Introduction:
The subframe is a separate component from the bodywork to which parts of the suspension are mounted, such as the control arms, the anti-roll bar and the steering rack. The subframe absorbs the forces acting on the car while driving. Many cars have a subframe at both the front and the rear. The subframe is attached to the body with bolted connections.
Usually, there are silent blocks between the subframe and the body to damp vibrations. After loosening the subframe bolts, the car often needs to be realigned, because it may have shifted relative to the adjustment points. When locating dowels are used, the subframe can only be mounted in one position. In that case, realignment is not always necessary, provided no other adjustment points have been loosened.
Front subframe:
The front subframe is also referred to as the front axle carrier. To attach the suspension components to the subframe, it is fitted with threaded and non-threaded holes and recesses with mounting points.
The following images show a subframe from a BMW 3 Series (E90). The first image shows a loose subframe without attached components. The second image shows the subframe mounted under the body of the BMW. Here, the subframe has been partially loosened to create extra space for removing the engine. The points where the mounting bolts go to secure the subframe to the longitudinal members are indicated with red arrows.
Rear subframe:
Cars with independent rear suspension also have a rear subframe. As with the front subframe, the suspension components such as the control arms and anti-roll bar are mounted to it. In vehicles with rear-wheel drive, the rear axle differential (propshaft drive) is also mounted 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 attached components. The second and third images show the subframe in removed and installed condition, viewed from the top (removed) and from underneath.
Subframe inspection points:
During a service or MOT, the subframe must be inspected for the following points:
- condition of mounting points: rust and cracking;
- condition of mounting rubbers: cracking, drying out and splitting;
- condition of engine mounts: cracking, drying out and splitting;
- condition of control arm mounting points: rust and cracking;
- condition of steering gear mounting points: rust and cracking.
In the case of light rust, a subframe may be welded because it is not a wheel-guiding element. Consult the MOT manual for the current rust grading. If the subframe shows severe rust damage, has cracks or is distorted due to an accident, it must be replaced.