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I am currently testing a bunch of lights for my (slowly) upcoming head lights review. Several of the lights can be mounted to a helmet using a mounting strap, like this:

helmet-mounted light

I'm wondering if having a light mounted on the helmet impacts its effectiveness in protecting from impacts. It seems that if the light hits the ground first, it might cause the rider's head to torque at a weird angle, or rotate the helmet causing the rider to hit their head when they might not otherwise.

Is there any research or data on this, or are there any anecdotal accounts of helmet lights affecting helmet safety?

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    Yeah, the danger is that the light could catch on something or, in a head-over fall, torque the head back. Mostly this would just displace the helmet from it's optimal position, but the head torqued back (face plant) is a very dangerous fall mode with high probability of neck injury. However, the odds of such an event are very low, unless you're a gonzo rider.
    – Daniel R Hicks
    Mar 13, 2013 at 11:45
  • @DanielRHicks That all depends on where the light is mounted.
    – Mark W
    Mar 13, 2013 at 12:35
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    It was suggested to me that I do some testing with a "crash test dummy or some annoying siblings". If there's no preexisting info out there, I may have to find some way to test it out and see.
    – nhinkle
    Mar 13, 2013 at 15:25
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    I don't have time to put together a proper answer based on this, so maybe somebody else does, but... Some of the helmet testing standards (SNELL, AUS/NZ, etc) include provisions for any protrusions to break off, not impact into the helmet too much when hit straight on, etc... I think that's mainly intended for visors/peaks, but covers anything that the helmet manufacturer puts on the helmet. A light attached to the helmet by the consumer isn't being tested against that standard and probably doesn't meet that standard, rendering the helmet+light non-conformant to the safety standards.
    – freiheit
    Mar 13, 2013 at 18:40
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    @freiheit: Great response. I think a more accurate statement is "the combination has not been tested to determine that it is conformant to standards, therefore it should be considered non-conformant."
    – mattnz
    Mar 13, 2013 at 21:08

4 Answers 4

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I found something on the homepage of the Bicycle Helmet Safety Institute. They have an article about bicycle helmet lights, listing the pros and cons of them:

Summary: Lights on bicycle helmets can be useful, but must have a breakaway mount.

Especially about the breakaway mount:

The importance of breakaway mounts

The first and most important rule for mounting a light on your helmet is that it must break away readily when you crash or catch an overhanging obstacle. If it does not, you risk having your neck jerked when it snags on the pavement or tree. Besides jerking your neck, that can add to the g's of the shock to your brain when you hit pavement.

Although there are no studies mentioned how dangerous it is when the light does not fall off during a crash, some manufacturers test their lights to break away during a crash:

Only one helmet manufacturer we have spoken to provided their lab test levels, Uvex, proving that they actually have an internal standard. And the light manufacturer Jet Lites has a standard requiring their mount to break away when loaded with a 5 pound weight.

This shows that it is important that the light falls off during a crash, but unfortunately most manufacturers do not care. Which seems to be a problem:

Some manufacturers use hook-and-loop straps to hold their lights on. We have seen some that wrap through the vents and under that seemed unlikely to detach when they should have. But again, there is no standard for that.

Source: http://www.bhsi.org/helmetlights.htm

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Answer: Yes - anything on your helmet may affect its ability to function.

Remember the skiing accident that gave Michael Schumacher brain damage?

http://www.telegraph.co.uk/sport/motorsport/formulaone/michael-schumacher/10640839/Michael-Schumacher-skiing-crash-did-helmet-camera-cause-head-injuries.html

That was not cycling, but he managed to run his helmet into a rock, and the impact was focused and exaggerated by the gopro camera mount on his helmet.

Corollary: taking things off your helmet also interfere with its function.

The best example here is when the plastic outer layer gets a bit tatty or peels off completely. Some riders may elect to remove it all, leaving a foam/polystychrene head-cup that looks as before, but will fail badly.

This looks okay, but if your head is sliding down the road, the foam will simply tear rather than absorbing and time-stretching the impact.

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    I rejected a gopro for kayaking as the camera on top of the helmet combined with the rather solid mount makes it a snag hazard on the rather tree-lined rivers I'm used to. Most light mounts, cheap camera mounts etc are much more likely to break in an impact. Of course that might not help much depending on the direction of the impact.
    – Chris H
    Apr 15, 2016 at 6:14
  • @ChrisH Agreed - every body mount has a risk. A chest mount would be bad in a frontal impact, a shoulder/arn/wrist could catch. I suspect a good bike/kayak mount will be safer. Can use a secondary lanyard as well, with a good mounting position.
    – Criggie ♦
    Apr 15, 2016 at 6:29
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    On a white water kayak, mounting on the boat mostly sees water, but I was happy with a chest mount (which would require a very upright position on a bike). Now I have a camera on the side of my helmet with breakaway mounting (including for the backup lanyard). The same camera has a handlebar mount I never get round to trying. My helmet light uses elastic and the thinnest cable ties I could find (I snapped one catching the light on a bike rack while wearing the helmet).
    – Chris H
    Apr 15, 2016 at 7:10
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It is certainly true that adding bulk to your helmet has some drawbacks in crashes and such, and consequently warrants some concern. In the incidence of a crash, solid, small objects that are fixed to a specific area of the helmet increase the distribution of force on mounting area, instead of spreading it away from the impact site.

However, my justification has always been this: a light can prevent just more crashes than not having a light will save you once you have crashed. Essentially, by using a head mounted light, you have more freely accessible light and can therefore be more aware of your surroundings. Equally, your surroundings will be more aware of you. These two facts combine to reduce the likelihood of crashing in the first place.

Also, I like to think that landing helmet first with a light attached is just like landing helmet first into a branch or small rock. These are also likely occurrences in MTB riding, so I would assume helmets are designed with at least some thought on this sort of situation. It mightn't be the case in reality, and I haven't confirmed this with standards, but hopefully saying this will give you peace of mind.

So yes, as far as personal experience has told me, a fixed light on your helmet can potentially do some harm in the event of a crash, but in the rest of the time spent riding, lights make helmets safer, and therefore makes riding safer. Seems like a good idea to me!

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    The evidence you've presented does not allow you to conclude that helmet-mounted lights make cycling safer overall. You've argued that they make crashes less likely but more severe. That could make cycling safer (e.g., if they made crashes much less likely but only slightly more severe) or more dangerous (e.g., if they made crashes only slightly less likely but much more severe). And, frankly, you've not even demonstrated that they make crashes less likely: a helmet-mounted light makes some kinds of accident (e.g., getting snagged on trees) more likely.
    – David Richerby
    Mar 29, 2017 at 14:17
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I think that's harsh. It's never going to be possible to exactly weigh up the balance between the additional hazard of the light on the helmet versus the additional visibility given to other road users. My fitting has a plastic screw holding a plastic bracket which holds the light. I think it's going to make a direct hit more dangerous, but not much more dangerous. When I have the light on, especially at dusk or at night it really moves overtaking traffic away from me in a very noticeable way.

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