The challenges of testing and using higher weight-limit toddler booster seats

Our sled tests of toddler booster seats were conducted to evaluate each seat's performance in a simulated crash when used both forward facing with the internal five-point harness, and as a booster seat. Testing these forward-facing seats with higher harness weight limits presented interesting challenges in terms of installation. For example, two of the models we tested, the Graco Nautilus 3-in-1 and the Britax Frontier 85, allow the forward-facing harness to be used up to 65 and 85 lbs., respectively.

The weight limits of vehicle LATCH anchors

Our testers have extensive experience as child passenger safety (CPS) technicians (the people who install car seats at free clinics nationwide), and in their work evaluating each vehicle Consumer Reports tests for how well it accommodates child car seats (those evaluations can be found in our ongoing vehicle model reviews). That experience, plus a review of the lower LATCH weight limits, by vehicle manufacturer, published in the 2009 LATCH manual, show that for more than half of all vehicle makes, the lower LATCH use is limited to 40 to 48 lbs. But average consumers may not know that their vehicle has such limits, though some child seat and vehicle owners' manuals do mention it. The owner's manual for the Graco Nautilus 3-in-1 clearly indicates to "Use vehicle belt (not LATCH connectors) for installations with children weighing more than 48 pounds." But parents still may not be aware that the child seat must be reinstalled using the vehicle's safety belts after the child exceeds a certain weight.

Those limits also provide the basis for why our simulated crash tests for the higher weight seats with the larger 6-year-old-sized dummy were all conducted with belt installations only, using two-point (lap) and three-point (lap and shoulder) belts.

The weight limits of the vehicle tether anchors

A second source of confusion, and one that's potentially less clear, is that most vehicle manufacturers also have weight limits for the use of the top tether anchor. Nineteen out of 45 vehicle manufacturers listed in the 2009 LATCH manual indicate top-tether limits between 40 and 48 lbs. The owner's manual for the Graco Nautilus suggests that you "Always use tether if a vehicle tether anchor is available." The manual for the Britax Frontier 85 goes even further, indicating that "Britax recommends that the tether be used at all times when installing the child seat," and that installers should "Always use the Versa-tether for children weighing 65-85 lbs." But those conflicting recommendations and requirements raise the question: Whose limits do you follow, those of the child seat manufacturer or the vehicle manufacturer?

From our own testing

• In our most recent tests of convertible child seats, the seats offered a better potential of protecting their occupants during a crash in a forward-facing orientation when installed using the top tether than when they were not tethered. Those tests were conducted with 3-year-old-sized dummies weighing less than 40 lbs. Similar results can be expected for children in forward facing toddler-booster seats.
• In our tests of the Britax Frontier 85 toddler booster seat, installed with a lap belt and without the top tether, and tested with a harnessed 52-lb., 6-year-old hybrid III dummy, the dummy's head was at the limit for head excursion (forward movement) that would be allowed by the standard test. Additional forward head movement in a real crash can potentially expose a child's head to injury. When we tested the same seat with the top tether installed, and again with a 3-point safety belt, the head excursions were within the limits. Based on those results and recommendations from the Britax manual, we would not recommend use of the Britax Frontier 85 for older vehicles that have only a lap belt for installation and lack a top-tether anchor.

Concerns about LATCH and higher weight-limit toddler boosters

The 2009 LATCH manual, a publication used by child passenger safety technicians as a guide to LATCH use, dedicates an entire chapter to the issues and concerns mentioned above. Key items include:

• Currently there are as many as 18 child-restraint models that have harness weights of 60 lbs. or more.
• All recommend tether use, but only two require it for specified weight (such as in the case of the Britax Frontier in our tests).
• The instructions for these seats may not mention the limitations of a vehicle's anchor capacity.
• Data suggests that top-tether anchors can withstand more load dynamically (such as during a moving crash) than they can statically (such as in the stationary pull-test used to certify the anchors). That's due to the shorter duration of the forces on the anchor, rather than the sustained loads of the pull test.
• The likelihood of a top-tether anchor failing when anchoring a higher-weight child restraint is considered highly unlikely and has not been seen in real-world crashes. And if an anchor were to fail, it would still have provided some benefit in reducing forward movement and absorbing energy from the crash as it deformed.
• The LATCH manual concludes that the more anchorage points used to attach a child restraint, the better.

Despite all of this awareness, child restraint and vehicle manufacturers and the National Highway Traffic Safety Administration (NHTSA) have not yet come together to provide a consistent message for top-tether use and weight limits. Based on the points related to top tethers outlined in the LATCH manual and our test results, we believe that top-tether use for all forward-facing seats would be beneficial even for higher-weight child restraints whose harnessed limits may exceed the published top-tether anchor weight limits for the vehicle.