The Properties of Bicycle Helmets

1. The Physics Principles of a Bicycle Helmet 

A bicycle helmet is intended to be worn by a person while riding a bicycle. The main design purpose of a bicycle helmet is to translate impact energy on the helmet into various other energies that do not result into direct damage on the user’s cranium. Their secondary design purpose is to minimize potential side effects such as interference and peripheral vision. (Bicycle Helmet Types)

The following is a general list of design criteria for a bicycle helmet:

            1.1 Crash Energy Management

            1.2 Firm Straps and Fit

            1.3 Comfort

            1.4 Outer Shell

            1.5 Easy Identification

1.1 Crash Energy Management 

Crash energy management is the central purpose of the bicycle helmet which involves concepts of acceleration, time, force, change in momentum (or impulse), and distribution of force through geometric design and special materials.

A bicycle helmet does two important things:

-Increases the distance of deceleration, which increases the time of deceleration, which decreases the rate of deceleration, which decreases the force.

-Spreads the point of impact over a large area.

Essentially the impulse or change in momentum received by an object depends on the force applied to it and its duration. Based on the equation J=F*t, where J=impulse, F=force, and t=change in time, it can be seen that the longer a force acts on an object the greater the impulse. The corollary of this is that if the change in momentum is prolonged, the magnitude of the force decreases. This can be seen by analyzing the equation F=ma, where F=force, m=mass, a=acceleration. In this equation we want to focus on the variable acceleration. The equation for acceleration is a=(v_f-v­_i)/t, where v_f=final velocity, and v_i=initial velocity. Because time is in the denominator of this equation, a greater value for it would result into a smaller value of acceleration.

With a smaller value of acceleration the value of force will also decrease because in the equation F=ma, force and acceleration share a direct relationship. By increasing the time it takes to decelerate the force of an impact, it becomes less severe. All bicycle helmets try to use this physics principle by incorporating a foam skeleton into the helmet, which would crush and deflate when experiencing a collision. The particles in this foam essentially translate the impact energy into a mechanical energy, which increases the time it takes for deceleration. The time is usually increased by a fraction of a second, but the crash energy is less severe.

The following data was collected by Brad W. who did an experiment on “The Comparison of Bicycle Helmets’ Abilities to Absorb Crash Impact.” (Bradley)

Basically Bradley measured the max acceleration on various helmets when a 2kg sandbag was dropped on these helmets two meters above.

First let us analyze the graph. For the No Helmet results the acceleration reaches 210 m/s² at 0.001 s and then the acceleration steeply decelerates. Since there was no helmet involved, the time taken to decelerate was essentially decreased because there was no distance increase for the impact to travel upon. Meanwhile, the maximum accelerations of the other test helmets were all significantly lower at an average of about 128 m/s². For all of these test helmets it is clearly noticeable that the slope of deceleration afterwards is much gentler than the steep deceleration slope of no helmet. This is because the helmet essentially increased the distance of impact, which increased the time for deceleration. Overall the curve for no helmet is very steep and narrow, while the helmet curves are smooth and bent, signifying deceleration over a greater amount of time, and hence a lower impact force.

Let us compare the impact force values of each helmet

Impact Force Value Summary

Notice that the impact force for no helmet is greatest at 420 N, while the other helmet impact forces are significantly lower. This clearly shows how an increase in time of deceleration consequently decreases the force of impact.

The following graph summarizes the inelastic properties of helmets and their role in impact energy management. (Foams Used in Bicycle Helmets)

In inelastic collisions that utilize helmets, the final kinetic energy of the moving objects is not the same as the initial kinetic energy. Rather than returning all of the potential energy (stored as part of the initial contact between the objects) into the motion of objects, some of this potential energy transforms into sound, thermal, or another form of energy. The result is that although momentum and energy is conserved, the system transfers some of the energy away from the moving objects into other parts of the system. Let us consider this in context of a helmet collision. (Hirsch)

The helmet essentially “cushions” the impact. The foam inside crushes and does not bounce back. The foam just gets thinner and slows down gradually which increases the time taken to decelerate. For a three foot head drop, the time taken by the helmet to bring the head to a stop is 6 milliseconds, while the time taken for the head to come to a stop without the helmet is 1 millisecond. These two scenarios are shown in the graphs above. As you can see, the case with the helmet involves a smooth bent curve that distributes the energy over 6 milliseconds. However for the no helmet scenario, you see a sudden spike that occurs within 1 millisecond. This sudden dose of energy will be fatal and will cause brain damage. This shows how the energy is translated into mechanical energy through the compression of foam particles. But energy is also translated into sound and thermal energy upon impact. When the exterior of a helmet skids on pavement, friction produces heat, and sound is also produced from the skidding. Hence there are many ways a helmet disperses the impact energy. (Foams Used in Bicycle Helmets)

