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The argument of whether or not aftermarket harnesses are safe to use in a street-driven car has always been a tumultuous topic. Are they legal? Will they mess with factory safety equipment? Do I need a roll cage? There’s a lot to this topic, with plenty of negatives extinguishing some noticeable positives. If you’re considering installing a harness, we have some information to present, mostly erring on the side of why not to. Here are some things to keep in mind if you’re thinking about upping your retention game.
Why It’s a Consideration
If you’ve ever considered putting a four-, five-, or six-point harness or harnesses in your car, you’ve most likely come to this decision because you want more retention. A harness firmly holding you down in the seat allows you to focus more on performance driving—the lateral g-forces aren’t making you brace your knee on the door card, press firmly into the dead pedal, lean into long sweepers, etc. This is prevalent in spirited street driving, but more so on track. With the added comfort and retention, you can focus, zone in, and start dropping lap times.
Though, replacing a worn factory belt might also be a consideration. If you’re in a position where your factory three-point belt is very worn and most likely not going to do its job in the event of a crash, sometimes it's cheaper to buy a harness than an OEM replacement. There are a lot of super inexpensive four-point harnesses out there, and if extra straps mean more safety, why not? However, of utmost pertinence, there are immense safety and legal factors to consider.
Why It’s a Bad Idea
Just because there are more straps on a belt doesn’t mean it’s safer. OEM seat belts are tested extensively for their longevity and effectiveness, and they’re designed to stay safe for the vehicle’s entire lifespan. Sure, they sometimes do wear out and need replacing, but they’re not considered a normal wear item. The cheap harnesses that you’ll find on eBay, Amazon, etc., are not strenuously tested like OEM belts, or any belt that’s DOT-, FIA-, or SFI-approved. You should also always make sure the belts you’re considering buying don’t have fake DOT, FIA, or SFI tags on them. When replacing a normal, factory three-point belt, source it directly from the automaker or a reputable retailer.
Then there are the legal ramifications. Only four-point harnesses that are DOT-approved, such as the Schroth Rallye and Takata Drift (which is essentially a re-branded Schroth product) should be considered for street-driven cars. However, there is a long list of requirements to ensure they’re mounted up correctly and safely, as detailed extensively by Schroth. Schroth also makes DOT-legal harnesses, the QuickFit—which is like their line of Rallye belts—but specifically designed and engineered for particular chassis and OEM seats.
What makes these legal is their push-button release and Anti-Submarine (ASM) loop (more on that in a bit), but only if they’re installed in addition to the OEM three-point belts.
Why It’s a Worse Idea
Three-point belts are engineered to prevent submarining, which is when the body slides under the waist belt in a head-on collision. Seats will flex and contort under extreme forces to prevent this as well, as will some vehicles' airbags. But generally, a three-point will allow the body to fold to disallow submarining, as well as increase head clearance in a roll over. The ASM loop on Schroth and Takata (not Tanaka, that's a cheap knock-off brand) belts breaks open under extreme g-force and allows the body to do the same. Certain Schroth retailers claim that an ASM-equipped harness will also prevent the body from coming too close to the window during a rollover, too.
Cheap harnesses don’t do this, and also don’t provide enough initial restraint in the first place due to being of cheap/poor quality.
OEM belts are designed to stretch out to a certain homologated point and slow down the body under extreme g-forces. Then, they allow it to be cushioned in reassuring comfort by the supplemental restraint system's airbags. In modern cars, they’re also designed to tighten, but it all depends on the kind of collision that’s happening within the given fraction of a second. This is also why I’m not a fan of aftermarket steering wheels in street-driven cars that were originally equipped with airbags, but that’s a blog for another day.
A non-ASM/DOT, yet FIA- or SFI-rated four-, five-, or six-point harness will hold the body in too well by either not allowing enough movement, causing you to get crushed if you don’t have added rollover protection, or putting too much strain on the neck and head, which could lead to the brutal killer of many race car drivers, the basilar skull fracture. I’ll link its definition because just thinking about it makes me queasy. After all, it’s why the Hans device was invented and should be used with a racing harness. For those who are curious, the SFI Foundation has a guide on how racing harnesses must be set up.
As far as preventing submarining with non-DOT five- and six-point belts, most put a sturdy piece of the harness webbing right in front of the crotch. Certain sub-belt designs are better at not crushing this region of the body than others; some six-point designs are routed around this region.
If You Still Aren’t Detracted
If you’re game for DOT-approved four-point belts, they are great for retention on track and help provide a better connection to the chassis. Street driving, however, makes them huge pains in the neck (pun intended). They restrict movement for checking blind spots, going through drive-thrus is annoying, and you might not be able to reach certain controls when fully belted in. If you’re thinking, “ah, well, I’ll just run them loose,” this completely negates their effectiveness—you might as well not wear any belt at all.
There are certain DOT harnesses that are easy to detach for street driving, and DOT-approved ones are fine in general, but they make life harder. Like any aspect of running performance driving equipment on the street, it's a balancing act of trade-offs.
There’s also a lot of engineering guidance and governmental regulation to take into consideration, such as mounting angles, seat design, mounting points, and more. Are they being run with an FIA-approved seat that won’t break under pressure? Are the belts contacting the harness holes before they’re contacting your shoulders? They shouldn’t; they should always contact your shoulders first, at the correct angle. Plus all of the other requirements for safely running them. Is the buckle sitting right on your waist and not up high on your belly button? Does your local track day organization or company even allow them in a non-caged car?
There’s a lot to think about and plan for—do your research and figure out what's best for your application and comfort level. But one universal truth, is it's a good idea to avoid non-DOT-approved/non-ASM belts in any car if you're not going to run a roll bar, helmet, and neck restraint as well. That's right, helmet and hans on the street, which is incredibly inconvenient and generally illegal in its own right. The whole thing is just not worth the risk. Again, these harnesses hold you in too well, and it either leads to being crushed or putting way too much strain on the back of the skull, neck, and so on.
If all this sounds like too much, or you aren’t sure, just stick with OEM seat belts that are in good condition. You don’t want to be inconvenienced, crushed, or suffer a basilar skull fracture, potentially all during the same impact. Speaking of neck protection—the Simpson Hybrid S, while a bit pricey, helps fill in the gaps of protecting your neck in the event of an incident on track while wearing three- (that’s right, they work with OEM), four-, five-, or six-point belts.