What is brake judder?
A major cause of brake judder is disc thickness variation (DTV). This is often misinterpreted as ‘warped discs’. Disc thickness variation (DTV) is the term used for the variation in thickness on the friction faces of the brake disc. DTV occurs when the Brake Disc becomes worn in a single area of the disc surface.
This wear is caused by excessive run-out during installation of the disc due to corrosion of the hub face and /or wheel studs, elongated bolt holes or the presence of foreign matter or debris, for example those found in copper based grease products.
What are the symptoms of brake judder?
Initially none, but ‘run-out’ will cause DTV and then judder and vibration during braking, typically after approx. 2000-4000 miles.
Although excessive run-out is a major factor in the generation of DTV It is equally important to check the following:
- Check the brake caliper pistons for any signs of sticking or seizure.
- Check the brake caliper slide pins and bushes, where fitted, for excessive wear or stiction.
- Where pad guided calipers are fitted, check the caliper mounting bracket pad abutment faces for wear.
- Check all steering and suspension joints / bushes for excessive play.
- Check the front wheel bearings for correct adjustment and excessive play, ( Refer to manufactures’ workshop manual for adjustment procedure).
- Check for damaged wheels / tyres, incorrect wheel nut / stud torque and wheel imbalance.
Installation tips to avoid brake judder
- Always replace discs in axle sets to ensure even braking.
- Clean away debris trapped between hub and disc mounting faces which will cause run-out, leading to DTV (thick/thin) and Judder.
- Check that the disc thickness is not less than that indicated on the edge-marking of the disc.
- Check the braking surface of the disc for damage or scoring – replace as necessary.
- Thoroughly clean the faces of the new discs to remove the protective coating (not painted discs).
- Ensure that the caliper carrier is clean, and that the pads are able to move freely in the caliper.
- Check discs for run-out with a DTI.
- Avoid heavy braking for the first 200 miles to allow your new pads to bed in and maximise their performance.
When the Beach Boys were singing about ‘Good Vibrations’, they definitely weren’t referring to the braking system. When vibrations occur between the brake pad, brake disc and caliper, the resultant brake squeal can make any driver wish they too could be on the beach, far away from that irritating high pitched noise.
Causes of brake noise
There are many causes for brake squeal. Excessive corrosion, seized or bent location pins, partially seized calipers, built up dirt and brake dust or worn brake discs can cause vibration between the brake disc and the brake pad under braking conditions. Any vibration within the brake set up will result in noise being generated, which is often referred to as brake pad squeal. Glazing of the friction material surface may also contribute to brake noise. Manufacturers of braking systems and friction materials have taken extensive measures to suppress brake noise in recent years.
Improvements to reduce brake noise
Demands for higher performance and overall weight reduction on modern vehicles have led to material changes. This has resulted in the increased use of supplementary processes to counteract vibrations. Noise fixes include the addition of shims, slots, chamfers and underlayers within the friction material. For example, Delphi carefully selects friction materials, using more than 19 different compounds in their brake pad product line to ensure the closest possible match to OE comfort characteristics and performance. These compounds are developed to provide consistently high levels of performance.
All Delphi brake pads also incorporate underlayer technology. This is a process used by many OE manufacturers to dampen noise-producing vibrations and ensure a higher bonding strength with the pad back plate. Delphi brake pads also feature shims, slots and chamfers that match OE specifications and provide superior performance and durability.
Brake installation tips
Whilst we’re on the subject of braking, here are some handy installation tips.
- Ensure that all corrosion, built up dirt and brake dust is removed from the caliper. Ensure that moving parts of the caliper are free to slide.
- Always clean exposed caliper piston surfaces before retracting the pistons. Ease piston retraction by opening the bleed nipple. Retract piston(s) with a suitable tool. Never lever against the disc friction face.
- Thoroughly clean pad contact points in the caliper.
- Check pistons, seals, boots and sliding elements on the caliper to ensure that they are free from damage and corrosion.
- Always check the disc for minimum thickness and uneven wear when fitting new pads.
- Never use clamps on brake hoses. Hoses contain multiple layers of braiding which give them their structural strength. The hose may become damaged or crushed, leading to hydraulic issues such as blockage or fluid leaks.
- Never use mineral oil based lubricants on parts with rubber seals, this will cause the seals to swell.
- Always fit a new pad fitting kit.
There are typical symptoms a customer may come into a garages that suggest faulty or worn brakes, here are the six main symptoms to spot and ask a customer if they are experiencing them. Some of the below symptoms may also be caused by the vehicle’s suspension system.
1. Vibration on the steering wheel
This is where the steering wheel shakes while braking or shakes all the time while the car is in motion. The reason it happens is because the brakes shudder when the pad meets the disc. The disc could have different thicknesses around the diameter or they could be warped from overheating.
How is it fixed?
Check for different thicknesses around the diameter of the discs. If the discs have varying thicknesses, both discs on the axle should be replaced. While the discs are being replaced, your mechanic should also be changing your brake pads to ensure peak braking performance (It is not safe to only replace discs).
2. Noise from brakes
The noise from your brakes could be high pitched squeal or a it could be a grinding noise. This could be because your brakes could be worn to the minimum where the back-plate is grinding on the disc or the screech ware indicator, the shim (which covers the back-plate of the brake pad) could be missing or damaged or the calliper is sticking.
How is it fixed?
Replace the brake pads (and the discs if they’re damaged) and also service the caliper.
3. Pulling to the side upon braking
When you brake there may be a pulling to the left or right (this is different to poor wheel alignment where the car pulls to one side while the car is in motion). The brake pads may have been contaminated by oil or grease with inefficient braking on one side. Or the caliper could be sticking – the brake pads on one side are unable to brake effectively.
How is it fixed?
Check the brake pads and replace them if necessary (both sides) and check the caliper for efficient braking and service if required.
4. Long or spongy pedal stroke
This is where you’d feel as though you have to press the pedal quite far down to achieve deceleration or the pedal literally feels spongy or softer than usual when you depress it. There are several reasons why this happens:
- Your car may have a brake drum system and the brake shoes are poorly adjusted
- The brake fluid has either air in the system, leaking or is in poor condition
- Load sensing in or load proportioning valve has seized
- Brake hoses are leaking or ballooning
How is it fixed?
Depending on the problem, do the following:
- Re-adjust the brake shoes
- Check the quality of the brake fluid, bleed the system and replace with new brake fluid
- Check the master cylinder and wheel cylinders for leakage and replace if required
- Check the brake hoses for any leaks or ballooning and replace if any faults are found
5. Hard pedal
This is when you depress the brake pedal and feel very little “give” it is literally hard to press. This could happen either because the brake pads are “glazed”, the friction material cannot cope with the braking demands. The caliper is sticking holding the pads to the brake disc, the brake service unit is leaking, or has poor vacuum supply leading to low force applied to the brake system.
How is it fixed?
Check the pads for glazing and replace them, service the caliper or check the servo and replace or repair as necessary.
6. Brake roughness
You’d usually feel brake roughness if the car has been parked for a while and you brake for the first time, there is a slight grinding. This happens because the brake disc becomes corroded, usually in the winter with salted road or near the sea. The metal in the pad has rusted and adhered to the disc – the rough wire-brush type sound happened as the corrosion is cleared off the disc.
How is it fixed?
The corrosion usually clears from the disc after a few light brake applications. If the corrosion is severe, the brake pads and discs should be replaced.