Have you used a helmet before? They don't track the heads movement very well. It's not that they lag behind or anything, they just move in completely different directions willy nilly. Even tight ones will have much sharper acceleration changes as they bounce around.
I regularly wear a full face helmet with tight fitment and straps and it is moving way more than my head is.
Actually the whole point of having a helmet is to not have your head track its movements - if the acceleration transferred to your head would be the same as that suffered by the helmet, you might as well not wear one at all. Same as with old cars with a stiff body, where in an accident the passengers were subjected to almost the same deceleration as the front bumper. Using the front portion of the car as a crumple zone to reduce the deceleration of the passenger cabin was one of the first major safety improvements...
Newer helmets are designed to be loosely coupled in twisting motions as well as direct impacts too, reducing the ability for the helmet to transfer angular momentum (at least that's the stated goal)
Yeah, for anyone interested there is a technology called MIPS in newer helmets that is intended to handle this better. There are some explanations and diagrams on the site https://mipsprotection.com/
It would depend on the material on the helmet, some are softer, some are more rigid.
Yes it does correlate directly, as in, a high acceleration will correspond to a high acceleration (maybe lower intensity, maybe low-passed in frequency) but it is a direct correlation
Yes it would be better to measure internal accelerations, but I wouldn't feel comfortable wearing a helmet that's being subject to 80G's
The helmet and head are too loosely mechanically coupled.
Let's say you have acccelerometer data from a sensor on the helmet and you pass it through an LPF:
If you have the helmet bobbying at really high forces you over-represent real G forces and you are right that you could negate that with filtering.
_However_, if you have really high forces transmitted from the bobsleigh up to the torso/neck/head it is possible that the inertia of the loosely coupled helmet makes it not register the full force as propagated through the tightely coupled torso/neck/head and you underrepresent the real G forces experienced by the brain/skull.
Accelerometer sensors should be coupled with the skull for proper risk/hazard assessment.
Please take this dismissive Wiki-linking clap-back garbage back to reddit. I come to HN to read insightful, diverse, thought-provoking discussion. It's definitely been slipping, but it's one of the few places on the internet that at least still tries.
You could filter it and make it look like what you think it should look like, but it would still be a measurement of the forces applied to the helmet not the head.
Maybe, but if I had the option of recording something with noise, then filtering it out, and recording it without or with less noise to begin with, I'd always go for the latter. Never shun improving the initial data, regardless of what neat things you can do after that.
