Ask an Engineer: Are Differences Among Webbing Types Important?

This article originated in the March/April 2018 issue of Safe Ride News.

Harness Webbing

The following guest article was inspired by an enlightening conversation between SRN editor Denise Donaldson and Dave Sander, CPST-I and engineer (formerly with Evenflo, but employed elsewhere at the time this article was written).

Have you ever given close attention to the webbing used for car seat harnesses, LATCH straps, or vehicle seat belts? If so, you may have noticed that some are wider or feel thicker, smoother, or rougher than others. You may have also noticed that some have stripes (actually called panels), and that those panels vary in appearance and number.

If you have noted these things, I congratulate you on your keen sense of observation! These differences are not random or decorative; each detail in webbing has been intentionally designed to affect how it will perform, especially in a crash.

FMVSS 213 stipulates certain webbing characteristics of CRs. It defines the minimum width of the webbing used in harnesses, tethers, and LA straps. It also says that new webbing must meet a minimum strength requirement of 11,000 Newtons for harness webbing and 15,000 Newtons for LA and tether webbing. To get an idea of how strong that is, you could basically pick up a Honda Accord with a strap made out of LA or tether webbing!

CR manufacturers purchase this strong webbing, and most also do their own internal testing to doubly ensure compliance with the standard. FMVSS 213 specifies that this be done using what’s called a quasi-static test. This is simply a test in which the webbing must not break, at the specified load, when a device attached to the ends pulls it apart at a slow and steady rate.

The quasi-static test is beneficial as a consistent benchmark for measuring performance criteria among all the different webbings that a company might use. However, CR manufacturers also assess webbing during dynamic testing of car seats during sled tests run at a very high rate of speed. Quasi-static results typically do not match these high-speed results, in that the amount of elongation (or stretch) seen during the quasi-static test is likely to differ from the amount during a sled test—it could be more or less. Since the amount of stretch is a key characteristic with respect to how webbing manages crash forces, it is helpful to know the results of both types of testing.

Now back to the guts of the story. We’ve observed that webbing comes in different styles with varying construction. Why? Because, depending on the configuration of the fibers (threads), webbing will stretch to varying extents when loaded by crash forces, such as in a sled test or actual car crash. Rather than considering one type the best, engineers make use of this variability.

Like car seats, webbing types can perform differently in FMVSS 213 crash testing, and the actual car seat it is attached to will further differentiate the results.  Sometimes the webbing selected during car seat design may even cause the CR to crack during the development and testing phases. By simply making a better choice for the type of webbing, the same car seat may pass testing without any other changes to the CR being needed. When looking at the performance criteria in FMVSS 213, differences in harness webbing can influence the results of the test dummy head injury criterion (HIC) score and Chest G injury criteria, as well as the head and knee excursion (forward movement).

As CPSTs know, the management of crash forces requires give and take.  While one goal is to hold a CR in place, injury may result if the body isn’t allowed to slow down gradually enough. Therefore, while webbing used for LATCH installation must be strong and hold the car seat in place, car seat engineers carefully select the kinds of webbing used for a particular car seat model to balance the CR’s overall performance.

For instance, some car seat models may have tether or lower anchor webbing that has a relatively high elongation in order to enhance the performance of the CR structure.  While this would increase some excursion measurements (a negative effect), this might be a net-positive tradeoff if it lowers the dummy HIC or chest Gs enough (a positive effect). In fact, because tethers do such a good job of supporting a CR and controlling head excursion, there is usually some room to use webbing that stretches more if the overall effect is a more structurally sound CR that measures better HIC and chest Gs in testing—a tradeoff that is likely to translate to better outcomes for real children in crashes.

Webbing variations can also be especially useful to engineers in the late stages of CR development. To CR engineers, these final stages are all about tweaking or “turning the dials” until you get the best performance possible in all the measurable categories: HIC, chest G’s, head excursion, knee excursion, and structure.  What does turning the dials mean?  Well, before a car seat is even made, developers use a variety of tools, like computer-aided design programs and 3-D printed models, to predict a CR’s fit, performance, and function, because changes made after a CR is molded are very costly. But, until it has been physically made, it is difficult to really know for sure how a CR will perform in every test configuration. So CR manufacturers have a few go-to ways to tweak performance during the final development stage. Having a wide selection of webbing to try is an important one of those, giving them so-called “dials” to turn. By matching the right webbing to a CR, manufacturers can fine-tune it so it performs to its best potential.

I hope this sheds some light on how CR manufacturers choose webbing, just one of the many factors that can influence the performance of a car seat. In particular, consider this when asked why owners are prohibited from swapping components of different car seats, even if the parts are from the same manufacturer. When it comes to webbing (and other parts, as well), rest assured that there were important reasons the CR developers used the particular type that they did for each model. So, even if parts seem similar to the untrained eye, making changes to a CR that are not approved by the manufacturer can truly have negative consequences on performance.

Dave Sander—Dave Sander