Rick's Free Auto Repair Advice

Brake pulsation and judder caused by DTV

What causes brake pulsation and judder and everything you need to know about DTV and BTV

Brake pedal pulsation and judder are the #1 causes of brake complaints. The pulsation and judder are almost always caused by sloppy brake installation practices. Brake pedal pulsation is caused by disc thickness variation (DTV) due to lateral runout, not “warped rotors”

FACT versus Fiction: 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

Lack of parallelism is the most common cause of brake pulsation

If the brake rotor doesn’t sit perfectly parallel with the wheel hub it will rotate in a wobble pattern. All it takes is as little a .002″ of lack of parallelism to cause lateral runout. 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.

Depending on the type of brake pad material, the rotor-to-pad contact can cause either a buildup of friction material on the opposite side of the rotor or a worn spot on opposite sides. So the rotor becomes thicker in spots or thinner in spots and this condition is referred to as Disc Thickness Variation (DTV).

DTV is felt in the brake pedal rising and falling with each rotation of the rotor. This type of brake pulsation is different than judder described below that’s felt in the steering wheel or the driver’s seat.

1) Rust on the wheel hub is the #1 cause of lateral runout that causes DTV and brake pulsation

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, DTV, and brake pulsation

There is no way you can achieve even lug nut torque without a torque wrench. Uneven lug nut torque causes lateral runout which then causes disc thickness variation.

3) Worn wheel bearings cause lateral runout and DTV

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

Judder: Pulsation or vibration felt in the steering wheel or the driver’s seat vibration is caused by brake torque variation (BTV)

Brake torque variation (BTV) 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 a 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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