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Having a rather "pointy" head (think of something like Patrick Stewart's head), all bicycle helmets I've every tried on tend to have contact with only one point at the top of my head.

Now my idea was to cut a little bit away from that rigid foam at that location. Let's say 1-2 millimeters at the size of a dime.

My local bike dealer told me that the whole helmet has a fine-tuned structure that completely breaks down when removing even the slightest piece of rigid foam, no matter how small.

While this sounds strange to me, I'm confused right now.

Therefore my question is:

Does a bicycle helmet still has protection when slicing some 1-2 mm from the inner rigid foam at a size of a 1-2 cm circle?

(I'm from Germany, in case this matters; maybe our helmets here are different)

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    I would not do it. But you can do the reverse, get a helmet that's slightly bigger and alter the padding. Often helmets are sold with pads of different shapes and thickness for fitting purposes.Put in thicker padding at the circumference and thinner at the top of the head.
    – Carel
    Aug 29, 2017 at 19:10
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    By rigid foam you mean the polystychrene? Or the open-cell foam stickers that help it conform to your head shape ?
    – Criggie ♦
    Aug 30, 2017 at 7:51
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    I have a fat head - and helmets are hard to find. The best answer is talk to an LBS and tell them what you need. Don't shop on-line for helmets, and only buy one if you can try it on first. (a hairnet may be required for "trials" in the shop - some of them are picky.)
    – Criggie ♦
    Aug 30, 2017 at 7:53
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    @Criggie Yes, I do mean polystyrene. I.e. what we call "Styropor" in German slang.
    – Uwe Keim
    Aug 30, 2017 at 8:56

7 Answers 7

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1-2 millimeters the size of a dime is not going to change the fit of a helmet.

Ventilated helmets have holes. Removing a dime from rigid foam is not going to cause a helmet to fail.

Consider an accident where you first hit and crush a dime size piece of foam. If the entire helmet failed from isolated damage then not a safe helmet. Helmets are designed to take damage.

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    "Helmets are designed to take damage." They're also supposed to be thrown away after being damaged.
    – David Richerby
    Aug 30, 2017 at 20:52
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    This answer offers dangerous advice. While some helmets might offer enough protection if you remove some foam, you simply don't know if your helmet will be ok. And the only way to find out is to crash it. "Helmets are designed to take damage..." once. Multiple sources agree on this. Please, for your own safety, do not deliberately damage your helmet unless you are immediately going to throw it away. It is simply not worth the risk.
    – jimchristie ♦
    Sep 1, 2017 at 19:32
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Unbeknownst to many consumers, different helmet manufacturers use different molds for their helmets. Just like some shoe brands are known to be thinner or wider than other brands, some helmets are more oval, some more round, and some pointier. Even from the same manufacturer, their racing models might be different from MTB, skateboarding type, and general lines. And there might be a helmet with a vent hole in exactly the right place out there.

It's important to find a helmet that fits correctly and comfortable. It might take having to go to different stores and trying different brands and types (or even to a store in a different country, if you have the opportunity).

That being said, through, if it were my head and I had to decide between a helmet that was partially shaved to fit -- or a larger helmet with some extra padding to fit; I would choose the latter. The helmet manufacturers provide extra padding pads of varying thickness in the box because they know they can't produce a helmet that fits perfectly on everyone's head. As long as the helmet is snug on your head and not uncomfortable, you should be good to go. Remember that many cyclists wear beanies in the winter and cycling caps in the summer under their helmets -- something that is well known to the helmet manufacturers.

Remember the worst helmet is the helmet you don't wear.

