Is Your CR Hanging Out Too Much?

Understanding CR Overhang

This article originated in the May/June 2020 issue of Safe Ride News.

Explanation of CR overhang.

The newly released version of the National CPS Certification Training curriculum does more than earlier versions to introduce the concept of overhang by including a slide and explanation in the technician guide.  It tells students to ensure that the base (footprint) fits on the vehicle seat by checking the CR owner’s manual to learn about overhang, and it says to use the “80/20 guideline” if instructions don’t give other advice.  It points out that some manufacturers require 100% of the footprint to rest on the vehicle cushion and that some vehicle seats are too shallow for some CRs.

What do “footprint” and “overhang” mean?

The words footprint and overhang don’t appear in the curriculum’s glossary, but they are key terms to understand and to teach to both technician candidates and caregivers.  To assess overhang, one must first know how to measure a CR’s footprint.

A person creates a footprint when stepping onto sandy ground; think of the part of a CR that would form such an impression in sand as its footprint.  Since it’s the bottom, supporting part of a CR that forms its footprint, one might think of the footprint as the CR’s “base.” However, be careful not to confuse this with the term already used for the stay-in-car part of RF-only CRs.  Even a RF-only CR’s base should be assessed for its footprint, as should any type of CR or booster.  Multimodal CRs, like convertibles and all-in-1s, might have only one footprint, but it’s also possible to have different footprints for different modes, depending on how the model is designed.

Overhang refers to the amount of this footprint (or, specifically, the fore-aft measure of the footprint) that extends beyond the vehicle seat cushion.  The overhang is a safety consideration because a CR’s crash performance can be degraded when too much of the CR is not supported by the vehicle seat cushion.

When must a CR be checked for overhang?

Overhang should be checked and considered for every CR or booster in any vehicle.  While overhang issues may be more common to rear-facing CRs, which jut toward the front of the vehicle, consider this aspect of vehicle fit for all types of CRs, whether rear or forward facing.  Also, be sure to note the vehicle seat cushion dimensions before installing a CR; many back seats have cushions that are shallower in the center than in the outboard positions, and this may not be apparent once the CR is installed.

Where is information about overhang allowance located?

The CR owner’s manual is the first place to look to see if overhang information is provided.  While this topic is not always addressed in instructions, more manuals include this point now than in the past.

If the manual doesn’t mention overhang, check the manufacturer’s website.  FAQ sections sometimes address this topic.  If necessary, contact customer service for guidance.

If these sources do not say anything about overhang, the curriculum now says to follow 80/20 overhang guidance.

How do I follow the 80/20 guideline?

To follow 80/20 overhang guidance, ensure that no more than 20% of the footprint (fore-to-aft) extends beyond (overhangs) the front edge of the vehicle seat cushion (see diagram).  This is the most common advice from manufacturers, and, according to the current CPST curriculum, the fallback position to use in the absence of manufacturer guidance.

To eyeball this percentage, estimate the halfway point of the fore-aft measurement, and then halve the outer half (the half farthest from the vehicle bight) to mark the spot that would be about 75%.  If it’s clear that this much and a bit more of the footprint is supported by the seat cushion, then the 80/20 guidance is met.  However, if results are unclear using this estimating method, use a tape measure and do the math to be sure.

What other overhang guidance exists?

This article (and the CPST curriculum) carefully avoids the term “rule” when referring to the 80/20 guideline because, while common, the guidance should not be mistaken as a universal rule.  Always check instructions.  Some manufacturers state a number of inches that may overhang.  A few have put a marker on a label to indicate the maximum point of overhang. (So far, this label has been added to a few models with leg-area extenders; a universal application of this type of straightforward guidance would be helpful.)

Overhang is more commonly allowed for rear-facing CRs than it is for forward-facing CRs.  BRITAX has clear instructions that allow 3 inches of its forward-facing CRs to overhang (the same as when they’re in rear-facing mode). But many manufacturers require 100% of a forward-facing CR’s footprint to be supported by the seat cushion, even if some overhang is allowed for RF models (or, even, the same model in RF mode).  Some manufacturers, such as Diono, require 100% support of the footprint for installations in all modes.

Do any CRs allow > 20% overhang?

In rare situations, manufacturers allow more than 20% overhang, although there is always a limit to how much more.  When a model has a relatively long fore-aft footprint, such as some RF-only bases with an anti-rebound bar, it’s a good idea to check with the manufacturer to see if there are special instructions regarding overhang allowance.

Also, consider asking the manufacturer about overhang if the CR has a load leg (aka, foot prop, stability leg).  When the leg is deployed, it extends to the vehicle floor, providing extra support for the part of the CR that hangs over the seat cushion. CYBEX and Clek reported to SRN that their CRs with this feature may overhang by more than 20% when the leg is properly deployed; check with the manufacturer for confirmation and specifics on how much more overhang is allowed. Evenflo says to follow the 80/20 guideline, whether or not a load leg is in use.  Nuna has issued more specific guidance for all its CRs with a stability (load) leg:  When the leg is not used, follow the 80/20 overhang guidance; when the leg is deployed, up to 40% of the footprint may overhang the vehicle seat.  (This Nuna permission, given to SRN in writing, was not yet in manuals as of press time, but was planned to be soon.)

Knowing about these load leg options that allow for more overhang may be especially helpful when working in vehicles with shallow seat cushions (like the back seats of some pickups or SUVs).  However, always be certain that there is no hollow space under the floor, such as a storage compartment, that would prohibit the use of the leg.