Time to grow up: FIM rated helmet choices

[tommymck]

I don't know If I can answer those questions directly, but I can tell you WHY I can't answer them (can you tell i'm a career bureaucrat?):

the various testing methodologies, and the helmets that adhere to them are unforunately strongly segregated by region. ECE and FIM are firmly planted over in the EU. they have a completely different helmet market over there. You still get ECE ratings on pretty much all good helmets sold in the US, but the ECE ratings are secondary pursuits. Snell is something that is almost exclusively tested for in North America, and it's whole philosophy of testing is different as a result. Again, the helmets we use over here are built for a different market because of this. SHARP is just a british government thing.

The result of this is that the scientific waters are badly muddied. You might say there are more or less fatalities of permanent brain injury with Snell or ECE helmets, but the data is corrupted, because those statistics are segregated by region. Riders in the EU and riders in the US are completely different animals with completely different training and completely different attitudes. Any difference in fatality statistics would be worthless scientifically because of this. There is, however, a whole lot of science out there on the most common causes of death in motorcycle accidents.

". Seventy-five percent of all fatal motorcycle accidents involve head and brain injury, with rotational forces acting on the brain the primary cause of mortality. Current motorcycle helmets are reasonably effective at reducing head injuries associated with blunt impact. However, the mechanism of traumatic brain injury is biomechanically very different from that associated with focal head injury." -- John D. Lloyd, PHD/CPE.

I'm putting that quote up there because the standards and practices of North American safety testing organizations (Snell, DOT) are so radically different from European testing (ECE, SHARP, FIM) that you can draw a clear line on this one issue: Rotational Inertia. The number one type of impact that causes fatality or permanent injury to the head/neck is utterly ignored by Snell and DOT. It's not a huge leap to conclude that helmets designed to mitigate this issue are going to most often serve to prevent serious injury or death. This speaks to your question by saying that helmets that focus on stopping rotational Inertia from getting to the brain/neck (FIM/SHARP) are less likely to be found on the heads of people who suffer traumatic brain injury/death. I can see how that's not the most conclusively scientific viewpoint, but I think it's all we've got until helmet construction and standards becomes a whole lot less regional.

When you say that the primary concern is “stopping rotational inertia”, this isn’t entirely clear to me. So let me ask to make sure:
we are looking for helmets which during any type of accident “will not force your head to twist or turn around any axis”, or ... keep induced torque minimal or zero, correct ?
If this is what is meant, wouldn’t this imply that all it takes to build such a helmet (or find and buy it) is to make sure that it’s as round and flush as possible. In other words ... all the complicated testing for impact might not be so important after all ?

When you say that the primary concern is “stopping rotational inertia”, this isn’t entirely clear to me. So let me ask to make sure:
we are looking for helmets which during any type of accident “will not force your head to twist or turn around any axis”, or ... keep induced torque minimal or zero, correct ?
If this is what is meant, wouldn’t this imply that all it takes to build such a helmet (or find and buy it) is to make sure that it’s as round and flush as possible. In other words ... all the complicated testing for impact might not be so important after all ?
The term "rotational inertia" doesn't make any sense in this context. The FIM tests contemplate somebody rolling around on the ground after they crash the bike. As the rider is rolling around, their head is repeatedly bashing around hitting the ground. This argues for a softer shell that absorbs the impacts while you are rolling around so that it doesn't get transmitted to your brain. There is little likelihood in this scenario, unless you hit a big rock while rolling around, that you'll have some huge high-g impact, where a harder shell would be better. The analogy here is that your head is like an egg, and the last thing you want to do is to stick an egg into a hard shell container and shake it around a bunch.

The Snell tests contemplate you bashing your head very hard repeatedly against the same rock or other hard object (or maybe more realistically, hitting something very hard once.) This argues for a hard shell so that the full g-force impact doesn't make it fully to your head. The analogy here is that you are going to hit the container with the egg with a hammer. A hard shell container would protect the egg, and the soft shell container would allow the force to be transmitted to the egg, cracking it.

"Rotational Intertia" is mass*radius^^2. It's a scalar quantity that depends on the mass, and where the moment of inertia is located relative to the axis of rotation. It's not something that a helmet shell can change.