Question Why are RF weight limits so much lower than FF limits?

happymom · May 10, 2013

I'm just curious more than anything, but how are these weight limits determined?

It seems like in the event of a crash a RF child would exert less energy on the car seat than a FF child - yet FF a seat claims to be able to support children with higher weights. It seems to me (from a very non-technical point of view) that the RF weight limits don't make a lot of sense.

I also realize that MOST kids outgrow their RF seat by height before weight and that is more of a guide to go by....but what makes a convertible seat that is FF able to withstand more weight than the same seat in a RF position?

I know that these weight limits are set in stone and if your child passes them clothed they must either be turned from RF to FF or in the event the child is already FF, the child needs a new seat. It's just not clicking in my head and I was wondering if anyone with a more technical background could explain it to me.

Thanks in advance! (Sorry for posting so much lately, I am car seat obsessed)

LISmama810 · May 10, 2013

Rear-facing an forward-facing seats behave much differently. In a forward-facing seat, the shell sort of acts as an intermediary between the seatbelt and the seat's harness system. Although the seat's shell takes force, it's really the harness doing the restraining.

In a rear-facing seat, it's the shell that takes most of the force. The weight of the child causes the seat to rotate down and toward the front of the car. The heavier and taller the child, the more the seat rotates, all while pivoting around the seatbelt toward the back of the car. A child who's too heavy could cause the seat too rotate too far backwards.

I'm probably not doing a good job explaining it, but I'm sure some others will do a better job.

ketchupqueen · May 10, 2013

Yes, overrotation is the primary concern (and head and spinal injuries from impacting vehicle interior features when overrotation and ramping up occur.) The main reason we can't have higher limits here, at least without ridiculously low height limits for the shell height (like, requiring 10 inches over the head or something- so either a very short kid, or a very tall seat that wouldn't fit in most cars) is that our test bench doesn't accommodate foot props or braced seats, which prevent the overrotation of seats and allow for larger children to be restrained with their head further up the shell without risking this type of injury.

joolsplus3 · May 10, 2013

I thought my favorite youtube page Crashnet1/ikea55 would have some plain old frontal crashes, but I can only seem to find side impact tests. So, I'll fall back to the old technical encyclopedia, even though it's outdated, the single picture of the seat dumping back during the crash pulse kind of gets the information across...http://www.carseat.org/Technical/tech_update.htm#angleRF
There's just only so much that can be done in the confines of the FMVSS213 rules... the seat has to be installable with just a lapbelt, on a fairly squishy bench seat. The heavier the kid gets, the more the seat dumps back, the higher the risk of the head and spine getting injured if the head comes out of the top of the seat (as mentioned above by the PP's).
The first answer I thought was, 'because that's the size that fits the dummies'

http://en.wikipedia.org/wiki/Hybrid_III ---little dummies go rearfacing, big ones go forward facing.

happymom · May 10, 2013

Thanks you guys! Which leads to my next question, my son's RF seat is tethered under the front seat, my car doesn't have any tethers on the roof, but it has tethers behind the head rests.

Looking at that picture, it seems like tethering above the seat would do more good than tethering to the floor. Does it matter?

safeinthecar · May 10, 2013

Playground physics explanation.

FFing is like Kindergarteners playing tug of war with a NFL linebacker . You need an awful lot 5 year olds to win.

Rfing is like a teeter totter with an NFL linebackers on each end. It's a balanced lever, so one moderately strong 5yo can go back and forth lifting each guy in turn.

*In car crashes, the car crash is playing the part of the kindergartener.

LISmama810 · May 10, 2013

kjandbaby said:
Thanks you guys! Which leads to my next question, my son's RF seat is tethered under the front seat, my car doesn't have any tethers on the roof, but it has tethers behind the head rests.

Looking at that picture, it seems like tethering above the seat would do more good than tethering to the floor. Does it matter?

I'm not sure what picture you're referring to (I'm on the mobile app, so I might be missing something). I have a feeling you might be talking about Swedish vs Australian tethering, though. Swedish is where you tether a rear-facing seat to the front seat track (or similar) and Australian tethering is where you bring the tether over the RF seat to the tether anchor usually used for forward-facing.

These serve two different purposes. Australian tethering reduces over-rotation. Swedish tethering reduced rebound (the secondary motion when the seat moves back toward the seatback after moving forward in a crash).

We really don't have much evidence on tethering rear-facing, so I'm not sure anyone knows whether Swedish or Australian is better. Rear-facing is already so safe, I'm not sure it really matters.

My personal feeling is that I prefer the reduction in over-rotation that Australian tethering provides, but it's more of a PITA than Swedish. Personally, I rarely tether my rear-facing seats

HaileysMommy · May 10, 2013

safeinthecar said:
Playground physics explanation.

