The brief
The S320 had developed a vibration through the steering wheel at higher speeds, clunks over bumps, and the front tyres were wearing unevenly, the kind of thing that says the front geometry isn't being held steady. He brought it in. The front control arms locate the front wheels and hold them at the angles the geometry is set to, pivoting on rubber bushes and ball joints. On a big, heavy car like this those parts work hard, and when the bushes soften and the joints develop play, the wheels can move around more than they should, which is the vibration at speed, the clunk over bumps, and uneven tyre wear because the wheels aren't sitting where they're aimed. Worn control arms only get worse, and they take the ride, the handling and the tyres down with them.
The diagnosis
On the lift a pry-test on the front control arms confirmed it: the bushes were deflecting past the service limit and the ball joints had play, with the rubber cracked, on both sides. One arm was no better than the other. When both are tired together you do them as a pair. Fitting one fresh arm against a worn one leaves a stiffness mismatch front to front, and you'd be back to do the second soon anyway, so it was both, with a four-wheel alignment after to reset the geometry on the new parts.
The work
The front control arms came off, and a matched pair of genuine Mercedes-spec replacements went on with new bushes and ball joints, every fastener torqued to the manual figures. Then the car went onto the alignment rig for a four-wheel set-up. A road test confirmed the steering vibration was gone, the clunk was gone, and the front felt solid.
The outcome
No steering vibration at speed, no clunk over bumps, the front solid, and the alignment back on spec, so the tyres will wear evenly. The S320 went home with the front suspension back to how it should ride. Control arms carry the front geometry, and once their bushes and joints wear the ride and the tyres pay for it, so a fresh matched pair and a reset alignment put it all back where it belongs.