The short answer
Most bike helmets are generally designed to protect during a single significant impact. After a fall in which the helmet struck the ground, a vehicle or another hard object, the usual guidance from manufacturers and safety organisations is to replace it, even if it looks undamaged. The exact advice depends on the helmet, so the most reliable source is always the instructions that came with yours, or the manufacturer’s customer service.
This article is general information. It does not name brands, and it cannot judge the condition of a particular helmet. If you are unsure, replacing it is the cautious choice.
How a helmet works, briefly
Most bike helmets have a firm outer shell and a layer of foam, commonly expanded polystyrene or a similar material, beneath it. In an impact, the foam is designed to crush and compress, absorbing energy and spreading the force over a longer time than a bare head would experience. That crushing is what gives the protection, and it is permanent. Once the foam has compressed, it does not spring back to its original shape.
Because of that, a helmet that has absorbed a big hit has used up part of its capacity to protect. The next impact may meet foam that has already been compressed or cracked. Some helmets, such as certain skate-style ones, are designed to cope with several lighter impacts, but they still have limits, so check what the maker says.
Why damage can be hard to see
You cannot always tell by eye whether a helmet is damaged. Compression of the foam may be under the outer shell, the shell may hide cracks, and the inside lining may seem intact. Possible signs of damage include:
- Cracks, splits or dents in the shell.
- Crushed, flattened or cracked foam, sometimes visible when you look at the inside.
- Straps, buckles or fixings that are frayed, torn or no longer work.
- A deformed shape, or a fit that has changed.
- Stickers or labels that have been peeled off or damaged.
But the absence of such signs is not proof that the helmet is fine. For this reason, many manufacturers advise replacing a helmet after any crash in which it was struck, whether or not damage is visible.
What counts as an impact
It is not always obvious. Think about whether the helmet hit anything hard.
- A clear impact. Your head or helmet struck the ground, a kerb, a vehicle or another object. Replacement is the standard recommendation.
- A hard fall with no obvious head contact. Whiplash-type movements or a slide can still stress the helmet, and you may not remember exactly what struck. If you hit your head or feel any symptoms, seek medical attention and consider replacing the helmet.
- A drop from a low height. Dropping an empty helmet from a bench or off the bike onto concrete may or may not matter. Some manufacturers suggest that a drop onto a hard floor can be enough to damage it, so check the instructions or ask the maker.
- Knocks in storage or transit. Squashing it in a bag or storing heavy items on top could cause damage that is hard to see.
If you are unsure, contacting the manufacturer is sensible. Some offer assessments or replacement schemes, but these vary and you should not assume one exists.
Do not try to repair it
Helmets are not designed to be repaired at home. Glue, tape, paint and solvents can affect the materials, and cracks in foam cannot be properly fixed. A helmet that has been repaired may not behave in a future impact as intended. If it is damaged, replace it rather than patching it.
Ageing and wear
Helmets can also wear out without any crash. Foam and plastics can degrade with sunlight, heat, sweat and chemicals, straps stretch, and the fit pads compress. Many manufacturers give a suggested lifespan, and the usual recommendation is to check the maker’s guidance for yours rather than relying on a general rule. A few things help helmets last:
- Keep them out of hot cars and direct sun for long periods.
- Clean with mild soap and water rather than strong solvents, unless the maker says otherwise.
- Do not attach stickers, paints or accessories that the maker does not approve.
- Inspect straps and buckles from time to time.
What about secondhand helmets?
Buying a used helmet is risky because you cannot know its history. It may have been dropped or crashed, or stored badly, without any visible sign. A new helmet from a reliable source is the safer choice.
Replacing it: what to look for
When you buy a new helmet, the advice on choosing a helmet applies: look for a recognised safety standard label for your country and type of riding, and check the fit carefully. A replacement does not have to cost a lot, since the label and fit matter more than the price. Do not simply buy the same model without checking that it still fits and suits how you ride.
Cost versus risk
It can feel wasteful to throw away a helmet that looks fine. But the helmet exists for the moment when something goes wrong, and an already compromised one may not do its job. If money is tight, weigh that against the potential consequences, and remember that outcomes in a crash are uncertain whatever you wear. Some regions run helmet discount or exchange schemes, so it may be worth asking at a local bike shop or cycling group.
After the crash
If you have had a fall, the helmet is just one part of it. Check yourself first and seek medical help if you have symptoms, particularly after a knock to the head. Then look over the bike, since crash damage can be hidden there too. The wider steps are covered in the article on what to do right after a bike crash.
Key points
- Most helmets are designed for one significant impact.
- Foam compression is permanent and may not be visible.
- Follow the manufacturer’s guidance for your helmet, and replace it if in doubt.
- Do not repair, and avoid secondhand helmets.
- Helmets also age, so check the maker’s advice on lifespan.