The brief
The M4 had developed clunks over bumps and a ride that had gone rougher than an M-car should ever feel. On a chassis tuned this tight, a worn part shows up far quicker than it would on a softer saloon, so the owner brought it in without waiting.
That combination, the knock and the lost composure over rough surfaces, points at the front anti-roll bar drop links. They are the short rods that tie each front strut to the anti-roll bar and keep the car flat and tied-down through corners. When their ball joints go loose, the bar knocks against the suspension instead of working with it, and the precise feel an M4 is built around starts to slip away.
The diagnosis
A pry-test at the front confirmed it. Both drop links had play in their ball joints, the rod able to rock in the socket rather than holding firm, and one of them had a split rubber boot that had let grit into the joint and worn it faster.
On a chassis like this one, a fresh link paired with a worn one would leave the front feeling lopsided side-to-side, which matters far more here than on a regular car. So this was a pair job, both front links replaced together so the front works as a matched set again.
The work
Both front drop links came off, and a matched pair of new BMW-spec links went on, with the ball-stud nuts torqued to the manual figures.
With the front suspension disturbed, the car then went onto the alignment rack for a settle cycle and a check of camber and toe, so the geometry was confirmed square before it went back to the owner.
The outcome
The clunks over bumps were gone, and the ride came back to proper M4 character, firm but composed instead of firm and rattly.
The M4 went home feeling like an M-car again, planted and precise through corners. For the owner that is the whole point of the car, restored. And with the alignment confirmed in spec, the front tyres will wear evenly across the tread rather than scrubbing on one edge, which on M-compound rubber is real money saved.