Why the 20 degree-angle for rear caliper pistons?

I previously posted wondering about alternative sources for rear calipers for a csi. I was having problems dislodging the pistons on one caliper and expected more of the same, but given enough air pressure and pop pop. I found the plating had come off of one of the pistons and just decided to use the pistons from a donor solid rotor-caliper along with new rebuild kits and everything is now operational.

Here is my real question: Many 2 piston Ate rear calipers have pistons with a "C" instead of an "O" surface area that meets the brake pads. Some BMW (like E9's), Porsche and Mercedes manuals state the "C" should be placed at a 20 degree angle using the appropriate measuring tool. Its not difficult and if any of you have read your manuals, you know what I am talking about. What purpose does this really serve? How would being off 10 or more degrees change anything? I mention this since I have rarely seen these calipers (when used) displaying the correct angle on the caliper pistons.

Thanks in advance.

Limits taper wear of the pad on fixed calipers.

Matt is correct, the cut out is to reduce tapered wear in the pads. The cut out must face the direction of rotation of the disc and the angle is not absolutely critical but it has to be close to the 20degrees

never really thought about it.

project E9 said:

The cut out is to reduce tapered wear in the pads. The cut out must face the direction of rotation of the disc and the angle is not absolutely critical but it has to be close to the 20degrees

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Not disagreeing, but why don't the front caliper pistons have a cut out? They are fixed too. I believe some of the later 5 series used two piston front calipers but I don't recall seeing the cut outs on those either.

Never really thought about it.