Why does my car judder when I brake?

What is brake judder?

Most drivers experience brake judder at some point in their motoring lives. This juddering can range from the mild to the extreme.

It occurs when there are issues within the braking system that cause irregularities in the braking process and can be felt through the steering wheel, brake pedal - and perhaps the entire cabin.

Is brake judder dangerous?

While not an immediate threat, brake judder signals potential issues within the braking system that can compromise safety. The vibrations during braking may reduce control, particularly in emergencies, and prolonged exposure can accelerate wear on brake pads, leading to costly repairs.

Prompt attention by a professional is essential to maintain optimal braking performance and ensure safety on the road.

What causes brake judder?

Here we run through some of the most common causes for brake judder. Unless you are a skilled motoring mechanic, the best course of action is to take your car to the garage:

• Hub and/or disc run-out: Brake judder may be caused by a poorly-fitted brake disc that is running out of alignment with the hub/calliper. Possible reasons for this are:

• Dirt/rust on disc: If dirt or rust has built up on the disc, it makes contact poor. This is remedied by dismantling the disc and cleaning both surfaces.

• Excessive tightening torque: If the position screws on the brake pad have been tightened too much, juddering can occur. In this case, the brake discs should be replaced but the position screws should not be tightened too much.

• Distorted hub: This is less common, but can happen. Once the disc is fitted, a dial gauge should be used to check for disc run-out. It should be re-fitted if it’s not within tolerance. Hub maintenance is needed if there is still run-out.

• Incorrectly fitted alloy wheels: Many alloy wheels are of the "one size fits all" variety. With the same wheel type being used for all kinds of different hubs and sizes, spigot wheel location spacers tend to be used by installers. A damaged/lost spacer can mean an incorrectly centred wheel. Your garage can fix this by measuring the run-out, and modifying as appropriate. However, the wheel may need to be changed.

• Extreme disc overheating & distortion: If excessive and regular heat is applied to disc brakes, they can overheat, resulting in irregular contact - and juddering. However, this is likely a problem faced by rally drivers - or those who try to drive like them! Blue spots on the disc suggest overheating. In this case, brake pads should be replaced. Poor quality brake pads can overheat quickly, causing them to warp.

• Disc Thickness Variation (DTV): Brake discs should be of the same thickness all around. Juddering will occur if they are not even. You can help prevent this by being gentle on your brakes when you first have new discs fitted. Hard braking should be avoided for the first 100-150 miles of motoring. A seized calliper, often caused by dirt or rust, can also lead to DTV. Good maintenance can prevent it. Corrosion or dirt on the brake pad can also cause DTV, a situation that often results from using cheaper/low quality brake pads.

• Holding the brake pedal down when the brakes are overheating can cause pad material to weld into the discs, resulting in DTV. In this case your mechanic can sand off the pad 'imprint'.