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    I think you're right (so +1) but the adjustment padding has a lot of give in it, and by using too much you can end up with a helmet that settles easily into a poor position. You have to be careful not to start too big.
    – Chris H
    Aug 30, 2017 at 12:12
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    The padding kits are supplied in the box with the helmets (whereas the no manufacture will tell you to cut into the helmet for fitting). Second, many of us in winter months wear beanies or in summer months, cycling caps under our helmets.
    – RoboKaren
    Aug 30, 2017 at 15:31
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    If you just use the padding kit supplied you should be fine, unless it's really too big or a poor fit anyway. I'd be wary of "extra" padding in addition to that, which was how I read your 3rd para initially
    – Chris H
    Aug 30, 2017 at 15:41
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Does a bicycle helmet still has protection when slicing some 1-2 mm from the inner rigid foam at a size of a 1-2 cm circle?

You don't know. It's as simple as that.

The amount of material you're talking about removing is a tiny fraction of the helmet's overall volume but you simply don't know what effect that will have. You have no way of knowing if the material you're removing had some structural purpose. You have no way of knowing how badly you're damaging material around it. You have no way of knowing how the helmet will behave after alteration.

Some analogous examples.

  • If you're careful, you can stack a few bricks on top of an empty coke can. But deform the can slightly and it'll be instantly crushed. The inside of your helmet is a smooth curve but it stops being smooth when you remove some material. The thinned part is more flexible, which could affect the whole helmet's behaviour under the load of your head hitting the deck.

  • Aircraft have rounded window corners. Squaring off those corners would only remove a tiny percentage of the aircraft's hull but it would cause stresses to concentrate at the corners leading to planes breaking up in flight. You don't know what effects removing your tiny fraction of material will have.

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    I don't think your analogies work too well (especially the second). The moulded foam often has some hard-edged cutouts (e.g. where the adjustment for a support at the back of you head attaches)
    – Chris H
    Aug 30, 2017 at 12:15
  • As I commented on Daniel Hick's answer, I don't think it's as black and white as a coke can crushing or a plane breaking up in flight. Rather, it would be about the reduction in alleviation of force and increase in risk. If slicing up the helmet caused its ability to reduce impact to go down from 10G to 9G -- and increased the risk of brain damage by 5% (all figures are theoretical), would one still risk it? I personally wouldn't.....
    – RoboKaren
    Aug 30, 2017 at 15:56
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The main function of a helmet is to absorb energy during an impact, I you think about it, the material that is between your head and the object being impacted, is compressed or crushed during the impact.

In that regard, the more material there is to compress/crush, the more energy can be absorbed before it is too much for your skull to handle.

This crushing also diminishes the deceleration for your head, thus preventing your brain from hitting the inside of your skull too hard. In the same line, the more material there is in the helmet, the more time to decelerate your head.

Another function of the helmet is to spread the load over a greater area. For example if a rigid flat object hits your head, only a tiny "contact patch" of your head takes the whole impact. With a helmet, the impact is spread over all the areas of contact between the helmet and the head (in the direction of the impact, of course). For this function the helmet depends on it's structural integrity. In the case of a vented helmet, the material is similar to beams or trusses of a construction. If you shave part of those, you are weakening the structure, thus a softer impact will be able to deform or break it.

So, it is a terrible idea to remove material from a helmet.

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This is a touchy issue. In general, unless the helmet is one that was very precisely designed for the lightest possible weight and the smallest dimensions, you should be able to cut away about 1/4 of the thickness of the liner in a small area without seriously reducing protection. It could be a bit tricky to do this over a larger area without endangering the integrity of the liner, however, from the standpoint of it fragmenting in an accident.

Probably better is a slightly oversize helmet with additional padding added. But one must be careful to use appropriate padding material -- not too hard, not too soft. A too-hard liner would transfer too much stress to the skull, while a too-soft liner would allow the head to "bang" against the real liner in an accident. And an added liner is apt to reduce ventilation and make the helmet less comfortable.

It's not really rocket science, but you have to have some understanding of the forces involved and how a bike helmet works.