FFing is like Kindergarteners playing tug of war with a NFL linebacker . You need an awful lot 5 year olds to win.

Rfing is like a teeter totter with an NFL linebackers on each end. It's a balanced lever, so one moderately strong 5yo can go back and forth lifting each guy in turn.

*In car crashes, the car crash is playing the part of the kindergartener.

That makes absolutely no sense to me.

Stelvis · May 11, 2013

HaileysMommy said:
That makes absolutely no sense to me.

Me either.

LISmama810 · May 11, 2013

safeinthecar said:
Playground physics explanation.

FFing is like Kindergarteners playing tug of war with a NFL linebacker . You need an awful lot 5 year olds to win.

Rfing is like a teeter totter with an NFL linebackers on each end. It's a balanced lever, so one moderately strong 5yo can go back and forth lifting each guy in turn.

*In car crashes, the car crash is playing the part of the kindergartener.

When I said I hoped someone else would come along and explain it better, you were one of the people who came to mind, because you always have such a great way of describing things.

But...I didn't get this one either :duck:

I still love you, though.

HaileysMommy · May 11, 2013

I'm so glad it wasn't just me! Lol

happiness-and-balance · May 11, 2013

safeinthecar said:
Playground physics explanation.

FFing is like Kindergarteners playing tug of war with a NFL linebacker . You need an awful lot 5 year olds to win.

Rfing is like a teeter totter with an NFL linebackers on each end. It's a balanced lever, so one moderately strong 5yo can go back and forth lifting each guy in turn.

*In car crashes, the car crash is playing the part of the kindergartener.

Huh?

Carrie_R · May 11, 2013

I pondered this for a while this afternoon and I *think* I understand what Kimberly is saying. She is 110% allowed to come in and correct me if I didn't get it!!

This assumes that everyone understands the visual of a linebacker playing tug-of-war with a kindergartener, as well as the physics principle of how two linebackers, balanced on a teeter-totter, could be easily pushed up and down by a kindergartener.

In car-seat terms, the forward-facing seat is the tug-of-war. If you think of the seatbelt & shell as the linebacker, the harness as the rope and the kid as, well, the kid - it is going to take a lot of kindergarteners (instead of multiple kids, the kid's weight is multiplied by crash forces) to tug the rope out of the linebacker's hands (ie, rip though the shell.) It is not prohibitively difficult for manufacturers to just make a "stronger linebacker," as it were.

Rear-facing seat is the teeter totter. So, the belt path is one linebacker, "down," and at the top of the seat is the other linebacker (top of the shell, unsecured to anything.) The pivot point is the seat bench, which in FMVSS testing is quite squishy. Kid in the seat is going to require a lot less force to tip the "up" linebacker to "down," thereby causing over-rotation. (Over-rotation then means that the head of the kiddo - who is now laying down past horizontal - scoots out the top of the seat and hits... stuff... which is exactly what we're trying to avoid by RFing them in the first place. In addition to the whole neck strain thing.)

To drag it out further, if someone takes a broom handle and puts it under the "up" linebacker's side, propping the teeter totter up, then it takes a lot more "kid force" to push that linebacker down, despite the teeter totter being balanced. That's essentially what the foot prop does on Euro seats, and why they have been able to certify their seats to higher weights. But because we can't use any sort of anti-rotation (foot prop, Australian tethering) to meet the standards here, the seat must pass on its own. No tethering, no foot prop, nothing but seat + lap belt. Those CAN be used on the seat, they just cannot be used to pass FMVSS testing.

It really is a testing vs. capability thing. CAN the manufacturers make seats that go above 50lb rear-facing? Yes. But they have not yet figured out how to do so without the "broomstick" propping up the "teeter-totter."

I hope that makes sense. If not, hopefully someone else can take a stab at it, or Kimberly can come make it make sense to all of us! LOL!

HaileysMommy · May 12, 2013

Ok I get it somewhat now. And I pray that's what she meant bc if not my head is going to spin and pop off.

Carrie_R · May 12, 2013

Ha ha!

Remember... this is why we have engineers to figure this out for us. This is why we follow seat labels, even if it seems stupid (different height limit in Canada vs US, different weight limit based on seat's trim level, etc.) It IS complicated and most of us are not physicists or engineers. There is a reason why most of the world is not either... it's complicated & confusing!

joolsplus3 · May 12, 2013

kjandbaby said:
Thanks you guys! Which leads to my next question, my son's RF seat is tethered under the front seat, my car doesn't have any tethers on the roof, but it has tethers behind the head rests.

Looking at that picture, it seems like tethering above the seat would do more good than tethering to the floor. Does it matter?

