Inflatable Helmets

 Inflatable Bike Helmets

When I was in Copenhagen, I noticed that most people wore bike helmets that inflate upon impact rather than standard bike helmets. Upon further research, I discovered that these helmets are advertised as much safer than standard helmets:

We know that standard helmets work by the helmet refill increasing the time over which the impulse acts. Simultaneously, the outer shell spreads the force to a larger area. 

The inflatable helmet uses artificial intelligence algorithms to know when to inflate. The process in which the collar is converted into a helmet takes 0.1 seconds. 

To determine which helmet is safer, I will compare the force upon impact when cyclists are going at the same speed, one wearing a regular helmet and one wearing the Hovding inflatable helmet. 

The average speed of an amateur cyclist is 18mph or ~8.0 m/s. 

So, if someone is biking at 8.0 m/s and gets into an accident, what is the force upon impact? 

Let’s say the cyclists are both exactly 70 kg.

𝐹∆𝑡=∆𝑝⃑=𝐼𝑚𝑝𝑢𝑙𝑠𝑒

∆𝑝⃑=mv= (70kg)*(8.0m/s)= 560 Ns

Time for a standard helmet= 6 ms

Time for inflatable helmet= 36 ms based on Stanford data: “We found that airbag helmets, with the right initial pressure, can reduce head accelerations five to six times compared to a traditional bicycle helmet (Kubota, 2016). 

The force upon impact for a standard helmet: ∆𝑝⃑/∆𝑡= F= (560Ns)/(6s)=93.33 N

The force upon impact for the inflatable helmet:∆𝑝⃑/∆𝑡= F= (560Ns)/(36s)= 15.56 N

Therefore, the force upon impact is ~6 times less with the inflatable helmet than with the standard.

How it works:

There are two aspects to the helmet’s function: the transformation from a collar to an airbag, and the functionality of the airbag itself. Researchers use an Artificial Intelligence algorithm for the helmet to detect when to inflate based on thousands of crash test videos. Inside the collar (helmet before inflation), a small gas inflator uses helium to inflate the airbag. The airbag stabilizes the neck and provides shock absorption. The pressure remains consistent so the airbag can sustain multiple head impacts during one accident, before it deflates. 

Rotation:

We know that oblique impacts result in rotation of the head, to which the brain is most sensitive. Linear acceleration is mainly associated with the risk of skull fracture, but angular (rotational) acceleration and rotational velocity are associated with brain injuries. The airbag in the inflatable helmet creates a flexible area between the head and the ground that effectively absorbs the energy that causes harmful rotational force.

To test how different helmets perform when rotational force is involved, researchers compared the translational accelerations (g), rotational (angular) velocities (rad/s), rotational accelerations (krad/s^2), and strain (%) for different helmets on the market. Strain is the brain tension ratio caused by rotation of the brain (generated by computer simulation). A strain above 26% corresponds to a 50% risk for concussion. Diagrams and explanations of each of the four tests (shock absorption, oblique impacts 1,2, and 3) conducted are below: 

Results of tests:

• The Hövding 2.0 helmet performed almost three times better than all the other conventional helmets (48 g vs other helmets that were around 175 g for translational acceleration). 

• In the oblique impact 2 test (side of helmet), the inflatable helmet had the lowest rotational acceleration of 1.5 krad/s^2. 

• In the oblique impact 3 test (upper part of the helmet), the inflatable helmet had the lowest rotational acceleration of 1.7 krad/s^2. 

• For oblique test 4 (impact on the rear of the helmet), the inflatable helmet had the lowest rotational acceleration of 2.8 krad/s^2, with the average being 12.0 krad/s^2. 

Overall, the inflatable helmet performed much better than other helmets in translational and rotational (linear and angular) collisions. This reduction in acceleration occurs because when the helmet is inflated, “the exterior fabric can slide sideways in relation to the fabric on the inside against the head, and two shearing layers are created that considerably reduces the rotational acceleration” (Olsson et al., 2017). 

Sources:

https://hovding.com/2021/09/06/have-you-tested-a-hovding-we-have/

https://news.stanford.edu/2016/10/03/stanford-researchers-show-air-bag-bike-helmets-promise/

https://www.cpsc.gov/s3fs-public/CP-18-1-Petition-Requesting-Rulemaking-to-Exempt-Head-Protection-Devices-from-Bicycle-Helmet-Standard-022118.pdf