Reduction of NVH (Noise, Vibration and Harshness) is a very important issue for modern cars. Conventional suspensions are mounted directly to the chassis (though via rubber bushing) so that NVH can be easily transmitted to the cabin.
One of the popular solutions is to mount the suspension onto a sub-frame (still via bushing), which is usually made of Aluminium alloy or is produced by hydro forming to minimize the addition of weight. The sub-frame itself can absorb some of the NVH. It is in turn mounted to the body by more bushings, thus reduces NVH further. The picture shows Porsche 993's rear suspension with sub-frame. Today, sub-frame mounting is no longer exclusive for high-price cars. The latest Opel Astra and VW Golf have sub-frame mounting too, so do many GM mainstream models.
This is distinct from the sub-frame assemblies on such stuff like minis where the sub frame was welded to the main chassis and provided a convenient mounting point for everything else. In the Mini's case the sub frame provided much of the strength of the chassis and a bad/rotten one like many were, was an immediate MOT failure
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