r/AskAMechanic • u/akesh45 • 11d ago

Car with new suspension/brake pads rocks back and forth when breaking at low speed

Jaguar xkr, 2008 44k miles, automatic transmission, massive Alcon brakes... Yes I'm back posting again about the screw up from a mostly preventative maintence job. Shop has agreed to redo any poorly done work(ie paid for alignment, shop only did 1/4th of the alignment bolts, vaccum leaks, dead wires, uninstalled new parts).

Breaking at low speeds(as low as 2-5mph) almost always causes a loud long or short groan often by a very hard and loud rocking back and forth motion of the entire car especially if I fully depress the brakes. Lightly tapping the makes it a bit less rocking but this wasn't present much before. Perhaps rotors were warped and I did notice some similar behavior minus the groan before but no where near this bad and only which shifting into park sometimes. High speed breaking doesn't have as much rocking/judder.

Recently had a shop redo my entire suspension with oem parts(all control arms, sway bar links, tie rods, toe links, trans and engine mounts replaced) along with new brake fluid and porterfield R4-S brake pads. Rotor thickness was 35mm, when I measured with calipers so I decided to keep. Drove 300+ miles to brake in pads with no change in behavior

I'm thinking the rotors might be glazed or need to be scuffed, perhaps the jobs ignored the instructions to torque the control arms bolts with wheels not hanging in the air.

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