Can we talk about head excursion testing?

Gypsy · Feb 11, 2008

I don't think it's well known how head excursion is measured in testing. I think knowing this, will help a lot of people make informed decisions on the seats they choose for their children.

Head excursion is measured from a fixed location behind the test bench seat. It is NOT measured from where the child's head starts in the event of a crash.

So, lets talk about this.

The Ride Safer Travel vest has excellent head excursion tests. It has no shell, so of course it's going to have good numbers.

The Radian has excellent head excursion results, even without a tether -- again, thin shell improves numbers.

The Regent tested better with the recline bar -- what the recline bar does is bring a child's head back more towards the vehicle seat, it does NOT mean the head excursion *for a child* are any better, just that from that fixed location behind the seat, the numbers are better. The Regent has a very thick shell.

Why is head excursion measured in this manner? shouldn't it be measured from the front of the seat -- where the child's head would be?

LuvBug · Feb 11, 2008

thank you for starting this. This is something that I have been going over in my head for a while and actually bothers me. I'll be keeping an eye on this thread.

Gypsy · Feb 12, 2008

LuvBug03 said:
thank you for starting this. This is something that I have been going over in my head for a while and actually bothers me. I'll be keeping an eye on this thread.

It really bothers me too. The numbers really don't mean much because the shell of the seats are all different. One could, in theory adjust the numbers on their own, from how thick a seat is, but who does that (other than a safety geek like me)??

I don't want to debate the merits of each seat. The three I used in the OP are the 3 seats known for different head excursion reasons -- either excellent (with little to no shell) or better w/recline bar for the thickest shell on the market.

CDNTech · Feb 12, 2008

This is a really good point... so, anyone wanna go adjust those numbers.

Beckers · Feb 12, 2008

Thank you! I have been wondering about this since I learned what head excursion was! It just didn't seem to make much sense...I will definately be looking for this thread when I get home from work today. I can't wait to see what all you guys have to say!

arly1983 · Feb 12, 2008

I have been puzzling this over in my head for a long while and I really feel like it should be measured some other way.

Defrost · Feb 12, 2008

I'm so glad you posted this! It's been bugging me ever since you mentioned it the other day. I wonder if we could get someone from Britax to chime in - they must have considered this at some point in all the testing they do, right?

o_mom · Feb 12, 2008

I think that it does make sense, somewhat, to measure from the fixed point. In your car, the front seats are a fixed distance from the back seats (other than moving on the rails). If we measure from, say, the back of the child's head, then a seat that "passes" may actually be more likely to allow the child to strike the front seats.

For example, say a vest type product had an excursion of 28" from the back of the child's head. Then you have a seat with a 4" thick shell, but only allows an excursion of 26". Based on head excursion numbers, you would say that second seat is safer (26" vs 28"). However, if you only have 29" between the back of the child's head and your front seat, the second (better) seat will allow the child's head to strike the front seat where the first (worse) seat would not.

Remember also, that head excursion is not the only measurement. Head Injury Criterion is important also. It is based on how quickly the head stops - stopping too quickly will cause injury to the brain, even if you don't strike something. I think that you would find that as Head Excursion decreases, the HIC may increase, so you need some give there. This is the basis of the SafeStop on the Radian. Allow slightly more head excursion (without going over the limit), but reduce the HIC.

Starlight · Feb 12, 2008

You know, in my tech class, when I asked why such large numbers were allowed - 28 & 32, would, in most cars, be hitting the front seats - I was by the instructor that it wasn't that bad, because they measured from the child's butt up. So, head excursion is essentially the child's torso length plus whatever. :dizzy:

I do think the fixed point makes sense, because it also encourages manufacturers to make a seat that keeps the childs head pretty far back, which keeps it further away from those front seats.

o_mom · Feb 12, 2008

starlightCPST said:
You know, in my tech class, when I asked why such large numbers were allowed - 28 & 32, would, in most cars, be hitting the front seats

Even with the approx 5 inches back from the seat bight, the 32 in measurement can be close even in a minivan, especially if the seats are pushed forward or back all the way. I measured in my Odyssey and suddenly 32 inches didn't seem very far at all.

Starlight · Feb 12, 2008

I think that the limits should be less... I've never measured the distances in my van, but I am fairly certain that 28/32 would be whacking right into the seatbacks, especially in the 3rd row. I might go play with a tape measure today.

We have tethered Radians for our 3rd row (3 tether anchors) and that is (IMHO) ideal... I have an email from Russ that states, IIRC, that untethered Radians tested "better" than 25 inches.

Defrost · Feb 12, 2008

o_mom said:
Even with the approx 5 inches back from the seat bight, the 32 in measurement can be close even in a minivan, especially if the seats are pushed forward or back all the way. I measured in my Odyssey and suddenly 32 inches didn't seem very far at all.

Yeah, I play a really weird game with moving the vehicle seats around depending on who's riding! But then... what's risk of head injury from hitting a vehicle seat? Do they tend to hit the headrest on the seat in front of them?

Starlight · Feb 12, 2008

We took our headrests off... the only headrests remaining are the ones that have DVD players attached to them. LOL Not something I want the kids to hit.

o_mom · Feb 12, 2008

Defrost said:
Yeah, I play a really weird game with moving the vehicle seats around depending on who's riding! But then... what's risk of head injury from hitting a vehicle seat? Do they tend to hit the headrest on the seat in front of them?

