The brief
Mr Foong's X4 had a knocking sound from the front, the kind you hear over bumps and feel on turns. He brought it in for us to track down. A front-end knock means something in the suspension has play in it, and play that's left alone wears its neighbours and eventually affects how the car steers and brakes. The usual suspect on a BMW front end is the lower control arm bushes. Each front control arm locates the wheel and has a rubber bush at the inner end, and that bush takes every pothole and every bit of cornering load for years until the rubber cracks and separates from its sleeve. Once it does the arm moves when it shouldn't, you get a knock over bumps and a vague feel through the wheel, and a worn bush doesn't recover. Control arms come as a left and a right, so they get done as a pair.
The diagnosis
On the lift the front got a proper shakedown. Both front control arm bushes were worn, the rubber cracked and the arms showing play under a pry bar, which is exactly the knock over bumps and the loose steering feel. The drop links, the strut mounts and the rest of the front held up fine. When the bushes are gone you replace the control arms as a pair, fitting one fresh arm against a worn one just leaves the knock and uneven wear, so the call was both sides.
The work
Both front control arms came off and new genuine BMW-spec arms went on, every fastener torqued to the manual figures and the suspension bolts done up at ride height so the new bushes aren't pre-loaded. With both arms in, the car went on the alignment rig and the front geometry was set back to BMW spec, because new arms change the numbers. A road test confirmed the knock was gone, the steering tight, and the car tracking straight.
The outcome
No knock over bumps, no clunk on turns, tight and precise steering, even tyre contact, and the car tracking straight. The X4 went home with the front end solid again. Worn control arm bushes only get louder and start eating tyres and pulling the alignment about, so doing both arms and resetting the geometry put the whole front right in one go.
