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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 '13 at 11:45
    
@DanielRHicks That all depends on where the light is mounted. –  Mark W Mar 13 '13 at 12:35
    
@MarkW - Yep, there are a lot of variables. –  Daniel R Hicks Mar 13 '13 at 15:13
    
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 '13 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 '13 at 18:40
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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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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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