So - a note I found on ECS regarding rotors:
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HELLA Pagid High Carbon Brake Rotors are manufactured to exceed original equipment specifications to be superior to most OE-style and performance rotors on the market. High carbon content means a denser rotor, for longer rotor life with less propensity to warping, along with better heat transfer for a cooler brake system, which is key in eliminating brake fade (and silent braking operation). These rotors feature a 2-piece fixed design with an aluminum hat for weight savings and improved cooling - just as BMW intended.
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This is an OE disc from Pagid which seems to preserve "two piece" like the original. If the hats on the genuine BMW discs are aluminum then that is a notable difference in construction compared to most of the OEM/OES options out there.
The notes from the Zimmermans on the same site:
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Want to restore your stopping power? Replace those old rotors. 1-piece construction.
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One piece options seem to be priced at around 50-60% of the price of two piece options.
I will be going two piece. It's not about the brand, it's about the product specification and construction.
Found a PDF from Pagid on 2 piece construction:
https://www.hella-pagid.com/hellapag...hicle_3.17.pdf
It has a section that says more or less what I was saying about thermal stress issues
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When subjected to a high level of thermal load, such brake discs are susceptible to deviations like variations in thickness, axial misalignment and in shielding.
In these cases two-piece brake discs, also called composite brake discs, offer distinct advantages. Thanks to the diverse materials and special binding processes used in their manufacturing, these discs allow a decoupling of the heatflow to the wheel hub
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