HELMETS - Off-center Impacts
By Mark Taylor
For over two years, the BMC Technical Committee's Helmet Testing Program has conducted much useful research into the level of protection one can expect from the various types of climbing helmet on the market. However (as is inevitably the case with these things!) the investigations have thrown up some important new questions and areas of concern. Here, Mark Taylor of the University of Leeds (head of the testing program) looks at one such issue - the case of off-center impacts - and discovers some surprising facts.
At the end of the year 2000, the BMC climbing helmet testing program found itself with several unanswered questions amongst the mass of useful data it had generated, but one issue in particular stood out above the others. Testing had shown quite clearly that in an impact on the crown of a helmet, a traditional hard-shelled helmet with a textile cradle offers unparalleled protection, but as we all know, this only reflects one of the hazards that a climber faces where a helmet may provide protection.
When the original UIAA standard (on which the current EN standard is based) was written, these hard shelled helmets were all that was available, and the writers considered that the major risk to climbers was from falling objects. As a result, the major pre-requisite of the standard was that helmets for climbing and mountaineering should provide adequate protection against impacts on the crown of the head. Unfortunately, this means that requirements in both these standards for minimizing the effects of impacts elsewhere, or for preventing major injury if the climber hit the ground headfirst are minimal.
After several conversations with climbers about the effects of hitting the deck or swinging headfirst into a route we decided to look further into this crucial area. A new comparison test was devised whereby the front of a helmet (inclined at 45° from the vertical - see figure 1) would be subject to an impact equal in energy to the crown impact test stipulated in the EN standard - 98 Joules (or a 5kg weight dropped from 2m in real language!), using a at striker. Three common types of helmet were to be compared:
A comparison of these results found and the standard crown impact results for similar helmets is shown below, and makes pretty disturbing reading. As we can see quite clearly, the helmets utilizing foam as an energy absorber offer far better protection in the frontal impact situation, with the thick foam/soft shell helmet * showing forces transmitted very similar to those experienced in the crown impact test. If the three types of helmet are carefully examined it is not too difficult to see why the results are this way:
Thick foam/soft shell* - The thickness of foam is constant throughout the whole shell, and as this is the main energy absorbing layer, it is clear that the helmet will be effective wherever an impact occurs.
Thick foam/hard shell - With these helmets the foam is concentrated around the crown, and thins out towards the edges or simply stops. Again the foam is the main energy absorber, and as it is thinner at the edges, the transmitted force is much greater for an impact in these areas than one where the foam is thickest.
Traditional - The main energy-absorbing component in these helmets is the textile webbing cradle, and the crucial clearance distance between this and the shell. This webbing is anchored to the shell at the rim, which means that the nearer the rim an impact occurs, the lower the energy absorption will be - in this case the force transmitted is over four times the maximum allowed for a comparative crown impact by the EN standard.
In light of these findings, we should remember that most incidents reported to the BMC do involve impacts in the crown area, and on the whole the helmet does its job more than adequately. In the past 20 years there have only investigated two instances that we are aware of where injuries were sustained to the forehead or back of the head whilst wearing a helmet. So, what does this mean for the user? As usual it all comes down to choice, preference and the type of climbing you will be doing, but the above results provide a couple of clear pointers to bear in mind when buying a new helmet:
For general rock climbing, you should be looking for a lightweight helmet that offers good all round protection (i.e. from impacts from all sides) with good ventilation to help keep the old noggin cool. Modern foam/shell combinations are a good choice.
For alpine and ice climbing, good top impact performance is more important along with good resistance to penetration from sharp falling objects. Traditional shell/cradle models are more appropriate for this use.
(Ed. Note: for caving choose either a traditional hard-shelled or a modern thick foam/hard-shelled helmet, NOT a soft-shelled helmet)
* (Ed. Note: thick foam/soft-shelled helmes are not suitable for caving due to easy damage to the soft-shelled covering!)
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