Oh geeze, more random thoughts by SPB

[southpawboston]

this time about forward excursion, belt webbing, and LATCH bars.

so, the idea of a LATCH belt or seatbelt stretching during an impact has been brought up before. someone here long ago said something about up to 12" of stretching can occur in a really severe impact, allowing the carseat to move forward several inches.

but stretching is a function of belt length. the longer the belt, the more stretch that can happen, since there is a certain amount of stretching that will happen per unit length of belt. so, if you have 24" of path length between two lower anchors (accounting for all the routing through the carseat), you can expect twice as much stretching compared to if the path length was only 12".

i noticed when i test fitted a BV in my car, and later, my signo, that the path length of webbing between the lower anchor and the LATCH bar was only about 4". since there is a LATCH bar in each side, that's 8" of webbing path length for the whole installation. that means that the amount of stretching that can occur is significantly less than if a normal LATCH belt is used. so this would mean that the carseat would move forward much less in a severe impact.

so how does this affect the whole ride-down hypothesis regarding the belt helping the carseat "ride down" during an impact by abosrbing some crash energy through stretching?

and which is better? to reduce the amount of forward movement of the carseat by having less stretch, or to allow more forward movement in the name of energy absorbtion and decreasing the rate of deceleration by stretching?

and how does rigid LATCH rank in this regard? obviously it allows no forward movement at all, except for some forward rotation since no top tether is used.

and finally, the argument against using LATCH and seatbelt simultaneously, which states that using the two together doesn't allow enough stretch for the ride-down... how you do reconcile that argument with rigid LATCH and also now LATCH bars?

[katiebug]

SPB,
If only I had gotten a degree in engineering at Purdue instead of a Liberal Arts degree... oh the conversations we could have ;)

[Mom to a few]

Are you counting both sides of the loop of webbing on each LATCH bar?? I just went out to my van to look at my Blvd, and if I put the tip of my finger on the red release button, then the top of the loop of webbing on the LATCH bar is at about my wrist....and then you have to multiply that by 2...on each side of the carseat. Does that make sense??

I don't know the answer to all your other questions, but I'm thinking there is more webbing there than you are figuring.

[southpawboston]

good point, i hadn't figured the looping. but it wouldn't get multiplied x2, since the looping doens't go all the way to the red button, only to the adjuster plate. but the looping does add length. i'll have to go out to the signo and take some measurements :)

[Mom to a few]

I just went and looked again, and on mine there is webbing on both sides (although sewn together at some point) on both sides of the loop, all the way down to the red release button.

And my dh keeps laughing each time I walk out to the garage :D.

[safeinthecar]

I knew better than to click on this thread. Really I did. So I guess it's my own fault that the room is spinning now, huh?

* going to get sleep now, will attempt thinking later*

[skaterbabscpst]

:yeahthat:

Did I mention that I only passed physics in high school before the teacher felt sorry for me? :whistle:

[CDNTech]

Another one posting to say it's late... I'll make my brain work tomorrow and kick in my thoughts... that is, if the boys will let me. ;)

[southpawboston]

glad i'm forcing a good think on the matter ;) that's a good thing, right?

okay, i measured the distance between the red button and the LATCH bar, and got 5.75". multiple by 2 for the looping and that's 11.5". multiple that by 2 for both sides of the seat and you get 23" of webbing. i wonder how long a typical belt path is for a regular LATCH belt, from button to button. i'm going to venture somewhere around 22-27", so not much more webbing than a LATCH bar setup. does anyone want to go out to their como and measure for me??? :) oh well, i thought i was on to something!

http://homepage.mac.com/tutter/carseats/New_1.jpg

still, how do you reconcile ride-down theory of belt stretching and rigid LATCH??? :p

[Mom to a few]

still, how do you reconcile ride-down theory of belt stretching and rigid LATCH??? :p

Well, that rigid LATCH hook can be released/extended in order to hook it to the car....Maybe it allows a little extension to help absorb impact during a crash?? Maybe :confused:.

