Hundreds Of Products Ready Today. Dispatch/Collection Customer question: Can I Fit a Crutch Strap to my 4 Point Harness To Make it a 6 Point Harness Belt?
Short answer: No, you cannot simply add a crutch strap (crotch strap) to a 4-point harness to turn it into a 6-point harness. This is due to strict FIA homologation rules and safety considerations. In this blog post, we’ll explain why adding a sub-strap to a 4-point belt is not allowed under current regulations, why 4-point FIA-approved harnesses aren’t designed for that modification, the safety risks of trying to mix and match harness parts, and what you should do if you want a proper 6-point harness setup. We’ll also clear up any confusion about harness compatibility across brands and models, referencing our previous Ultimate Guide on crutch strap swaps for additional context.
The Question: Upgrading a 4-Point Harness with a Crutch Strap?
At GSM Performance, one of the most common customer questions we get is: “Can I fit a crutch strap to my 4-point harness to make it a 6-point harness belt?” It might seem like a straightforward idea, after all, a 6-point racing harness is basically a 4-point harness with two added anti-submarine (crutch) straps, right? You might think you can save money or time by just adding those straps to your existing 4-point belt. However, the reality is that the FIA (motorsport’s governing body) does not allow this conversion. Harnesses are homologated (approved) as complete systems, and you must use them as they were designed, without modification. Let’s break down exactly why you cannot upgrade a 4-point harness to 6-point just by adding a crutch strap.
FIA Homologation Rules: Why the FIA Says No
The FIA homologation rules are the first and most important reason you cannot add a crutch strap to a 4-point harness. In motorsport, any FIA-approved harness is tested and certified as a complete unit, meaning the shoulder straps, lap belts, and crotch straps are designed and tested together under one homologation number. You are not allowed to mix and match parts or alter the configuration of that harness, because doing so immediately invalidates the FIA approval. In fact, FIA guidelines explicitly state that a safety harness “must be used in its homologation configuration without any modifications or removal of parts,” in line with the manufacturer’s instructions. Adding a crutch strap to a harness that wasn’t originally manufactured with one counts as a modification, which is strictly forbidden for FIA-compliant gear.
Regulation changes in recent years have made this even clearer. The latest FIA harness standard (FIA 8853-2016) introduced significant changes, one of which is that all FIA harnesses must now be six-point, 5-point systems (which have a single sub-strap) are no longer legal for motorsport use. In other words, any new harness intended for competition needs dual crutch straps as part of the design. As of 2023, six-point harnesses are effectively compulsory in all cars for FIA-sanctioned events (with 4-point belts only still seen in some historic or certain national-level scenarios). This means if you have an older 4-point harness, it’s already not acceptable for most current competition. Simply strapping on an extra belt or two won’t magically make it legal, you’d still lack an official homologation for a 6-point harness. The harness would no longer match its certification label, and any scrutineer (race inspector) would fail it on the spot for being an altered safety device.
To put it plainly: a 4-point harness is homologated as a 4-point harness. You can’t turn it into an FIA-homologated 6-point harness by adding parts later, because it wasn’t tested or approved in that configuration. The only (rare) exception is if the harness’s manufacturer offered a specific brand-approved conversion kit for that exact model, and even then, it must maintain the original homologation and be installed per manufacturer guidelines. Such cases are uncommon nowadays (more on that later), and they still must meet FIA rules. In general, if you want a 6-point harness, the FIA expects you to buy a harness that was built and homologated as a 6-point system from the start.
Harness Design Differences: 4-Point Belts vs. 6-Point Belts
Apart from the rules, there’s a very practical reason you can’t just fit a crutch strap to a 4-point harness: the hardware and design of a 4-point belt typically won’t accommodate it. FIA-approved 6-point harnesses use a central buckle (often a camlock “twist-release” mechanism) that is designed to secure six straps, two shoulder, two lap, and two sub (crutch) straps. In modern 6-point designs, the crotch straps are usually integrated into the buckle assembly from the factory. Brands like Sabelt, Schroth, OMP, TRS, etc., build their camlock buckles with the sub-strap connectors as a permanent part of the mechanism. You cannot purchase those crutch straps separately or clip them on later; they’re literally built-in and non-removable. This integration prevents tampering and ensures the anti-submarine belts always work correctly as part of the whole system.
By contrast, a 4-point harness (especially any FIA-homologated 4-point) will lack the attachment points or slots for sub-straps. The central buckle on a 4-point belt may have only four insertion points (for the two shoulders and two lap belts) with no provision for additional straps. Even if the buckle looks similar to a 6-point’s, internally it may not have the receptors for extra belts. In some older or road-car harness designs, the buckle might have an unused opening, but if the harness was sold and homologated as a 4-point, it likely did not include the necessary hardware for a sub-strap connection. There also might be no anchor mounting points provided for crotch straps in your car if it was originally set up with a 4-point harness. (Most 6-point harness installations require mounting the sub-straps to the chassis floor or a bar under the seat with proper eye-bolts or brackets. A car running a 4-point belt may not have had those anchor locations installed.)
In essence, trying to retrofit a crotch strap onto a 4-point harness is usually physically unfeasible without significant changes. You’d need to somehow attach the new strap to the buckle or lap belts securely. If your harness uses a different style latch (for example, some older 4-points use a latch-and-link mechanism instead of a camlock), it’s even more incompatible with adding a sub-strap. And even if you jerry-rigged a way to bolt in a sub-strap to the car and loop it to your 4-point harness, it would not function the same as a real 6-point harness. The lengths and adjusters might not position correctly, and the belt geometry could be wrong, which leads to the next critical point: safety.
