ASE A4 Suspension and SteeringSuspension System Diagnosis and RepairHard
A vehicle with a multi-link rear suspension exhibits a noticeable 'crabbing' sensation, where the rear of the vehicle feels like it's trying to move sideways, especially during lane changes or cornering. The customer also reports accelerated and uneven tire wear on the outer shoulders of the rear tires. A thorough inspection reveals no loose components, but a four-wheel alignment shows that the rear thrust angle is significantly out of specification. Which of the following is the MOST likely cause?
- AA faulty rear ride height sensor causing incorrect suspension calibration.
- BExcessive positive camber on both rear wheels.
- CWorn rear stabilizer bar bushings causing excessive body roll.
- DAn improperly set or bent rear subframe leading to a misaligned thrust angle.
Show answer & explanationAnswer & explanation
Correct answer: D. An improperly set or bent rear subframe leading to a misaligned thrust angle.
A 'crabbing' sensation and uneven tire wear (outer shoulders) combined with an out-of-spec rear thrust angle strongly indicate that the entire rear axle assembly (often mounted on a subframe) is not perpendicular to the vehicle's centerline. This is commonly caused by an improperly set or bent subframe, often from impact.
Why the other options are wrong
- A. A faulty ride height sensor would affect ride height and possibly camber/toe, but typically not the thrust angle directly, which is a structural alignment issue.
- B. Excessive positive camber would cause wear on the outer shoulders, but without a significant thrust angle issue, it wouldn't typically cause a pronounced 'crabbing' sensation during maneuvers.
- C. Worn stabilizer bar bushings would cause excessive body roll and potentially clunking, but not a 'crabbing' sensation or a misaligned thrust angle.
Rear Thrust Angle Misalignment
Thrust angle is the angle of the rear axle relative to the vehicle's geometric centerline. A misaligned thrust angle causes the vehicle to 'crab' (drive slightly sideways) and can lead to uneven tire wear and unstable handling.
- Indicates if rear wheels are tracking straight.
- Often caused by bent subframe or control arms.
- Causes 'crabbing' and steering wheel offset.
- Impacts tire wear and vehicle stability.
Memory trick: Crabbing and thrust angle? Think 'BENT SUBFRAME'!