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A friend of mine gave me a foam bike helmet had the plastic covering removed from it. Besides for the plastic being removed, the helmet is in excellent condition. Is there any issues with using this helmet for riding locally as far as safety is concerned? I think the plastic cover was thin to begin with so I think it wouldn't make much of a difference.

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    Better than no helmet – paparazzo Aug 22 '17 at 20:14
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    Not sure it is Pap, gschenk's answer addresses my concerns. – alex Aug 23 '17 at 2:37
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    @alex - addressing gschenk's answer only, I expect a helmet with padding but no low-friction coating is still better than a skull with no padding and no low-friction coating. – AndyT Aug 23 '17 at 8:34
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    It's good enough for a ride to the store to buy a new helmet, but that's about it. – stannius Aug 23 '17 at 17:34
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    If the plastic covers came off, it either is a very damaged helmet or it is a very cheap or very old helmet that is not in-molded. Even decent helmets are now very cheap (≈15 €£$). It would be better to do as @stannius said and replace it. – gschenk Aug 24 '17 at 6:19
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The foam protects the head from impacts by deforming. The plastic cover obviously protects the foam from wear and aging. However it also serves safety aspects in most helmets:

A good helmet has a smooth outer shape, preferably close to a ball, to allow it to slide over tarmac or other hard surfaces in an accident. The hard and smooth plastic outer shell helps this by reducing friction and it prevents snagging.

A reason why we want the helmet to slide rather brake ones horizontal movement is to prevent rotational acceleration of the skull. Due to its inertia the brain resist that acceleration. This in turn exerts a force on the brain (warning, oversimplification the brain is in large parts mechanically decoupled from the skull by a gap filled with fluid). This so called rotational impact is thought to be an important contribution to concussion (cf Kleiven 2013).

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    The outer shell isn't just about the brain, it's about the neck. When foam helmets first came in (with cloth covering) there was a noticeable increase in neck injuries. They discovered (as you pointed out) that foam on its own catches in the ground and this was causing neck injuries. Thus the plastic shell helps your head slide and avoid a broken neck. – Niall Aug 23 '17 at 11:24
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    @Niall thanks. Would you be so kind to add this as an answre as well or if you happen to have sources add these to this answer? – gschenk Aug 23 '17 at 12:51
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    Another important (if obvious) point is that the foam probably can't take much friction before it starts to break apart. I'm not sure if one could hit hard enough, or slide far enough, for that to make a difference, but ... – jpaugh Aug 24 '17 at 8:06
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One thing the plastic is designed to do (and for which it doesn't need to be thick) is to have low friction with the road. So if you come off your head slides rather than snagging. The friction that the plastic stops would injure your neck, and also lead to very fast abrasion of the foam,perhaps just in time for it to be destroyed before you hit something solid. Even a fairly thin layer backed by foam also spreads the impact of a point over a larger area. Flexible plastic does a very good job of holding the helmet together, so that it absorbs more energy if it splits and still has some use after the first hit (one crash can easily cause multiple head impacts)

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    In addition the outer shell helps retain foam pieces in case of multiple impacts. A substantial impact could break the helmet in half, leaving nothing to protect the head for a subsequent impact (think tumbling) – Rider_X Aug 22 '17 at 23:59
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    This is the correct answer. I remember in the 80s doing science in high school, and one of the tests was to drag a large block of polystychrene foam across the concrete with various loads on top, and again with a piece coated in plastic. From memory, we had two or three school kids standing on the block of plastic covered foam and ~20 people dragging it, with only scratches. The naked foam grated like cheese on a cheese-grater and then snapped after partially eroding. Then we had to pick up all the loose pieces :-\ – Criggie Aug 24 '17 at 4:45
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You are correct that the impact protection comes from the foam. The plastic covering is there to protect the foam from dents and gouges. Now that it is gone (which I doubt could be done without damage, most helmets are directly molded into the plastic), it will be easily damaged.

Fun fact: Early non-racing helmets didn't have the plastic cover. They generally didn't last long.

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    As the other answers make clear, the plastic shell is not just there to reduce degradation of the foam. – David Richerby Aug 24 '17 at 13:00
  • Perhaps you can explain how vents and straps do not snag to ground. – ojs Aug 24 '17 at 13:15
  • @ojs what I am concerned with in answer is not so much dragging over the tarmac but oblique impact. Where a smooth surface might reduce momenta. Perhaps a helmet without vents, where a larger area would interface might reduce these; perhaps it would not. Simulations and experiments will be needed to determine that. However, a helmet without vents has only very little use for most cyclists. – gschenk Aug 25 '17 at 14:58
  • There is the issue of neck injuries, @niall raised above. I don't know if snagging matters for that. I hope someone will a address this in another answer. – gschenk Aug 25 '17 at 15:05
  • @ojs Polystyrene grips against rough surfaces; harder plastics don't. Straps can't drag on the ground unless your skin is already on the ground. Vents have a rounded profile to reduce snagging. Seriously, try dragging some polystyrene over any sort of rough surface. – David Richerby Aug 25 '17 at 22:51

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