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    " you should be able to cut away about 1/4 of the thickness of the liner in a small area without seriously reducing protection." [citation needed] This seems like very dangerous advice, to me.
    – David Richerby
    Aug 29, 2017 at 23:48
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    @DavidRicherby - If losing 25% of the thickness of the pad is going to produce a serious reduction in protection then it's not very well "over-designed", is it? Over-design is a necessary feature of safety devices of this sort.
    – Daniel R Hicks
    Aug 30, 2017 at 0:59
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    25% is a massive fraction, which could hugely compromise the structure of the helmet. And there's much more to consider than just the volume of the material. For example, early jet planes crashed because their square window corners concentated stress and caused them to fail. How do you know you're not saying, "Squaring off the corners of the windows of a 747 will remove much less than 1% of the plane's skin. C'mon, it'll be fine!"
    – David Richerby
    Aug 30, 2017 at 9:19
  • @DavidRicherby - I said, "you have to have some understanding of the forces involved and how a bike helmet works".
    – Daniel R Hicks
    Aug 30, 2017 at 12:11
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    I'm more worried about the stress fractures or other structural weaknesses that amateur carving would create. Furthermore, you're aware that most helmets bought these days come with padding kits of various thicknesses that allow you to adjust their fit, right?
    – RoboKaren
    Aug 30, 2017 at 15:27
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It largely depends on what type of helmet you have. If it is a fancy "aero helmet", it is likely already very thin, and I would not feel comfortable taking any material off it.

But it should be safe to remove material from a "regular" bike helmet. Most helmets are simply foam with a plastic cover on top, and can take quite a bit of abuse.

The purpose of helmets is to absorb the impact when your head hits the ground. But very rarely do riders crash so badly that they use of the full absorption potential of the foam in their helmets. More often the helmet simply distributes the impact over a larger area of the head.
Imagine the difference between dropping a 1kg bag of flour on your head vs dropping a 1kg rock on your head; the force is the same, but the flour has a larger contact patch than the rock. By giving your helmet a better fit, you will allow the impact force to be spread over a larger area.

When it comes to removing the foam, don't cut it out. Use a small fingernail file (the type you would find on fingernail clippers). This will prevent you from cutting out more foam than you need, and it will also allow you to "shape" the indentation so it fits your head better.

I would not recommend buying a larger helmet as Carel suggested. Ideally a helmet should make contact evenly with your entire head when you tighten the straps. Padding would likely prevent this. A good way to ensure you have a well fitting helmet is to try to wiggle it around when you have fully tightened the straps. The helmet should not move around on your head.

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    "But it should be safe to remove material from a 'regular' bike helmet." [citation needed] This seems like very dangerous advice, to me.
    – David Richerby
    Aug 29, 2017 at 23:47
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    Cutting a 2mm out of a 20mm wide circle equates to removing 1/8 of a teaspoon of material. That is going to have a negligible impact on the structure of the helmet and it's ability to absorb impact.
    – sam
    Aug 30, 2017 at 0:00
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    2mm out of a 10mm thick foam helmet is 20%, and it forms a weak spot. Even if the helmet was an inch thick its almost 10%. Sorry but I can't agree. Your answer is well written though, so please don't take it badly, and have a go at some other questions on the site.
    – Criggie ♦
    Aug 30, 2017 at 7:50
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    I'd be worried about the reduction in material -- but even more so, that in the process of cutting the foam that you inadvertently create score marks or cracks/weakness in the foam that would cause it to break on impact along those areas.
    – RoboKaren
    Aug 30, 2017 at 8:16
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    @Sam Have you tested that? Have you done any analysis of how your modification will affect stresses in the helmet? Remember that early jet planes crashed because their square window corners concentrated stress and failed. How do you know that your eighth of a teaspoon won't do that? How do you know is the material you removed was important?
    – David Richerby
    Aug 30, 2017 at 9:21
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They are designed to make money ..

ESP helmets do nothing to protect from concussion .

They are safer the more you add fault lines .. so they can crack at lower stress levels .

As tested they only absorb motion after 200 plus G s ... while 50 plus G's while cause CONCUSSIONS.

Dealers will try and scare you into buying a replacement if yours has a fingernail scratch inside...

Yet a few knife slices will actually lower your chances of concussion..

See David Camarillo TED.COM talk on why helmets don't work .

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