Are M5 bolts enough?
Has anyone ever had rack bolts shear off? What did you do about it?
I have seen quite a few broken rack bolts over the years and I would say most, if not all, were due to corrosion or crash. The first is easy to fix. Use stainless bolts and thread locker. Unfortunately, although the first will help some, the second is sometimes unavoidable.
Carrying a few extra bolts is always a good idea. If your clever you can find places on your frame to store them (like threading one in from the back side of a braze-on). Also, just carring some zip ties, quality cordage, and good old duck tape will get you out of most any jam.
I wound up helping a guy out not to long ago with a broken bolt. He had tryed to remove the lower rack bolts to create clearance for his new Bob trailer and the bolt snapped off. I tried a few common broken bolt removal tricks, including an EZ-out, to no avail. Unfortunately he had failed to properly treat the threads and the bolt had seized. If this had been a do or die case I would have drilled out the old bolt and either tryed to salvage the old threads, used a helicoil, or worst case put a button head bolt through the other way, locked out his A cog (to stop his chain catching it) and put a nut on the outside. But given the new trailer he was happy to just make use of that.
I second the spare bolts advice. I haven't seen broken bolts, but I have seen quite a few come loose, and I think it"s more likely to happen. The context was always fully loaded touring with a rigid frame though, so in a bikepacking rig I think it's even more unlikely to have broken bolts.
As big papa nut said, stainless bolts and thread locker should put your fears to rest. And to be completely on topic, yes, M5 are ok. M6 on older steel frames were a lot better though, and I'm not sure why frame manufacturers went for the smaller bolt. Most rack eyelets still fit an M6 bolt.
A riding partner's rack bolt holding the left rack leg came loose and fell off while on tour on rough roads. We heard a clunking noise that didn't sound too threatening, so we assumed that rocks thrown up from the unsurfaced road were to blame. Instead, it was the rack moving and hitting the frame with the loose leg. And yes, the rack was so sturdy that even an unsupported leg and around 60lbs of luggage weren't enough for it to break or buckle and catch the wheel, on a dirt road full of holes. However, you shouldn't count on having that much luck. I thought it was a miracle that he was able to ride like that, and it made me see racks made of large diameter aluminium tubing in a whole new light.
If you use a longer bolt from the rear of the brazeon with a locknut on the outside - your rack bolt is less likely to jump ship and should it break you are not left with a tiny stub stuck in the brazeon that needs a heroic effort to remove.
You can also replace the craptacular bolts that frequently come with a rack with some high grade bolts while you are at it.
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