Originally Posted by closetbiker

well, that's a problem now, isn't it?

Manufacturers keep a close eye on this but don't want to give out information on it as it can reflect badly on the protective qualities of their product. Certainly we know it happens and manufacturers try to prevent it, but fail to do so. Letting people believe broken helmets are helmets that have "worked" works in their favor. Telling them that when a helmet splits, cracks, or comes apart the helmet isn't useful doesn't help them at all.

I've had a Bell representative tell me that a helmet splitting is a valid form of attenuating energy, which of course is technically true, but misleading in the extreme. Helmets work via compression, not by splitting.

Measurement rates of energy that coincides with the splitting of helmets is also extremely difficult to come by but we know at what rate of impact a helmet is tested to and that rate does not correspond to real life serious injury or death situations. Most conditions that result in such serious results are far beyond the limitations of helmets. Manufacturers are careful to say their product cannot be relied upon to prevent these types of injuries and that serious injury can occur to any wearer of their products by any impact.

Collisions, falls, and impacts to the head is an extremely complicated issue and not one that will be controlled by a single, simple, mitigating factor. Despite what emotion Helmeteers bring to the table, placing a helmet on a head does little to change much.

Originally Posted by meanwhile

I do. I've read papers and articles by professional helmet engineers. The chances are that if a helmet fails that it provided about zero meaningful protection.

Why?

Helmets absorb energy when the foam liner compresses. But to compress properly the liner must have a shell to brace against - think of it as being like a fluid. If it is confined, then compressing it takes a lot of energy. If it isn't, the liquid flow freely at hardly any energy cost.

When helmets split they do so because the shell has failed. This takes place BEFORE compression. So meaningful energy absorption hasn't taken place - the helmet might have absorbed something 1/10 to 1/100 the energy it is designed to. To put this in perspective, that's take the 1/10 figure and call that amount of energy absorbed a "splat" and consider a 24mph crash:

- Helmet absorbs 1 splat

- Would have absorbed 12 splats in 12mph (design limit) hit

- Actual crash energy 48 splats (kinetic energy is a square law)

Important note: impact speed is the component of speed at right angles to the impact surface. If you hit a road and you were cycling at 30mph, then hopefully your impact speed will be vertical only. In theory. (In practice, some helmet shells may shatter from the "belt sander" effect of the 30mph collision - this isn't part of the helmet cert test, and in the real world helmets have an extremely high failure rate.)

Originally Posted by meanwhile

I doubt that any helmet company will go on the written record with that claim: the legal consequences would be disastrous.

In the UK the case law accepts that helmets have no possible benefit at over 12-14mph - repeated attempts to argue that cyclists deserve reduced damages for not wearing a helmet have been rejected for this reason.

Originally Posted by meanwhile

The problem with this "experiment" is that it is stupid. It isn't enough for a helmet to do a tiny, pointless amount of energy absorption - that's like trying to justify sunscreen as a defense against nuclear weapons, a small parasol as a replacement for an effective parachute. To be worthwhile a helmet has to have some chance of reducing a dangerous head impact to a less dangerous one. Changing a 48 splat to a 47er (see above) isn't doing that.

To discuss the stupidity of the above further: even failed helmets increase the total head size (making head impact more likely) and rotational damage. These are significant dangers - so you can't even assume that negligible safety benefit of a helmet isn't outweighed by the negatives if you crash outside its design limits.