2013 DB9 CCM Brake Shimmy - Diagnosed and (Mostly) Resolved

[andratch]

Posting this in case it helps anyone chasing a similar issue on a VH platform car with carbon ceramic brakes.

The car

2013 DB9, 15,000 miles, factory CCM brakes.

Symptoms

• Speed-dependent steering shimmy that worsened under braking
• Constant pull to the right (car tracked straight if I held the wheel straight, but it would drift right on its own)
• Low-speed steering jitter during turns, sensitive to bumps
• One spot in the right front wheel rotation that was slightly harder to turn by hand when lifted off the ground
• Audible once-per-revolution change in brake squeak under braking (the only time the CCM brake squeak helped!

What I ruled out

• Alignment (verified on a Hunter rack — perfect)
• Wheel balance (done, no change)
• Tires (issue persisted across two sets)
• Bent wheels (no runout when spinning on the balance machine)
• Inner/outer tie rods (no play)
• Front suspension bushings (visually inspected, no visible deterioration, no movement observed under braking load in slow-motion video)
• Break pad material transfer (did several high speed braking runs to clean the surface, no change).

What fixed the cruise shimmy and pull

Pulled both front rotors, cleaned the hub faces and rotor hat mating surfaces with a Scotch-Brite pad, and swapped the rotors side to side. The cruise shimmy and pull to the right both disappeared immediately. I believe the root cause was hub face contamination on the right front creating rotor runout, which also caused uneven pad contact over time and potentially started my uneven CCM wear (more in "What remains" below).

What fixed the low-speed steering jitter when turning

A power steering fluid flush with Pentosin CHF 11S. The jitter went away almost completely. My car would have had CHF11S from the factory, however the fluid in my system was very clearly just standard PS fluid...the yellow stuff. Someone changed it out improperly in the past. If your fluid has never been changed, this is worth checking and doing regardless — it's cheap and the difference is noticeable. Aston1936 has a video on it on youtube.

What remains

A mild shimmy felt only under braking. I measured DTV with a digital micrometer (0.00005" resolution) and found 0.015mm on one rotor and 0.010mm on the other. The worse rotor correlates with the braking shimmy. This is likely uneven pad material transfer baked into the CCM surface. I'm exploring buying a used one from e-bay (with free returns, if the problem persists) as an alternative to a $6,000+ replacement rotor, however I may just live with it since it's pretty minor.

Key takeaways

• Hub face corrosion can cause significant shimmy on CCM-equipped cars, even at low mileage. Low-mileage cars that sit between drives may actually be more susceptible. A Scotch-Brite pad and 20 minutes of labor fixed what could easily have been misdiagnosed as a $$$ rotor replacement.
• CCM rotors don't warp from heat, but they absolutely develop DTV from uneven pad material transfer. The "warped rotor feel" is real even on ceramics.
• Power steering fluid condition matters more than you'd think on these ZF racks. Aston Martin issued SB-11-0300 about this exact issue.
• Buy a micrometer before buying rotors. A $30 tool saved me from guessing with $6,000 parts.

Happy to answer questions if anyone's dealing with something similar.