SHARP, the Safety Helmet Assessment and Rating Program, is a helmet testing entity that outlines a rigorous and multifaceted set of standards aimed at ensuring that helmets are thoroughly tested to pass a litany of qualifications which will ensure, as much as possible, the maximum safety of a rider who may be in an accident while wearing them. SHARP tested helmets are purchased from the same retailers as consumers assuring that the testing sample is relevant to products purchased by riders.
The helmet undergoes a series of tests and is assessed with a star rating with 5 stars representing those that have tested the best. The rating is based on the predicted number of fatalities for those riders who wear the particular helmet. Those helmets that do not qualify for the 5-star safety rating are subjected to additional testing criteria.
Every model of the helmet that is rated with SHARP testing is subjected to 32 tests. 30 of these tests evaluate the linear absorption of impact and the other two test oblique surface friction. The tests are performed on a spectrum of helmet sizes.
The impact testing is performed with the use of impact anvils. The methodology
incorporates the helmet size with helmets dropped from a precalculated distance at various impact velocities on all areas of the helmet. The different types of impact anvils used are flat, kerb, or oblique. Each helmet is tested by dropping the helmet on different anvils to make an impact on a
*Flat and Kerb impact anvils depicted to the right*
The following helmet test matrix outlines the testing for the 32 linear and oblique tests:
The linear impact absorption tests against a flat anvil are also used to evaluate the helmet performance for oblique impacts. The following table outlines how linear impact velocity correlates to the oblique impact velocity:
The testing of peak vertical acceleration from linear impact absorption tests against a flat anvil is utilized in calculating the peak accelerations at higher impact velocities for oblique impacts. Between the 30 linear impact absorption tests against the three anvil types and the 15 oblique impact tests, 45 results are provided to the testers.
Each of these 45 results is then used to calculate the risk percentage of fatal injury based on a peak acceleration and risk of fatality comparison matrix that follows:
Represented below are three line graphs that represent various levels of injury – slight, serious, and fatal. As the peak acceleration grows, the risk of a slight injury goes down and eventually flatlines, the risk being serious holds steady, then rapidly drops off, with the fatality risk maximized as the acceleration increases. In other words, the faster the acceleration leading to the eventual impact, the higher the risk of fatality regardless of helmet quality.
For each of the 45 different impact configurations, an impact weighting factor is calculated. This is the probability of an accident occurring in the particularly tested scenario. These are calculated by multiplying the probability of the impact site by the probability of head impact velocity, and then by the probability of an impact surface. The probabilities for each configuration scenario are laid out in the following table:
Each of the 45 impact confirmations then has a weighted risk of injury calculated by multiplying the impact weighting by the helmet and configurational situations rate of fatality of the rider. These calculations are then added together to provide the full weighted injury risk for every helmet model being rated with the testing schematic.
The weighted injury risk is finally multiplied by the Department for Transportation (DfT) derived accident statistics-based exposure population (7,078) to determine the number of fatalities predicted overall. It is then this predicted number of fatalities are used to determine the safety rating attributed to each model by SHARP. The safety ratings are attributed based on the helmet’s predicted number of fatalities being below a particular threshold. If the final result of the calculation shows that the projected number of fatalities falls under 155, the helmet gets awarded a 5-star safety rating.
The rating system by ranges of predicted fatalities breaks down as follows:
Helmets that come in over the 155 estimated prediction of fatalities, require the weighting of additional criteria. This includes the helmet performance at a peak acceleration in linear impact testing against a flat anvil. If the peak acceleration is less than or equal to 300g, the helmets qualifies for a 4-star rating.
In-Depth Look at the Testing Equipment
Particular testing equipment is utilized to perform both SHARP’s linear and oblique tests.
*Head forms used for testing*
Linear impact testing is performed using twin, stainless steel guide wires, 4mm in diameter and positioned 455mm apart. These are secured at their upper end so that the effective length can be used to test the various degrees of impact velocity.
A control arm can hold the helmet securely at any point along the twin wire lengths. The arm can be remotely lowered or raised and can trigger the release of the helmet during the impact testing process. The supporting arm’s design permits it to test the helmet’s impact on any of the tested sites without needing to modify the helmet’s construct or size in any way. The arm is designed in such a way so that the center of gravity and corresponding counterparts of the apparatus can reside within a cone of 10-degree incline with a particular vertical axis at the point of impact. In short, the design is such that it allows the dropped helmet to land with equal efficacy on any tested sides or portions of it, with the testing part of the helmet being the main center of gravity.
For oblique testing, the requirements are similar to those of linear testing. More specifically, they require an anvil at the base of the testing apparatus, a free fall guiding system, as well as a mobile system that supports the helmet, an energy-absorbing base, and a net to catch the helmet to prevent any damages short of those being tested during the impact test. Finally, the system must be adjusted to correspond with rotating the helmet, as well as angling it in different ways to be able to guide it in different angles upon the anvil’s face.
More Testing Aspects
Head forms for linear testing are geometrically shaped forms made of metal that do not present a resonance frequency under 3,000 Hz. These will be utilized in helmet testing to view the potential damage to the head form from drops at varying velocities.
Every test will also record a time history for both linear and oblique impacts. The time history will be done to measure impact across certain anvils. The data collected is also managed to assure that every test is afforded a passing qualifier. The testing equipment is checked and calibrated as needed following the procedures as detailed by the manufacturers at predefined periods of time.
Before testing, helms are exposed to 25 degrees C temperature (+/- 5 degrees) for a minimal time of 4 hours. The helmets are also marked to designate specific areas for the tests. The helmet is positioned on a head form, centered to coincide with the head form’s center. The procedures are conducted consistently across all helmets, with variables only in helmet size, weight, the position of impact, and the anvil type.
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 The Safety Helmet Assessment and Rating Programme, Laboratory Test Procedures, Department for Transport, available at https://sharp.dft.gov.uk/wp-content/themes/sharp2017/pdfs/SHARP-laboratory-test-procedures.pdf