Brake System Engineering Helps with Brake Squeal
Picture this: It’s midnight and your six month old daughter is not in the mood to sleep. She does what she does best when not sleeping—she cries! You decide to take her for a ride in your car. It’s really quiet and peaceful outside and within a few minutes your baby is sound asleep like, well,…a baby! Time to go back home and put her to bed. But wait, not just yet. As you pull your car into your driveway and hit the brakes, BAM! That high pitch, annoying brake squeal sound. Now you are back to square one with a crying baby.
Moments like these underline the importance of silent brakes. You start wondering what caused the brakes to squeal, even though your car was serviced just last month. Brake squeal can occur because of two reasons. First, it could be a maintenance problem (worn out pads, glazed pads and rotors, broken anti rattle clips, etc.) or second, it could be a design issue in the brake system. Even though brake squeal noise can’t be eliminated completely, it can be minimized to a great extent if enough care is taken while designing the brake system.
So how does one design a “squeal-proof” brake system? Let us first understand what brake squeal is and when it occurs. Brake squeal occurs when energy of motion is converted into heat and low-amplitude vibrations, which then dissipates throughout the brake system. Brake squeal is a result of a condition known as “stick-slip,” causing microscopic vibrations that create noise which is amplified by the rotor – the rotor ends up resonating and acting like a speaker. Brake squeal is caused by friction-induced dynamic instability. All components vibrate at their natural frequencies. When assembled together, similar frequencies may couple. In certain cases, these coupled modes result in instabilities and give rise to brake noise.
These problematic modes are known as “unstable modes” and can be identified using the 3DEXPERIENCE platform. Not only can engineers identify potentially unstable modes, but they can also determine numerically whether this instability increases, decreases, or disappears entirely from one design iteration to another.
The 3DEXPERIENCE platform offers several design and simulation apps which makes life easier for brake designers and analysts. Let’s quickly go through the process of brake squeal simulation on the 3DEXPERIENCE platform.
Once the brake design is released by the designer, one can very quickly perform the model build and scenario definition (loads and boundary conditions) of the whole brake system on the platform. It took me just 6 hours to complete the model build and generate a simulation-ready model. If one were to use other tools, it would typically take three days for similar operations. And what’s more amazing is whenever there is a need to replace the part because of design change, this can be done with a few clicks. No need to repeat the meshing and connections on a new part, just click on “update.”
As braking conditions differ across a number of different situations, brake squeal analysis has to be carried out under a variety of conditions, such as varying fluid pressures, coefficient of friction, and disc velocity. There are multiple simulation scenarios which need to be carried out and which may result in significant solution times. To minimize the solution time, a standard practice is to complete the first part of the simulation which includes applying pretension loads on the bolts and pressure on the pistons. One can then perform a restart analysis to complete the subsequent scenarios and resultant eigenfrequency steps (across different rotation speeds, coefficients of friction, and pressures). The Process Composer app on the 3DEXPERIENCE platform lets one automate these simulations. The user just has to run the process, which will subsequently take care of running all simulations in parallel.
Once all of the simulations are completed, frequency vs. damping ratio results are plotted. For such “instability analysis,” a negative damping ratio is an indicator of unstable modes, and a primary variable is identifying and understanding these unstable modes. A high (in absolute value) negative damping ratio denotes a high likelihood of instability in the system. These frequencies at which multiple negative damping value data points are present across multiple scenarios are frequencies of high likelihood for brake squeal. Further mode shapes can be studied to understand the system behavior at those frequencies. Referring to the image below, rectangular boxes (2360 Hz, 4900 Hz and 5800 Hz) highlight the brake squeal frequencies.
So back to the main question: How does one get rid of those frequencies/higher damping ratios? This is not always straight forward. There are many factors that impact brake system instability. Brake disc stiffness, coefficient of friction, stiffness of the backing plate, and piston pressure are just a few.
Using the same process composer template, one can add parameters for Young’s modulus of backing plate (to represent material changes), rotor disc, thickness of rotor disc, etc. and repeat the brake squeal analysis and arrive at more robust designs with minimized brake squeal propensity.
Based on this example study shown above, one can arrive at the following observations:
- Coefficient of friction: Higher COF between the brake pad and rotor disc increases friction induced instability in the system and thus brake squeal propensity increases. (This observation is well-known to anybody who is already familiar with this subject). One can reduce friction coefficient to reduce brake squeal but this of course will reduce the braking performance hence it is generally not preferred.
- Stiffness of rotor disc: Stiffness of rotor disc can be increased either by increasing thickness and/or by choosing a higher Young’s modulus material. We changed the thickness of the rotor disc (since the rotor disc CAD model was parametric, it took just few minutes to change it). The results showed that the negative damping ratio (absolute value) decreased significantly indicating diminished instability in the system. So higher disc stiffness helps in reducing brake squeal, an observation which is again consistent with expectations.
- Backing plate stiffness: Since the brake pad material attached to the backing plate is relatively soft, increasing backing plate stiffness increases uneven deformation and magnitude of vibrations in the brake pads. This results into an increase in the negative damping ratio. Hence Young’s modulus of backing plate was reduced, which resulted in decreasing the negative damping ratio – this leads to a brake system with a lower brake squeal propensity, but may have other performance consequences.
For this study, we increased the rotor disc stiffness and reduced the backing plate stiffness. This helped us to reduce the brake squeal propensity at 4900 Hz and 5800 Hz. In many automotive environments, modification to the disc design may not be possible. But this same approach could instead be used to explore changes to caliper, brake knuckle, or even brake pad designs which could similarly reduce brake squeal, often without compromising other key performance attributes.
Thanks to the 3DEXPERIENCE Platform, I was able to design silent brakes for my car and provide a nice, peaceful sleep for my child!
See our brake simulation solutions by visiting: https://www.3ds.com/products-services/simulia/solutions/transportation-mobility/brake-system-engineering/