Cunobelin wrote: pjclinch wrote:
The question still remains, common sense says that if you are using "safety equipment" then it should be used within the proven design limits
The design limit is that the vertical component of your fall will be of the order of 12 mph. What percentage of falls do you feel this is unreasonable assumption, roughly?
There is the matter that helmets are pushed in a generic sense as lifesavers well beyond the above, but the actual design spec is the above, and is fair and reasonable, and the actual people making and selling the helmets aren't the ones claiming miraculous powers for them. Their legal folk are far too canny to let Marketing say they'll save your life, and why would they when so many cyclists are doing it for them in an unregulated and unactionable manner?
Once again, 12 mph design constraint is not an issue that relates to your forward speed on a bike, and banging on about it is unhelpful.
You believe that there is only one element to the impact energy, and that it will always be less than 12 mph.I presume that this will also be on teh point of the helmet used in the tests as well?
I will continue to accept the reality that there are other components in the impact speed including a contribution from the forward speed. Feel free to be deny that element
Of course the "claim" that there is only a vertical element will also disprove rotational injuries, inquiries caused by the helmet sliding on the head )or being ejected and a multitude of other issues that should be considered in the effectiveness of cycle helmets
Unless you hit something sideways then the vertical speed is the only one that actually has any significance in the direct KE absorption... That's basic kinetics.
There are two main cases where the 12 mph is the falling speed from upright is going to be exceeded.
- Any time you hit anything other than the ground (a vehicle, a wall, a tree, a rock, a lamppost)
- Any time you are thrown from the bike (high side, vehicle impact) and therefore start your fall from above 6'
The first of those is highly likely when doing downhill or mountain biking, and somewhat likely in an urban environment (there are just so many obstacles around).
The second is likely in any vehicular collision... the 12mph is based on a static fall...
Or is it?
A static fall from 6' would take (s=ut+1/2at^2) .37s, resulting in an 8mph collision
To fall at 12mph requires a starting height of 2.7m.
Of course the human body isn't a point mass, and so it's much more complex than that, with KE transfers as well as self protection being significant factors. In all the falls and collisions I've had I haven't hit my head, though I have seen it done.
The third issue is where horizontal velocity isn't retained by virtue of frictional forces. By definition those will be at the edge of your (now enlarged) head, and so will exert quite a significant torque to the neck, and induce a significant rotational acceleration in the skull. Both of those are known to cause very severe trauma, and for that reason motorcycle helmet specifications include (I am led to believe) elements designed to prevent such 'snagging'. Cycle helmets have no such design criterion, and the 'lightweight', 'vented' designs are pretty much designed to provide snagging points.
At least most helmets I see aren't worn properly, so they'll just rotate around the head or fall off...