Why do nuts and bolts keep coming loose on my motorcycle?

Why do nuts and bolts keep coming loose on my motorcycle?

Why do nuts and bolts keep coming loose on my motorcycle?

Nuts and bolts were constantly loosening or falling off his bike… including the license plate bolts, just blocks from the dealership where he purchased it!

His tools filled a surplus World War II backpack he kept strapped to his rear fender, and their bounce probably negated some of the tightening he did with them. The loose bolt phenomenon will be familiar to those of you on thumpers or big twins, although it can happen to any bike. Why does this happen?

It's not entirely accurate to say that fasteners are loosening . Leave your bike parked and they just sit there, right? Instead, it's the interaction between the vibrations of your bike and the natural frequencies of the fasteners that causes them to loosen. Expose a threaded fastener to a particular vibration for long enough, and it will move a little, then a lot, and then you'll be wondering where your bolt went. Motorcycles with naturally unbalanced engines, such as narrow-angle V-twins or large singles without counterweights, generally suffer more vibration than naturally balanced engines, such as the inline sixes found in BMW K1600s or older Honda CBXs. Prolonged periods of high-rpm riding often accelerate loosening.

A German engineer named Gerhard Junker studied the effects of vibration on fasteners in the late 60s and found that transverse loads (perpendicular to the length of the fastener) were much more likely to shake things than axial loads (in line with the bolt). He went on to design a means of testing a fastener's resistance to vibration, called the Junker test. Insert a fastener, tighten it to the desired setting, and the testing machine tortures the fastener with simulated shaking. Researchers can then see what it takes to overcome the fastener's clamp load. When the clamp load fails, the fastener can no longer do its job, and engineers typically want to find those points of failure long before a motorcycle, bridge, or skyscraper does.

Putting on lab coats and watching bolts come loose might seem a little dry, but that's why our motorcycles wear all kinds of lock washers, bend tabs, spring washers, lock nuts, jam nuts, castle nuts with cotter pins , rubber insulators, handlebar ends weights and thread locking compounds, just to name a few tricks used to keep fasteners tightened. Trackday and race organizers usually go a step further by requiring a safety cable for critical parts such as oil filters. And for the crucial things that never need to fall apart, well, there's always crossthreading...

So what can we do to combat loose bolts that could spill our saddlebags full of roller skates onto the highway at any moment? Anticipate the problem by following all tightening schedules in your motorcycle manual. If test riders found that some bolts were trying to jump ship every 5.000 miles, your bike probably has the same problem. This goes double for wobbly singles and twins. Keep that torque wrench handy! You may need to re-tighten some fasteners after you get “settled” during those first few miles.

For pesky fasteners that won't stay put, you have a few options. First, inspect the fasteners themselves. They may be worn, and Loctite says a typical bolt can only have 15% surface-to-surface contact to begin with. Threadlocking adhesives, like Loctite, come in a wide variety of strengths, so a little browsing on the forum should find you another cyclist with the same problem and a liquid solution. Finally, a good old-fashioned cable lock is always an option. Secure fasteners like a pro with a little practice and a few basic supplies.

For the truly fast-paced nerds, here's an old NASA manual that covers the entire gamut of fastening technology. My favorite section is about the split-ring style of lock washers: "In short, such a washer is useless for locking purposes." Damn, NASA, tell us how you really feel.

Loosening fasteners isn't exactly the biggest danger motorcyclists face today, especially with so many smooth-running motorcycles available, all designed with a better understanding of the effect of vibration than in the pre-Junker days. Long story short: Don't be late to the skating rink because the right vibrations at the wrong time sidelined your bike. Take everything your manual recommends and move on.

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