The lower ball joint connects the lower control arm to the steering knuckle — the part that holds the wheel hub. It does two jobs at once: it's the pivot that lets the steering knuckle turn left and right when you steer, and it's the joint that lets the wheel follow the suspension's up-and-down travel over bumps without the connection binding.
Two Jobs, One Joint
Steering pivot. When you turn the wheel, the steering knuckle has to rotate around a vertical-ish axis to point the tire in a new direction. The lower ball joint (working with the upper joint, or with the strut on MacPherson-strut cars) is one of the two points that axis pivots around.
Suspension follower. As the lower control arm swings up and down with the suspension, the steering knuckle needs to stay correctly positioned relative to it — tilting and shifting slightly without losing its connection. The ball-and-socket design is what allows that multi-directional movement instead of the rigid connection a simple hinge would give you.
Does It Carry the Vehicle's Weight? It Depends on the Suspension
This is where sources disagree, and the disagreement is worth untangling rather than papering over with a one-line answer.
- In a short-long-arm (SLA / double-wishbone) suspension, the lower ball joint genuinely is the load-carrying joint — it supports a real share of the vehicle's weight, which is a direct reason it wears faster than the upper joint on this suspension type.
- In a MacPherson strut suspension (the layout on most modern sedans and crossovers), the coil spring and strut assembly — not the lower ball joint — carries the vehicle's sprung weight. The lower ball joint in this design is more accurately described as a follower: it locates the wheel and handles the side loads from cornering and braking, but it isn't what's holding the car up vertically.
You'll see both "the lower ball joint carries most of the vehicle's weight" and the more precise strut-vs-SLA distinction stated online, sometimes even in the same article. The blanket version isn't wrong so much as it's a simplification that works for SLA suspensions and gets imprecise on strut cars — worth knowing which one your vehicle actually has before you take "load-bearing" at face value.
What both suspension types agree on: the lower joint takes more continuous stress than the upper one, which is why it's usually the one that wears out first. We cover the full upper-vs-lower comparison, including which typically shows symptoms sooner, in How Many Ball Joints Does a Car Have?
What Happens When It Wears Out
A worn lower ball joint typically shows up as clunking or knocking over bumps, steering that feels loose or wanders, vibration in the steering wheel, or uneven tire wear from the shifted alignment. If the joint fails completely rather than just wearing loose, the wheel can separate from the suspension — that's a safety-critical failure, not a comfort issue, and it's the reason a clunk from the front end shouldn't sit on a to-do list indefinitely.
Related reading: What Is a Ball Joint on a Car? · How Many Ball Joints Does a Car Have? · How Much Are Ball Joints?