Overrotation is already prevented by the seat design and the testing (and in real life, in most cars, the front seatbacks being right there to help hold things up). Australian isn't needed to keep the seat from dumping back too far, it will already be taken care of by the rest of the seat design. Swedish prevents rebound, which is the after-effect of a frontal crash, or what happens in a rear impact. Britax prefers Swedish, it prevents the head of the seat flying too far back and side, and may prevent such things as too much force on the legs (There is a big push for seats to have rebound control, in Canada it's mandatory, in the US more seats are meeting Canada standards... tethering is kind of rudimentary and a pain (on Britax and Radian, because there's basically no such thing as a top tether in a car for RF), but seats like the True Fit and NextFit are designed at the foot end to prevent rebound without any extra work/rigging on the part of the person installing.

happymom · May 13, 2013

Haha, I agree with this! I always like to know WHY we do something.....but this seems a bit over my head.

Carrie_R said:
Ha ha!

Remember... this is why we have engineers to figure this out for us. This is why we follow seat labels, even if it seems stupid (different height limit in Canada vs US, different weight limit based on seat's trim level, etc.) It IS complicated and most of us are not physicists or engineers. There is a reason why most of the world is not either... it's complicated & confusing!

Thank you for the explanations though they certainly give me a little more insight and help me be sure that my kid is safe as can be in the car.

safeinthecar · May 13, 2013

Carrie_R said:
I pondered this for a while this afternoon and I *think* I understand what Kimberly is saying. She is 110% allowed to come in and correct me if I didn't get it!!

This assumes that everyone understands the visual of a linebacker playing tug-of-war with a kindergartener, as well as the physics principle of how two linebackers, balanced on a teeter-totter, could be easily pushed up and down by a kindergartener.

In car-seat terms, the forward-facing seat is the tug-of-war. If you think of the seatbelt & shell as the linebacker, the harness as the rope and the kid as, well, the kid - it is going to take a lot of kindergarteners (instead of multiple kids, the kid's weight is multiplied by crash forces) to tug the rope out of the linebacker's hands (ie, rip though the shell.) It is not prohibitively difficult for manufacturers to just make a "stronger linebacker," as it were.

Rear-facing seat is the teeter totter. So, the belt path is one linebacker, "down," and at the top of the seat is the other linebacker (top of the shell, unsecured to anything.) The pivot point is the seat bench, which in FMVSS testing is quite squishy. Kid in the seat is going to require a lot less force to tip the "up" linebacker to "down," thereby causing over-rotation. (Over-rotation then means that the head of the kiddo - who is now laying down past horizontal - scoots out the top of the seat and hits... stuff... which is exactly what we're trying to avoid by RFing them in the first place. In addition to the whole neck strain thing.)

To drag it out further, if someone takes a broom handle and puts it under the "up" linebacker's side, propping the teeter totter up, then it takes a lot more "kid force" to push that linebacker down, despite the teeter totter being balanced. That's essentially what the foot prop does on Euro seats, and why they have been able to certify their seats to higher weights. But because we can't use any sort of anti-rotation (foot prop, Australian tethering) to meet the standards here, the seat must pass on its own. No tethering, no foot prop, nothing but seat + lap belt. Those CAN be used on the seat, they just cannot be used to pass FMVSS testing.

It really is a testing vs. capability thing. CAN the manufacturers make seats that go above 50lb rear-facing? Yes. But they have not yet figured out how to do so without the "broomstick" propping up the "teeter-totter."

I hope that makes sense. If not, hopefully someone else can take a stab at it, or Kimberly can come make it make sense to all of us! LOL!

I could have sworn I typed in an explanation of that thought process.

Carrie has it right though. The basis of rfing seats is that it rocks on a base (the vehicle seat) and that it can't go too far, but that it takes a lot less weight to make it over-rotate than it does to rip out the shell or harness of a ffing restraint.

Question Why are RF weight limits so much lower than FF limits?

happymom

New member

LISmama810

Admin - CPS Technician

ketchupqueen

CPST and ketchup snob

joolsplus3

Admin - CPS Technician

happymom

New member

safeinthecar

Moderator - CPS Technician

LISmama810

Admin - CPS Technician

HaileysMommy

New member

Stelvis

New member

LISmama810

Admin - CPS Technician

HaileysMommy

New member

happiness-and-balance

New member

Carrie_R

Ambassador - CPS Technician

HaileysMommy

New member

Carrie_R

Ambassador - CPS Technician

joolsplus3

Admin - CPS Technician

happymom

New member

safeinthecar

Moderator - CPS Technician

Car-Seat.Org Facebook Group

Sponsors

Forum statistics