Safety helmets for motorcycle riding are produced from a variety of different materials. Carbon fibre and fibreglass are two of the most common materials in commercially available helmets. They have different physical properties and different strengths and weaknesses.
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Manufacturers make fibreglass and carbon fibre helmets in a fairly similar way. The only real differences are the chemical compositions of the solutions used to build the shell. Strands of fibres are gradually built up in layers inside a mould. The manufacturer seals each layer in place with a resin, which is allowed to dry before pasting the next layer over the top. The fibres in each layer run at different angles to each other, creating greater torsional strength between the layers.
Carbon fibre is significantly lighter than fibreglass. It's common in aircraft, sports cars and military equipment for this reason. A lightweight helmet improves rider safety because it reduces fatigue. Helmet design is usually a trade-off between the different properties of a material. The lighter a helmet is, the easier it is for the rider to wear, but if it's too light, the strength will be compromised.
Fibreglass helmets are strong, and have been proven as effective motorcycle helmets over many years of use. However, fibreglass is brittle compared to carbon fibre, and tends to dissipate impact energy by shattering. For most impacts, this is not a problem; the helmet absorbs the impact and rider is OK. But when a rider suffers two impacts in a crash, for instance, falling off the bike then sliding into an obstacle, The helmet could be compromised by the first impact, allowing the second to harm the rider. Carbon fibre helmets have significantly more flex and "give" in the outer shell, offering protection from accidents of this type.
Fibreglass has always been the cheaper option for motorcycle helmets. Carbon fibre is a "prestige" material and commands a high price tag due to its higher costs of production and manufacturing. In some ways, this reflects the appropriate market for each type. The high-priced carbon helmets are more likely to be bought by people riding high-performance race-ready machines, that are more likely to be involved in high-speed crashes that would require the additional protection. Fibreglass helmets are more than adequate for those riders on slower machines, more likely to be involved in minor spills.