Now we shall consider the helmet’s role in spreading the impact over a large area. As previously mentioned, helmets involve crushable foam such as Expanded PolyStyrene. This foam is generally shaped into a round structure that encompasses the head. Plastic is usually placed on the outside of the foam in order to keep the foam from breaking up and to make sure that a collision will result into sliding of the head on pavement, not a sticking jerking motion on the neck. Essentially the rounder and the smoother the helmet, the better it will slide. (Bicycle Helmet Types)

The diagram above also demonstrates the structural importance of helmets in distributing impact force. Essentially helmets can be compared to being stepped on by a snow shoe, while no helmets can be compared to being stepped on by a high heel. In the case of the snow shoe, the snow shoe has a greater surface area hence distributing the force, and reducing the pain, while the high heel has less surface area hence concentrating the force, and increasing the pain. The helmet’s round structure essentially distributes the force much like a snow shoe with a lattice structure, instead of concentrating it on the impact surface. This is another reason why a geometrically round helmet is preferred for safety reasons. (Mills)

1.2 Firm Straps and Fit 

It is important for a helmet to firmly fit a user’s head because a first collision is often followed by a second collision. If the helmet falls off due to insecure straps, then the user will be helmet-less for collisions that occur after the first. This is why a helmet must have a firm strap that encompasses the chin, and have a proper fit. Due to the variegated head shapes of human beings it is often very difficult to mass produce a one size fits all helmet, but it is necessary for greater security. (Foams Used in Bicycle Helmets)

1.3 Comfort 

There are two key factors involved in the comfort of a helmet

            -Weight

            -Ventilation

In order to satisfy the helmet’s requirement of a low weight, the helmet usually uses lightweight crushable foams such as EPS, and also lightweight plastics for the exterior. Generally current helmets usually weight between 10 to 12 ounces which is extremely light. (Foams Used in Bicycle Helmets)

In order to satisfy the helmet’s requirement of ventilation, the helmet also incorporates elongated ventilation holes into the exoskeleton and cover of the helmet. (Bicycle Helmet Types)

A ventilated bicycle helmet

1.4 Outer Shell 

As previously mentioned, generally it is preferred that the outer shell be formed in a round shape in order to distribute impact force. However, at professional levels, this can become a hindrance due to air friction which reduces a cyclist’s time. As such, professional cyclist’s often wear a helmet that is sleek and pointy much like the nose of a fighter jet, which greatly reduces security, but also greatly reduces air friction by cutting through air more efficiently. (Bicycle Helmet Types)

Road racing helmets (elongated design, sleek, pointy, and sharp)

Commuter helmets with rounder design

Children’s design with extremely round design for maximum safety

Skate helmet with optimal round design and smooth/hard plastic exterior

1.5 Easy Identification

It is also important for helmets to clearly identify the user, especially for cyclists that travel on busy roads, in order to decrease the chances of a collision with a moving vehicle. This is especially important for cyclists that travel during the night.

Often times helmets incorporate neon color, LED flash lights, and reflective tape to satisfy such safety demands. (Bicycle Helmet Types)

Helmet that incorporated an LED light system

2. Bicycle Helmets and Impact on Society 

Generally it can be seen that the use of helmets have had a positive impact on society. Although some claim that helmet safety is just a myth, helmets do use sound physics principles that can save one’s self from most treacherous falls. However helmets also have a tricky relationship with society. Helmet companies often have an extremely difficult time reaching out to the cycling market for a variety of reasons. (Contradictory Evidence about the Effectiveness of Helmets)

            2.1 Helmet Impact on Society: An Analysis of Statistics

            2.2 Helmets and the Market

2.1 Helmet Impact on Society: An Analysis of Statistics 

Bicycle deaths by Helmet Use in the United States

The above chart clearly indicates that most cycling deaths are correlated to cyclists that do not use a helmet. As such it can be generally concluded that helmets are important for the safety of cyclists. (Helmet Related Statistics)

