A quick guide on Disc Thickness Variation (DTV) issues and causes

9 June 2020

 

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.

Brake pad material deposits are often mistaken for DTV. The key differences to be observed are:

• Pedal pulsation is typically associated with DTV.
• Clearly visible smearing of pad material on the friction surface is associated with material deposits. Vibration increases with temperature. Both issues may cause steering wheel vibration.

Worn or damaged suspension bushes can also simulate the vibrations associated with DTV, as the loose suspension assembly oscillates back and forth under braking.

 

 

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 resulting in DTV 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.
• Extraction screw pressure/damage.

Once DTV is correctly identified, disc rotors may be repaired by means of re-machining only, but only if adequate material is remaining on the disc rotor surfaces.

Note that mechanical or fitting issues must be addressed to eliminate re-occurrences.

 

Test discs for problem with vibration

1. Clean mounting face of all rust and debris (with 3m resin disc - not wire)
2. mount correct mandrel on vertical lathe
3. face true mandrel
4. clamp disc on mandrel in vertical lathe
5. take 5um (.005mm) depth of cuts to ascertain the cause of the vibration
6. 15um (.015mm) depth area not touched by pad has clean up (witness mark) within tolerance
7. 25um ( .025 mm) DTV has become visible with both discs
8. if no DTV, disc will clean up on full face and no sunken area will be visible. If this is the case, then the issue is probably the pads.

Jacking screws can be just part of the issue. 

Ultimately extreme damage or mild, will eventually have the same result and taking care when removing the discs is paramount. More importantly check the discs. This cannot be stressed enough.

The best result will come if you

1. Clean the damage with a hone stone.
2. Clean the hub with Emery paper.
3. Mount the new disc with the extraction holes in the same position on the hub.

In doing so if there is any damage it will be covered by the 8 mm hole and not cause the disc to run out.

Remember - Not cleaning debris from the hub and inspecting for extraction screw damage before installation can be a root cause.

 

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