The sides – or sidewalls – of car tyres have a tough job, supporting the weight of the car, flexing against kerbs and speed bumps and absorbing the impact from potholes while keeping the tyre sealed to the wheel rim.
They’re strengthened for these ordeals, but can still pick up cuts, cracks and bulges. Ignoring such damage increases the risk of tyre failure at some point in the future, possibly disintegration of the sidewall and a blowout.
As you’re advised to regularly look over the tyre tread for wear, this is also the time to get a look at the sidewall. Remember to check the inside of the tread/sidewall as well. Where it’s safe to do so, lie on your back and use a torch to help you.
How to check for sidewall damage
A tyre sidewall is not always perfectly even; sometimes there will be dimples and indentations. Indentations are harmless and aren’t detrimental to either driving or safety characteristics. The dimples are superficial. You’re looking for bulges and splits.
At the MOT test, as well as checking for the minimum legal tread depth across at least ¾ of the width of the tyre (1.6mm) tyres are also expected for visible damage.
For cuts, the government’s vehicle inspection manual advises testers to fail if any ply or cord (the inner construction of the tyre) can be seen without touching the tyre or it can be seen by folding back rubber or opening a cut with a blunt instrument.
When assessing lumps or bulges testers are advised to distinguish between normal undulations resulting from manufacturing, and lumps or bulges caused by structural deterioration.
A pronounced external bulge on the sidewall of the tyre indicates that strengthening cords inside have been destroyed within the carcass. This is a fail and will need replacement.
When you’re checking the sidewall, continue over the top to the tread and look for objects such as nails or glass sticking out of the tyre. You can expect some pieces of gravel to lodge between the treads, but sharp stones can also lead to a slow puncture.