Why not use both LATCH and seat belt install systems?



I have two Marathons, for two almost-4 year olds, that are pushing 40 pounds each. They are installed in a 2002 Honda CRV. I am planning on switching from the LATCH to the shoulder seat belt strap-in (sorry for the un-technical terms :lol)

The car seat manual says you should NOT use BOTH the LATCH connectors & the seat belt strap-in systems for car seats. Would someone please explain why? I would appreciate a detailed, physics based reply. My DH is an engineer and a "because the manufacturer says so" it not a good enough explanation for him; he figures that two restraints are better than one.


Moderator - CPSTI Emeritus
Unfortunately, we don't know why - no one has ever done an independent study and told us.

There is plenty of speculation about why, of course, but I'm not sure that would benefit your husband. How about asking HIM why? If he's so smart ;) he should be able to figure out most of the same theories we carseat-junkies have come up with, right? Make HIM prove it! :D

What we do know is that they say "no," and when they say "no," that means it was either never tested that way,

OR it means they did test it that way and it failed for some reason.

That's a pretty big risk to take (to assume that they've just never tested it rather than that it failed testing) for something that's so unnecessary. We know that they've done a LOT of tests with just the seat belt, or just the LATCH installation, and we know the carseats performed at least according to the minimum regulation. Why risk going against something that has been very thoroughly researched?


New member
I am not a tech... but I think I know the answer...

Well because the manufacturer says not to, it means that it either was not tested on the carseat, or it was tested but it failed. Doing what the manufacturer says not to you would be making your child a crash dummy.

Using just seatbelt or just the latch are made so in a crash it distributes some of the force to the seatbelt or latch (it stretches) instead of all on the carseat and child. If you use both the seatbelt and the latch, it might not distribute as much force and it could put most of it on the carseat and your child.


There is plenty of speculation about why, of course, but I'm not sure that would benefit your husband. How about asking HIM why? If he's so smart ;) he should be able to figure out most of the same theories we carseat-junkies have come up with, right? Make HIM prove it! :D

But he is arguing to use both. He cannot fathom why you would only want to use one or the other. And he would love to see the theories you expert-carseat-junkies have come up with.

Please help me, because i am going to lose this battle without some plausible-even-if-flawed-theory against using both :(


Admin - CPST Instructor
Staff member
The LATCH strap and the seatbelt are each designed to stretch up to 20% in a collision. Same with the harness, but that has no bearing in this conversation. That increases the "ride down time", how long it takes for the stop to happen. You don't want to stop immediately. You want to slow it down. It's like slamming into a brick wall versus a padded wall. The padding slows you. You'd much rather hit that than straight brick.

Ok, back to why. Each of those are designed to take the crash energy and stretch with them. If they're both in use then NEITHER of them may stretch properly. Back to baseball, have you ever seen two people chasing the same ball? "I got it!" "I got it!" Neither ends up with it. However, the energy must go someplace as it can not simply be destroyed there, and the next place it'll go is into the carseat. Well, the carseat may absorb it without a problem. It may not be able to absorb it and it may crack in half, or it may simply transfer that force to the next thing in line, which would be your child. So instead of hitting a nice padded wall your child has been slammed chest first into a brick wall. One that unfortunately will allow the head to slam right over the top of it. So all of that energy goes into the weakest point, which is the neck.

So instead of the LATCH or seatbelt stretching you may end up with your children's neck stretching. With their skeleton not yet fully ossified that may result in either a broken neck, or something called internal decapitation, where their head comes off of their spine, but it's not removed from their body. In lucky cases that results in paralysis. In EXTREMELY lucky cases it results in nothing more than a couple of months of recovery and rehab (this just happened to a four year old I know of on my local mommies board, she was in a booster and "dislocated" her spine). In most cases it results in death.

OR, if the carseat cracks you now have that behind your child, riding down the forces on two pieces. At the very least I would expect severe bruising and cuts from the plastic on the child's back. In the worst I would expect a broken back.

All of this can be prevented by simply using the seatbelt (continue to use the top tether) and using the seat as the manufacturer has tested it.

Your husband can also feel free to call Britax and chat it over with them. :) Maybe he'll find out the actual reason why. But these are our theories and concerns about it.

Thanks for this post. My daycare girl's mom installs with both and keeps assuring me it is safer(oh and having your straps loose with the baby falling out of the seat :rolleyes:). I am going to read her this.


Admin - CPS Technician Emeritus
Staff member
As an engineer, perhaps your DH could relate to the fact the seats were designed and tested by engineers, who more than likely tested them out with both installation methods simultaneously, and found that it either adversely affected crash performance in some way, or it added no additional benefit. Trust me, I've been in your shoes with the DH situation. :) My DH has the engineering mindset, likes to take things apart and analyze everything, and left grad school a dissertation away from becoming a theoretical physicist. He's satisfied with the above explanations. :)


New member
I just had to laugh reading your post... my husband is in the same line of work as engineers and they just don't get it.

I agree with what Wendy said... just to add to that; take a look at your Marathon, the LATCH connectors are located on bars running along the side of the seat and to install with the belt the vehicles seat belt goes through the back of the Marathon (called the forward facing belt path). In a collision this would put 2 very different pressure points on the Marathon. Plain and simple it isn't designed to be used that way and who knows what would happen or how the seat would perform.

Car-Seat.Org Facebook Group

Forum statistics

Latest member

You must read your carseat and vehicle owner’s manual and understand any relevant state laws. These are the rules you must follow to restrain your children safely. All opinions at Car-Seat.Org are those of the individual author for informational purposes only, and do not necessarily reflect any policy or position of Carseat Media LLC. Car-Seat.Org makes no representations as to accuracy, completeness, currentness, suitability, or validity of any information on this site and will not be liable for any errors, omissions, or delays in this information or any losses, injuries, or damages arising from its display or use. All information is provided on an as-is basis. If you are unsure about information provided to you, please visit a local certified technician. Before posting or using our website you must read and agree to our TERMS.

Maxi Cosi is a proud sponsor of Car-Seat.Org!Graco is a Proud Sponsor of Car-Seat.Org!Nuna Baby is a Proud Sponsor of Car-Seat.Org!

Please  Support Car-Seat.Org  with your purchases of infant, convertible, combination and boosters seats from our premier sponsors above.
Shop travel systems, strollers and baby gear from Britax, Chicco, Clek, Combi, Evenflo, First Years, Graco, Maxi-Cosi, Nuna, Safety 1st, Diono & more! ©2001-2021 Carseat Media LLC