I'd like to understand "why" or "why not" the last poster (and the manuals!) says "The tether has no functional purpose from a crash perspective with a booster seat." That is, I'd like to understand the basic physics, etc. -- on a level that a layman can understand!
A booster only has 1 function - it positions the child so that the adult seatbelt fits correctly. From a protective standpoint, it's the seatbelt restraining the child - all the booster does is make sure that the seatbelt is low on the hips/across the upper thighs, and that the shoulder belt is positioned comfortably between the shoulder and the neck.
The booster is a positioning device basically. It's a very specifically designed positioning device, that can make the difference between life and death for a child restrained by the seatbelt, but that's it's only job.
In a ff'ing restraint the top tether reduces how far forward the top of the restraint and therefore the child's head moves in a collision. The child is restrained in the seat and for the purposes of explaining I'll just use the word "attached" to the seat. So preventing the carseat from moving forward too much also reduces how far forward the child moves. It's important because it reduces the likelihood of the child contacting the vehicle interior and thereby the risk of injury.
In a booster the child isn't restrained against the shell the same as in a harnessed seat, so holding back the top of the booster would make no difference in terms of the movement of the child - the child would continue to move forward until the seatbelt prevented further movement. So when I say it would have no functional purpose from a crash perspective, it's because preventing the booster seat from moving forward in a collision isn't going to reduce the movement that the child experiences in a collision - whether the booster stays still in the vehicle or whether it moves with the child during a collision, it's still the seatbelt that stops the movement of the child.
Hopefully that makes it a little easier to understand.
As the last poster said, "As to why it's not allowed, it's just a matter of that being how the manufacturer has directed the seat be used. Whether it's because tethering adversely affects the function of the booster, or because they just didn't test it that way there's no way to know..." Again, I would really like to understand the reasoning behind all this!?
Thanks!
I'm afraid I don't have a good explanation on the reasoning of why some manufacturers approve top tethers in booster mode and some don't. The best I can do is give you some background on the approval process and how manufacturers bring seats to market...
Transport Canada sets standards that all manufacturers must meet. Booster seats in Canada aren't required to be crash tested - they have to meet standards in terms of compression, materials used, labeling, and that kind of thing - the technical term is static testing. As long as they comply with all of those criteria, they can be sold in Canada. It's up to the manufacturer how they want to design the seat, and what instructions they send with the seat. The reason that boosters aren't crash tested in Canada is because the booster isn't restraining the child - the seatbelt is.
Now just because CMVSS standards don't require crash testing doesn't mean the carseat manufacturer doesn't do crash testing. Pretty much all boosters available in Canada are also available in the US, and FMVSS standards do require crash testing - which translates to the seats and designs that are in use in Canada still having gone through crash testing.
When a company designs a seat, they basically start with a blank sheet. They have standards that must be met, but it's up to them to determine what they want for the design and what features/components they want to include - as long as the seat meets standards. They may have research giving important points for what they want to include in the seat, but at one point it starts off as a design. Manufacturers do multiple crash tests during the design stage, and they base their instructions on the results of the crash tests and the design of the seat.
Specifically in regards to tethers, it could be that the company research says there's no benefit to top tethering a booster, so they don't test it. Or it could be that they crash tested it with the seat top tethered and the structural integrity of the seat was affected or some other thing. There's just no way to know without being able to sit down with the engineer of the seat and discuss the how's and why's.
So that's why I say it could be because it wasn't tested that way, or it could be because it failed in some manner when tested that way. The public doesn't know results, or the combinations that tests were run in, so it's just impossible to know the reason why the manufacturer doesn't allow it. When it comes down to it, we aren't always able to understand the reasoning a manufacturer has for a specific instruction, but we still have to follow it.
For a young child, a harnessed seat is obviously preferred and you'll find that recommendation pretty much straight across the board here, but there comes a point where a child does not fit a harnessed seat anymore and HAS to move to a booster seat. A booster is not a restraint, it's a positioning device. Seatbelts are designed to restrain passengers, and they do a good job of that. Using a booster allows the adult seatbelt to fit a child who is out of a harnessed seat, but doesn't fit the seatbelt properly. They fill a very critical role in keeping children safe in vehicles and absolutely save lives and prevent and/or reduce injury.
