Brake disc runout diagnostics — which symptom is most directly associated with lateral runout of a brake disc (rotor) during normal service braking?

Introduction / Context:
Brake disc lateral runout is the wobble of a rotor as it turns, measured as side-to-side deviation relative to the hub plane. Even small runout can push pads in and out, causing thickness variation over time and transmitting vibrations to the chassis and steering wheel. This question asks for the most characteristic symptom that a driver perceives when disc runout is present.

Given Data / Assumptions:

• Hydraulic disc brake system with floating or fixed calipers.
• Rotor exhibits measurable lateral runout beyond specification.
• Vehicle is otherwise in serviceable condition with adequate friction material.

Concept / Approach:

Lateral runout causes periodic pad knock-back and nonuniform contact as the high spot of the disc sweeps through the caliper. The driver feels this as brake judder or pedal pulsation, especially at speed. Over time, runout can lead to disc thickness variation and uneven pad imprint, reinforcing the vibration. While braking force may remain adequate initially, the vibration is the hallmark indicator that prompts inspection with a dial indicator and specification comparison.

Step-by-Step Solution:

Verification / Alternative check:

Workshop procedure measures lateral runout with a dial gauge and correlates excessive values with customer complaint of judder or vibration under braking, confirming the causal link.

Why Other Options Are Wrong:

Ineffectiveness of the brakes is too general and not diagnostic of runout alone. Localized or rapid pad wear can occur secondarily but is not as direct or perceptible a symptom to the driver as judder.

Common Pitfalls:

Confusing thermal judder from hot spots with geometric runout; overlooking hub face contamination or improper wheel nut torque that can induce runout.

Final Answer:

Judder during braking