There are many many factors that have contributed to SK allowing the top slots below the shoulders. Since questions regarding these factors are causing so much controversy on the boards, I thought I'd share some of the info I have.
The first issue I would like to address is the idea that measuring the spinal compression is not possible because we have no dummies that test for it. This is not correct.
SID II(s), which stands for Side-Impact Dummy, version two, small, is representative of a 5th-percentile female. Like her Hybrid III sister, she is also 5 feet tall and weighs 110 pounds. This dummy is a derivative of the original SID, a 50th-percentile male dummy that was developed in the late 1970s by NHTSA. Because humans who are in a side-impact-type of collision are likely to suffer different types of injuries than those in a front-end collision, side-impact dummies have ribs and a spine that flexes differently in a crash. They are built specifically so that researchers can measure the risk of injury to the ribs, spine and internal organs, such as the liver and spleen.
SID II(s) was built as a special project by a group of researchers working with a group of automakers. Currently, she is only being used in the side-impact testing conducted by the IIHS, although NHTSA is considering a proposal to put her into use in its program. The IIHS began using her in its early developmental side-impact testing in 2000 and now uses her in all its side-impact crash tests both in the driver seat and in the rear seat behind the driver. "The 5th-percentile female is also similar in size to the average 12- or 13-year-old child, who is most often in the rear seat," explains the IIHS' Lund. "This allows us to measure the potential of injury to those youngsters as well."
It is my understanding per disscussions with SK that this is the dummy used to test the upper weight limits of the seat.
In addition to the placement of the shoulder harness, the effectiveness of pelvis restraint plays a large part of how the rest of the body reacts in a crash.
In Nascar testing, it was found that seats and restraint systems that did not provide adequete protection against submarining allower an average of 5 inches of spinal and chest compression. Seats and restraint systems that do prevent submarining allowed an average of 1.5 inches of chest and spinal compression.By restraining the pelvis, there is less energy for the shoulder harness to absorb, keeping the chest from being compressed.
Nascar goes on to explain the seat belt geometry that best restrains the pelvis. The attachment points must provide the optimum geometry to minimize movement of the belts. Lap belts perform best when they act at an angle between 45 and 55 degrees relative to the longitudinal axis of the vehicle. This angle permits the lap belt to react to the upward pull of the shoulder harness. A system installed with a shallow belt angle permits the shoulder harness to pull the lap belt up off the pelvic area and into the abdominal region, which greatly increases the chances for internal injury.
Shoulder harness placement also, of course, affects spinal compression. The following is Nascars guidelines.
The end attachments of the shoulder harness must also be installed at appropriate angles. The ideal position is anywhere between five degrees below and 30 degrees above the driver’s shoulder.
If the upper attachment point falls significantly below the driver’s shoulder, then a spinal compression injury is likely to occur. In an accident, the shoulder belts pull down and back on the torso as they resist the forward motion of the driver. The resultant restraint force compresses the spinal column and will add to the stresses in the spine already caused by the force of the crash impact.
This is why the general rule is at or above ffing. Official Nascar rules thus state
The shoulder harness cannot be
secured more than 1" below the driver's shoulder.
Just as a point of interest, although it is not the focus of this conversation, Nascar also states
In the opposite situation, if the trailing ends of the harness are too far above the shoulder (greater than 30 degrees), then two problems can occur. First, tension in the shoulder harness is increased and undue stress is applied to the harness and its structural attachments. Second, excessive angle will cause excessive motion. If the harness belts are too far above the shoulder, they will provide little resistance to forward motion of the driver’s upper torso. The result is an impact with the steering wheel and the possibility of neck injury. The shoulder straps should also be three to six inches apart behind the driver’s neck to prevent slippage off the shoulders.
If you measure the angle between where a child's shoulders would be if 2 inches above the top slot (this is where my ds shoulders are if I stick books under his bum to make his ears even with the top )and where the harness passes through the back of the shell, you have approximately a 7 degree angle.
The low profile and close set crotch strap and 50 degree angle between the attachment points for the hip straps and the buckle are right in line with Nascar's recommendations for best pelvis restraint. This would result is a great reduction in chest and spinal compression.
Just so you all know, I do NOT like the Radian, even though I own 2 of the things. I believe it is a great nitch seat, ( I really need two seats that fit this "nitch" otherwise I would have ditched these things already) It is hard to install, the instructions are needlessly inflexible, and it is a total PITA to to adjust the straps. But for me, the options are limited to the Radian or putting my 5 and 8 year old children in a lap only belt.
There is an AMAZING amount of engineering that goes into any car seat design. It is way more involved than the hard and fast set of rules that have been drummed into our heads. There HAS to be innovators in the field willing to figure out ways to do what we have all been told is impossible or we will never make progress. It wasn't all that long ago that we didn't have HWH at all because everybody said that it just wasn't done. The first HWH seats on the market (Fisher Price Futura, and Super Elite) didn't sell well at all, and one reason was consumer distrust. other innovations such as reverse belt paths and infant cs bases also were met with high levels of distrust.
As for me, I will without a seconds hesitation use my Radians with shoulders above the top slots. Both because I trust that SK is not under any cercumstances willing to open themselves up to the huge lawsuits that would result if a child was injured because they allowed something so basically against the rules without doing their homework first ( and honestly, the ruling fits with the practices of the leading source of crash data, NASCAR), and because I have done my own homework. This isn't to say that a year down the line SK and I won't both be proved wrong. But for now, this is the best information I have to go on.
I understand that my children would be safer if the harnesses where above their shoulders, but I also believe that they are safe with them below in the Radian 80. Whether or not you choose to follow SK guidelines, or go with the time honored ones is up to you. It is truely a parental choice to make. I don't advocate my choices to anyone, )this includes my seat choices, erfing, ehnessing, chest clips, convertables vs infant seats, etc) that is not my job. My job is to educate. I can and do offer my opinions, but I also try to accurately represent both sides of the issue whenever possible.
That is what I am trying to do now, fill in the missing pieces of info. We all know why over the top slots isn't allowed in general. We all deserve the other pieces of the story so we can EACH make informed choices.
Kimberly
Mom to Becky, 11 yrs old, 90 lbs in adult belt most of the time, Danny, 8 yrs old, 52ish lbs in a Husky and a Radian 80(1 inch below top slots) and Hope 5yrs old 60 lbs in a Radian 80( 2 inchs below top slots)