Rick's Free Auto Repair Advice

Brake pulsation judder caused by DTV

Car vibrates, brake pulsation caused by DTV

If your car vibrates when braking, you’re in good company; brake pedal pulsation is  the #1 cause of brake complaints. The vibration is almost always caused by poor/sloppy brake installation practices that results brake pulsation due to disc thickness variation (DTV).

Do you feel the vibration in the brake pedal or seat/steering wheel only when braking?

Brake pedal vibration or pulsation is most often caused by disc thickness variation (DTV)

The #1 cause of a “car vibrates when braking” complaint is caused by disc thickness variation (DTV) due to lateral run-out. In other words, the brake rotor face is not spinning perfectly parallel with the wheel hub. It’s rotating in a wobbling movement. As the high point of the wobble hits the brake pads it pushes the caliper piston away from the rotor and that pushes your brake pedal up slightly. Then, as the low point approaches and the piston moves out, the drop in pressure caused the brake pedal to fall slightly.

It only takes only .006″ later runout to cause DTV.

Steering wheel or seat vibration is caused by brake torque variation (BTV)

Brake torque variation is often referred to as brake judder. It occurs when there is a variation of torque across the face of the rotor that causes the rotor to slip and catch as brake pads pinch the rotor. BTV may be caused by an inconsistent rotor finish, metallurgy flaws or poor metal quality, patches of corrosion on the rotor face, or uneven deposits of friction material on the face of the rotor (the transfer layer). The varying rotor surface condition causes the brake pads to grip and then slip.

What causes lateral run-out and disc thickness variation?

Poor brake installation practices

• Reusing old rotors without first inspecting them for lateral run-out
• Not cleaning corrosion off the wheel hub and mating surface of the rotor, resulting in non-parallel operation
• Improper or uneven lug nut torque, resulting in non-parallel operation
• Improper brake pad bedding procedure, resulting in an uneven transfer layer*

DTV is a topic unto itself and since it causes the vast majority of brake noise problems you should learn more about what causes DTV and how to fix it. See this post on DTV.

Other things to check when you have vibration when braking, brake pedal pulsation or vibration in the steering wheel or driver’s seat

Check all of these possibilities before replacing any brake parts:

• Check for wheel bearing wear
• Check the rotor face for hot spots. Blue spots on the disc surface are a sign of rotor overheating. Hot spots change the friction characteristics of the rotor, causing the brake pad to slip/stick as it passes over the hot spots. Hot spots cannot be machined out. The rotor must be replaced.

overheated brake rotor

An overheated brake rotor will cause variation in braking grip which causes vibration

 

 

 

 

 

 

 

 

• Check for overheated brake pads. Overheating from continued heavy braking can cause brake friction material to overheat and glaze. This causes slip/stick conditions.

glazed brake pad

• Check for proper caliper slide pin operation. Caliper slide pins must move freely in their bores. If caliper slide pins stick, the caliper and brake pads can’t apply and release evenly, causing pad glazing and disc thickness variation.

Corroded brake caliper slide pin

Corroded caliper slide pin

• Check for proper caliper piston movement and retraction. Corroded caliper pistons or worn or degraded square-cut O-rings prevent proper piston movement, resulting in pad glazing and disc thickness variation.
• Check for corrosion in abutment areas that can prevent proper brake pad backing plate movement.
• Check for pad imprints on the rotor. If you’ve performed heavy braking and left the overheating brakes applied for a long period, the pads can deposit a coating of the friction material on the face of the rotor. For a while afterward, the brakes will judder. Use 150 sandpaper in a circular motion to remove pad imprints.
• Remove the brake rotor and remove rust and corrosion on the hub surface.

rust on wheel hub

Corrosion on the hub prevents parallel rotor mating and that causes lateral runout and disc thickness variation

• Machine the brake rotor to remove thickness variation or replace it with a new rotor.
• Use a dial indicator to check the lateral run out on the hub. If the hub is out of spec, rotate the rotor to clockwise to the next stud location and repeat the dial indicator. Repeat until you obtain the lowest lateral run out reading.

If you can’t correct lateral run out by rotating the rotor, use Brake Align™ – Runout Correction Plates or replace the hub.
Clean wheel mating surfaces. Remove corrosion and oxidation from wheels where they mate to the rotor hat to ensure proper torque
Install lug nuts with a torque wrench. Tightening lug nuts unevenly can introduce lateral run-out. Always use a torque wrench set to the proper torque when installing wheels.

* transfer layer. For more information on transfer layer, see this post

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat



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