[Jeanum]

My only experience with rigid LATCH so far was with DD2's Baby Trend Latch Loc 22 infant seat, which has rigid LATCH for installing the base (it's also possible to install the base with the seatbelt, just not as friendly to install that way). It's true the rigid LATCH bars extend from the base to aid in hooking up the LATCH anchors, then they must be shortened up and retracted into a locked position in the base to properly secure the base in the car. I don't believe the rigid LATCH bars were designed to lengthen or give at all in a crash. Between the rigid LATCH anchors, and the fact that the metal base is very heavy and almost tailor made to be anti-rebound, I wouldn't expect it to budge much in a crash. I suppose it's possible the base could pivot some at the LATCH anchor points, depending on the vehicle seat contouring, severity and type of crash. :twocents:

[southpawboston]

see, that's the thing jean. since the latch loc infant seat is designed to become essentially "one with the car" and not move at all in an impact, then i don't know what to think of the whole ride-down theory of belt stretching. i do agree that by the very nature of the belt stretching, energy is being absorbed and deceleration rate is being decreased. however, i wonder how beneficial that is versus having a seat that remains in its exact location during a crash (which minimizes excursion in any direction). which is better?

[Defrost]

My thought on the whole stretching = ride-down:

Is this just something like "cocooning," where it happens anyway so "they" (whoever "they" are) decided it was okay/beneficial? I mean, I don't think anyone really knows how much "ride-down" is ideal, right? When you consider that there's the internal harness that's going to stretch, plus the vehicle seat belt/LATCH that's going to stretch... how much ride-down does a child need, and why wouldn't the ride-down provided by the stretching harness be enough? It's enough for adults in vehicle seat belts, right?

[Synchro246]

My thought on the whole stretching = ride-down:

Is this just something like "cocooning," where it happens anyway so "they" (whoever "they" are) decided it was okay/beneficial? I mean, I don't think anyone really knows how much "ride-down" is ideal, right? When you consider that there's the internal harness that's going to stretch, plus the vehicle seat belt/LATCH that's going to stretch... how much ride-down does a child need, and why wouldn't the ride-down provided by the stretching harness be enough? It's enough for adults in vehicle seat belts, right?

That's sorta my thought too. When top tethering was new wasn't there similar debate about ride-down, yet top tethering is safer than not. . .

I think it might depend too on RF vs FF, I don't know.
If RF, I think it'd be ideal for the seat to be one with the car. If FF my brain has a harder time imagining the ideal. Increased forward movement of the seat--what effect does that have on the child? I'd think that it'd be good at first because the whole body could theoretically move forward for a bit so it wasn't just the head moving forward--that's ride-down, right? BUT, I think about how the actual numbers on how top tethering reduces head excursion so forword movement of the seat might just be more mass moving forward. . .I don't know. LOL maybe I shouldn't have posted.

[skaterbabscpst]

I saw a neat website about ride-down and seatbelt recently, here's the link:
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

It's done in frames, so I can't link to to the specific page. :rolleyes: You'll have to scroll down the sidebar on the right until you see "seatbelt function" and click the link.

It really is a neat site thoughg.

[joolsplus3]

how much ride-down does a child need, and why wouldn't the ride-down provided by the stretching harness be enough? It's enough for adults in vehicle seat belts, right?

Now THAT I like :)

But it makes me wonder why we need rip stop tethers and safestops...? Maybe we DO want it as tight as possible at the base yet want some 'give' in the head for increased ride down? Maybe rigid latch with no top tether is actually safest? (I have no idea, but it's something to ponder...)

And yeah, an RF seat is very different in function from an FF seat...the only ridedown you need is energy managing foam for the head, yeah?

[beeman]

When you mention the stretching of the straps, the first thing that comes to my mind is tow straps. Part of the theory with them, is that the strap stretches, and then pulls back releasing the energy, and multiplying the force. So now with this idea, lets take that over to tether straps.

The vehicle hits something and decelerates very quickly.
The latch strap stretches allowing the child restraint and child to continue forward motion
The child restraint comes to the end of the tether strap, and the strap pulls back, causing the child restraint to decelerate

Now the question is, would the initial shock of the crash be greater than the shock of the pull back of the strap? Ideally, we would choose the one with the least shock, right? So would the initial stretch still allow the seat to slow enough, so that the rebound would be less than the initial shock?

The way I see it is this: For the initial force, the seat would stay one with the passenger of the restraint, until the rebound of the strap, where it would essentially pull the seat right out from under them, putting all the weight and forces onto the harness straps of the seat. This may be advantageous by allowing the passenger to decelerate before the impact on the straps, or it may be to a disadvantage, by having the force multiplied by the rebound of the strap.

There you go. If Southpaw didn't make your head spin, that probably will :p !