Originally Posted by
mconlonx
I find it hard to believe and laughable that a self-professed helmet proponent expert is telling people they can modify their helmets by hitting them with a hammer.
If you are prone to wearing a helmet and find that you are a hard fit, before you go taking a hammer to a new, potentially expensive piece of gear, try on different models from different companies. Very often, variance of fit between companies and models is large enough to accommodate nearly every head size and shape.
As a PT bike shop employee who sells helmets, I would never advise that a customer modify their helmet like this, and I believe my boss would cringe from a liability standpoint to hear me voicing such instruction.
And, also:
The conclusion is a little misleading IMO, perhaps not fully thought through. If you strike a hundred helmets with a hammer, you'll surely have a number of them having fractures in the crush foam. The fractures, which are not necessarily apparent to visual inspection, seriously degrade the impulse reduction. Impulse, the change in momentum (of our brain) with respect to time, is the main thing we're trying to reduce with helmets.
To the extent that polystyrene has a linear response in accordance to Hooke's law, the impulse during crushing will be roughly inversely proportional to the thickness. So the shaving of a quarter inch from three inch thickness, would result in about 1/12 greater impulse from sufficiently strong impacts.
Partially crushed to the same thickness on the other hand, having greater density, you wouldn't expect it to have a large effect on
peak acceleration, but you'd expect it to be
less protective of lesser impacts than the shaved helmet.