Engineering: Materials Selection: Bike Helmet Safety
Choosing materials to protect the head during impacts
Engineering: Materials Selection: Bike Helmet Safety
Choosing materials to protect the head during impacts
Engineering - Grade 6-8
- 1
A bike helmet has two main parts: a hard outer shell and a foam liner. Explain the job of each part during a crash.
Think about which part touches the road first and which part changes shape.
The hard outer shell spreads the force over a larger area and helps the helmet slide on the ground instead of catching. The foam liner crushes during the impact, which increases the stopping time and absorbs energy to reduce the force on the head. - 2
Engineers often use expanded polystyrene foam, called EPS foam, inside bike helmets. EPS foam is lightweight and crushes when hit. Why is crushing a useful property for a helmet liner?
Crushing is useful because it absorbs energy from the crash and increases the time over which the head slows down. This lowers the peak force on the skull and brain. - 3
A helmet liner material has a density of 0.05 g/cm³. Another material has a density of 0.20 g/cm³. If both liners have the same volume and similar safety performance, which material is better for a comfortable bike helmet? Explain your choice.
For the same volume, lower density means lower mass.
The material with a density of 0.05 g/cm³ is better for comfort because it would have less mass for the same volume. A lighter helmet is usually more comfortable to wear. - 4
A student says, "The strongest material is always the safest helmet material." Explain why this statement is not always true.
The statement is not always true because a helmet material must do more than resist breaking. It also needs to absorb energy by deforming, fit comfortably, be lightweight, and protect the head without transferring too much force. - 5
During a drop test, Helmet A produces a peak force of 900 N on a sensor. Helmet B produces a peak force of 650 N on the same sensor in the same test. Which helmet performed better in this test, and why?
In helmet safety testing, lower force on the head is usually safer.
Helmet B performed better because it produced a lower peak force on the sensor. A lower peak force means less force would be transferred to the head during the impact. - 6
Look at the material data: Material X costs $2 per helmet liner, has mass 180 g, and has impact force 720 N. Material Y costs $4 per helmet liner, has mass 130 g, and has impact force 610 N. Give one reason to choose Material X and one reason to choose Material Y.
One reason to choose Material X is that it costs less, so it could make helmets more affordable. One reason to choose Material Y is that it is lighter and produces a lower impact force, so it may be more comfortable and safer. - 7
A helmet shell can be made from polycarbonate plastic or thin metal. Polycarbonate is tough, lightweight, and does not rust. Thin metal can be strong but may dent and may be heavier. Which shell material would you choose for a youth bike helmet? Support your answer with at least two properties.
Consider safety, weight, weather, and comfort.
Polycarbonate plastic would be a good choice because it is tough and lightweight. It also does not rust, which helps the helmet last longer in wet conditions. - 8
A helmet foam liner is tested at three thicknesses. A 15 mm liner gives a peak force of 850 N. A 25 mm liner gives a peak force of 620 N. A 35 mm liner gives a peak force of 590 N but makes the helmet bulky. Which thickness is the best design compromise? Explain your reasoning.
The 25 mm liner is likely the best design compromise because it greatly reduces the peak force compared with 15 mm while avoiding the extra bulk of 35 mm. The 35 mm liner is only slightly safer in this data but may be less comfortable. - 9
Explain the difference between a constraint and a criterion in an engineering design problem. Then give one constraint and one criterion for designing a bike helmet.
Constraints are must-haves. Criteria are ways to compare designs.
A constraint is a limit the design must follow, while a criterion is a goal used to judge how good the design is. A constraint could be that the helmet must meet a safety standard. A criterion could be that the helmet should be lightweight. - 10
A company wants to make a low-cost helmet for students. List three material properties engineers should still test before choosing the cheapest material.
Engineers should test impact energy absorption, strength or toughness, and density or weight. They may also test durability, water resistance, and comfort because a cheap material still needs to protect the rider. - 11
A helmet absorbs 45 J of impact energy in a test. A second helmet absorbs 60 J in the same test before the force reaches an unsafe level. Which helmet has better energy absorption, and what does that mean for safety?
More absorbed energy usually means less energy reaches the head.
The second helmet has better energy absorption because it absorbs 60 J instead of 45 J. This means it can take in more crash energy before the force becomes unsafe. - 12
A design team tests three liner materials. Foam A has low mass, low cost, and medium impact protection. Foam B has medium mass, high cost, and high impact protection. Foam C has high mass, low cost, and low impact protection. Rank the foams from best to worst for a safety-focused helmet and explain your ranking.
For a safety-focused helmet, Foam B is best because it has high impact protection. Foam A is second because it has medium protection and good mass and cost. Foam C is worst because it has low impact protection, even though it is low cost. - 13
Why do bike helmets need to be replaced after a serious crash, even if they do not look broken from the outside?
Some damage is hidden inside the helmet.
Bike helmets need to be replaced because the foam liner may have crushed or cracked inside. Once the foam has absorbed energy in a crash, it may not protect as well in another impact. - 14
A helmet has vents to keep the rider cool. Adding too many vents can remove foam and shell material. Describe the trade-off engineers must consider when designing helmet vents.
Engineers must balance comfort and safety. More vents can improve airflow and make the helmet cooler, but too many vents can reduce the amount of protective material and may weaken the helmet. - 15
Design a simple fair test to compare two helmet liner materials. Include what you would keep the same, what you would change, and what you would measure.
A fair test changes only one main variable at a time.
A fair test would use the same helmet shape, liner thickness, drop height, test weight, and sensor for both materials. The only thing changed would be the liner material. The team would measure peak force, energy absorbed, and visible damage after the impact.