Dangerous Seat Belt Extenders

Why Seat Belt Extenders Should Not Be Used with Carseats and Boosters

seat belt extender in Tesla Model XHave you seen the recent story in the news about Ethan, the 6 year old who was severely and permanently injured in a crash? Because his seat belt buckle was flush with the vehicle seat, his father used a seat belt extender to make buckling his booster seat easier and it failed in the crash. What is a seat belt extender and why should it NEVER be used with kids in carseats?

Seat belt extenders are designed for ADULTS who are too large to fit into seat belts. This means that they have pulled the seat belt out all the way and it still doesn’t allow them to buckle it, not that it’s too difficult for them to buckle (my mom is obese and she struggles greatly to buckle the seat belt in my car, but that doesn’t mean it’s too short). There are different kinds of extenders: rigid, which have a metal stalk, and flexible, which have a webbing stalk, and they come in differing lengths. They also come in different colors and some are designed to be bolted at the seat belt anchor instead of buckled at the buckle.

If an adult needs an extender, they should choose the one that is the shortest that will accomplish the job of buckling the seat belt. Seat belt extenders change the geometry of the seat belt on the body. Ideally, the lap belt should lay low on the hips and the shoulder belt should cross the chest and touch the collar bone midway (use the vehicle’s shoulder belt height adjuster to change this, if there is one). Using an extender will cause the shoulder belt to shift off the collar bone because the buckle moves the latchplate and buckle over the body. No hardware should touch the body because it would cut into it in a crash; the seat belt will slice through the body until it reaches bone. In fact, the instructions will tell you exactly where the extender buckle will be placed in relation to the center of your body (your belly button).

What makes seat belt extenders dangerous? They are specific to the vehicle and seat belt into which they buckle and not every vehicle manufacturer makes them. NHTSA, the government agency which crash tests vehicles, hasn’t safety tested extenders. Just because a generic extender you bought off Amazon or some other website buckles into your car’s buckle doesn’t mean it will hold in a crash. And the extenders that bolt at the seat belt anchor? The instructions don’t even specify the amount of torque to be applied to the bolt!

Word of mouth makes seat belt extenders popular. I’m sure all vehicle groups discuss them at some point, but I’m most familiar with the Tesla forums since we’ve been early adopters of all 3 models—the Model S, the Model X, and the Model 3. Tesla parents are frustrated that their kids can’t easily buckle themselves into their boosters, or if there’s a 3-across situation, they can’t buckle themselves next to a sibling’s carseat because the buckle is flush with the vehicle seat.

Now would be a good time to talk quickly about crash forces. A (granted overly simplistic, but easy to understand without a scientific background)* way to figure out how much force you’ll put on a seat belt (or harness for a child) is to multiply your weight by the speed you’re going. So, if you’re going 30 mph, which is the standard crash test speed for a carseat, and you weigh 150 lbs., 30 x 150 = 4500 lbs. of force against the seat belt. Can you imagine having the hardware (latchplate and buckle of the extender) on your hip bone or belly in a crash? Only do it if you absolutely must, not for convenience; it’s better to be restrained for sure.

*Yes, we know that the actual equation is force = mass x acceleration, but then we also have to get into Newtons and that really muddies the math for folks who don’t have that background. So we keep it simple 😃.

Why shouldn’t kids use a buckle extender when using a booster? For a variety of reasons.
  • Increasing the length of the seat belt increases head excursion and neck flexion. What does that mean in layman’s terms? A higher possibility of head and neck injury and a much greater risk of the child hitting the inside of the vehicle. Having a pre-tensioner to spool the extra webbing back during the crash cycle may help, but those are only just now coming out in back seats.
  • seat belt extender booster Tesla Model XThe extender may place the buckle right under the arm of the booster, which can then knock the latch release in a crash, leaving your child unbuckled.
  • It may place seat belt hardware over your child’s soft belly. The whole goal of a seat belt (and booster seat!) is to protect your child in a crash. Changing the geometry so that the buckle is near, or over your child’s abdomen, increases the injury potential.
  • It’s infuriating as a parent to know that someone is trying to make a buck off our children’s safety by selling a quick-fix solution targeted at unknowing caregivers.
What should you do instead of using a buckle extender?
  • Use a harnessed carseat as long as possible. It’s much easier to buckle a harness, which is up front and easily seen, than to struggle with a seat belt buckle that may be hidden.
  • Pull the booster forward on the vehicle seat slightly, buckle the child, then slide the booster and child back into position. Don’t forget to snug up the seat belt! This requires some strength, though.
  • If the buckle stalk is floppy, cut a short portion of a pool noodle and wrap it around the stalk.
  • Use a booster without arms, if you can. This isn’t always possible, unfortunately. Some vehicles have head restraints that jut forward and push the headwings of the booster forward. Most booster manufacturers won’t allow this. I’ve contacted our Maxi Cosi rep and she confirmed they don’t allow the RodiFix to be used with the Tesla Model X, which is unfortunate because it’s often mentioned as a solution for that vehicle. There’s too much of a gap for Maxi Cosi to feel comfortable that the RodiFix will perform well in a crash. You’ll want to check for that gap in other Tesla models since the vehicle seats are similar amongst the newer models.
  • Use a narrow booster or one that tapers toward the back. This means, of course, that your child needs to be narrow too and we know that kids come in all shapes and sizes, so that won’t always work.
  • Keep the booster buckled and have your child slide in and out of the seat belt. This takes a mature kid who won’t pull the length of the seat belt out and is fairly flexible. The trick is to put the shoulder belt behind the child’s back first, then slide down under the lap belt. Reverse the process to get into the booster.
  • In the future, look for vehicles that don’t have buckles that are flush with the vehicle seat. While this type of buckle actually improves belt fit and keeps the latchplate out of the belt path for carseat installation, if you’ve had a kid struggle to buckle a seat belt with one, you know the seconds tick by like hours until you get that tickle of anxiety in the pit of your stomach and grab the latchplate from them in a fit of “let me do it for you so we can get out of here!”

There are certainly alternatives to using a seat belt extender. It’s an easy out and usually easy tricks like that and carseats don’t mix. You don’t want your child injured and you certainly don’t want the hospital bill that comes with it!


  1. Patrick May 3, 2022
    • Heather May 4, 2022
  2. Victor November 27, 2021
    • Heather November 27, 2021
  3. Linda April 2, 2021
    • Heather April 3, 2021
    • Tess July 29, 2021
      • Heather July 29, 2021
  4. Mike November 27, 2020
    • Heather November 28, 2020
  5. Lori Jacobs January 28, 2020
    • Heather January 28, 2020

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.