My AM DB9 has been slightly juddering whenever I braked between 50-60 MPH, below and above this speed range all was normal.
My research and a comment by a mechanic all pointed to warped discs, something I found hard to reason with so I had a very experienced engineer test drive the car. He could not feel it, he did however say that as well as the discs it could be a hub. This reminded me that I had hit a pot-hole some time ago - which before I became aware of the judder, I then connected the pot-hole with the issue.
The engineer offered to check both the disc and hub run outs for £140.
Knowing that two discs are only £210, I decided to check the run outs myself.
I found a new Katsu Dial Indicator Test Gauge on eBay for £8 and made up a flexible but sturdy support from pieces in my garage, see photo

Aston Martin's workshop manual guidance states that the run-out should be <70 μm measured @ 2 mm in from
the brake disc edge, i.e. no more than 0.07mm. From my setup I was able to measure the offside disc at 0.04mm and the nearside at 0.10mm, so here was my problem, 3 hundredths of a mm over the max! Swapping discs around i was surprised to find that not only was the NS disc warped so was the hub, quite a pot-hole! I guess that a warped hub will then warp the disc too.
One new hub (£210) and two new discs fitted and both disc run outs now measure well inside max - and NO MORE JUDDER!
Peter.

 

Good information and thanks for sharing.