Cycling UK Forum

stoobs wrote:

MartinC wrote:[quote="stoobs
It's precisely because their design parameters have been exceeded, and that they have absorbed their designed energy that they break.

Can you offer any reasoned justification for this assertion?

Basic engineering, and the papers that I've quoted. I've already explained and you still don't get it. Your homework now beyond looking up fracture toughness, is to look up yield, yield stress, ultimate tensile stress and failure mechanisms.

You still haven't told us what your qualifications are. What are they? Tell us. Go on![/quote]

stoobs, you're apparently claiming that an overloaded helmet will only break when it's already met its design target for energy absorption. I've never come across anybody else making this claim so I'm interested to see what it's based on.

So far, in support of this, you've quoted a magazine article, the entire Snell website and now basic engineering. None of this is specific enough to understand what your claim is based on. What I was hoping for was a reasoned summary of the rationale behind your view.

[quote="stoobs
It's precisely because their design parameters have been exceeded, and that they have absorbed their designed energy that they break.[/quote]

Can you offer any reasoned justification for this assertion?

If the middle gear is set up at about the same ratio as your singlespeed then there'll be no extra drag 'cos it's direct drive - the gears in the hub are only used for the low and high ratios. You'll have some drag when you change down or up - but I guess it won't matter then. Just get the ratio between the chainring and sprocket the same as your SS and you'll have a 'normal' gear with the same efficiency and the option to change up or down.

Aha, that's the cause of the current disagreement. The opposing views are that a helmet that has broken into pieces did one of:

• Crushed to absorb energy, and only then suffered fractures so that it broke up.
• Fractured on initial impact, leading to limited energy absorption an minimal crushing.

Yes, I think this is the nub of the current disagreement. I think there can be a wide spectrum of outcomes between these 2 extremes. They will be dependent on many interacting factors outside of the material used - like the construction of the helmet and the nature of the impact. One thing I'd be very interested to know more about about is how the behaviour of EPS varies with the force and velocity of an impact.

Few helmets are now manufactured to the benchmark Snell B90 standard and even fewer to the higher B95 standard. This suggests to me that the standards are hard to meet and that there's little spare capacity even if the standard is met.

Of course, not having any specialist medical training in head trauma some people may not consider me qualified to comment on helmets!

You can secure an up and over garage door with a Garage Defender http://www.pjbsecurity.co.uk/products.asp?id=7 - I've seen them on sale in B&Q too. B&Q also sell very good chains with ground anchors too (Masterlock) for securing Motorbikes - they're good for bikes too.

Yes, you might find a combination of Chainwheel and Sprocket that gives you the right chain length. Using a half link in the chain should make this possible but you'd have to scour the net to find them - Sheldon Brown's shop used to sell them I think.

I agree that a chain tensioner isn't elegant and must add some friction to the transmission but I've often thought about fitting one to my hub gear bike - it avoids moving the wheel as the chain wears and upsetting the brake block alignment.

I've seen some that extreme MTB'ers use to avoid unshipping the chain that fit very closely to the chainrings. Again you'd have to scour the net.

I'd endorse CJ's comments aboutthe Sachs 7 - I've been running one for 8 years and it's still faultless.

stoobs wrote:While not the main energy absorption method, cracking is still a viable method of energy absorption:

http://cyclehelmets.org/papers/c2023.pdf

I don't believe I've paraphrased you unfairly.

My assertion was that a helmet that's cracked in an impact has failed. They're designed not to crack so if it's cracked it's failed to meet it's design objective.

Cracking (whether brittle or not) won't absorb as much energy as crushing. You've offered no convincing counter case.

Because a bicycle has no hard shell and contains large voids (ventilation) cracking allows the EPS to be displaced and contribute little or nothing to energy absorption.

It would seem clear that a helmet that cracks cannot offer it's optimum level of protection. Helmet performance is severly constrained by the design criteria (mostly weight and ventilation) and any decrease in the protection is a significant variation in effectiveness.

Suggesting that cracking is a viable way for them to operate is incorrect.

Your pitch was basically that the energy absorbed by the cracking in a bicycle helmet provided a viable level of protection. You've quoted the Snell site in support of that.

My assumption is that in the reference quoted they're talking about the EPS liner in a hard shell helmet not any soft foam comfort lining. If the EPS starts to crack then it's held in place to absorb energy by being crushed. So I think the statement about cracking is true in the context of a hard shell motorcycle helmet but not for a cycle helmet.

I agree with your interpretation of the CPSC advice. A small crack will have created a stress riser and further impact may cause this to propagate quickly and significantly compromise the protection the helmet provides. This must be true whether the crack happens pre the crash event or during it.

