[peanutaxis]

I have a small crack in the polystyrene of my helmet on the 'rim' just above where my left eye would be. I can spread the crack about 1mm but I have to pull quite hard to spread it. My bike mechanic of course says I should replace it. I asked him why and got a fuzzy answer.

So my question is, can anyone give me a blow by blow account of what might happen during an accident such that the crack will compromise the helmet doing it's job.

thanks

 

[Oh My Sack!]

Your helmet is DONE. Replace it.

What will likely happen is if you hit that same area or near it, it will not perform as designed by absorbing the impact.

Bike Helmets, not unlike motorcycle helmets, are a one hit deal. Once the EPS liner is struck, it will not react as designed for a second shot.

How did it get cracked?

 

[Rob-Bob]

Any damage to the helmet can effect how it will react during an impact.. How did you crack it.

 

[peanutaxis]

So my question is, can anyone give me a blow by blow account...

Your helmet is DONE. Replace it.

What will likely happen is if you hit that same area or near it, it will not perform as designed by absorbing the impact.

Bike Helmets, not unlike motorcycle helmets, are a one hit deal. Once the EPS liner is struck, it will not react as designed for a second shot.

How did it get cracked?

Any damage to the helmet can effect how it will react during an impact.. How did you crack it.

That would be two "no"s. My skepticism increases.

 

[Rocky Mtn]

To me it would just seem more likely to crack further and deeper upon impact.

To me you are risking your brain and saving $130.......

 

[Oh My Sack!]

What more of a blow-by-blow account do you need? :skep:

Perhaps you should do a little studying on the science behind moto and bicycle helmets and how EPS liners are designed to react to impact. If that doesn't sink in, pose your question to the manufacturer of the helmet...or ANY manufacturer of helmets. You will see that your helmet is DONE, plain and simple. They are not designed for multiple impacts. The answer you seek is very common knowledge. It's not because someone is trying to sell you a new helmet.

Then again, you can just take your chances and see what happens if you smack your noggin in that area and hope for the best.

btw....I'm not sure if this totally equates to mtb helmet blows but decades of studying moto helmet blows, the quadrant of the forehead above the eye is the most impacted area in a fall of all crashes. Something more to consider, perhaps.

 

[Procter]

Agree with others - replace it, but, if you're really interested, the best blow-by-blow I could give is the following:

A) It probably indicates that there has been some prior impact in this area. Bike helmets are made to take up impact and dissipate energy exactly once (in any particular area). This is done mostly through compression of the internal Styrofoam but also through bending and tearing of the outer shell. It could be that, whatever cracked your shell in that area, also compressed the internal styro in that area, reducing the helmet's ability to dissipate energy in that location in a future accident, increasing the brain injury that will occur. Its not 100% but its a chance. Its also generally a sign that it may have had its share of dings elsewhere and may just be old and past its useful lifespan.

B. ) Even if the internal styro isn't compressed at all, a small crack in the shell could reduce its energy dissipation capacity in that location. It takes a certain amount of energy to crack & tear the outer shell. Have a friend hold a piece of paper and try to punch through it or push through it. When you do this, the paper resists your fist as it tears. If you were to do a formal experiment measuring the resistive force of the paper, you'd find it's higher before the first tear occurs, and, after a tear forms, the paper's resistance to your force decreases significantly. The shell of the helmet has this same effect, translating impact energy to tearing energy or general plastic deformation. If it is already torn, this area will tear easier and hence provide less energy dissipation in a crash, increasing risk of brain injury. I have no idea how much of an impact is taken by the shell vs the polystyrene, it could be 10% of the energy dissipation, could be 20%. But you're talking about your head here.

(source: BS Mech Eng and Matls Sci, though, I never practiced and went straight into software so take it for what its worth)

 

[CheapWhine]

I am not sure what you mean by blow by blow. You have a crack in the helmet. It is well known that forces are concentrated at the end of a crack. This is why a small crack in a piece of glass suddenly expands. When you get in a crash, the forces will be concentrated at the tip of the existing crack, potentially causing a catastrophic failure as some of the energy is used to expand the crack through the rest of the helmet and the remainder is absorbed by your head. Without the crack, there is no focal point to dissipate the impact's energy, so it is dissipated throughout the helmet.

Of course, it is only your brain. Would you be able to get a replacement for less than the cost of a new helmet?

 

[Blatant]

I love pseudo-engineers (or, worse, real engineers) on the Internet.

A blow-by-blow? How about either replace it or don't. It's your head. The impact lining of these helmets is designed to be a one-hit wonder. If it's cracked, it's done. Period.

I replace a helmet any time it takes a significant impact. My skull is worth the $100.

