Does the padding inside of a bicycle helmet contribute to the overall security of the helmet? I.e. do they cushion a crash in some way, or are they just there for comfort?
Because there is an optimum placement position for the helmet on the head, having padding that helps keep the helmet in place, or makes the helmet more comfortable to wear, indirectly contributes to the safety of the helmet. But aside from a very minor absorption of force during the moment of impact the padding really has no direct impact on the safety a helmet.
Modern bike helmets are designed to deform in a crash. Basically, the hard foam and plastic that the helmet is made out of, crumples during an impact. This dissipates the force of impact during a crash. For this reason it is important to replace your helmet after any significant impact. The padding is mostly there for comfort, and to make the helmet fit slightly better.
From the Bicycle Helmet Safety Institute "The spongy foam pads inside a helmet are for comfort and fit, not for impact protection"
2Minor detail: It's not that the hard foam "crumbles", but that it smashes, while absorbing a considerable amount of energy. This (relatively) slow smashing of the foam helps decelerate the skull slowly enough that the brain is much less likely to bang the inside of the skull. It's the brain hitting the inside of the skull that causes the damage. Mar 19, 2013 at 20:02
3Yes crumples rather than "crumble", is quite similar to smashes.– WadelpMar 19, 2013 at 20:52
1@DanielRHicks - Minor quibble with your quibble. The foam doesn't absorb energy per se, it extends the duration of impact which reduces peak force experienced by the brain (the total energy experienced by the brain remains the same)– Rider_XNov 2, 2015 at 19:41
1@Rider_X - Well, the foam does absorb some energy -- how significant the amount is, relative to the total impact, is hard to say. But, as you say, the main intent is to stop the skull (and hence brain) slowly -- it's not the fall that hurts, but the sudden stop at the bottom. Nov 2, 2015 at 20:27
Wadelp's post is the correct one -- the pads add nothing to crush/impact damage.
There is one factor that is worth considering and that has recently received more attention -- especially in the sports world -- and that is twist damage -- also known as rotational or shearing damage. That is, if you get a glancing blow from an object or if you hit the road and slide, your head and neck can receive considerable twisting (torsional) force unless the helmet can deflect it. Inside the skull, your brain can twist and tear at the brain stem.
What you want with glancing blows (or sliding on pavement) is the helmet to slide (which is one purpose for the external hard plastic layer) but the part next to you to not twist too much. The pads help increase friction between you and the polystyrene impact layer.
For example, this is an advertisement from a helmet maker (I occluded the logo mark of the manufacturer) that talks about how they are working on reducing rotational forces:
As the technology improves (based on the not-inconsiderable amount of money the NFL is putting into helmet technology), we'll see better designs on the market that take into account all sorts of different damage scenarios.