Why a 40 pound weight limit for boosters?

[cat mommy]

What is the reasoning behind the 40 pound weight limit for boosters?

And why do some boosters have a lower weight limit?

Is there really a big difference between, say a 38 pound, 4 year old and 40 pound, 4 year old in a booster. Is 40 pound kid just as safe in a booster as 42 pound kid of the same age? Is it better to wait to booster a child, of appropriate age, just because he is only 40 pounds?

 

[MommyShannon]

There is a higher risk of submarining under the seatbelt for smaller kids. Age/maturity is a big factor too. Most 4 year olds are not mature enough to sit correctly 100% of the time. That is often listed as a minimum age, but in my experience, kids aren't mature enough until 5 or 6 years old. I know some techs will have some data to share. Canada has a 40lb minimum for all boosters because of the risks.

 

[thepote]

As I understand it, the 40lb limit is to help ensure:

-That body size is within an acceptable range for the belt geometry to fit -most- kids.

- That hip development is within an acceptable range for the protrusions at the top of the hips to have enough margin of error where it is likely the lap belt will catch on bone with -most- seats.

-That -most- kids will hit 40lbs when they are old enough that they will sit correctly all the time, and won't turn into noodles when they are sleeping.

 

[Patriot201]

I'm glad that some boosters do have a lower weight limit because there are those kids who are seven or eight years old and who only weigh 35-40 pounds. Those kids, despite their small size, do still have the bone maturity of an average child their age. While a harness might still be a good idea, there needs to be the option for a booster, as well.

 

[carseatcoach]

Is there really a big difference between, say a 38 pound, 4 year old and 40 pound, 4 year old in a booster. Is 40 pound kid just as safe in a booster as 42 pound kid of the same age? Is it better to wait to booster a child, of appropriate age, just because he is only 40 pounds?

Just a note that many 4yos are not developmentally ready for boosters, regardless of weight.

I'd rather see a 38# 6yo in a booster than a 42# 4yo (assuming, of course, that the booster is rated for <40#).