Why Do Brakes Squeal/Squeak?

We’ve all been there – you pull up to a stop sign or intersection slowly, press on the brakes slightly and hear ‘SQUEEEEEEEEEEAAAAAAKKKKK!!!’…embarassing to say the least as it seems everyone in the same zip code turns their head to look at your vehicle, wondering what is wrong with your car and why you haven’t fixed it yet. Well, todays the day, lets get into why this happens and what you can do about it…

The Physics of Noise

To understand why this is happening, its useful to understand the basic building blocks of noise, especially squeak type noises. Sound is produced by virbating objects – in a radio speaker the speaker cone vibrates, producing the sound which we hear. A symbol in a drum set vibrates when a drummer hits it with a drumstick, making the very identifyable symbol ‘crash’ noise. Guitar strings, same thing, vibrate when plucked, producing the notes for music. The speed of the vibration determines the pitch – slower vibrations create deeper sounds, faster, tighter vibrations produce high pitched sound.

The higher pitch sounds also travel farther than the lower pitched sounds – physics – which is why sirens or other important warning systems use high pitched noises: they can be heard clearly over greater distances and areas. This makes high pitched noises more noticable.

Your Braking System

Lets apply the understanding of how noise is made to how a sqealing brake system sounds. These squeals are high pitched and very noticable, even to those who are quite a bit distanced from the offending vehicle. We know that noise is made by vibrating objects, in this case a high pitched noise is made by an object vibrating very quickly.

Your braking system is made up of many components, but the main items in disc brakes are the brake rotor and brake pad. The rotor is a large, pretty solid piece of cast metal, so it won’t be able to vibrate at the speed required to make that high pitched noise. That leaves the brake pad as the primary suspect.

Putting it all together we can say this: brake squeal is caused by a brake pad vibrating at high frequency.

Holding the Brake Pad

The design of your brake pad and how it is attached to your vehicle contains a bunch of design elements to prevent this vibration. Stainless steel clips on the carrier arms, spring clips on the outside of the caliper, clips and/or shims on the backside of the brake pad itself.

All the design elements though rely on a healthy overall braking system. As the braking system ages those systems start to break down and can cause a variety of issues for how the brake pad contacts the brake rotor. Any time the brake pad does not contact the brake rotor correctly, all the design elements to prevent the pad from vibrating are unable to work and the result is brake squeal.

The Checklist

If your brakes are squealing running these checks can help:

  • Check the brake rotor for “grooves” – if the surface of the rotor is not smooth, and has rust, grooves or other corrosion, it is an indication that the brake pad is no longer making correct contact
  • Check the brake rotor for rust at the edge – as rotors age they will start corroding fastest at the edges, where the pads don’t make contact. As ruse builds it can start contacting the edges of the pad, causing the pad to vibrate.
  • Brake pad even wear is vital, too. Peer between the spokes of the wheel and see if the brake pad is wearing evenly. If the pad is wearing unevenly that means that there is something preventing part of the pad from squeezing the rotor evently at the top and bottom.
  • Keep and eye out for smoking or metal-burning smell – this can be an indication that the pad has been locked agaist the rotor and has overheated and glazed. Glazed pads will be very susceptible to squeal.

Prevention is the best Medicine.

Whenever you install new brakes on your vehicle, you’ll want to be sure all the braking components are healthy in order to limit the chances your brand new brakes will squeal. Remember, when you’re putting on new rotors and pads you’ll be resetting your calipers, and, setting your calipers and carriers at a place that they haven’t been in years or even a decade. So even if your brakes weren’t squealing before, they may squeal after a brake job if the whole caliper and carrier wasn’t fully serviced.

Here is a checklist of things to do during your brake job to ensure that your new brakes don’t squeal:

  • Use a wire brush to remove any built up rust or corrosion on the caliper carriers – while the caliper squeezes the pad, the pad actually sits on and is held in place by the caliper-carrier. The carrier is often made of cast steel, so will rust and corrode over time. It is vital that any rust or corrosion is fully removed, and that any machined surfaces are back to their like-new smoothness. Any rust or corrosion can cause the brake pad to stick or get hung up as it is squeezed repeatedly while driving down the road.
  • Inspect all clips, especially for bends or corrosion, and replace if necessary many brake systems use stamped stainless steel clips to provide the brake pad a smooth surface to sit and move on. These must sit fully in their intended place on the caliper-carrier and be completely flat. You will need to remove any rust or corrosion buildup on the carrier before reinstalling the clips. Some high quality pads come with new clips, and you should use those if available.
  • Remove all guide pins to clean and regrease them – every caliper mounts using guide pins which allow the caliper to slide in and out as you apply and release the brakes. These guide pins are greased at the factory to ensure they operate smoothly. Over time this grease breaks down and road salt and grime can build up on the guide pin. You’ll need to remove all the guide pins and use a wire wheel or a brillo pad to remove any buildup, then regrease them using a proper high-temperature grease for brake applications – ceramic based grease or “sil glyde” are good options – before installing.
  • Clean Debris from Caliper Piston Boot & Seals – carefully remove any buildup around the caliper seals or the caliper boot. If the boot cannot collapse properly it can fold over on itself and can get caught between the piston and the brake pad. This will prevent the pad from squeezing properly and can lead to boot failure.
  • Ensure brake pad can slide easily without resistance – the brake pad should be able to slide on and in and out along the arms of the carrier, if it can’t you’ll have problems down the line. If you’ve fully cleaned the arms of the carrier and ensured the clips are aligned properly but the brake pad still has resistance you may need to slightly file down the brake pad’s metal backing plate. Most backing plates are coated with a rust-resistant paint and at times that paint may be slightly too thick. A quick metal file will help things go together smoothly.
  • Buy High Quality Components – rotors with plating or rust resistant coatings will last longer while pads that are made with champfered edges and come with rubberized shims will also fair better in the battle against noise.

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