Maintenance
Why a Bolt Comes Loose and What Threadlock Actually Does
Vibration does not unscrew fasteners by magic. It relaxes the clamping force that was holding them, and then the thread has nothing left to grip.
The points below about fastener loosening are ordered by how much difference they make, not by how often they get repeated.
What matters most
- A tightened bolt is a stretched spring holding two parts together.
- Loosening usually starts with the clamped joint settling, not the thread.
- Threadlock fills the thread gaps; it does not replace correct torque.
A bolt is a spring under tension
Tightening a bolt stretches it slightly, and the tension in that stretched shaft is what clamps the two parts together. The friction produced by that clamping is what actually stops the joint moving, not the threads themselves gripping each other. As long as the tension stays high, the thread has no opportunity to rotate and the joint stays where it was put.
Everything that makes bolts come loose works by reducing that tension until the clamping force is no longer enough. This is why a bolt that was never tightened enough is far more likely to loosen than one that was tightened correctly.
How the tension gets lost
Surfaces that look flat are microscopically rough, and those high points flatten under load in the first hours of use, which is called embedding. Paint, anodising and grease films between clamped parts compress and creep, which has the same effect over a longer period.
In the saddle, repeated loading in the direction that makes the parts slide sideways is the most effective way to walk a fastener loose. Temperature changes alter the dimensions of aluminium and steel by different amounts, which slowly cycles the tension up and down. The practical consequence is that new assemblies need rechecking after a short period of use, which is why bike shops ask you to come back.
What threadlocking compound does
Threadlocking compound is a liquid that flows into the tiny gaps between engaged threads and then hardens in the absence of air. That fills the space the thread would otherwise need in order to rotate, and it adds friction across the whole engaged length.
It does not add clamping force, so a bolt tightened too little and glued in place is still a loose joint. Different strengths exist, and the stronger grades can make removal difficult or impossible without heat. On a bicycle the medium strength grades cover almost everything, and the strongest are rarely appropriate.
Where it belongs and where it does not
Small fasteners subject to vibration with nothing else retaining them are the classic case, such as rack bolts, bottle cage bolts and disc rotor bolts. Disc rotor bolts in particular are worth treating carefully, since a rotor coming loose is a brake failure.
The bit that actually matters: it does not belong on anything relying on a controlled friction interface, such as a seatpost clamp or a stem face plate. It is also inappropriate on fasteners you expect to adjust frequently, where you will simply end up fighting it. Threads must be clean and dry for the compound to work, since oil in the thread prevents it curing properly.
The alternatives that also work
Spring washers and serrated washers provide some resistance to rotation and are common on rack and mudguard fittings. Nylon-insert locking nuts grip the thread with a deformable ring and are reusable a limited number of times.
By the second week of riding, correct torque is itself the most effective retention method available, and it is the one most often skipped. Anti-seize compound is a different product entirely, intended to prevent corrosion between dissimilar metals rather than to retain fasteners. Confusing the two produces either a bolt that will not stay put or a bolt that will not come out, both of which are avoidable.
The fasteners worth checking regularly
Rack and mudguard bolts vibrate loose more than anything else, and their failure drops hardware into a moving wheel. Bottle cage bolts are harmless when they fall out and annoying because the thread in the frame can be damaged if ridden empty. Stem, handlebar, seatpost and brake fasteners are safety-critical and have specified torque figures that are worth respecting.
Mid-traffic, torque figures on lightweight and carbon components are not advisory, and guessing them is how parts get crushed or slip. If you are unsure about a safety-critical fastener, that is precisely the situation where a mechanic with a torque wrench is worth the visit.
Everything above, in order of what to do first
- A bolt is a spring under tension. Tightening a bolt stretches it slightly, and the tension in that stretched shaft is what clamps the two parts together.
- How the tension gets lost. Surfaces that look flat are microscopically rough, and those high points flatten under load in the first hours of use, which is called embedding.
- What threadlocking compound does. Threadlocking compound is a liquid that flows into the tiny gaps between engaged threads and then hardens in the absence of air.
- Where it belongs and where it does not. Small fasteners subject to vibration with nothing else retaining them are the classic case, such as rack bolts, bottle cage bolts and disc rotor bolts.
- The alternatives that also work. Spring washers and serrated washers provide some resistance to rotation and are common on rack and mudguard fittings.
- The fasteners worth checking regularly. Rack and mudguard bolts vibrate loose more than anything else, and their failure drops hardware into a moving wheel.
The takeaway
A bolt stays put because it is stretched, not because it is glued — get the tension right and retention becomes a backup.
Most of riding well is being boring and predictable to everyone else on the road.
Questions readers ask
Does threadlock replace tightening properly?
No. It stops a correctly tightened bolt from rotating; it does nothing about a joint that was never clamped hard enough. Torque first, retention second.
Which bolts on my bike come loose most often?
Rack, mudguard and accessory bolts, because they are small, they are subject to constant vibration and nobody checks them. They are also the ones whose failure can put hardware into a wheel.
