Brake System NVH Woes: Addressing and Avoiding Noise, Vibration and Harshness Issues
We’ve all seen the television commercial where the guy applies his brakes and they squeal, causing everyone to look at him. Then, the announcer goes through the sales pitch, including a low price and everyone is happy. Happy, that is, until the brakes begin to pulsate, squeal, shimmy, pull or worse a month or so later. The fact is brake comebacks are not fun for anyone. The customer isn’t happy, and you shouldn’t be either. Here we offer you some great tips to eliminate brake NVH problems.
The usual complaint after performing a brake service is typically going to involve noises — squeaks, squeals, squawks or vibrations. You name it, brake noise under moderate to light applications is a common complaint, but what causes it? And why don’t they make noise under a hard application?
In a nutshell, the cause is vibration, where the rotor or drum act as the speakers. The actual brake material is vibrating; most commonly it’s the pad assembly, but shoes can do it, too. This noise is most commonly made during light application because there isn’t enough force keeping everything tight in the abutment bracket assembly as it would be held under a hard stop.
Brake part replacement and services are something that we all do regularly, and most of us do it routinely without issues. But we have all had to deal with the fact that brake jobs can often lead to comebacks from noise and vibration that may or may not have been created by us, the techs who performed the job. We usually are quick to blame the brake component manufacturer, the pad material type or the customer’s driving habits, but in many situations it was something we overlooked and let go. It takes more than just slapping in a new set of pads to prevent these noise issues, but with attention to detail and doing a complete and thorough job the first time, we can prevent a number of self-induced comebacks.
Starting the brake job: disassembly
Almost all light-duty vehicles today are using a floating caliper design, so the issues and problems are fairly similar across the board and across manufacturers. The first step is disassembly, which sounds easy enough, but there are a few steps to which we need to pay particular attention.
After removing the caliper, don’t let it hang by the flex hose, as this can easily stress and damage the flex hose. Things have never been lighter or smaller than on the modern vehicle and a flex hose can easily be damaged.
Now that we have removed the caliper and its bracket from the mountings on the spindle or axle assemble, it’s time to formulate a plan of attack for proper service that will give us the absolute best chance of this brake job not coming back with a noise or vibration complaint.
A careful visual of the abutment bracket is the next step. This is the piece that holds the pads and this bracket needs particular attention. These brackets contain major areas of concern that need to be addressed during proper brake service. They can be a significant cause of noises and issues if not serviced properly.
In the area that the abutment pad resides, the guide pins/guide pin boots and guide pins themselves all require attention. This bracket is subject to all the heat and forces that are generated during a stop; they gather brake dust, road grime and salt (I’m from the North, and this is a huge issue) and other road-clearing winter chemicals. Removal of the abutment pads will expose rust and debris that needs to be thoroughly cleaned.
This is critically important. The area is designed to have just enough space to allow free pad movement but also keep the pad tight enough to absorb the pad noise that can cause vibrations. The buildup of corrosion and rust often locks the pads in place causing accelerated wear, pad failure, uneven braking, uneven wear and noises.
There are a number of ways to perform this operation from wire wheels, gentle filing, sand or glass bead blasting, if available, to replacement in extreme cases of corrosion or wear.
The abutment pads are a throwaway piece and should be replaced any time pad replacement takes place. These complicated little pieces of metal are designed to provide a flat, smooth area for the pad’s backing plate edge to ride on, absorb vibration and provide anti-rattle properties. Some even help push the pads back slightly to reduce drag (more on this in a bit).
All of these properties will have been compromised and damaged by rust, heat and fatigue during their life on the bracket, so replacement is paramount.
The next concern involves the guide pins and their boots. The guide pins locate the caliper to the abutment bracket and must provide free and equal movement for optimum braking and even pad life. But again these components are subject to heat, fatigue, the elements and road debris, so these parts have to be carefully examined and replaced if necessary.
The guide pins must be clean, rust free and straight, but the boots and seals that the pins slide into have to be scrutinized as well. They shouldn’t be cracked, torn or hardened, as they are designed to allow movement yet provide protection against the elements.
In extreme but not rare cases the guide pins can be stuck or frozen in the bracket and replacement may be the only option, but if they are able to be removed, they need to be properly cleaned and new lubricant applied. The pin holes need to be cleaned of the old lubricant that has gathered dust, moisture and road grime that could impair the pins’ free movement.
The boot attachment areas should be clean and rust-free, providing a sealing surface that prevents foreign material from entering the guide pin’s hole. In some cases these boots are also a bushing that the guide pin rides on and after many brake applications they deform into an oval or egg shape that can allow rattles by not holding the caliper securely.
After cleaning and visual inspections for wear and tear, the guide pins and boots should be reinstalled or replaced and properly lubricated with the correct lubricant. This is an area that can’t be discounted.
