Vibration felt on the steering wheel and suspension when applying the brakes, known as brake judder, is one of the most common problems experienced by vehicle drivers. But it IS possible to put the brakes on shudder! Here’s how…According to Federal Mogul, the international manufacturer and distributor of Ferodo brake products in South Africa, there are a number of ways to diagnose and solve this problem.
Hub and disc run-out is the most common source of brake judder and is caused by incorrect fitting of the brake disc that pulls it out of alignment with the hub or calliper. To remedy the situation, check for rust or dirt on the hub surface, which will cause poor contact between disc and hub. Dismantle the hub and clean the surfaces of both hub and disc to eliminate pollutants.
Next, check if the hub contact surface has been distorted by excessive tightening torque. Overly severe tightening torque on the positioning screw leads to vibrations from initial brake application, and onwards. Replace the discs and avoid excessive tightening torque.
Check for distortion of the hub itself, a fairly rare occurrence. Bolting a disc to a warped hub will cause vibration. After fitment use a dial gauge to check disc run-out. If the run-out is out of tolerance, refit the disc in another position until the run-out is within tolerance.
If alloy wheels have been fitted, check they have been correctly secured. The incorrect fitment of one-size-fits-all alloy wheels causes disc run-out. As the same wheel is used for multiple hub types and sizes, installers use location spacers on the wheel spigots, but sometimes the wheel cannot be correctly centred on the hub. Place the run-out gauge on the back of the disc while fitting the wheel, and measure the run-out once the wheel is fitted and tightened. Change the wheel. If the problem persists, change all four.
Overheating can cause distortion in different areas of the disc. These hot spots create waves on the disc’s outside diameter, which causes intermittent contact between the pad and disc.
Check for any signs of brake abuse, the most common reason for overheating. Discs are designed to cool rapidly between brake applications, but when the brakes are applied intensely in quick succession, the discs do not have enough time to dissipate the heat. Blue or dark spots on the disc surface indicate overheating. Replace the discs, as well as the brake pads.
Poor quality pads overheat easily, especially during heavy braking with excessive heat resulting in disc warping. Check for blue spots on the disc surface and replace the brake pads and discs if spots are present.
Disc thickness variation (DTV) indicates unevenness on the friction surface of the disc. For effective braking, discs should have the same thickness throughout. When DTV is present and braking is applied on one or more of the wheels, the brake pad loses and regains contact with the disc as it turns, which causes brake judder.
Check that the brakes have been properly bedded-in. Applying only moderate pressure on the brake pedal during the first few brake applications and the first 200 km ensures that an even layer of friction material is transferred from the brakes to the disc surface, which improves safety and prevents DTV-related judder.
Dirty or corroded disc surfaces will cause uneven wear on the brake pads. Some of the pads’ friction material transfers to the disc during braking. Especially with poor quality brake pads, the deposits of friction material can stick to the disc unevenly, which changes the disc’s thickness and parallelism. If only minimal DTV is present, remove the deposits with a brush or sandpaper. If this does not realign the disc surfaces, the pads and discs need to be replaced.The discs should be checked for pad imprints. By holding down the brake pedal when the brakes are overheated, weld pad material may become imprinted on the disc. DTV will often be visible as the outline of a brake pad on the disc surface. Removing the pad imprint with a brush or sandpaper usually solves this problem.
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