Food for thought on rebounding

so i've been thinking a lot lately about the rebound phenomenon with RFing carseats. some say it's good, some say it's bad, some say it's just something that happens and it's neither good nor bad. it is what it is.

there are also pro-RF tethering and anti-RF tethering attitudes out there. britax obviously endorses the practice, as does sweden. on the other hand, recaro's stance is that it's just not standardized enough in its implementation to be reliable and there are too many vehicle-specific variances.

however, i was thinking that when is comes to anti-rebound designs, there's more than one way to skin a cat. the britax companion base has a rebound bar, which functions to minimize rebound, but by a completely different mechanism than an RF tether. it uses compression against the vehicle's seat back to counter rearward rotation.

this got me thinking, why don't all RF seats have some sort of bar, similar to the companion? that would take away the variability of an RF tether and the questionable choices of suitable anchor points. then it dawned on me... some RF seats are inherently better than others at preventing rebound, and they are this way for the same reason that the rebound bar on the companion works. they compress into the vehicle's seat back.

everyone here knows that a graco snugride and an RF scenera are very easy to rotate rearward, no matter how tightly they are installed. we've had both seats, and i've been able to rotate either seat at least 10-15 degrees without even trying, with minimal force. but, our evenflo triumph never rotated easily. and now, our new signo barely budges even an inch if i try to apply force on the top of the shell to try to rotate it rearwards.

it seems to me that the difference between the seats that can almost freely rotate and the ones that are more solid is whether they have a tall base, with a tall front edge of the seating area pushed into the vehicle seat back. the scenera is base-less. there is virtually no part of the seat that compresses into the vehicle seat back. same for the snugride base. but if you look at any pictures of an RFing britax, you will see that the base is relatively tall, and above the base, the front seat edge pushes very prominently into the vehicle seat back. the taller the seat (and therefore the farther away the compression into the seat back is from the hinge point, which is essentially the attachment point, or roughly, the seat bight), the more force it will require to rotate the carseat rearward. this is simple physics (work = force x distance). the work it takes to rotate the seat rearward is proportional to the force (compression against the seat back) times the distance (distance the seat compresses into the vehicle seat back; this distance is proportional to the height of the seat, relative to the seat bight).

and if you look at any comparison pics of a como/signo with a britax, you will see that the signo sits up even higher.

okay, i have to admit, i was kind of bored last night. the kids were asleep early, and we were waiting for the next netflix DVDs to arrive, so i fired up my CAD software and whipped up drawing to help illustrate this concept.

in (A), we have a typical infant seat base. notice how it doesn't compress greatly into the seat back. at least that is how our snugride base fit. look what happens when you rotate the seat (i've chosen 45 degrees of rotation for the illustration). there is very little resistance to rotation about the attachment point, because the base does not compress much into the seatback (focus on the area inside the gray circle). the small red line represents the belt path, and the green lines depict the pivot point (such as the lower anchors).

in (B), we have a base similar to the companion, with a rebound bar. notice how the rebound bar does its job by pushing against the seat back. think about how much force it would take for it to compress as far back as it is shown in the illustration. chances are, it would hit the metal frame of the seat long before it rotated that far.

in (C), we have a typical base-less seat like a scenera or uptown (or radian without the foot, which i believe is not allowed). again, there is little compression into the seat back, and if there is, it is not at a very far distance from the pivot point (seat bight). so the force required to compress into the seat back is less than if the compression were occurring farther away from the pivot point.

in (D), i've tried to illustrate how a tall convertible such as a britax or como/signo with a large base sits. notice how high the seat compresses into the vehicle seat back. it's almost like the rebound bar on the companion! and like the companion, if the seat were rotated 45 degrees, look how far the seat would have to intrude into the vehicle seat back. again, it would stop as it hit the vehicle seat frame.

so this is my deep thought for the day. of course these are only generic illustrations, but i would like to start comparing pics of different RF seats to see how high they sit and how deeply the squish into the vehicle seat back. i am also showing a pick of the signo installed in my car so illustrate how high the compression is relative to the seat bight.

my whole point here is that RF tethering serves a purpose, but there may be other design differences among seats designed to accomplish the same thing.

