Subframe

Subjects:

  • Preface
  • Subframe front
  • Subframe rear
  • Subframe checkpoints

Preface:
The subframe is a separate part of the body where parts of the suspension are mounted on, such as the wishbones, stabilizer bar and the wheelhouse. The subframe absorbs the forces that act on the car while driving. Many cars have both a front and rear subframe. The subframe is bolted to the bodywork.
Silent blocks are usually located between the subframe and the body to dampen vibrations. After loosening the subframe bolts, the car often needs to be aligned as it may have shifted over the adjustment points. Where dowel pins are involved, the subframe can only be mounted in one way. In that case, alignment is not always necessary if no other adjustment points have been loose.

Subframe front:
The front subframe is also known as a front axle carrier. To fix the suspension components to the subframe, it is provided with threaded and unthreaded holes and recesses with mounting points.

The following images show a BMW 3 Series (E90) subframe. The first image is of a separate subframe without attachments. The second image shows the subframe under the BMW body. The subframe is hereby partly detached to obtain extra space when disassembling the engine. The points where the mounting bolts come in to attach the subframe to the side members are indicated with red arrows.

Subframe front without attachments and attachments
Subframe partly disassembled seen from the engine compartment

Subframe rear:
Cars with independent rear suspension also have a rear subframe. As with the subframe at the front, the wheel suspension parts such as the wishbones and stabilizer bar are mounted on this. On vehicles with rear-wheel drive, the rear axle differential (cardan) is also attached to the subframe.

The following three images show the subframe of a BMW 5 Series (E60) M5.

The first image shows an overview of the attachments. The second and third images show the subframe removed and installed from the top (disassembled) and bottom.

Rear suspension subframe from above
Subframe rear suspension from below

Subframe checkpoints:
During a service or MOT, the subframe should be checked on the following points:

  • condition of attachment points: rust and cracking;
  • condition of suspension rubbers: cracks, dehydration and cracking;
  • engine suspension condition: cracks, drying out and cracking;
  • condition of suspension arms attachment points: rust and cracking;
  • condition of steering attachment points: rust and cracking.

A subframe may be welded in case of slight rust formation as it is not a wheel guide element. Consult the APK manual for the current rust grade. If the subframe shows serious rust damage, has cracks or is damaged by damage, it must be replaced.

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