ROI SMR Mar-Apr 2023

Around the Wheel

14

Subframes

There are generally three basic forms of the subframe. 1. A simple "axle" type which usually carries the lower control arms and steering rack.

The purposes of a subframe are to spread high chassis loads over a wide area of relatively thin sheet metal of a monocoque body shell and to isolate vibrations and harshness from the rest of the body. In a car, with its powertrain contained in a subframe, forces generated by the engine and transmission can be dampened enough that they will not disturb passengers. As a natural development from a car with a full chassis, separate front and rear subframes are used in modern vehicles to reduce the overall weight and cost.

What causes subframes to fail?

2. A perimeter frame which carries the components in box 1 but in addition supports the engine.

In a word – corrosion. 99% of subframe components are made from pressed streel plates welded together. All subframes have drain holes, however this doesn’t allow for any form of effective cleaning and leads to a heavy build up of dirt, road grime and salt. This material can rot the frame from the inside.

All the corrosion shown in the image has happened due to the road and weather conditions. The best way to check is round the drain holes in the frame, taking special attention to the areas of the frame where multiple layers of the steel meet and are welded together.

3. A perimeter frame which carries the components in box 1 and 2 but in addition supports the engine, transmission. (Front wheel drive cars)

When found the corroded part needs to be replaced and the only option is a new replacement. The average cost from a main dealer can be in excess of £700, and then other fitting costs can easily raise the cost of completing the job to over £1000. The subframe cannot normally be welded or repaired as they subframes are high stress components.