Generally about 15% of bicycle deaths occur for youth aged between 0-14. It is estimated that in the United States only about 15% of children in this age group regularly use helmets. (Helmet Related Statistics)

About 40% of bicycle deaths occur for adults aged between 16-35. The helmet usage of this age group varies widely across the United States, but most white collar workers do wear helmets regularly, while young adults and other labour workers show lower percentages of helmet usage. (Helmet Related Statistics)

It is estimated that increased helmet usage can save Americans 1.8 million dollars per year in head related injuries that were caused by the cycling related incidents. (Helmet Related Statistics)

As such helmets do have a positive impact on society, and helmet usage should be encouraged for all cyclists. Currently most countries enforce a helmet usage law for any cyclist up to the age of 16. Some countries even enforce such laws for all age groups.
However such laws are rarely enforced or followed by citizens. Both state and federal governments around the world should tightly enforce helmet usage for all cyclists for all age groups in order to improve cyclist safety, and road safety. (Helmet Laws)

2.2 Helmets and the Market 

Generally helmet companies have had a difficult time promoting truly secure helmets to the cycling market due to a variety of reasons. (Foams Used in Bicycle Helmets)

1. Secure helmets often have an obtuse and round shape that is not considered fashionable by the market. Sleeker designs have been criticized as insecure and worthless.
2. Bright colors which are considered safer for cyclist identification are also not considered fashionable by the market. Current industry designs that are selling well usually incorporate darker colors with sleeker and elongated racing structures.
3. Helmet companies are unable to indulge in marketing campaigns due to fears of being sued by cyclists involved in accidents even with a helmet. Helmet companies are unable to claim improved safety or improvement from previous designs due to such fears. As such, helmet companies have a hard time reaching out to the market, and the market remains unaware about the importance of helmets.
4. Generally research and development in this industry has been limited especially due to the limited potential for marketing the product. As such revenues are limited and consequently funding for better materials and designs for a helmet are severely limited.

3. Helmets and New Technologies 

-Ring fit systems for “one size fits all”: Straps that tighten around the circumference of one’s head. Greatly simplifies manufacturing requirements to accommodate the variegated cycling market and also reduces responsibility of consumer to find the right size. (Bicycle Helmet for the 2008 Season)

-EPP (Expanded PolyPropylene) Foam: EPP is equivalent to EPS foam but has a slight rubbery feel. EPP has the unique characteristic in that after a crash EPP will over time recover its original uncompressed shape. EPS foam does not have the ability to uncompress from a compressed state after a crash. (Bicycle Helmet for the 2008 Season)

-Carbon fiber is also being used for the outershell of a helmet in replacement of plastic. Carbon fiber is stronger and lighter, but the amount of it used is so minimal that the benefits are outweighed by the cost of the fiber itself, compared to plastic. (Bicycle Helmet for the 2008 Season)

References

Bicycle Helmet for the 2008 Season. (2008). Retrieved October 17, 2008, from Bicycle

Helmet Safety Institute Web site: http://www.bhsi.org/

Bicycle Helmet Types. (2008). Retrieved October 17, 2008, from Bicycle Helmet Safety

Institute Web site: http://www.bhsi.org/

Bradley, W. (2008). Comparing Bicycle Helmets’ Ability To Absorb Crash Impact.

Retrieved October 17, 2008, from Selah K12 Science Projects Web site:

http://www.selah.k12.wa.us/soar/sciproj2005/BradleyW.html#Results

Contradictory Evidence about the Effectiveness of Helmets. (2008). Retrieved October

17, 2008, from           Helmets Web site:http://www.cyclehelmets.org/1052.html#A

Foams Used in Bicycle Helmets. (2008). Retrieved October 17, 2008, from Bicycle

Helmet Safety Institute Web site: http://www.bhsi.org/

Helmet Laws. (2008). Retrieved October 17, 2008, from Bicycle Helmet Safety Institute

Web site:http://www.bhsi.org/

Helmet Related Statistics. (2008). Retrieved October 17, 2008, from Bicycle Helmet

Safety Institute Web site: http://www.bhsi.org/

Hirsch, A. (2002). Physics 12. Toronto, ON: Nelson.

Mills, N. J. (2008). Protective Capacity of Bicycle Helmets. Retrieved October 17, 2008,

from University of Birmingham Web site: http://www.perg.bham.ac.uk/pdf/ProtectiveCapacityOfBicycleHelmets.pdf