Q for TECHS pls, Head excursion and tether Q's....

[Mama!]

Ok, so a friend and I are reading about head excursion (she was reading about the new Radian XT and we weren't 100% sure about Safestop) and she found this :

http://edocket.access.gpo.gov/cfr_2007/octqtr/pdf/49cfr571.213.pdf

Is that really true? FMVSS213. says 32 in / 13 mm's is the limit for head excursion????????

We're trying to read this, but its really technical. It says this:

Harnesses, backless booster seats and
restraints designed for use by physically
handicapped children.
Sarah M.: those excursion limits are 813mm for head and 913mm for knee, attached with lap belt; in addtion, if a tether is provided, it is attached

are we nuts? I'd think that 32 inches of excursion would just destroy a child's head/neck/brain. :(

And surely a child's head in a seat like a Regent that was tightly installed with tight harness couldn't go that far?

BTW, this was spawned by the 3 inch reduction of head excursion mentioned by Radian here:

Radian80: In crash tests with a 10 yr old, 80 lb test dummy (lap belt only and no top tether) resultant excursion
values were approximately 3” under the limit established in FMVSS213

And then we searched for the limit in FMVSS213 and so forth.

[An Aurora]

Yup, that's why top tethers are so important. They reduce head excursion by 4-6 inches.

[southpawboston]

Ok, so a friend and I are reading about head excursion (she was reading about the new Radian XT and we weren't 100% sure about Safestop) and she found this :

http://edocket.access.gpo.gov/cfr_2007/octqtr/pdf/49cfr571.213.pdf

Is that really true? FMVSS213. says 32 in / 13 mm's is the limit for head excursion????????

We're trying to read this, but its really technical. It says this:



are we nuts? I'd think that 32 inches of excursion would just destroy a child's head/neck/brain. :(

And surely a child's head in a seat like a Regent that was tightly installed with tight harness couldn't go that far?

BTW, this was spawned by the 3 inch reduction of head excursion mentioned by Radian here:

Radian80: In crash tests with a 10 yr old, 80 lb test dummy (lap belt only and no top tether) resultant excursion
values were approximately 3” under the limit established in FMVSS213

And then we searched for the limit in FMVSS213 and so forth.

sorry, i'm not a tech, but i think i may know why 32 inches seems a lot. (techs, please correct me if i am wrong).

i think FMVSS213 head excursion limits are based on distance of the head from a fixed point, which may be somewhere behind the child's head or behind the carseat. that point is not necessarily the head of the child. in other words, seated correctly in a correctly installed carseat in a car at rest, it is possible that the child's head is already measuring 10-15" of excursion, depending on where that fixed point is. the implication here (at least as far as i can see) is that a big, fat, honkin' seat that places the child's head farther away from the fixed point to begin with must pass FMVSS testing with even less movement than another seat that is very shallow and places the child's head closer to that fixed point. does that make sense? for example, a big britax or recaro seat may position a child's head 10" from that invisible reference point (hypothetically), meaning that those seats must prevent the child's head from moving 22" to pass FMVSS. another, slimmer seat may position the child's head 5" from that invisible reference point. in that seat, it would pass testing even if the child's head moves 27" does that make sense?

again, techs correct me if i am wrong.

[An Aurora]

Sort of, SPB.

Before September 1999, the limit was 32 inches forward of a point located about 5 inches rearward of the seat bight. Since that date, nearly all forward-facing CRs have to meet an additional limit of about 28 inches (720 mm) from that point, but they can use a tether to do it. Retaining the old limit provides a guarantee that the CR is structurally sound and will perform as well as older models, even if the tether is not used.

ETA: link (http://www.carseat.org/Technical/tech_update.htm#headexcur)

[Jeanum]

Here's how head excursion is measured (From http://74.125.95.132/search?q=cache:Vi6T28OWfoEJ:www.cpsboard.org/pdf/techreport/techreportapril01.pdf+how+is+head+excursion+measur ed&hl=en&ct=clnk&cd=3&gl=us)

Head excursion is measured not from
the seat back but from the point where
the test-sled seat back pivots. This is
about 5 inches behind the seat bight
(where the back and bottom cushions
meet) on the test sled. To get an idea of
approximately how much head excursion
space there is in a particular vehicle, sub-
tract 5 inches from the 28-inch or 32-inch
allowance, measure that distance forward
from the bight, and then imagine a verti-
cal line upward to the level of the child’s
head and shoulders. The availability of
the maximum head excursion space (27
inches) in the rear seat depends a great
deal on how far back the front seat is po-
sitioned and reclined.

[serabi]

Yup, that's why top tethers are so important. They reduce head excursion by 4-6 inches.

they just REDUCE it though, not limit it to only 4-6", right?

(I'm the Sarah M. up there :o)

[An Aurora]

they just REDUCE it though, not limit it to only 4-6", right?

(I'm the Sarah M. up there :o)

Nope, just reduce. :thumbsup:

[serabi]

Nope, just reduce. :thumbsup:

ok!! thanks. that helps!

[safeinthecar]

I know the distance seems like a lot, and it is, but keep in mind the engineers are walking a fine line between keeping head excursion down to a distance that will ensure the child doesn't slam into stuff, and avoiding stopping the child's body so abruptly that their internal organs and brain slam around inside their bodies.

Head excursion is only a very small piece of the equation.

[Gypsy]

A child's head isn't all that's moving...

The seatbelts stretch and the child restraint moves forward...

The harness on the carseat stretch and the child moves forward...

The child's body reacts to the crash and the furthest point the child's head reaches is what's required to be under that limit.

So... seatbelt stretches a few inches, harness too - keep in mind the pre-set location for the beginning of the measurement is BEHIND the vehicle seat... it's not nearly as bad as it sounds now, right?

[Mama!]

Yeah, its not so terrible horrible now :o

[joolsplus3]

http://carseat.org/Technical/tech_update.htm#toptether
This has a picture....

(you'll have to scroll up, that page doesn't work well with my browser...ugh).