Recently, DBA has been receiving a lot of inquiries about brake shudder or pulsation. DBA’s chief engineer shares his knowledge on DTV – disc thickness variation.
DTV is a variation in thickness of the disc rotor braking surfaces as it rotates on the axle. The hydraulic pressure behind the caliper pistons force the brake pad to maintain consistent load on the rotor surface. When the thickness of the disc rotor varies by as little as 20 microns, the pads oscillate back and forth causing a pulsation in the hydraulic circuit back to the brake pedal.
This brake pad oscillation also causes a variation in brake torque as the brake pads grab and release over the higher portion of the rotor thickness. This is typically noticed in the steering wheel as the brake torque from the left and right axles is different and out of sequence.
Common Causes of DTV
DTV is the end result with many causes behind the generation of this issue. A few of the more common causes are;
- Wheel nut torque. Variations in torque values causing distortion of the disc rotor and hub. Typically associated with pneumatic rattle gun misuse
- Worn or damaged wheel bearing assemblies
- Not cleaning the hub mounting faces. Scoring or corrosion on the mounting face
- Bent or damaged wheel studs
- Irregular thickness of disc rotor anti-corrosion coatings on the mounting face
- Poor quality rotors. Out of tolerance spec
Friction Material Deposits
Another common problem drivers experience is friction material deposits. Friction material deposits are a common cause of steering wheel vibration when braking. This form of brake shudder is the result of intermittent grabbing and releasing of the brake pads on the disc rotor surface. Brake shudder due to friction material deposits is frequently mistaken for disc thickness variation (DTV) or the common industry term warped discs.
Friction material transfer is related to the organic elements component of the brake pad material. While an evenly deposited friction material transfer layer is a critical component of modern braking mechanics, it is when this layer becomes irregular in thickness that problems occur. The irregular thickness of material deposits causes alternating, amplified torque levels in random locations around the braking surfaces. Out of sequence and random brake torque amplification will cause the steering wheel to vibrate back and forth, sometimes violently in extreme cases.
How Does This Occur?
The most common cause of irregular friction material deposits is poor selection of the friction material and disc rotor combination for the intended application. All friction materials have a maximum operating temperature. When this temperature is exceeded, the material may begin to break down and release large deposits rather than the typical fine dust generated by braking friction.