I don't have figures for the relative amounts of energy absorbed by brittle fracture, ductile fracture and compression of EPS. My understanding from reading the literature is that the first 2 are substantively less. If you've got more information here then I'd welcome you sharing it. It may change my view.

Stoobs, I'm sorry to labour this point (I'll explain why in a minute) because I think we agree about a lot of this. It's actually a pleasure to discuss helmets with someone as knowledgeable and sensible as yourself.

You're quoting the Snell website. I guess you're referring to the "How Helmets Work" section where it mentions "The foam liner then starts to crush and break which uses up a lot of the energy". You need to remember that Snell deal in many helmet standards (bicycle, ski, skateboarding, horse riding but primarily motorcycle). I believe this wording relates to the crushing of the foam liner of a motorcycle helmet within it's structural hard shell. The use of the word "liner" here is key, bicycle helmets don't have foam liners - the foam is effectively all. Motor cycle helmets are very different - the hard shell forms a protective shield within which the foam liner can't be displaced.

The Snell site also contains a copy of the CPSC Bicycle Safety Tips document. This says "Even very small cracks in the helmet may greatly reduce a helmet's effectiveness in preventing injury."

I find that the whole helmet debate is marred by the exaggerated and wildly optimistic claims about their effectiveness and implicit assumptions about their ability to protect. I'm, by no means, accusing you of this. I think fracture, whether brittle or ductile, is a serious weakness in bicycle helmets and I'd challenge anyone who wants to make a reasoned case otherwise.

Clearly cracking must absorb some energy - that's the laws of physics. Bear in mind that Snell B90 calls for a helmet to absorb only 100 Joules in the flat anvil test. I think the generalisation that a broken helmet has failed is more accurate than the generalisation that cracking absorbs viable amounts of energy.

stoobs wrote:[ the crushing (and cracking!) reduce accelerations through the absorption of energy

Helmet manufacturers go to great expense to avoid helmets cracking. They bond microshells to them, put resin in the EPS to bond it better and put plastic or carbon fibre frames into them. They know, like many of us and I suspect you too, that cracking, even if it's ductile, can only absorb a fraction of the energy that compression will and has a significant potential to totally compromise any protection the helmet can offer.

aesmith wrote:Energy absorption isn't the end in itself for a helmet, surely. The aim is to reduce deceleration shock to the head. I don't believe that's the same thing in all cases.

Tony S

Aaaargh! You're clearly right but trying to evaluate the first was hard enough!

stoobs wrote:Do me a favour and read what I said.

Check the SNELL website

Too many assumptions! Your post wasn't there when I started mine!

Yes the Snell site list most (all?) of the Specialized helmets as currently certified assuming the 7/2/2008 date is American.

More interestingly it asks you to contact them if you find a helmet on the list that doesn't have a sticker. My Air 8 is on the list but doesn't have a sticker.

Well here's the answer. On the website it says:

"All Specialized helmets comply to one or more of the following standards:

C.P.S.C
(US Consumer Product Safety Commission)
For more information on the CPSC helmet standard, visit the Consumer Product Safety Commission web site

SNELL, B-90A, B-90C, B95 (Snell Memorial Foundation)

CE, EN1078
(Europe Safety Standard)

AS/NZS 2063
(Australia/New Zealand Safety Standard)"

It says the same when you look at the individual helmets. I guess you need to look at the stickers in an actual helmet to find out what standard it's certified to.

Basically they all used to be Snell B90, now they're not.

I bought a Specialized helmet last week. Part of the rationale was that I believed them to be certified to Snell B90. Previous ones certainly were. The new one had a Snell logo on the box but there's no certification sticker in the helmet any more. Hmmmm..........

OK, thanks - I was looking for a pdf page 42!

I find it hard to reconcile "cracking is still a viable method of energy absorption" with the reference "Movement of a helmet about the head and breakage of the helmet shell also assist with the reduction of some energy." (my bold).

Certainly any fracture of the shell will absorb some energy whether it's brittle or ductile. The amount will be less than that absorbed through deformation/crushing. How much of a contribution this makes is impossible to evaluate because you need to know at what point in time the crack occurs - before or after how much crushing. Helmets struggle to provide any meaningful energy absorption because of the constraints they have to work under - weight, ventilation etc. Cracking will reduce this amount. Helmets are designed to meet the standards by being crushed not through breaking. I think it's fair to say that a broken helmet has failed. I can't see any justification for claiming that cracking provides viable levels of energy absorption.

The mention in the reference of helmet movement absorbing energy is interesting. Helmet manufacturers have put a lot of effort in improving harnesses to make them fit better and allow the helmet to move less.