Brake squeal after replacing front disks and pads

Tiny
GARY BANKS
  • MEMBER
  • 2001 MAZDA MX5
  • 1.6L
  • 2WD
  • MANUAL
  • 118,000 MILES
So I replaced the front disks and pads on my vehicle, now when I turn left the right side squeals quite loud, and it is only when turning left. I made sure to copper grease the backs of the pads and the edges before putting that back plate on. I must have looked it over four times by now and I still cannot figure out what is causing it. Any idea's would be grateful.
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Saturday, March 9th, 2019 AT 9:03 AM

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Tiny
CARADIODOC
  • EXPERT
Are you saying you greased the edges of the pads on the lining material or the metal backing plates? If any contamination, even fingerprint grease, got on the friction surfaces, that has to be washed off with brake parts cleaner before those parts get hot. Once hot, any contamination will soak into the porous rotors and linings, and will never come out. The fix for that is to replace those parts again.

If there's no contamination on the friction surfaces, the next common cause of a squeal is failure to prepare the linings. We used to grind the trailing and leading edges to a 45 degree angle on the bench grinder, then using a flat file to break the sharp edges was found to be good enough. Over the years I figured out it was sufficient to just drag those edges on the concrete floor. That is to remove the "fingernails-on-the-blackboard" type squeal. It seems that is enough to prevent the squeal during the break-in period, and if that is prevented, the squeal does not return once the linings have worn down beyond that ground-off part. Some pads come out-of-the box with those edges tapered as much as an inch into the contact area. We must not do that on our own. The friction coefficient of all linings is very carefully designed-in to match that of the originals, and those were carefully designed to produce balanced braking power front-to-rear. The tapered linings' material is constructed to keep that friction constant as they wear down. The problem with doing this is you lose the squeegee action after driving through deep water. Sharp edges on the linings that scrape that water off is one reason disc brakes are less susceptible to one form of brake fade. We don't want to take that away.

You'll find a lot of disc pads for import cars have a cut through the middle of the friction material. The engineers know pads are going to squeal. The thinking is cutting them creates two smaller pads side-by-side, and the natural frequency those will vibrate at will be too high for us to hear. Some brake system instructors show how to make that cut in new pads when nothing else has solved the noise, but my feeling is if the original pads didn't need that, replacements shouldn't either. I've always found some other solution.

When the noise only occurs during periods of uneven loading, or pressure on the wheels, such as when turning, look first for mechanical issues. One common mistake is a lot of pads have metal tabs riveted on for wear indicators. The two inner pads are mirror images of each other and must go on the correct side. When they are switched, those tabs might contact the caliper or its mount, and that might only occur when a wheel is stressed from the forces of cornering or hitting bumps.

Look for a metal splash shield behind a rotor that got bent and is rubbing on the rotor. That typically causes more of a scraping sound due to rust on an old rotor. With a new rotor, it can be a high=pitched squeal.

Failure to lube the rotor's center hole where it mounts onto the hub can cause a crunching noise when cornering. This was real common on GM front-wheel-drive cars in the '80s and early '90s.

Pads are going to vibrate, and there's nothing we can do to stop that. The high-temperature brake grease you used is meant to allow the pads to slide and vibrate freely without transmitting that noise to the caliper where it would be amplified to the point it can be heard. Before that grease is applied, a flat file should be run over the piston and the fingers the outer pad butts up to. Those surfaces do not have to be shined up. The goal is simply to remove any high spots of rust or dirt so the backing plates sit firmly against those parts.

The mounting pads and / or bolts the caliper slides on must be greased too. When old rotors are machined to be reused, it is not uncommon, due to haste, to set the rotor up on the brake lathe such that it wobbles a little and machines in a warp. New rotors, especially those made in China, will develop a warp within three months that might be too insignificant to feel, but that, or if a small chunk of rust or scale got caught between the hub and rotor, will cause the caliper to slide back and forth on its mount with each wheel revolution. That can cause squeaking

Sometimes a clue can be found by looking for "witness marks" on the caliper or wheel. Those are areas that are shiny from scraping against something. I've already missed when installing caliper mounting bolts the caliper rides on. The caliper sits up too high and can rub on the inside of the wheel. Believe it or not, I've already seen an experienced brake system specialist install an inner pad backward so the metal backing plate was contacting the rotor.

Import cars are notorious for using all kinds of metal shims and anti-rattle clips and springs. Many of those have tabs that must be snapped into place. If not installed just perfectly, they can sit at an angle sufficient to cause them to scrape on the rotor.

Be aware too that very high-quality pads are more prone to squealing. Ceramic linings do this very often, especially in humid conditions. That squealing usually goes away when they warm up after a few miles of driving. A "higher-quality" lining will not stop the car any faster than the cheapest ones you can find. Their coefficient of friction still has to be the same as that of the originals. Some manufacturers use ceramic linings for their other advantages which could include less "off-gasing", which is another type of brake fade, or better wear characteristics. The point is, switching to more expensive pads in an effort to stop the squealing could actually make it worse.
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Saturday, March 9th, 2019 AT 5:45 PM
Tiny
GARY BANKS
  • MEMBER
Thanks for the information. I took it back apart and it was the clips that the pads sit in. I must have nudged it over slightly so it was catching the disk itself.
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Sunday, March 10th, 2019 AT 4:53 AM
Tiny
CARADIODOC
  • EXPERT
Welcome to our club. Half of professional mechanics have done the same thing at least once in their careers. The other half just won't admit it.

Happy to hear you solved it.
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Sunday, March 10th, 2019 AT 10:14 PM

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