Interesting article about rock solid installations

Mommy0608 · May 14, 2010

I just saw this article posted on Facebook by KDMF. Apparently, Sarah Tilton (Britax) and Vera Fullaway (Combi) had an update class for CPSTs. During the course, it was mentioned that "rock your shocks" installations aren't actually best, because if your seat is so tight that there is no movement at all, then the LATCH belt or seat belt is already stretched and won't be able to stretch more to provide ride-down time in a crash.

What do you think of this? I'm not so sure, myself. Is it humanly possible to tighten a LATCH strap or seat belt to the max, meaning it can't stretch any more in a crash? I thought that the stretching that happens with crash forces isn't something that can be replicated with just human strength. I do understand the need for the belt to stretch during a crash for ride-down purposes, but didn't think it was possible to tighten it so much that it impeded this function.

What are your thoughts? (If this needs to be moved, please do so, I wasn't sure where to post).

FWIW, my Frontier 85 is installed rock solid. My Symphony 65 is pretty nearly rock solid as well.

carseatcoach · May 14, 2010

I've heard that before. My own personal interpretation, based solely on speculation and conjecture, is that applies to installations where someone gets the seat good and tight, then tightens it a little more, then kneels in it to get it even tighter, and then jumps up and down on it a few times for good measure.

If the seat goes in rock-solid easily (and however else I feel about Britax, I give them credit for convertibles that do that!), I'm not inclined to loosen it just because. But once it feels secure, I also don't feel the need to make it rock the car.

Mommy0608 · May 14, 2010

I can see how that makes sense... maybe in the most forceful, extreme installations (I'm imagining a big football player crouched down, bouncing in the seat!

)

Mine are solid, but I certainly didn't jump up and down in them. My Frontier just happens to install beautifully in my van, without too much effort.

Angela · May 14, 2010

That was actually mentioned in our class too. They said that it compressed the cushioning on the seat too much so there's no 'bounce back'. I'm afraid that's the case with my FR85 since I literally have to put all my weight on the seat in order for it to buckle since I didn't have any seatbelt left. It leaves quite the indentations in my seat. There's really no other way for me to install it though.

Mags462 · May 14, 2010

Interesting..... makes sense, but i highly doubt my puny arms and all of my pulling could get it so tight it has zero more give/stretching.

BookMama · May 14, 2010

chickabiddy said:
I've heard that before. My own personal interpretation, based solely on speculation and conjecture, is that applies to installations where someone gets the seat good and tight, then tightens it a little more, then kneels in it to get it even tighter, and then jumps up and down on it a few times for good measure.

If the seat goes in rock-solid easily (and however else I feel about Britax, I give them credit for convertibles that do that!), I'm not inclined to loosen it just because. But once it feels secure, I also don't feel the need to make it rock the car.

I completely agree with Chickabiddy.

ajtastad said:
That was actually mentioned in our class too. They said that it compressed the cushioning on the seat too much so there's no 'bounce back'. I'm afraid that's the case with my FR85 since I literally have to put all my weight on the seat in order for it to buckle since I didn't have any seatbelt left. It leaves quite the indentations in my seat. There's really no other way for me to install it though.

What do they mean by bounceback? And how is it good/necessary?

nak

cowgirlsmommy · May 14, 2010

I think it can be true if it's a crazy tight install. My husband actually installed our Marathon so tight before that the base couldn't stay clicked into the correct position.

amelia222 · May 14, 2010

Mags462 said:
Interesting..... makes sense, but i highly doubt my puny arms and all of my pulling could get it so tight it has zero more give/stretching.

My thoughts exactly. Unless you're Superman, I really doubt it's much of an issue. Looking at how much the belt stretches in crash test vids, and the inch or so less of belt between barely tight enough and 'rock your shocks' isn't going to make a difference IMO

Angela · May 14, 2010

BookMama said:
What do they mean by bounceback? And how is it good/necessary? nak

No idea... that's all that was said about it. I was going to ask more about it, but we moved on too fast.

yetanotherjen · May 14, 2010

Just wanted to say that I know Vera

She's awesome :thumbsup:
That is all lol

joolsplus3 · May 14, 2010

My instructor for my recert class said she installed some seats for a crash test, and the nice tight tether she did had way better performance than a 'slack gently removed' tether...much lower Head Excursion, I guess.

So I'd wonder if this warning about too tight is aimed at the three firefighters climbing in your seat or people cranking down a mighty tite, and not the average knee of the average tech tight?

I've seen some dads even really overdo it...my seats are always rock the car solid, but yeah, the 'doesn't stay in the proper recline position' problem is definitely a case of it being too tight.

