Head rest design

Gents, I'm working on my headrests for my car, with two concerns:
1) keeping heads away from the roll hoop
2) providing enough room front to back when bare headed, or wearing a helmet.

The backer plate/support is about 3 1/2 inches behind the seat back.

Current thinking is to bond 2 inches of sfi 45 conforfoam to a thin plywood backer, and use a small screw in the center to attach it to the aluminum backer. I think that will offer good support and energy absorption when wearing a helmet.

When without a helmet, I'd like the padding to project closer to the head. thinking about unbolting the pad and slipping a layer of rigid foam insulation (extruded polystyrene) between the plywood and aluminum backers, then bolting all together again. Has the benefit of bringing the pad closer to the bare head, offering an addition layer of energy absorption, and keeping the head further from the roll hoop.

Down side is that the polystyrene is not supported around the edges, so it may fracture under a hard impact. May also encourage the plywood backer to fracture, since the foam underneath can deform locally.

Any thoughts/criticisms appreciated.

fracturing is not a bad thing in that it does remove energy during the process. My bike helmet is essentially that philosophy. One crash use, and you replace it. Great post, as I'm working through the same issues. My smaller wind screen still deflects air over to the top of the helmet, but a behind-the-head support structure that's adjustable is desired. I'm looking at a cantilevered arm from side or below, that allows adjustment fore/aft, that would use a shear pin for adjustment in two fixed positions at the attachment point on the framing.

thanks for your reply. revised thinking is to build a few head rests with different thickness than I can swap out in about 5 minutes - going to the track, install the thinner one.
Your point about the bike helmet is exactly what I was thinking, it's the material they install in Snell helmets. Extruded polystyrene, the stuff you find in rigid home insulation panels, seems like it might work in situations where there is no SFI approval required. As an informal test, I punched a 3" thick panel backed by a rigid steel wall. Knuckles dented the surface about 1/16".
Fracturing may not be optimal however, I understand it absorbs energy, but it's not great if it blows out around the perimeter before it has had an opportunity to collapse and reduce acceleration.
The other issue is that if you are using it in conjunction with Confor foam, as I am, you want the EPS layer separate from the unit which contains your backer board, Confor foam and upholstery, for the simple reason that when the EPS takes a hit, it needs to be replaced. Sandwiching between the backer board and the support structure seems like the right location, but it slightly complicates my mounting system; you either velcro the backer board to the EPS to the support structure, or you bolt from support structure through the EPS to the backer board. In which case, the bolts need to be able to back out of the support structure in a hurry, rather than hanging up and punching into the headrest. My current set-up won't accommodate that, but I can change it.
Re your set up, the cantilever will need to be beefy - a 10G stop will put a roughly 1100 pound load into it if my calcs are correct.

Yep on your last statement. I can't realistically build it for 10G (or 30G), but it can support my head, provided some level of deceleration if I back into anything fast, and if designed properly (or as properly as I can), won't present additonal risks aside from nothing existing at all. The key is a maximum survivable deceleration until you stop, or until it fails.

As I recall, about 7G is the threshold for jet pilots to start blacking out, so 10G doesn't seem like a huge number to design around. Instead of doing a cantilever, is it possible to tie into the roll hoop with a flat plate like I did above? That way, the anchors serve to support the plate when it starts bending (which also provides some "cushioning"). One of the reasons I did mine this way is that the diagonal support brace in my roll hoop is probably 3-4 inches below the center of mass of my skull, so unsupported, it will bend in a crash. I really wish I could drop my seat position, but I'm already only about 15mm above the chassis, and I DONT want to sit directly on it in case something hits the underside.
Anyway, here's my progress so far using 2M 74 foam adhesive, and conform foam. I've built one 2 layer headrest, and one 3 layer headrest for the passenger side. Have to do a little sanding on the backer board for the driver side headrest to make sure all the wax is gone from the release layer for the reinforcement. 74 spray is seriously aggressive, once the parts touch each other, they are STUCK. No repositioning possible.

I used the same 3M adhesive to apply black cloth (a thin type of canvas that was easy to clean/vacuum), to my foam 1/2" cushion seat inserts and lumbar support.

Here for foam material

Spud, that 7G figure for blacking out is for a continuous force. Football players, etc routinely suffer 50G+ impacts but those are only for a few milliseconds.

manta22 said:

Spud, that 7G figure for blacking out is for a continuous force. Football players, etc routinely suffer 50G+ impacts but those are only for a few milliseconds.

Click to expand...

When I flew and crashed my Cobra, the calculated G Force was 42G and I didn’t black out. When the car slammed into the tree, the car went from 70mph to zero in 23 inches..

manta22 said:

Spud, that 7G figure for blacking out is for a continuous force. Football players, etc routinely suffer 50G+ impacts but those are only for a few milliseconds.

Click to expand...

Understood and agree. I raised the point to argue that a cantilever structure should be able to support a significant load in case one backs the car into a barrier, to help prevent whiplash etc.

Big-Foot said:

When I flew and crashed my Cobra, the calculated G Force was 42G and I didn’t black out. When the car slammed into the tree, the car went from 70mph to zero in 23 inches..

Click to expand...

My Cobra replics was "nudged" off the road into a ditch of wet grass...you can imagine that the brakes were useless. My undercarriage suffered significant damage...bent axle housings and the shafts inside them. My head (no helmet, it was just a "pleasure cruise") hit the roll bar and it took 9 staples to close up the laceration on the back of my head. When I woke up my daughter (who saw the whole thing happen) said I had been talking real crazy while I was out. When I get the car fixed (maybe in a year) I'll never drive again without a helmet.

Glad you didn't suffer worse injury.
I'm a bit incredulous when I watch custom car TV shows, and, without fail, they install seats with no headrest. I understand these high dollar cars are not being raced (despite having ungodly power under the hood), but shit happens...
Regarding helmets, I tried contacting a vendor in India that makes carbon fiber novelty polo helmets that I think would be pretty good for cruising (def NOT for track work). Was inquiring to see if they could build the shell with some kevlar in the layup to help with integrity, but they never replied. Still exploring similar ideas.

YerDugliness said:

My Cobra replics was "nudged" off the road into a ditch of wet grass...you can imagine that the brakes were useless. My undercarriage suffered significant damage...bent axle housings and the shafts inside them. My head (no helmet, it was just a "pleasure cruise") hit the roll bar and it took 9 staples to close up the laceration on the back of my head. When I woke up my daughter (who saw the whole thing happen) said I had been talking real crazy while I was out. When I get the car fixed (maybe in a year) I'll never drive again without a helmet.

Click to expand...

SFI roll bar padding would have helped, too.

YerDugliness said:

My Cobra replics was "nudged" off the road into a ditch of wet grass...you can imagine that the brakes were useless. My undercarriage suffered significant damage...bent axle housings and the shafts inside them. My head (no helmet, it was just a "pleasure cruise") hit the roll bar and it took 9 staples to close up the laceration on the back of my head. When I woke up my daughter (who saw the whole thing happen) said I had been talking real crazy while I was out. When I get the car fixed (maybe in a year) I'll never drive again without a helmet.

Click to expand...

Ouch.... Glad you survived that!