It would come as no surprise that we like cargo mounts on our frames and forks here at Curve Cycling. Famously, the first iteration of our SEEK 430 fork had six(!) of them on each fork leg because… why not?!
First of all, what is a rivet nut? Put simply, it’s a tubular shaped metal fastener with internal threads. Rivet nuts are also known commercially as Rivnut™ and Nutsert™. In this article I’ll use the generic name “rivet nut” or “insert” to refer to classic cargo mounts.
When it comes to adding cargo mounts and bidon bosses into a carbon fibre structure, the rivet nut solution has largely been the de-facto industry standard. You won’t see many carbon fibre frames that do not use these in various forms to attach bottle cages to. The same goes for “adventure ready” forks, including the ones we’ve been selling for many years. Cargo forks will be the main topic for this deep-dive into the do’s and don’ts of rivet nuts.
While rivet nuts certainly work well for the most part, they’re also the single most common thing to fail or become problematic on forks and frames. There are many videos on YouTube dedicated to repairing or replacing them in bicycle frames. There are various ways and reasons that a rivet nut can fail. Anything from less-than-ideal installation from the factory (yep, let’s get that one out of the way right now. If it’s our fault that one of our cargo mounts fails, we’ll take care of it - just get in touch), to overloading, over-torque, cross-threading, riding with one or more bolts loose etc.
Failures like the above can be avoided to a large extent if you treat your inserts well and know their weaknesses. Over the years I’ve come up with a list of best practices for how to use them, and I’ve not had one fail (yet?) on any of my personal bikes. Hopefully, that is a sign I know what I’m on about. But maybe a decent portion of luck is involved too. Who knows?
Firstly, let’s dive in and tackle the most common failure we see - spinning rivet nuts.
Spinning Rivet Nuts
To get a good understanding of why this happens, let’s first go through the principles of how a rivet nut works. It’s basically an internally threaded cylinder with an outside lip (the head) that is installed in a hole and then pulled tight to the point that the deformation zone mushrooms out and crimps the rivet nut so that it’s held tightly in place in the substrate. In the context of this blog post, the substrate is the carbon fork leg.
Some rivet nuts have a smooth outside surface and some are knurled. The knurled version is better since the knurling will provide extra resistance to twisting or spinning.
Spinning happens when the friction between the bolt thread, bolt head and/or the accessory you’re attaching exceeds the friction between the insert and the fork leg. This force is called “breakaway torque” or “spin-out torque”.
When installing a bolt in a rivet nut on its own, such as when you’re just plugging the thread hole to keep water and debris out, it’s very easy to exceed the breakaway torque. This is because as soon as the bolt head touches the rivet nut head, the friction between the two parts increases exponentially as you keep adding torque. Soon all the torque you add will be transferred to a twisting motion of the insert itself and it will start turning relative to the fork leg. It usually doesn’t take much more than a couple of Nm to exceed the breakaway torque.
The same principle applies if you use threadlocker (i.e. Loctite), a bolt with threadlocker already applied. The locking compound will increase the friction between the bolt thread and the rivet nut thread, and thus you risk exceeding the breakaway torque as you turn the bolt. In some cases this will not be a problem until you try to remove the bolt.
Similarly locking washers and/or bolts with serrated heads have the same effect (see image below). Even more so when undoing bolts. This is because of the way the serrations are shaped, they bite into the head of the rivet nut when turning the bolt counterclockwise to undo it. Serrated head bolts are commonly used for 6-bolt rotors for example.
If you were to accidentally cross-thread a bolt into an insert, or use a bolt with a damaged thread, or if the bolt seizes in the insert due to corrosion, that can also result in spinning.
The best way to avoid exceeding the breakaway torque is to simply use a cage that spans between at least two mounting points. When you do this, the bolt head will tighten up against a stationary surface, the cargo cage, and friction from the bolt head will not transfer twisting torque into the insert. In fact, as you add torque to the bolt, the insert will instead be “loaded up” with a compressive force, helping keep the insert crimped tightly to the fork leg.
I’ve done in-house testing applying torque to a bolt directly threaded into a single rivet nut compared to having a bottle cage, attached at two points, in between the bolt and the rivet nut. The spin-out torque for the bolt directly mounted in the rivet nut was well below 5Nm. But with a bottle cage in between, I was able to add so much torque that I risked snapping the bolt or stripping the thread without any sign of the rivet nut twisting even a little bit.
As such, systems like the Gorilla Cage II from Free Parable that uses three “cleats” attached to one mount each can cause problems as the risk is that the cleat will start spinning with the insert as you add torque to the bolt to secure the cleat. The cleat is round, so there’s no good way to hold it to keep it from turning when tightening the bolt.
Loose Bolts
The second most common failure mode is riding with loose bolts. Gravel and bikepacking bikes like the ones we design and sell will naturally see their fair share of riding on rough surfaces with vibrations as a result. The vibrations can cause the bolts you use to shake loose and if you don’t immediately tighten the bolt, chances are you will damage the insert and in many cases also the underlying carbon structure.
Riding with loose bolts will cause whatever cargo cage or bottle cage you use to suffer excessive rattling and the force from this rattling can damage the thread and also break the insert away from the fork leg. When an insert breaks away from the fork leg, it means that it no longer has sufficient grip for a secure attachment. If you continue riding with a loose bottle cage or spinning insert, chances are that the relative movement between the insert and the carbon will over time wear away at the carbon, probably more rapidly than you’d think, thus enlarging the hole or even fraying the carbon. That type of damage is not easily repaired and can make your fork unsafe to ride. However, in most cases if addressed quickly, a loose insert can be re-crimped or replaced without affecting the integrity of the fork.
Best Practices
The best way to avoid spinning or damaged rivet nuts is to follow a few simple guidelines.
- Only tighten bolts finger-tight if just plugging the hole
- Never use Loctite or any bolts with threadlocker applied
- Never use locking washers or serrated head bolts directly onto the head of the rivet nut
- Lubricate the bolt thread with waterproof grease
- Never force a bolt into a thread that seems tight
- Remove cargo cages and bottle cages every now and then to clean the threads and reapply grease
- Ideally use all three mounts at the same time for cargo cages (the load rating on our forks applies when all three mounts are used together)
- Avoid attaching cargo mounts to a single rivet nut (such as Gorilla Cage cleats)
- Avoid cargo cages that do not contact the head of the rivet nut
- Do not use devices such as the B-RAD Double Bottle Adapter from Wolftooth (it voids our warranty)
Can I repair a damaged or loose rivet nut?
The short answer is yes, in most cases you can.
A repair is actually quite straightforward with the right tools (we have all that’s needed here) and it usually takes us less than an hour to complete a repair. The key to a successful repair is early detection, as the longer you wait, the risk of damage to the carbon increases and with that, the prospects of a successful repair decreases.
If an insert were to still work its way loose despite having followed all the best practice recommendations above, you should address the issue immediately. Do not continue to ride for hundreds or thousands of kilometres after an issue has been discovered. Continuing to ride may make the issue worse and render your fork unsafe and, ultimately, unusable. If you discover the issue partway through a long adventure and cannot stop riding, then reinforce your fork cargo with sturdy tape, a Voile strap or both! (Both of which should be standard long distance equipment.) This should enable you to continue safely and slow any additional damage before addressing it thoroughly.
This concludes my lengthy ramble about rivet nuts.
For the next instalment of Jimmy’s tech ramblings, we’ll discuss our new VGM mounts which is a Curve Cycling innovation in cargo mounts, and they make almost everything you’ve just read obsolete.
Thanks for reading!