A ball joint sits in a vehicle's front suspension, at the point where the control arm meets the steering knuckle (also called the spindle or hub assembly). It's a ball-and-socket connection — mechanically similar to a hip joint — that lets the wheel pivot for steering while still moving up and down over bumps. Most vehicles have one or two per front wheel, depending on suspension design.
Two Positions: Upper and Lower
Ball joints are named for where they sit relative to the steering knuckle:
- Lower ball joint — connects the bottom of the steering knuckle to the lower control arm. Every front-wheel-drive and rear-wheel-drive vehicle with an independent front suspension has one on each side.
- Upper ball joint — connects the top of the steering knuckle to the upper control arm. Not every vehicle has one; whether it exists depends entirely on suspension type.
Whether You Have One or Two Depends on Suspension Type
This is the part most quick answers skip, and it's the actual reason the count on your car might differ from your neighbor's:
| Suspension type | Ball joints per side | Notes |
|---|---|---|
| MacPherson strut (common on FWD cars/crossovers) | 1 (lower only) | The strut's upper bearing mount carries the load the upper ball joint would otherwise handle, so there isn't one |
| Double wishbone / short-long arm (SLA) — common on trucks, SUVs, and many 4×4/off-road platforms | 2 (upper and lower) | Both control arms need their own pivot point at the knuckle |
| Solid front axle (full-size 4×4 trucks, Jeep-type live axles) | 2 (upper and lower, per side) | No separate control arms — the axle housing itself pivots on the joints |
Across a whole vehicle, that works out to a total of two to four front ball joints, depending on which of these three layouts it uses. If you're trying to find yours, start by identifying which of the three you're looking at — a strut tower up top with no separate upper arm means you're only looking for one joint per side, low behind the wheel where the control arm meets the knuckle.
What You're Actually Looking At
Once you've got the wheel off (or are looking up into the wheel well), the ball joint itself is a compact housing — usually a rounded stud protruding from a metal shell — bolted or pressed into the end of the control arm, with the stud's other end clamped or threaded into the steering knuckle. A rubber dust boot typically covers the joint where the stud meets the housing; that boot is often the first thing you'll actually see before you spot the joint underneath it.
A Less-Common Variant: Rear Ball Joints
Nearly all consumer content on this question — and most of what a front-suspension search turns up — assumes you're asking about the front. That's the right assumption most of the time, since ball joints are overwhelmingly a front-suspension component. But some independent rear suspensions (including some double-wishbone rear setups on performance and off-road-oriented vehicles) use ball joints at the rear control arm-to-knuckle connection too, for the same reason: they need a pivot point that handles both vertical wheel travel and small amounts of toe/camber change. If you're chasing a clunk from the rear of a vehicle with an independent rear suspension, don't rule ball joints out just because most guides only talk about the front — check your specific vehicle's suspension diagram to confirm whether it applies.
Related reading: How many ball joints does a car have (Q1-02) · What does the lower ball joint do (Q1-04) · What are the two types of ball joints? · How to inspect ball joints?
