What is the function of a subframe?
A subframe is a structure within the vehicle which carries the load from components such as the suspension system, drive train and engine, depending on its position in the front or rear of the vehicle. This helps to improve driver comfort by minimising the transmission of vibrations from the road, engine and drive train into the cabin.
How is a subframe designed?
Subframes must carry heavy loads while adding little extra weight to the vehicle, and this is achieved by using a weight optimised design that uses specific material and manufacturing processes that result in a light yet strong structure. A subframe will typically be assembled using sections that are welded together, but the total number of sections are limited so welded joints can be minimised.
Why do subframes corrode?
Subframe corrosion is common especially in places with high road salt use and generally goes un-noticed because vehicles are fitted with undertrays which aren’t removed for normal servicing or health inspections, and corrosion is generally only identified when the tray falls off or is removed for access during jobs like cambelt changes or clutch replacements.
Corrosion in a subframe will eventually lead to the structure becoming unstable and at risk of fracture or sudden collapse. It will happen eventually to every subframe and repair is often not an option.
Can corrosion in a subframe be repaired?
The DVSA statement* is shown below, but we can explain it a little more clearly.
Subframes are normally made of sections welded together. If any severe corrosion is found, then the complete section it is on must be replaced and new welds made in the same location as the original factory welds. If this is done, then the original strength of the subframe is retained after repair.
*Taken from DVSA guide to corrosion; “Highly stressed components, such as a suspension arm or bar, cannot usually be repaired by welding unless they are made up of sections that are made up of sections that are welded together. In these cases, the repair should appear to be as strong as the original design.”
Should a subframe be repaired or replaced?
We at IM Axle Specialists recommend against repair to corroded subframes as it’s quite simply a better value option to replace it as our subframes can be delivered next-day ready to install with new subframe bushes pre-fitted.
You should keep in mind that any potential repair work would typically require the subframe to be removed from the vehicle, so at this point the new subframe could be fitted and the extra workshop time to repair the old subframe is completely avoided.
But if the repair were to be carried out, then these steps would be involved:
- Replacements for the corroded sections need to be sourced which will probably be from another old subframe
- Corroded sections carefully removed from the old subframe and the donor subframe
- Replacement sections welded in place
- Subframe recoated with a tough and durable paint