BRAKES: WHY DO THEY SOMETIMES SQUEAK AND SHUDDER?
The eternal question of the drivers – why do the brakes squeak or shudder after being changed
when everything was fine before? What went wrong and is this normal?
Under no circumstances should vibrations or noise be considered normal, especially if such a change has occurred after replacing (for example) the factory set of discs and pads from the "first installation". This problem affects both front and rear brakes, either disc or drum. Certainly, after the squeaking starts, we start to get well meaning advice from friends and acquaintances who also share their fears. They are wondering together with us: What have we installed, is the quality of the parts satisfactory, where did "maestro" go wrong…? The various "diagnoses" about the quality of materials, and good and bad manufacturers have become urban legends, but there is only one truth, and it is very simple. In this article, we will try to explain it as best we can, using front brake discs and pads as an example. However, everything we say below also applies to rear brake technology, whether disc or drum.
Is a perfect fit possible?
Braking, which is the result of friction between the fixed pads and the rapidly rotating steel disc, causes high temperatures, wear on the pad material and eventually on the disc itself. To ensure that the braking force is perfect and that these materials do not develop excessive temperatures, vibrations and noise (squeaking) on contact, they must fit together as closely as possible. That's the whole secret, because as soon as their contact is not even or at an angle, if the surfaces of the disks or pads are not flat, there is noise and vibration. As soon as the material of the pad and the disc touch, friction occurs, resulting in high temperatures. The harder the pressure of the pads on the disc, the greater the friction. This raises the temperature and the process continues until the force is strong enough to stop the disc completely.
The temperature generated by braking can instantaneously reach more than 500°C (!) and in extreme cases the disc can even catch fire due to braking. Such sudden heating and then cooling on the disc and disc material. Quite logically, it is therefore important that the pad and the disc contact each other over the entire surface. Otherwise, concentric traces appear on the disc, leading to deformation of the material. The disc becomes uneven and wavy, causing vibrations when braking.
Undesirable natural sprouting
Just imagine if water from the road splashed onto such a heated disc. The hot pad and disc with all the deformations cool rapidly and further deformations are inevitable. An additional problem is caused by the pads wearing unevenly and only touching a small surface area when the disc is first pressed, with that surface being a subject to high force and friction. This produces resonant high-frequency sounds and makes the brakes squeak. Although the materials the discs and pads are made of can withstand extreme temperatures and friction, they also have their limitations. The temperature rises to 500 °C due to the pressure of the pads on the disc to fit over the entire surface. Can you imagine how much higher it would be if the contact surface was smaller? In such cases, the materials are simply incinerated, so that the pad material is either crumbled or so altered that it can no longer perform its function. Even when properly installed, there is a slight vibration and sound generation, but due to the increased surface area of the contact, it instantly dissipates and passes into a frequency spectrum that is inaudible. The vibrations also go down.
How often is the material to blame?
Depending on the purpose, there are different materials and hardnesses available, so it's a good idea to fit exactly what the manufacturer has specified for your vehicle. Today's production system is automated for all disc and pad manufacturers, with continuous control of materials and finished products. The defects in the material or product that we often try to blame for shaking and squeaking are really negligible. After analysing these complaints, it turns out that more than 90% of the problems are elsewhere, so we'll explain a bit more. In order to achieve uniform contact between the disc and the pad, the position of the pads in the jaws is not fixed, but has a certain tolerance in the mounting method, as does the jaw itself. The discs "float" a little around the disc, which rotates on a fixed axis. This makes the jaws easier to fit to the disc and adjust the grip of the pads over the entire surface. For this purpose, as well as for sound stabilisation, they are fitted with insulation plates, stickers, etc., which make it more difficult to transfer unwanted high temperatures to other parts of the suspension, and also ensure that they move properly and freely in the jaws. Also, the disc must fit flat during mounting and the wheel bearing assembly and the disc carrier must not have any tolerance.
Where do problems arise during installation?
Unfortunately, a large part of the problem arises when we skip certain processing or control steps during installation. This results in a poor installation of the disc or pads. Too much grease on the disc seat can cause uneven loading and thus vibration. You should also use a measuring device to check that the disc rotates evenly after assembly. When reassembling the pads, be sure to replace all parts that come with the new pads and clean all parts of the jaws thoroughly. Pads are exposed to dirt and if they are placed on uncleaned surfaces, they can quickly become unsuitable for use. Correct installation and monitoring of all surfaces and checking rotation are prerequisites for proper brake performance. Unfortunately, it is the missed stages of correct installation that cause most of the problems that lead to squeaks and shakes later on. Mounting pads on discs already in use is perfectly adequate, as they are usually less worn than pads. But this is only possible if they are still good enough – not too worn and with no unevenness that would indicate a defect before the pads are replaced. In the past, discs were turned more often – their surface flattened on a lathe. This can also be done if the disks are not too worn out. A thinned disc can no longer withstand high temperatures due to lack of material, no matter how well treated. Today, the prices of new discs are so good that it is not worth investing too much in a used disc. There is no point fitting the old pads to the new disc, as it is best to fit the new pads to the new disc and then be sure that the brakes work properly.
This article was published in Bartog INFO No. 3, Summer 2022. Read the magazine HERE.