In a stock 135i, the rear subframe bushings are *very much* a part of the suspension.
1) They are used to absorb the road vibrations transmitted through the RFT tires.
2) They are claimed to place an upper limit on the maximum rear spring rate that can be used. (reference from TC Kline)
Only after you replace the bushings with a hard rubber/elastomer or solid metal component, then it can behave like a structural component.
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