesive can be tailored to be hard or soft. One disadvantage of using acrylic adhesive is that the adhesive will get oxidized at high temperatures of 250-300°C, whereby the material becomes brittle and the damping performance in the adhesive is lost. The standard acrylic adhesives have been on the market for many years and are typically used for aftermarket shims.
Figure 3a. shows a damping plot of a standard acrylic adhesive sandwiched between two steel layers. The main damping is located around +20°C in a broad frequency range (1 kHz – 12 kHz).
Figure 3a. Typical damping plot of a standard acryl adhesive.
Figure 3b. shows an adhesive with a cold noise-damping profile that offers excellent performance criteria such as compressibility. This type of adhesive is new to the market. The cold noise-damping adhesive has a main damping of around 0°C in a broad frequency range. This means that the adhesive will damp particularly well for brake systems where noise issues occur in this temperature range.
Figure 3b. Damping plot of a cold noise adhesive. Main damping is now at 0°C in a broad frequency range.
Figure 3c. Damping plot of a warm noise acrylic adhesive. The main damping is at 40-65°C in the frequency range <7 kHz.
Figure 3c. shows a new type of acrylic adhesive that recently entered the anti-noise shim market. The adhesive has a main damping in the region +40-65°C, which makes anti-noise shims using this type of adhesive particularly attractive for EVs operating in areas with a warmer climate.
Noise incidents significantly reduced with anti-noise shim technology
Figure 4a shows noise results from a noise dynamometer test (dyno test) of a brake system from a large EV platform. The purpose of a dyno test is to measure the noise level of a given brake system when exposed to different conditions. The result shown in Figure 4a is from a dyno test run at low temperatures, with a backplate and shim temperature range from approximately -15°C to +15°C (see left axis). The right axis shows the sound volume measured in dB. The blue lines (braking under forward driving) and red lines (braking when driving reverse) mark noise events above 70 dB. Approximately 60% of the stops have significant noise above 70 dB and going up to 110 dB.
Figure 4a. Noise dyno test of a brake system from EV platform. The test is run from -15°C to 15°C and noise incidents are shown as blue (forward) and red (reverse) lines. For this test, there was no use of an anti-noise shim on the brake pad.
Figure 4b. shows the noise results from the exact same test hardware, but this time, the brake pad was equipped with an anti-noise shim with cold noise adhesive. By applying cold noise adhesive, the number of noise brake stops was successfully reduced from 60% down to 4.2%.
Figure 4b. Same dyno test as in Figure 4a, this time with an anti-noise shim with cold noise adhesive applied on the brake pad. Noise incidents are significantly reduced using anti-noise shim technology.
Decrease in noise acceptance levels calls for novel noise-damping solutions In conclusion, although regenerative braking will be the most frequently used braking technique in EVs, there will still be a need for standard disc brakes. Considering EVs’ noise acceptance level being strongly decreased, there is an increased need to find novel solutions. It will require innovative solutions from the producers of brake systems, brake pads, and from anti-noise shim suppliers. The way forward for anti-noise shim suppliers will typically translate to broadening their product portfolio to be able to fulfill the OEMs’ requirements for low-noise vehicles.
Martin Søgaard Group R&D Manager Meneta