:twocents::twocents::twocents::twocents::twocents:

that's very interesting...i can't remember how high up the base
on the latch lock was, but when it was installed with latch there
was no way in heck that you could lift it up like the snugride base.

i wonder how the new combi w/anti-rebound would sit or the maxi
cosi (seemed like the base was pretty high on that one too)....

I was totally lost until I got to the pictures. Now I completely get it. Are you a physics teacher? You should be! Very interesting!

BTW from what I remember, no matter how tight the Snug Ride base was in I could lift it up an inch or two.

brightredmtn said:

I was totally lost until I got to the pictures. Now I completely get it. Are you a physics teacher? You should be! Very interesting!

BTW from what I remember, no matter how tight the Snug Ride base was in I could lift it up an inch or two.

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not a physics teacher, but took plenty of it

and if you installed the snugride without a base, you could almost rotate the seat all the way.

The Keyfit was pretty good since the base is a little higher against the back of the seat. I never tested the Safeseat though, I wonder how it compares to the Snugride?

The argument regarding RF tethering v. not tethering and rebounding gives me a headache. I personally like the RF tether and I use it. I feel more secure when the seat feels more secure to me.

But I really just wanted to thank you for your post since I work at a high tech company and spend way too much time here. You've just justified my time for me. Now I can honestly say I'm reviewing CAD drawings at work.

LOL Amy!

Interesting points, SPB; great food for thought.

I personally have tried to make a rf Britax w/o tether "rebound", and it went with just a little force. The seat was a BV and it was installed very tightly - I was showing my friends what the seat would do w/o the tether in a frontal crash when they asked me about the rf tether. Really, all I had to do was take my hand and push the car seat's back toward the back of the car, and it wasn't all that difficult to do. Just my 2 cents and experience.

monzogary said:

I personally have tried to make a rf Britax w/o tether "rebound", and it went with just a little force. The seat was a BV and it was installed very tightly - I was showing my friends what the seat would do w/o the tether in a frontal crash when they asked me about the rf tether. Really, all I had to do was take my hand and push the car seat's back toward the back of the car, and it wasn't all that difficult to do. Just my 2 cents and experience.

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hmm... interesting, it must really be dependent on the car then.

for me, both the britax and the recaro would not budge when i tried to force them to rotate, unlike the scenera. it probably has to do with several things like the angle of the vehicle seat back, the firmness of the padding, the angle of the carseat recline, and the placement of the LATCH connectors/seatbelt anchors. i suspect that the less recline you have in the installation, the more resistance to rebound there will be, when you think about the geometry of the carseat.

Interesting!!

Thanks! Love the pics

The pics really helped! Thanks SPB, it's definitely something to think about. I'm interested to see the anecdotal evidence on the amount of rebound in different infant seats. I know personally that the Snugride could be pulled up very easily, with or without the base, but I don't have any experience with other infant seats. I always sort of assumed they were all very similar to the Snugride in that respect, so I find it interesting to think that the Keyfit or the Latch Loc (or others) are different.

southpawboston said:

hmm... interesting, it must really be dependent on the car then.

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Yeah, I really think it would depend on the car. I'm thinking about my Mazda 5 and my attempts to install the Radian in the rear row... aidensmom has forward-of-the-bight buckles and was able to compress the vehicle seat enough install the Radian anyway; the Mazda 5's rear-row seats simply will not compress enough to do that. My 200-lb dh plus me pushing on the base as hard as I could wouldn't compress it enough to make a difference.

Anyway, my point is that clearly some vehicle seats are more easily compressed than others.

Interesting theory. I don't know that your models are applicable to true crash dynamics. Keep in mind that the LATCH connectors and seatbelts are going to stretch significantly in a crash. That would pull the seat back from the seat bight several inches or more and would minimize intrusion into the seatback while allowing it to rebound further. Try loosening the belts a few inches and see how far you can push it then. I imagine it would be a substantial difference.

