Appendix A: Structural integrity and corrosion
Assessing corrosion and methods of repair for motorcycle MOT tests.
The effect of corrosion on the safety of a motorcycle is a difficult matter to resolve since it depends not only on the extent of the corrosion but also on the function of the section where corrosion is found.
A small amount of corrosion which substantially weakens an important component or part of the structure would make the motorcycle unsafe, whilst significant corrosion of a less important part may be acceptable.
You should identify the important load bearing members and highly stressed parts of the motorcycle such as the frame, suspension components and reaction brackets. Then determine whether any of these parts are excessively corroded, firstly by visual inspection and then by applying pressure with finger and thumb.
If necessary, the corrosion assessment tool should be used to assess the extent of any corrosion by carefully scraping or lightly tapping the affected areas.
The corrosion assessment tool should be only used to ensure that the failure criteria are met, and not for poking or heavily scraping the corroded areas.
Excessively corroded metal, or metal treated with filler, emits a duller sound than unaffected metal. You must not hit or use a sharp instrument to probe at the frame or components.
After determining the extent of the corrosion, you must use your experience to judge whether the degree or position of the corrosion has significantly affected the strength of the part, considering remaining amount of sound metal.
You must consider if the corrosion is likely to make the motorcycle dangerous to use on the road under any condition of use, such as fast cornering, use on uneven ground and emergency braking.
Structural corrosion should not be regarded as defective if you consider the motorcycle safe to use. However, you should advise the owner of the presence and location of the corrosion.
If the corrosion has severely reduced the strength of a certain component, you can refuse to carry out a brake test. See ‘Refusal to test’ in Introduction.
The severity of general or local corrosion in highly stressed components, such as suspension arms, rods and levers can be assessed by lightly tapping or scraping with the Corrosion Assessment Tool.
In places inaccessible to the Corrosion Assessment Tool, an alternative blunt instrument may be used. A highly stressed component should be rejected if corrosion has resulted in serious reduction in the overall thickness of the material, or has caused a hole or split.
Welded repairs to highly stressed components are not normally acceptable, other than where the component is made up of sections that are welded together. In these cases the repair should appear to be as strong as the original design.
It’s essential that repairs to structural components are properly carried out and appear to be as strong as the original structure. Suitable materials of appropriate gauge or thickness must be used and any plating or welding extends to a sound part of a load bearing member.
Gas brazing, soldering, adhesive bonding, fibre reinforcement and body filler are bonding processes and are not regarded as strong enough for repairs to load bearing members, although they are normally adequate for other repair work.
Brazing, bonding and riveting are only acceptable when used by the motorcycle manufacturer. The standard of any such repair must be comparable to the original strength of the component.
Sometimes it’s difficult to identify the repair method, especially after the repair has been covered in paint or underseal. If you are not sure what the method of repair is, you should pass and advise.
Non-metallic materials, such as carbon fibre, may be used for load bearing structures and highly stressed components, such as a swinging arm. Repairs to non-metallic materials are not usually acceptable.
If a component has been replaced with one made from a different material, you should consider whether the replacement component is strong and stiff enough to perform its intended function.
If you are not sure about the effect of the modification, you should pass and advise.