Brakes - Judder/Steering Wheel Shimmy: Overview

Classification:
BR94-002a

Reference:
NTB94-012a

Date:
June 17, 1999

This amended version of NTB94-012 adds a caution and a note to the service procedure. Please discard all paper copies of NTB94-012.

This bulletin supersedes TS89-105/BR89-004

APPLIED VEHICLE(S):
ALL

BRAKE JUDDER/STEERING WHEEL SHIMMY

SERVICE INFORMATION

Brake judder/steering wheel shimmy is a condition which may occur in any vehicle when excessive rotor thickness variation is present on one (1) or more of the vehicle's brake rotors.





Rotor Thickness Variation (RTV) is the variation in thickness around the rotor. The RTV can be determined by measuring the rotor thickness with a micrometer at several locations around the circumference of the rotor. The RTV is the difference in a thickness between the thickest and thinnest points on the rotor.





Rotor Run-Out is the distance that the rotor surface travels in and out, in relation to the vertical plane of the hub, as the rotor turns with the hub. The distance is measured with a dial indicator.

NOTE:
Rotor Run-Out does not cause brake judder or steering wheel shimmy. However, excessive Rotor Run-Out does lead to rotor thickness variation as the rotor wears. It is the excessive rotor thickness variation that causes judder and/or steering wheel shimmy. This is described in more detail below.





Possible Cause of Brake Judder/Steering Wheel Shimmy

A brake judder/steering wheel shimmy incident may develop in any vehicle if the following series of events occur:

^ The brake rotor run-out exceeds specification limits.

^ As the brake pad rubs against the rotor while driving, the high spots on the brake rotor wear slightly.

^ Rotor thickness variation may develop as the high spots on the brake rotor wear. If this thickness variation becomes large enough, brake judder/shimmy may occur.