Granted, I don't disagree that the higher bases may somewhat reduce rebounding and will likely perform better than baseless seats. However, they can in no way remove it completely (and probably not even what I would call effectively).

crunchierthanthou said:

Interesting theory. I don't know that your models are applicable to true crash dynamics. Keep in mind that the LATCH connectors and seatbelts are going to stretch significantly in a crash. That would pull the seat back from the seat bight several inches or more and would minimize intrusion into the seatback while allowing it to rebound further. Try loosening the belts a few inches and see how far you can push it then. I imagine it would be a substantial difference.

Granted, I agree that the higher bases may somewhat reduce this and will likely perform better than baseless seats, but in no way remove it completely.

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right, i agree that it won't prevent rebounding altogether, but perhaps manage it a little better or reduce the severity. and i agree with your theory about the belt stretching and moving the seat away from the seat bight. granted, stretching of the belts to the point of moving the carseat several inches forward will only occur in very severe crashes, and vehicle seatbelts (non-load limiting ones, anyway) will stretch less than LATCH belts simply because they are tested to a much higher weight and are designed to contain adults weighing hundreds of pounds.

however, if that is in fact true that the belt will stretch enough to allow the carseat to move away from the seat bight by several inches, then that limitation to my theory would apply equally well to rebound bars, since they are operating under the same principle as my theory. the only way that a rebound bar could improve upon my theory of tall base convertibles is if the rebound bar was on a very strong spring mechanism that sprung the bar against the back seat even when the carseat itself is moving forward due to belt stretching.

WOW SPB!!! Very interesting thoughts. We talked about RF tethering very briefly in my CPST class. There were a couple people there who were getting re-certified and were so confused b/c they thought all RF seats are *supposed* to travel with the child and "cocoon" on rebound. They thought the ideas of rebound bars and RF tethers were crazy. Yeah - and there's me with DD's MA RF tethered! The instructors quickly dismissed the conversation because they had to stick to the curriculum or we'd be there 'til 7PM!!

The co-founder and president of Compass was in my class - I was bugging him about whether the True Fit will allow RF tether and he said probably not, but they are working on the possibilities of it having a rebound bar!!

We also talked about how keeping the handle in the "up" position on an infant RF only seat could potentially provide some "anti-rebounding" (if it doesn't crack off first!). [Compass' new infant seat coming out will allow the handle to be in the up position.]

I am SO interested in this because I want to do and teach what is safest. All methods have their validations. I guess the problem lies in that you just can't predict what kind of crash you may get in someday!!

southpawboston said:

and if you installed the snugride without a base, you could almost rotate the seat all the way.

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If you install a base-less infant seat the same way *I* do, it doesn't rebound at all unless you put a lot of force behind it. That's o Most people I haven't trained don't do it like I do though.

If you install a base-less infant seat the same way *I* do, it doesn't rebound at all unless you put a lot of force behind it. That's o Most people I haven't trained don't do it like I do though.

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Ooohhh... care to share your technique? I've found this soooo awkward and challenging.

Okay... here's something I've been thinking about.

I was unable to get a good install of my BV in the center of my Subaru Forester (2002, no side curtain airbags), so I moved it outboard with a rock-solid install. I also have it tethered RFing.

HOWEVER, in an outboard seat, if there was a side impact collision, maybe I wouldn't want it tethered? If it were untethered, wouldn't the seat travel away from the point of impact? If it's tethered, it's like it is being held directly in the path of the impact.

So I've been wondering if maybe I should not tether when I have the seat outboard. But then the whole reason I went with the Britax rather than considering the Recaro was so I could RF tether. I'm so confused!

mimieliza said:

HOWEVER, in an outboard seat, if there was a side impact collision, maybe I wouldn't want it tethered? If it were untethered, wouldn't the seat travel away from the point of impact? If it's tethered, it's like it is being held directly in the path of the impact.

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regardless of seating position, the seat (and any loose objects in the car) will always want to travel *toward* the point of impact, not away. an untethered seat may travel slightly more toward the point of impact than an untethered seat in a side impact, but not by much.