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I'm installing a rack for a new project.

The rack model I'm picking is rated for a load of 18 kg, and there are even models rated up to 27 kg!

Though, racks are usually fastened to the bike using just two tiny M5 bolts, on threaded holes in the frame, near the rear hub. There is also a fastening near the sear, but it's just to prevent the rack from rotating, and basically doesn't bear any load.

I am very umconfortable with the fact that these bolts receive so much load, especially as it is shear force. Near to 10 kg of shear force on a M5 screw, especially combined with the road bumps, really doesn't sound reassuring to me.

From experience I already know these bolts loosen very easily, so I have to use nylstops or nord-locks. In case of a loosen bolt, luggage weight combined with road bumps would damage the threaded hole very quickly. Provided that the screws don't break or loosen, the threaded holes should not wear over time.

On previous projects I was using stronger designs, suck as mounting brackets added to the seat stays. But here I have to use the classic fastening.

So for this project, I'm looking for the best material to pick for these M5 bolts, to minimise the risk of screw breakage.

  • For now, I am considering 304 stainless steel.
  • 316 would have been better, but it is hard to find. Edit: found a retailer for 316; but although 316 has better corrosion resistance than 304, it has roughly the same strength (strength varies more between manufacturers than between the two materials).
  • Alternatives could be 10.9 or 12.9 steel, but it may be counterproductive, as they are more brittle. And if I remember correctly, they are purposed for higher clamping force, but aren't really better with shear force. Additionally, they are not rust resistant.

So, I'm looking for your insights on this matter. Thanks in advance!

References:

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    All of 27 kg? I am used to seeing adults sitting on bike racks, 70 kg and over. Bikes should be/are usually engineered to take much more than the stated weights on the connections.
    – Willeke
    Feb 11 at 6:51
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    Racks survive for years being fixed on with the rubbish screws that come with them or other accessories, and properly torqued shouldn't loosen. And this includes riding my tourer/commuter heavily laden on gravel and rough roads for long distances. That said, I do buy stainless (usually A2) out because I don't want rust either in the head (making the tool not fit nicely) or where it meets the frame (which might be steel or Al. And a bit of grease on the threads.
    – Chris H
    Feb 11 at 7:35
  • If both the frame and the rack stays have enough metal around the mounting holes, the classic solution is to drill them wider and cut M6 thread in the frame. Feb 12 at 7:58

2 Answers 2

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The bolts are not taking any shear force (unless they are not done up properly). The fasters cause friction between the rack post and the frame. It is this friction that takes the shear force.

The bolts need to be the correct type, prepared and done up correctly. The prep is clean threads that are then properly lubricated. The bolt is then done up to the specified torque. Bolts are elastic and stretch, taking up changes caused by vibration and thermal expansion etc. This ensures the pressure on the interfaces is held under normal working conditions. Too much torque will exceed the yield strength of the bolt. At this point, it is no longer elastic, and is prone to loosening (Exceeding yield strength is permanent - bolt is now, as we say in my country "munted"). In design of things like bikes, engineers allow for "Gorilla with spanner uses cheap and nasty bolt", so there is a heap of scope for idiots to do the wrong thing and nothing bad happens.

Thats all along way of saying the correct M5 bolt is more than upto the job when used correctly and probably up to the job when used badly.

Choose quality, highgrade bolts, ideally from a reputable source. Grease the bolt thread and torque to the specification of the frame or carrier manufacturer (probably around 6Nm, do not over-torque). 304 Stainless have a similar tensile strength but a much lower yield strength. If using Stainless bolts, you need to be especially careful not to over torque them (easily done on M5 sized bolts.).

Loosened bolts are usually a problem of poor assembly, although paint between the rack and frame can cause things to loosen. If you are concerned about bolts loosening, a thread locker (Loctite Blue) will be more effective than nylstops or nord-locks. An advantage of nylstops or nord-locks etc is when the part is removed/reinstalled by people who might not reapply a thread locker.

Personally, I would use a high tensile steel bolt with Blue thread locker. If you use Stainless, you must use a lubricant on the threads, might as well use a thread locker.

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  • For bearings, I have heard not to use stainless steel bearings unless you know there will be a huge amount of water. Stainless is not as strong in that application. Can the same be said of this application?
    – Weiwen Ng
    Feb 11 at 21:48
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    SS is not as strong, but in this application, I would be very surprised if 304 SS is not strong enough.
    – mattnz
    Feb 12 at 1:12
  • Thank you, this answer made me understand much better, in particular that the bolts don't take the shear force. I also discovered that 304 SS has a catastrophic yield strength (205 MPa!), previously I believed it was at least as strong as the common steels, but no, it's much worse. I have a handful of 12.9 bolts around, so I'll use them. I also understood that 12.9 brittleness is not an issue, because what matters is to not exceed the yield strength. And all these steels have the same Young's modulus, so they require the same torque to achieve the same clamping, thus, 6 Nm should be enough. Feb 14 at 23:17
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As these bolts are likely to stay in place for a long time. They are exposed to water there. It is likely they can start rusting and eventualy might even get stuck, because of corrosion in the treaded part of your frame.

Worldwide m5 has been the standard for rackbolts for decades. If they where not up for the task, things would have changed. Use stainless and avoid a problem in a few years when the rack won't come off.

As anecdotical evidence, in 40 years on heaps of different bikes i always used m5 bolts. Occasionaly some get loose, like on an offroad loaded biketour. I have had persons on the rear rack multiple times, mostly short distances. Never had one break.

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