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Warped Brake Rotors? Addressing the Common Misconception

Published by Mark Link
Saturday, October 27, 2018

The Diagnosis: Warped Brake Rotors

A common diagnosis given when describing the issue of pedal pulsation and/or steering wheel shaking under braking is warped brake rotors. Warped brake rotors are typically diagnosed when thickness variation is found on the rotor that is thought to be a result of the rotors themselves being warped. However, while the term ‘warped” is commonly used by consumers, the correct terminology to describe this issue is excessive lateral runout and/or thickness variation (TV).

 

Causes of Disc Excessive Lateral Runout and TV

Excessive lateral runout is the wobbling of the rotor on its axis. While vehicles are manufactured to only have between one thousandths and two thousandths lateral runout, over time this runout can increase due to certain conditions described below. Thickness variation, on the other hand, is usually the uneven transfer of brake pad material onto the rotor. Related, both excessive lateral runout and thickness variation can cause pedal pulsation and/or steering wheel shaking under braking. This can be due to installation of the wrong brake pad compound for the car you drive and the type of driving you do, caliper malfunctions, incorrect rotor installation, or improper break-in of the pads and/or rotors.

 

Improper Brake Pad Compound

In many cases, it is actually the pad which causes the uneven disc thickness variation (and ultimately increased lateral runout). Depending on the composition, brake pads are designed to both wear the rotors and transfer a thin layer of friction material onto them for optimal performance. However, when the brake system is used to its limit and the brake pads are heated beyond their operational temperature limit, this wear or material transfer can occur unevenly. Uneven buildup is what results in thickness variation. This causes the pads to oscillate and displace the brake fluid which is what ultimately causes the common symptom of pedal pulsation/steering wheel shake. In severe cases, this can lead to the formation of cementite, a iron carbide that is extremely hard and abrasive. In the case of cementite formation, the rotor must be replaced. Thus it is important to install a brake pad compound with an effective temperature range that corresponds to the type of braking your vehicle undergoes.

 

Caliper Malfunctions

Additionally, caliper malfunctions can also cause lateral runout. If moisture gets into either the guide pins or the piston cylinder, rust can form that causes the brake pad to get “stuck” against the rotor, even when you are not braking. This can cause excessive heat that ultimately leads to the same disc thickness variation previously discussed. Furthermore, corrosion of the hub, where the rotor sits, can cause the rotor to rotate out of true on its axis. When installed properly, the hub and calipers should be thoroughly cleaned of any rust in order to avoid lateral runout and pedal pulsation. Additionally, the caliper should be inspected for proper piston movement and guide pin movement (guide pins should be cleaned and re-greased). Finally, it is also important to examine rubber seals on the guide pins and piston for cracks or tears and replace for the reasons described above.

 

Proper Installation and Break-in Procedures

As previously mentioned, incorrect installation of the brake rotors can also cause lateral runout. In line with manufacturer specifications, both hubs and rotors have some degree of lateral runout. However, during installation, sometimes if the runout from the rotor and the hub are “stacked,” the resultant lateral runout can be enough to fall outside of specification which can cause pedal pulsation and/or steering wheel shaking under braking. While most people do not use a dial indicator to position the brake rotor on the hub for minimal run-out it is proper way for a “best practices” brake job.  

It is also important employ proper break-in procedures to avoid variations in disc thickness and lateral runout. This is because too much heat applied on the rotor before they are “seasoned” can lead to disc thickness variation immediately upon installation of new rotor/pads. You can check out our article on brake rotor burnishing to learn how to break in your rotors for your specific driving style.

The importance of proper break-in procedures also applies to brake pads. Brake pads must be broken in such that they transfer an even layer of friction material onto the rotors. If not done properly, this can lead to symptoms associated with TV and/or excessive lateral runout. Refer to your brake pad manufacturer instructions for proper break-in procedures.

           

How We Can Help

Here at Frozen Rotors, we have had the opportunity to see the worst of brake rotor abuse. We want to help our customers diagnose their brake system problems and get them into the correct brake pads and rotors specific to their driving style and their vehicle. If you would like assistance choosing the right pads and rotors for your vehicle call us at 888-323-8456 for a consultation with one of our brake specialists.


 






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