Bolts for Bikes

For my geek bike project I needed to choose some fastenings to make the solar panel mount, and also for attaching carrier racks to the frame.

The choice is quite important and I have had problems with them before. On my last tour one of the bolts attaching the rear carrier to the frame fell out. I had to remove another one of these to fit the solar panel and almost couldn’t because it had rusted solid.

Size

The brazeons on my bike are M5 threads. This is the same as used in my SPD shoes, which saved me when I lost one last time, so I’m standardising on M5.

Drive

I want something common so I don’t need special tools and can find tools if I need to. This means slot, cross / phillips, or hex / allen key. Slot and cross heads can be deformed too easily - hex gives much more torque and is harder to damage. This is important if the bolt gets stuck.

Head shape

Some hex socket screws have a rounded off head and some are cylindrical. The later kind could be gripped with pliers if the socket was damaged so I favour these.

Material

For strength I think it has to be some sort of steel. The options are zinc plated, galvanised or stainless steel. Hot dip galvanisation gives too thick a layer of zinc for threads of this size, so the real options are zinc plated or stainless.

The screws in my rack which have rusted solid were zinc plated. Clearly this is inadequate unless you constantly attend to them by oiling and replacement. That leaves stainless steel. The specific grade of stainless steel is probably irrelevant for this application.

Galvanic corrosion

There is a possible problem with stainless though. When dissimilar metals are in contact in the presence of water they form an electrical cell which oxidises the less noble metal. My bike frame is made of aluminium, which is less noble than stainless steel but more noble than zinc. Stainless steel risks corroding the frame whereas zinc plated screws would not. According to the British Stainless Steel Association, the risk of problems from a small amount of stainless in contact with a larger piece of aluminium is quite low. Additionally, galvanic corrosion is only a problem if the parts are wet, and particularly with salt water. Most of the time I expect to be touring in warm dry weather (this is an advantage of being able to move all the time) so the risk seems quite low. Nevertheless, I could reduce this risk my adding some sacrificial zinc somewhere. An easy way to do this would be to use zinc plated washers.

Locking

As well as the carrier bolt which fell out, in testing my solar panel mount around town I lost another screw. I suspect this is because of the vibration, particularly from hard tyres I like to reduce rolling resistance.

The one which came out used a nyloc nut but the bolt was very short so it might not have reached the plastic. I think I will use nyloc again with the correct size of bolt and also use threadlock. I would like to use copper grease to avoid seizing but I’m not sure about how to use this and threadlock.

Someone told me you can use nail polish instead of threadlock. Presumably threadlock is better, but I don’t know. I also found the idea when searching about this to use nail polish to indicate movement of the bolt - put it across bolt and other part and it will crack if the bolt turns.

The value of lock washers seems to be debatable, so I think I won’t use them for now.

Conclusion

My standard bolt is an M5, cylindrical hex socket head, stainless steel. It should be used with threadlock, stainless nyloc nut and plain stainless or zinc plated washers.