Safety Risks: Why Mixing or Modifying Harness Parts is Dangerous
Rules and design aside, safety is the ultimate reason you shouldn’t add a crutch strap to a 4-point harness on your own. A racing harness is a life-saving device, and it’s engineered to work as a cohesive system. If you start mixing parts or improvising, you can seriously compromise its performance in a crash.
The short answer: No, you cannot mix harness components. Here’s why:
FIA Homologation Rules
FIA harnesses are tested and approved as a complete unit, shoulder, lap, and crutch straps must all match and share the same homologation number. Mixing parts immediately invalidates the approval.Incompatible Hardware
Even though many buckles look similar across Sparco, TRS, OMP, Schroth, or Sabelt, the internal locking mechanisms are brand-specific. Misaligned buckles could jam, not lock correctly, or even release under stress.No Swapping or Re-Webbing
FIA rules state that once any part of the harness is worn or damaged, especially the sub-strap, the entire harness must be replaced. Replacement or re-webbing is only allowed through the original manufacturer and is rarely supported.Expiry Dates Must Match
Even if one strap is still “in date,” it cannot be used with a different homologated harness. All parts must come from the same original set.
One of the biggest risks with adding a crutch strap to a 4-point harness is buckle compatibility. Even if a strap looks like it fits, there’s no guarantee it locks properly. Camlock mechanisms vary between brands and models, and even tiny differences in the tongue size or angle can cause a buckle to jam, fail to latch, or pop open in a crash. Unless every strap is from the same original harness set, tested together, you can’t trust the system to hold up under pressure. That’s why mixing components is strictly not allowed, including adding parts like a sub-strap.
There’s also the issue of load testing. Each harness is tested as a full unit. Adding a strap the system wasn’t designed for could create weak points or incorrect routing, which affects how the harness performs. Sub-straps are critical for preventing “submarining” (sliding under the lap belt), so if they’re not positioned or secured correctly, they won’t do their job. FIA rules say if any strap is damaged or worn, you must replace the entire harness, not just the part.
Lastly, harness sets have matching serial numbers and expiration dates. Adding a crutch strap from another harness, even if it’s still “in date,” won’t match the rest of your set and will fail scrutineering. Plus, you can’t be sure the webbing or adjusters haven’t degraded. Harnesses aren’t modular or interchangeable. They’re safety systems built to work as one, and cutting corners with mix-and-match parts just isn’t worth the risk.
FIA 8853-2016: What Changed?
The updated FIA 8853-2016 standard introduced some important changes to harness design:
All FIA harnesses must now be 6-point only — 5-point systems are no longer legal for motorsport.
Crutch straps are now permanently fixed to the camlock buckle.
You can’t purchase crutch straps separately — they’re built into the full system.
Brands like TRS, Sabelt, and Schroth integrate their sub-straps into the buckle housing, making them non-removable.
These changes improve anti-submarine safety and eliminate the chance of tampering.
What If My Crotch Strap is Worn or Broken?
You’re not alone; this is a common scenario.
Your options:
Replace the entire harness with a new matching set.
Contact the manufacturer directly — but replacements are only offered in rare cases where the serial matches and compliance isn’t broken.
Do NOT attempt to substitute parts from other brands, even if they look compatible.
Remember: the sub-strap is your last line of defence in a crash to stop you from sliding under the lap belt.
The Role of the Crotch Strap in Your Safety
Also known as the “anti-sub strap,” this part plays a critical role:
Keeps your body correctly positioned during an accident.
Prevents submarining under the lap belt.
Helps secure the central camlock and shoulder/lap belts in the correct location.
This isn’t just a “bonus” strap — it’s essential for your safety. Tampering with it by using a mismatched replacement can lead to failure during high-impact crashes.
So, Can You Replace Just the Sub Strap?
| Question | Answer |
| Can I use a crutch strap from another brand? | ❌ No |
| Can I replace just the sub-strap? | ❌ Only with an exact manufacturer match (rare) |
| Can I mix harness parts from different brands? | ❌ No |
| Can I upgrade from a 4-point to a 6-point system? | ✅ Yes, but only with brand-approved kits |
| Are 5-point harnesses still legal? | ❌ Not under FIA 8853-2016 |
Explore Our FIA Harness Range – Stay Compliant and Safe
At GSM Performance, we stock a wide range of complete FIA-approved harnesses from top-tier motorsport brands. These are all tested, homologated, and safe when used as complete sets.
Our Most Popular FIA 6-Point Racing Harnesses
If you’re ready to upgrade to a fully FIA-compliant 6-point harness system, we’ve got you covered. At GSM Performance, we stock the most trusted motorsport harnesses in the industry, all designed to meet the latest FIA 8853-2016 standards and deliver the best in driver safety and comfort.
Here are some of our most popular options from leading brands:
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Final Thoughts: Safety First, Always
To answer the question clearly: no, you can’t add a crutch strap to a 4-point harness to make it a 6-point. It’s not allowed under FIA rules, it’s not how the harness was designed, and it’s not safe. The only correct way to run a 6-point harness is to buy a proper FIA-approved 6-point model from the start.
Harnesses are critical safety equipment and should never be modified or pieced together. Mixing parts or adding straps that weren’t tested with the original system can lead to failure in a crash and will also make your setup non-compliant at events.
If you want the extra safety of anti-submarine straps, upgrade to a proper 6-point harness that’s built and certified for the job. It’s the safest and only legal choice. Always follow the manufacturer’s instructions and the motorsport regulations, and if you’re unsure, ask a professional. Safety isn’t something to take chances with.