 

[pharmaboy]

From reading everything I can get my hands on for this subject, evidence is brutally thin on the ground. Everyone who has ever crashed and broken their helmet however seems to be totally convinced that their helmet saved their life.

For xc, the main risk seems to be concussion and soft tissue damage and cranial fracture. Helmets used in cycling definately help the middle one and probably the last one. For road use it's different - cycling helmets are designed to help when you are smashed into by a a car at 30mph , flung 10 feet in the air and land head first into bitumen - for that they seem to reduce severe brain injury.

Your helmet was tested with an intact plastic layer and intact EPS. Now it may pass just as well or it may not. Only you would know what the impact was that caused the crack, and if it was small, you can at least surmise that if it was a small impact that the EPS hasn't crushed ( when testing helmets , the EPS has notable and obvious crush damage from the inside of the helmet) or if it was a big impact, it may have been.

Helmet manufactures are always going to recomend replacement - it is after all , strongly in their interest to do so.

 

[terrasmak]

From reading everything I can get my hands on for this subject, evidence is brutally thin on the ground. Everyone who has ever crashed and broken their helmet however seems to be totally convinced that their helmet saved their life.

For xc, the main risk seems to be concussion and soft tissue damage and cranial fracture. Helmets used in cycling definately help the middle one and probably the last one. For road use it's different - cycling helmets are designed to help when you are smashed into by a a car at 30mph , flung 10 feet in the air and land head first into bitumen - for that they seem to reduce severe brain injury.

Your helmet was tested with an intact plastic layer and intact EPS. Now it may pass just as well or it may not. Only you would know what the impact was that caused the crack, and if it was small, you can at least surmise that if it was a small impact that the EPS hasn't crushed ( when testing helmets , the EPS has notable and obvious crush damage from the inside of the helmet) or if it was a big impact, it may have been.

Helmet manufactures are always going to recomend replacement - it is after all , strongly in their interest to do so.

All EPS lined helmets do the same thing, motorcycle helmets are just designed to take a bigger hit with their outer shell staying intact.

When in doubt, change it out. Keep your old helmet for mild bike path rides with your kids, use your other one for taking on trails.

 

[Joules]

helmets absorb energy by breaking. No one can give you a blow by blow other than the fact that your helmets ability to absorb energy is basically gone. The nature of a situation where a helmet does its job is unpredictable.

If your brain isn't worth the cost of a replacement, by all means keep using it.

 

[LyNx]

This is a modern Darwin tool, idiots question them and don't wear them and those dumb enough who do, question when they should replace them Please don't replace it, help weed out the gene pool.

I love pseudo-engineers (or, worse, real engineers) on the Internet. A blow-by-blow? How about either replace it or don't. It's your head. The impact lining of these helmets is designed to be a one-hit wonder. If it's cracked, it's done. Period.

I replace a helmet any time it takes a significant impact. My skull is worth the $100.

So my question is, can anyone give me a blow by blow account...That would be two "no"s. My skepticism increases.

 

[RaythePedaler]

helmets absorb energy by breaking.

X100. They're designed to break to dissipate the energy of impact. A pre-broken helmet it's like a pre-used condom.

 

[vikb]

If you are going to keep that cracked helmet at least throw some Gorilla tape on the crack. Should be good as new then.

 

[J.B. Weld]

It's pretty obvious that helmets can prevent you from littering the trail with brains and blood, but 3 days of nausea and headaches after thumping my head on the ground last week left me wondering what, if anything helmets do to prevent concussions. Apparently 6 milliseconds can make a significant difference, and crushed or broken styro is probably going to reduce that number somewhat.

Helmets: How they Work and What Standards Do

 

[hillcountryav]

Bam!!!
High speed collision with a deer...Yes I believe my helmet saved me. This was a year ago this month and it still hurts to stand up straight, but with out a properly functioning helmet, who knows.
In a fraction of a second the liner broke through in four places and crushed the back.

 

[pharmaboy]

X100. They're designed to break to dissipate the energy of impact. A pre-broken helmet it's like a pre-used condom.

Maybe you should read some papers on what helmets do and what the design goals are, or in fact what the testing procedures are, because the helmet breaking is not how they work.

They are supposed to slow decceleration, that eps is special eps designed to crush during impact and provide extra distance for the brain to slow down.

A crack means the helmet would be more likely to break in a crash which reduces it's ability to do its job - ie stay together and absorb

 

[Procter]

Now you're just nitpicking - I think Joules and Raythepedaller were commenting that helmets are designed for one hit, not necessarily taking a stand on whether cracking itself plays a role in energy dissipation.

 

[pharmaboy]

Maybe you are right. Plenty of opportunity to read helmets .org and your post on how they work.