Have you ever taken your car or truck out on a beautiful sunny day just to have your drive ruined with a vibration when the brakes are applied? The vibration can be felt through either the brake pedals or the steering wheel. This terrible vibration or pulsation is called shudder (or “judder” depending on who you ask) and there are two main reasons as to why this may happen: lateral runout and disc thickness variation.
Brake rotor lateral runout is defined as the off-centre deviation of the rotor as it rotates around the hub. In simple terms, it is a measure of how much the brake rotor wobbles. Consider the following schematic.
A rotor with run-out will wobble when spinning on the hub. When the brake pedal is pressed, the caliper will exert a clamping force on a surface that essentially moves inwards and outwards the distance of the run-out. This is called lateral run-out.
All vehicles are designed to allow a small amount of run-out. When the actual run-out exceeds that tolerance, the driver will feel shudder or vibrations in the vehicle that can be barely noticeable or severe.
Why Does Run-out Happen?
- Rotors do not meet OEM tolerances: These days, manufacturing capabilities have improved such that most rotors you get through retailers and the internet meet OEM tolerances. However, not every batch of rotors are perfect and although this is highly unlikely, it can occur in rare instances.
- FIX: Buy rotors that meet G3000 metallurgy requirements and that are manufactured in ISO9001/TS16949 facilities
- Worn out Hub bearings: Hub bearings are an essential part of your vehicle’s motion. They carry a significant amount of weight and spin upwards of 750 times per minute on a typical vehicle. If they wear out and they start rotating unevenly, you can expect run-out to occur.
- FIX: replace hub bearings
- Unclean hub surface: The hub is a steel component of the vehicle and like most steel components, it is subject to oxidization (rust). Over time, debris and rust may accumulate behind the rotors and on the hub surface and when replacing the rotors, if this is not cleaned thoroughly, the rotors will sit unevenly on the hub and run-out will be out of tolerance.
- FIX: clean the hub surface thoroughly using a wire brush, abrasive pad or a hub cleaning tool prior to installing any new rotors.
- Incorrectly torqued bolts/nuts: believe it or nut, torquing down the nuts on your wheels can have an effect on the run-out. Incorrectly or over-torqued nuts/bolts will affect the way the hub assembly expands and contracts with heat and the forces of the nuts/bolts on the rotors may be uneven enough to push the run-out out of tolerance.
- FIX: Always be sure to torque your nuts to manufacturer’s specifications and in a cross pattern.
Disc Thickness Variation
Disc thickness variation is just what it sounds like! It is the change in disc thickness as you move around the rotors. This change in thickness of the rotors can cause a pulsing in the brake system and much like the symptoms of run-out, you can expect vibrations to be barely noticeable to severe.
As the rotor passes through the caliper, the variation in the disc thickness causes changes in the pressure and force exerted by the caliper. These changes will feed back through the brake lines and will result in pulsing and vibrations of the vehicle.
What causes Disc Thickness Variation and how to avoid it?
- Lateral Runout: Lateral runout can cause disc thickness variations though either excessive wear of the rotor or through the uneven deposit of brake pad material.
- FIX: See the previous section
- Uneven brake pad deposits: Uneven brake pad deposit is the most common cause of disc thickness variation and it is the most likely reason that a vehicle experiences shudder. Here are some ways in which uneven brake pad deposits occur.
- Incorrectly bed-in pads: incorrectly bed-in pads can leave deposits and uneven rotor surfaces when the brake pedal is used. These uneven pad deposits can lead to pad build up over time in those areas. As the material builds up, the thickness of the rotor in that section increases and the vibrations become more apparent.
- FIX: Bed-in your brakes whenever you change your brake pads, or your brake rotors, or both your pads and your rotors.
- Uneven clamping due to seized pistons: This most typically happens on multi-piston calipers. Debris and dust may build up at the piston seals and this may cause a piston to seize. Application of the brake pedal at this point will force the other pistons to work and the uneven pressure on the back of the pads will cause the pads to wear in a non-uniform manner. This may form hot-spots on the rotor and lead to the build up of pad material.
How To Fix Brake Shudder
- Practice prevention. Ensure you clean the hub, torque the lug nuts/bolts as per manufacturer’s specs, and bed-in your brake system.
- Resurface your rotor. If your rotor is still within tolerance of manufacturer’s minimum disc thickness, you may be able to resurface your rotor face. In doing so, you will be reintroducing a flat and uniform disc surface for your brake pads to make contact with. However, this will reduce the life of your rotors. Talk to your local mechanic to see your options here.
- Buy new rotors. In most cases if you have severe shudder, you may be better off just replacing the rotors.