The proper lubricant is critical. Using the old tub of water-attracting white grease simply won’t cut it anymore. The lubricant should be designed to be compatible with the rubbers and seals that it touches, silicone based to prevent water attraction and must be able to properly deal with the high temperatures involved.
Many brake lubricants now even contain PTFE (Teflon) to improve the lubricant’s performance. Anti-seize should not be used on the guide pins or boots; it is not a lubricant and will ball up, gather dust and cause a whole host of issues.
The brake rotor
In most cases a new rotor is going to be installed, but if we just machined it, either on the vehicle or a lathe, it is important to wash the rotor clean with mild soap and hot water to remove all the metal debris and graphite from the previous brake pads and machining process.
Yes, I said soap and water, the use of a brake cleaning solvent isn’t sufficient to properly remove all these leftover materials, as it evaporates too fast. It’s this leftover material that can ruin even the most thorough brake service. When not properly removed from the surface of the freshly machined rotor, materials left over from machining (too small to be visible to the eye) can be transferred to the new pads, and this can result in noise, uneven braking and a host of other issues.
Many times the problem is fixed by changing the pads that are contaminated with the small machining particles, with a new set of non-contaminated pads. But the actual issue was improper cleaning of the machined rotor.
Rotor run-out should also be checked. Just a few thousandths of an inch over the spec can cause uneven pad transfer to the rotor that results in uneven braking and vibrations, usually within the first 3,000 miles of driving. The softer pad materials that were used for years (late 1970s, ’80s and ’90s) were quite forgiving and didn’t as a rule generate much noise or cause excessive rotor wear. The problem was that they often contained asbestos, so the brake pad manufacturers changed to a semi-metallic or a ceramic compound that can cause noises and wear rotors, but these compounds require a very true rotor.
Ceramic pads are the worst for this, as a small amount of run-out will unevenly deposit transfer materials across the surface of the rotor, and this creates a slap-stick action that mimics a bad rotor.
Use the lug nuts to tighten the rotor to the clean, rust-free hub surface, and check the run-out with a dial indicator. If it’s over 0.002-inch you have a problem (the old spec was no more than 0.007-inch). Just try indexing the rotor a couple of times, and in most cases the problem is solved. Shims are available and many times the actual hub/bearing assembly is the root issue, not the rotor.
All the other components and parts
After all the cleaning, lubricating and installation of the new abutment pad hardware, the bracket is ready to be installed back on the spindle over the cleaned and run-out-checked rotor.
The fasteners require proper torque, thread locker materials and in some cases replacement. Be aware that some bracket fasteners are of the torque-to-yield style and will need to be replaced.
Stay away from the use of anti-seize.
Other components that will need to be installed such as pad anti-rattle clips should be replaced if not part of the actual brake pad. The same goes for any pad separators.
Pad separators or those spring-like pieces that are either attached to the pad or are part of the abutment pad shim aren’t found on every vehicle but they are becoming more common. These isolate noise-causing vibrations and also serve to push the pad back just far enough to increase fuel economy, increase pad wear and lower pad temperatures. They are just strong enough to push the pad back slightly without affecting the brake pedal feel. Again, after being involved in many heat cycles, these springs lose tension and need to be changed.
Brake pad shims are also subject to heat cycles, rust and corrosion. They can wear out and can separate from the brake pad. They need to be replaced and lubricated with the correct lubricant following the manufacturer’s instructions. Vibration dampers (Ford loves using these on their rear caliper assemblies) are also becoming more common and they have to be properly reinstalled or replaced if damaged.
The road test
Burnishing in the new pads is just as important as all the other steps that we have so carefully performed. This is a job that shouldn’t be left to the customer.
Proper burnishing of the pads involves about 10 stops from 40 mph down to 15 mph under gentle and easy brake pressure (not a panic stop that burns the pads and glazes the rotors), allowing the pads to cool between applications.
This procedure allows for proper pad-to-rotor brake material transfer and will provide optimal brake performance. If the new pads are not properly bedded to the rotors, pedal fade and noise can be a concern.
Finishing up
I am sure that many of you have skipped more than a few of these steps over the years and managed to get away without any issues. But with things getting lighter and smaller with every model, the smallest vibration or noise is now easily felt and heard, especially after the customer has paid you and had no noises before the brake job was performed.
It’s the same as most things — we always tend to take time to figure it out the second time. But if we pay a bit more attention to a few steps along the way, we can usually avoid countless issues that rear their heads and involve a comeback that we end up fixing for free. ■
Jeff Taylor boasts a 31-year career in the automotive industry with Eccles Auto Service in Dundas, Ontario, as a fully licensed professional lead technician. While continuing to be “on the bench” every day, Jeff is also heavily involved in government focus groups, serves as an accomplished technical writer and has competed in international diagnostic competitions as well as providing his expertise as an automotive technical instructor for a major aftermarket parts retailer.
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