Parts required are the 280mm diameter BMW Z1 rear discs and radial mounted Alcon 2 piston calipers (CRR280/11-35RT and CRR280/11-35LT), shown below:
These particular Alcon calipers employ 35mm pistons. Pads are available from Alcon, Pagid, Ferodo, Mintex etc in all the usual road to track flavours. The 280mm Z1 discs are a direct bolt on replacement for the 260mm diameter E30 discs. The curved lip of the heat/dust shield (located behind the disc) will, however, require some fettling to accommodate the larger disc. I took a hammer and dolly to mine and simply flattened out the curved edge. I did this without even removing the shield from the hub (its a fairly basic procedure TBH).
Two brackets are required to attach the calipers to the rear hubs.
I milled mine up from two pieces of 25mm x 25mm 7075 T6 aluminium square bar according to the attached drawing.
You can download the drawing of the caliper bracket as a PDF here.
I paid around £75 each for the OEM Z1 rear discs, and £210 each for the Alcon calipers. I would recommend that you have your two brackets milled up by a machine shop if you prefer not to do this sort of thing yourself. Some fettling with an angle-grinder/sander/dremel is required (to the hubs) where the brackets attach to the two mounting points/ears. This is because the brackets are square while the mounting points/ears are rounded along their outside edge. You'll see what I mean when it comes time to attach the brackets to the hub. This is also a relatively simple procedure. The calipers then bolt directly to the brackets with two stainless-steel M10 allen caps (65mm long if memory serves correctly).
Once fitted you should have something that looks like this:
BTW, for those who might be considering a brake upgrade (either front or rear), do consider the effect upon your cars brake bias. I have a pedal box with adjustable bias and NO servo on my car. This is, therefore, the primary reason for fitting larger discs and calipers to both the front and rear of the car (to regain the lost mechanical advantage of the removed servo, although improved wear and fade resistance is of course not to be discounted). Keep in mind that fitting bigger brakes doesn't necessarily improve your stopping distance since, ultimately, your braking power is limited by your tyre traction. If your existing brakes can already lock your wheels, fitting bigger brakes will only make it easier to lock them at lower pedal pressure. For the most part, messing with your brakes is downright dangerous due to the inevitable effect upon brake bias. I therefore offer the presented information (as with my earlier front brake upgrade) to those who a) know how to size a master cylinder to match their calipers, and b) primarily have the track (not the public road) as the intended place of application.
