Installation with seat belt and LATCH

[mlohry]

I know this is an unsafe practice and forbidden, but a friend told me today that she was doing it to keep the seat "really tight." Is it not ok just because the 2 may get in the way of each other not allowing the seat to be tight or is there other reasons, just wanted a good explanation.

[Baylor]

I was told once because it was not tested that way.
Maybe send her here.. But there may be a thread about it. It is meant to use either or. I swear there was one seat where it was okay though?? Or am I losing it more than I thought????

ETA: Found this

http://www.car-seat.org/showthread.php?t=8385

ETAA: From Wendy
"Because it's not been tested that way, or it's been tested and failed. None of us are willing to turn our kids into crash test dummies to see how it works. It may be the best thing ever. Since every single carseat manufacturer says don't do it, though, I have to guess they've tested it and it's not worked as well.

It may be like two outfielders. Each system is tested to take crash forces. But if you put them together you've got two ballplayers running toward the ball. In the end the ball ends up on the grass and your ballplayers have black eyes from slamming into each other. LATCH is supposed to stretch up to 20% in a collision. So is the seatbelt. If neither has the full crash put on it, neither may stretch. And now those crash forces, instead of being absorbed by the LATCH strap or seatbelt, need to go elsewhere. And the only elsewhere to go is into the seat where your child is sitting, magnifying crash forces and possibly causing more injury to the child in the seat.
"

http://www.car-seat.org/showthread.php?t=99413

[Brianna]

I like Wendy's baseball analogy http://www.car-seat.org/showthread.php?t=67995

(Unless of course your friend happens to be installing a Combi in a Ford- this is the only combination that allows for use of LATCH and the seat belt)

[Keeanh]

If "it wasn't tested that way" isn't enough, then maybe point out that seatbelts are designed to stretch a certain amount. By doubling up with a LATCH strap, the install might be TOO solid and the child would be subjected to greater forces.

If she HAD to bungee jump, would she rather jump with the proper stretchy rope tested for her weight? Or would she ask for 2 or 3 of them together? I think common sense would tell her that 2 together wouldn't stretch enough and she'd likely be injured or killed.