I found an interesting article: "HELMETS - Off-center Impacts" By Mark Taylor posted on the NSS website:
http://www.caves.org/section/vertical/n ... lmets.html
Here are some of the highlights
"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.
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!)"
Also, from the British Mountaineering Council website:
"Picking a helmet
All the evidence shows that in the event of a strike to the head, the wearing of any helmet can greatly reduce the severity of injury. Given this, the single most important criteria when choosing a helmet is that you actually like it and feel comfortable wearing it. What is the point in buying the best performing helmet going if it’s uncomfortable and you hate wearing it? As well as finding a comfortable helmet, it’s worth understanding the basic differences between the various types of on the market:
Traditional hard shell:
The main energy-absorbing component in these helmets is the textile webbing cradle and the size of the crucial clearance distance between this and the shell. The 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. This problem is often compounded as the clearance between the shell and the cradle is usually much less towards the rim of the helmet.
- Good for impact performance on the crown of the helmet (e.g. rock or ice fall).
- Less good for side impacts more common in fall situations.
- Score the best result for resistance to penetration from sharp objects.
- More suitable for alpine or mountain routes.
E.g. HB Olympus, Petzl Ecrin Roc.
Thick foam/soft shell:
Energy absorption comes from the thickness of the foam. If the foam is a consistent thickness throughout the design the helmet will be equally effective wherever the impact occurs. Examine the design closely though, because if the foam gets much thinner towards the rim, there will be the same loss of performance in side impacts as with traditional designs.
- Lightweight and with good side impact performance
- Less good in the penetration tests where localised pressures can build up.
- Often more easily damaged through misuse (stuffing into rucksacks)
- More suitable for general rock climbing where there is more concern about falling than rock fall.
- More likely to completely disintegrate in a major impact than traditional designs - not a major problem on a single pitch crag, but a cause for concern if you find yourself with several broken pieces on a big alpine route.
Eg. Cassin Astral, Grivel Cap.
Thick foam / hard shell:
A combination of the above two. The foam is concentrated around the crown and either thins out towards the edges or simply stops. Again the foam is the main energy absorber, and where it thins the transmitted force is much greater.
Depending on the exact nature of the design and the distribution and amount of foam, the performance of these designs may fall closer to one or other of the above types.
Eg Petzl Elios, Black Diamond Half Dome.
A couple of articles in recent years have suggested the top tip of using the space between the cradle and the shell of a traditional helmet to store spare kit such as your headtorch, spare batteries, ski goggles first aid kit or bivvy bag. However it is precisely this space that gives these types of helmet their performance, and taping anything into them can severely reduce performance. Filling the space with anything solid removes almost all point in wearing the helmet. "
The Elios is listed as one of the modern, thick-foam, hard shell helmets, so it should be fine for caving. It seems the Elios offers better protection for side impact and sufficient protection from top impact. The Ecrin Roc is definately more durable, but after a *serious* impact, even an Ecrin should be retired. Also, a not on the Ecrin: don't put your Sten battery or other junk inside your helmet!