No, it's actually not a big mystery at all. :-)
Here are the relevant facts:
Fact: Newborns MUST be fully reclined as the seat allows. Why? Because their tracheas are not fully developed as connective tissue, and they have lousy neck control. If their little heads tip forward, it actually pinches their tracheas off, and they don't have enough coordination/awareness to adjust their positioning, especially if they're asleep.
Fact: A more upright rear facing seat is always safer in terms of crash forces. Why? Because in a frontal impact (which accounts for 70% of impacts), when a seat is fully upright, more of the energy of the crash is diffused through out the shell before it is transmitted to the child. When the seat is reclined, more of the force of the crash goes into the harness as the child ramps up the shell, and thus is absorbed directly by the child's body.
Fact: A 45 recline is only a nod to the fragility of newborns, NOT safer in terms of crash dynamics.
Fact: Almost all restraint companies acknowledge this, and have created seats that can be installed at different angles depending on the age and maturity of the babe. Evenflo, Graco, Britax seats, for example, all may be installed between 45 degrees from vertical for a newborn, to 30 degrees for an older child.
Fact: The First Years observes a more conservative requirement. Their angle range is 45-35 degrees.
Fact: Until, well, NOW, actually, Dorel had a blanket rule of 45 degrees on all its seats, due to the newborn issue. They were catering to the lowest common denominator, to prevent stupid parents from killing their babies by misusing the restraints. A bunch of us wore them down, they finally confessed that yes, their seats pass testing just fine as upright as 30 degrees, and they are now FINALLY releasing seats with two different angle lines. 45 degrees for newborns and very young infants, 30 degrees for older children.
Fact: Sunshine Kids/Diono has no maximum upright angle. The seat may be 45 degrees to as upright as you can get it.
Fact: Swedish seats, which are designed to keep kids rear facing as safely and as long as possible, are installed at roughly 10 degrees from vertical...in other words, the same recline as a forward facing seat, but facing the other way. Swedes know what they're doing with rear facing technology.
What you need to know? YES! A Britax Marathon can be installed more upright than 45 degrees for a 6 month old infant. You're good.
Originally Posted by sunflwrmoonbeam
Called a tech already, he was completely clueless. Information in the internet for RF basically talks about how to do it with a newborn and how it's important to do it. I can't find any consistent info on recline angles for older children. Some sources say upright's fine or even safer, others say RF must be reclined.
I must conclude that no one f*@(ing knows.