I would think the headrest, the seat back or even those grab handles on some seats. Headrests and seats are harder than they appear and are not really designed to absorb impact (at least that's what Julie told me

).

Defrost · Feb 12, 2008

o_mom said:
I would think the headrest, the seat back or even those grab handles on some seats. Headrests and seats are harder than they appear and are not really designed to absorb impact (at least that's what Julie told me ).

Well, here's my situation - I can't take the headrest off of one of the seats, because the kid riding there needs it. But, I turn it around backwards on the other seat, so I don't have to bother storing it. Plus, without that headrest there, the shell of the carseat sticks up over the top of the vehicle seat - and I'd rather he hit the headrest than the carseat shell. But, it decreases the amount of head-excursion room the kid behind her has, by probably about 2 inches. BUT then I have her seat all the way forward (and upright), which gives the kid behind her quite a bit more room... I should go measure though, hm...

Okay, so here's the thing. The third row has DS2 in a booster with the 86Y, behind DD's carseat with the headrest on backwards. DS1 is in the third row with just the 86Y. Since a booster increases head excursion, should I switch them? (Oh, that'll be fun...) I can't swap the seats in the second row because I need the seat behind me empty for pizzas.

Ooo, my head is spinning now. We'll just WALK everywhere from now on!!

Gypsy · Feb 12, 2008

starlightCPST said:
I do think the fixed point makes sense, because it also encourages manufacturers to make a seat that keeps the childs head pretty far back, which keeps it further away from those front seats.

Why is it called head excursion & why does it pertain to the seat?

If a seat is being measured for safety, and crash tests are showing alarming images of children's necks being stretched, wouldn't it make more sense -- maybe -- for there to be two separate tests, or even better - two separate measurements? One from the fixed point behind the seat to see if it meets current guidelines, and one from the fixed point where the child's head is to see if it could meet new/better guidelines?

Everyone raves about the head excursion limits of the Radian, and "if you don't have a tether, get the Radian" but WHY are we making that recommendation? Does a child's neck stretch less in a Radian? probably not. A child's neck will stretch less with a tether, simply because of what a tether does. We tell people that if they have three seats, and 2 tethers, to leave the Radian untethered -- again, why? Other seats still pass tests without the tether too, their shells are just thicker and the child's head starts out further away from that fixed point, maybe those seats are really safer for a child's neck?

Defrost · Feb 12, 2008

Gypsy said:
Everyone raves about the head excursion limits of the Radian, and "if you don't have a tether, get the Radian" but WHY are we making that recommendation? Does a child's neck stretch less in a Radian? probably not. A child's neck will stretch less with a tether, simply because of what a tether does. We tell people that if they have three seats, and 2 tethers, to leave the Radian untethered -- again, why? Other seats still pass tests without the tether too, their shells are just thicker and the child's head starts out further away from that fixed point, maybe those seats are really safer for a child's neck?

The fact that it's not up on a base make a difference in head excursion, too, from what I understand, but I don't know if that's because of how they measure or if being up higher actually means more head excursion. If being baseless does actually decrease head excursion, though, then it would be better to choose to tether a Marathon, for example, over a Radian.

crunchierthanthou · Feb 12, 2008

Defrost said:
The fact that it's not up on a base make a difference in head excursion, too, from what I understand, but I don't know if that's because of how they measure or if being up higher actually means more head excursion. If being baseless does actually decrease head excursion, though, then it would be better to choose to tether a Marathon, for example, over a Radian.

right. the closer you are to the vehicle seat, the closer you are to the center of gravity. It's not just the thinner shell or lower base that contribute to the lower numbers. Sure, they have farther that they can move and still pass testing, but physics is playing a role, too.

CDNTech · Feb 12, 2008

Defrost said:
The fact that it's not up on a base make a difference in head excursion, too, from what I understand, but I don't know if that's because of how they measure or if being up higher actually means more head excursion. If being baseless does actually decrease head excursion, though, then it would be better to choose to tether a Marathon, for example, over a Radian.

:yeahthat: The low profile is why I recommend to tether other seats before the Radian.

joolsplus3 · Feb 12, 2008

Maybe 'from a point behind the vehicle seatback' is just the most objective measurement they could come up with? Why not 'the vehicle seatback', I wonder?

Most seats are well under 32 inches, even untethered, unless I"m remembering the NHTSA compliance data incorrectly...of course, given a 99% misuse rate, seats probably really ARE moving forward at least 32 inches.

Seatbacks do cause injury...it's not the flying forward that hurts (as much, obviously it's not *good*), it's the head contact with something else in the vehicle that hurts...but in a hard frontal crash, they will also fly forward a good deal and be going the same direction as the child's head (thus reducing the force of impact, or preventing impact entirely)

I don't know what Starlight's instructor was talking about, maybe they really didn't know how to express themselves, but they were really, really wrong, lol.

It's about risk. The risk of head injury presumably increases with higher head excursion. The Radian has that low objective number even without a top tether, so the risk of injury is just lower. There are no guarantees, of course.

Does anyone want to see if the LATCH manual has a chapter on this that explains it all?

Can we talk about head excursion testing?

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