Edit, Vera IS awesome, she was at the Combi booth at Lifesavers and super nice and smart and all that good stuff

crunchierthanthou · May 14, 2010

cowgirlsmommy said:
I think it can be true if it's a crazy tight install. My husband actually installed our Marathon so tight before that the base couldn't stay clicked into the correct position.

but that's a little different. The Britax recline mechanism breaks when the rear facing tether is over tightened. It's putting constant stress on the base. It isn't same as seatbelt or LATCH straps.

y_p_w · May 14, 2010

amelia222 said:
My thoughts exactly. Unless you're Superman, I really doubt it's much of an issue. Looking at how much the belt stretches in crash test vids, and the inch or so less of belt between barely tight enough and 'rock your shocks' isn't going to make a difference IMO

I think people would be surprised at how much something that seems stiff and unmoving might additionally stretch/deform when subjected to really high stresses. I mean, hold an adult-style baseball in your hand and you probably can't imagine that something that hard would compress/deform at all. Even fairly solid metal can have a trampoline like effect with sharp impacts.

rodentranger · May 14, 2010

I guess I'm thinking that *my* seat is firmly attached to the car for a reason, right? Don't I want my kid's seat to be solidly attached to the car too? I mean, if I do one of those awesome "I threaded the belt through the beltpath and buckled it" installations and have 4 inches of movement, I could justify it by saying , "I'm allowing more ride down" right?
I'm more comfortable with a rock solid installation if I can get it without having to stand in the seat. But I'm picky. I won't keep a seat if it doesn't install easily enough that anyone watching my kid can replicate it.

ketchupqueen · May 14, 2010

I think that ordinary human strength, even a strong person with lots of body weight and efficient at using leverage, is unlikely to completely overstretch a belt and leave no room for further stretch and ride-down. I think it is possible with a Mighty-Tite or other ratchet device, and have seen belts damaged by them. If seatbelts are fragile enough that that they can be damaged by ordinary human strength (I have seen it once, with 15+ year old car and belts), I would consider them probably uncrashworthy and would (and did) advise the driver to have the belts replaced.

LISmama810 · May 14, 2010

I don't know. I do know that I read in...Safety Belt Safe? Safe Ride News? (I always get them confused) that there was no difference in performance between seats installed super-tight and those with 1" of movement. Or maybe it was even more--like loosely (not ridiculously loose, but "eh, that's probably good enough") installed seats didn't show a decrease in performance. Or something.

A lot of us know Vera, and Sarah posts here now and then. Perhaps we can invite them to shed some more light on the issue? I can email Vera with a link to this thread if you want. (Or someone else can. But let's coordinate so we don't overwhelm her.)

Mommy0608 · May 14, 2010

I think it would be great if we could get Sarah and/or Vera to come give us a little more information. I'm afraid that this blog post is going to make people deliberately not install their seats tight enough. In my mind, it still seems safer to get it tight enough to have no movement at the belt path, using normal human strength (no big guys jumping up and down, no Mighty Tite, etc.) I'm a small person, and can tighten car seats without too much physical exertion in most cases because I think technique is more important than brute strength (of course there are always the exceptions, like using the long belt path when the seat belt is just barely long enough, etc.) I just have a hard time believing that, under normal circumstances, we could tighten seats so much that they wouldn't be able to stretch any more in a crash. Like someone else said, we've all seen crash test videos showing how much seat belts and LATCH straps do stretch, and I know that's not because there was that much slack in the installation.

LISmama810 · May 14, 2010

I'm emailing Vera. I don't "know" Sarah, though, so i'll leave that to someone else.

Mommy0608 · May 14, 2010

LISmama810 said:
I'm emailing Vera. I don't "know" Sarah, though, so i'll leave that to someone else.

Thanks! :thumbsup:

y_p_w · May 15, 2010

rodentranger said:
I guess I'm thinking that *my* seat is firmly attached to the car for a reason, right? Don't I want my kid's seat to be solidly attached to the car too? I mean, if I do one of those awesome "I threaded the belt through the beltpath and buckled it" installations and have 4 inches of movement, I could justify it by saying , "I'm allowing more ride down" right?
I'm more comfortable with a rock solid installation if I can get it without having to stand in the seat. But I'm picky. I won't keep a seat if it doesn't install easily enough that anyone watching my kid can replicate it.

I guess safety has to be a carefully matched.

I'm an engineer and remember taking a materials engineering class in college. There's a term called toughness. What it requires is a combination of strength and ductility. Materials can be made very strong, but without enough ductility, they will fracture over time. The resistance to long-term failure is toughness.

So there is probably some happy medium when it comes to securing a restraint. I guess there are a bunch of factors. One obviously doesn't want the child restraint flopping around the passenger compartment and slamming into hard objects, so the installation is supposed to be tight. On the other hand, one probably wants some parts to absorb as much shock as possible, such as the straps and energy absorbing foam.

I think some may be familiar with automotive crumple zones. The basic idea is that the front/rear are designed to sacrifice themselves by taking the brunt of the energy, but the passenger compartment is relatively stiff. I guess the straps and foam are the crumple zones, and the restraint shell is the passenger compartment.

Interesting article about rock solid installations

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