Rick's Free Auto Repair Advice

Brake pulsation judder caused by DTV

Car vibrates, brake pulsation caused by DTV

Brake pedal pulsation is  the #1 cause of brake complaints. The vibration is almost always caused by poor/sloppy brake installation practices.

Brake pedal vibration or pulsation is most often caused by disc thickness variation (DTV), not “warped rotors”

Brake rotors don’t warp. That’s a myth. It never happens. Yet even professional technicians refer to DTV as rotor warp. To understand why rotor warp is impossible, see this post

If the brake rotor doesn’t sit perfectly parallel with the wheel hub or the knuckle it will rotate in a wobble pattern. That will cause one side of the rotor to hit the inboard brake pad on every revolution, and the opposite side of the rotor to hit the outboard pad.

If the brake pads use adherent friction material that will result in the brake pads depositing more material onto the rotor when it hits, which then causes alternating high spots. If the brake pads use abrasive material, they will wear down the rotor, causing alternating low spots.

This is disc thickness variation and it causes brake pedal pulsation during brake applications.

Lack of parallelism is the problem

Brake rotors must be parallel with both the wheel hub and the caliper or you’ll get a wobble that causes DTV. Some carmakers require less than .002″ lateral runout, while others allow up to .006″. Here’s what causes lateral runout.

1) Rust on the wheel hub causes lateral runout

You must clean all the rust off the wheel hub to get

rusted wheel hub

Rust/corrosion on the wheel hub prevents the rotor from sitting perfectly parallel with the hub.

good rotor-to-hub contact. Rust accumulation causes lateral runout which then causes disc thickness variation.

2) Not using a torque wrench to tighten lug nuts causes lateral runout

There is not way you can achieve even lug nut torque without a torque wrench. Uneven lug nut torque causes lateral runout. Uneven lug nut torque causes lateral runout which then causes disc thickness variation.

3) Worn wheel bearings cause lateral runout

The caliper and caliper bracket are attached to the steering knuckle. But the rotor is attached to the wheel hub which is held in place by the wheel bearing. If the wheel bearing is worn, the rotor won’t run perfectly parallel with the caliper and will cause lateral runout and disc thickness variation

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.

corroded brake rotors

Patches of rust on a brake rotor cause a slip/stick condition that’s referred to as brake torque variation.

 

Other poor brake job practices that cause lateral run-out and disc thickness variation?

• Reusing old rotors without first inspecting them for lateral run-out
• 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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