I cracked yet another helmet at the weekend (after a 10ft plummet/slide down a bank).
The theory is, as I understand it, that the polystyrene compresses in absorbing the impact.
But I crack a helmet every few years, which means this is not working, I tend to wear a snug fit, should I go a size up?
No - the better the snugger the fit the better they work.
No compression of the foam at all? Pics? How hard did you thump it?
cynic-al - MemberBut I crack a helmet every few years, which means this is not working
I don't follow?
Can't see any...slid down a bank like I say, hard to say how hard.
Thing is my forheid is quite square so the front of the helmet (where they always crack) is a few mm out from my ponce, so an impact cracks it rather than compresses it.
I'm just thinking the "snuggest is best" logic may be flawed...as long as the helmet doesn't move on impact.
NW - if the helemt just compressed, it wouldn't crack. A crack means it has bent (or tried to).
teh snugger the fit the better they work the nore in contact with your head on impact the more the forces are dissipated
I will try to find the diagrammes
I understand that TJ, but it's not working in practice for me, maybe I need a better fitting helmet.
Here we go
The illustration above is designed to demonstrate the effect of a helmet which does not fit correctly. On the left, the helmet is not a snug correct fit to the head. On the right, the helmet fits snugly and touches the head at all points. The red areas denote the effective crumple zone in each case. As you can see, the area covered when the helmet is the correct size is quite noticeably larger than that in the helmet which does not fit. This in turn means that there is less material which can be crushed leading to less energy absorption.
http://www.whycycle.co.uk/safety_and_security/cycling_helmets/
If you crack one every few years, no problem, there supposed to be replaced thar often anyway.
Crossed posts - try a different shape helmet? I'd be interested in pic or seeing this one
In theory if there is a gap at your forehead in an impact on the forehead your head would move forward inside the helmet to take up the gap
crack is bang in the middle, no signs of compression.
IMAG0075.jpg by alan cole, on Flickr
Thats weird. The crack is even in a thick part of it. you are thinking its flexed in far enough to have cracked as the inside went into tension?
Ask the manufacturer?
I can't see what else would have done it...and that's happened to me before.
I don't imagine Giro would be that interested but it might be a laugh asking...
cynic-al - MemberNW - if the helemt just compressed, it wouldn't crack. A crack means it has bent (or tried to).
Well, not neccesarily- polystyrene can crack as it compresses, especially under a fast hard impact. Though I think you're right enough and that's not what happened here.
What should I get then?
My best ever fitting helmet was one of those 1990 Specialized ones.
I thought that the shell was also there as part of the shock absorption and that it cracking was taking energy out of the impact. In essence, with that sort of fall, I'd expect to have to replace the helmet.
Well I went looking for a better fitting helmet...and I found one, a £160 Specialised 
needless to say it wasn't bought.
All helmets have the roc-lok type band extending over the rear 50% of the helmet....many have a continuous loop around the whole helmet, meaning you have no idea how well the helmet conforms to the shape of your head.
So it seems the perfect fitting helmet has been given up on - even by Giro, whose original usp was 5+ sizes so you could get a perfect fit 
I hate buying helmets; I ALWAYS struggle to find a decent fit. The best was Cratoni, but I have such a big head that most brands physically don't fit on my head, especially if I've got my (thin) winter beanie on. I've cracked a couple too, but with pretty hard impacts. I must admit I always thought they were supposed to crack on impact.
My Met Veleno did the same as your pic above when I had a head on crash earlier this year.
I was surprised to see no sign of compression, just a large crack. I thought that the polystyrene compressed to absorb the impact, but I guess the crack means that energy was transferred into the helmet & stressed it enough to cause it to crack.
I expected to see a squashed section of helmet though.
I've never seen a helmet damaged by compression only and all mine have broken by cracking.
I have to say I think the diagram above and implication that compression is the way helmets absorb energy is dreamland.
Get an Urge Enduromatic, Al. You'd look well buff (like me). They fit snug all the way round. Most comfy helmet I've ever had.
And welcome back.
Helmet appears to have done its job. Cracking is a way of absorbing energy. So it might have a had a bash when you fell off. Happy days, it could have been a childs your face
TBH it looks like an obvious place for a crack to me.
a few mm out from my ponce
Rolling with laughter at that one. Obviously meant to be bonce. 
Sorry, not very helpful...
Have you tried the Kask helmets in TBC (if he still does them)? They seemed pretty good, although not quite right for me.
I ended up with a £100ish Spesh. Figure it's worth it though and they have a reasonable crash replacement policy.
Actually, if you wanted to stick with Giro they have a crash replacement policy too.
took a while but glad someone got it.
Anyway here's my updated pic of how helmets actually work, unless you are lucky enough to have one that fits your head perfectly:
helmet by alan cole, on Flickr
Cheers A, the £40 for my Hex is not bad but I think I'll go for something else, I thought it was a good fit but I think I'd get better protection from something a little bigger.
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