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Next job is to make the rear brake hanger plate from 10mm aluminium.
The S4 (and the M900) caliper mounting plates are cranked in towards the wheel about 12mm between the wheel spindle centre and the face where the caliper bolts to. Measuring the exact distance between the centre of the wheel spindle and the caliper bolt holes is therefore problematic (you are effectively measuring a hypotenuse between the centre of two holes one of which is 25mm diameter).
To get around this I think I will machine a 40mm long stub of 25mm aluminium round bar with a central 3mm thread and attach it to the 10mm plate by a 3mm set screw roughly in the position I will need to cut out the plate. This will act as a dummy wheel spindle.
The existing S4 hanger plate can then be pushed over the bar with the wheel spindle offset uppermost and clamped to the plate between the caliper mounting holes. I will then grind/machine a point on an 8mm bolt and thread it through the caliper mounting bolt holes on the S4 plate thereby making centre pop marks to drill my 10mm plate for attaching the caliper. I can then reasonably accurately measure the radius and prescribe an arc below the spindle intersectining with the centerline between the calipe bolt holes and the centre of the wheel spindle to locate the position of the reaction rod attachment point.
I have to cut a 52mm hole in the middle using a HSS holesaw (I have a 2 “ or 50.8mm one) and take the hole out to be a good fit with emery on a split arbor in the pillar drill.
All that is left then to do is drill the 5 or 6 (still undecided) holes in the hub and drill and tap corresponding holes in the plate. The attachment bolts will be countersunk set screws, I will have to wait to do this as I am awaiting delivery of metric countersink bits which are apparently 90 degrees and I only have imperial ones at 82.5 degrees.
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