Rusty bolts

[Profpointy]

Some bolts on bicycles are critical, others not. I consider the stem bolts that hold the handlebar in place as critical, as well as the seatpost bolt. I won't die if I break a crank bolt but if the handlebar comes loose whilst sprinting you will die. I know, I'm now on my second life.

Critical bolts are made from high-tensile steel and then chromed. It may seem obvious that the bike company should just have fitted stainless steel bolts in the first place but there is method to their frugality. Stainless steel bolts are not high-tensile and cannot withstand continuous cyclical stresses such as those experienced by seatpost clamps and stem clamps. A rusty head is a small price to pay for safety and reliability. The reason the heads are rusty inside where the allen key goes is because such a recess is a Faraday shield and the electroplating doesn't work so well in there.

I would replace high-ten bolts with stainless on my bike only if there is redundancy. Some stems have a front loader cap with 4 bolts. I may risk stainless there if they are four 5mm bolts in there but not 4 x 4mm ones. Seatposts, never, but that's just me.

Further, if you are going to replace paired bolts with stainless ones, make absolutely sure you torque them up incrementally and evenly. It is very important that in a stem clamp, all four bolts are evenly tightened otherwise one of them strains more than the other and fails quickly.

Not everything is what it seems to be.

whilst handlebar bolts are "critical" in the sense of holding the handlebar bolts on, bear in mind they're screwed into soft alloy, so the crappyest of steels will be far stronger than the allow it's screwed into. Thus, any reasonable stainless bolts will be fine.

 

[howard2107]

Apply a very small amount of copper grease to the bolt, it will never rust and the grease wont run when it gets warm unlike normal grease which will end up on everything. The next time you undo it, it will be a doddle.

Get some Balistol and give everything a good wipe and you will never see rust, it will lubricate the cables. Its excellent as a cleaner, lubricant and its awesome stuff in winter for locks, one spray and they will never freeze up.

It smells a bit when you first use it, but it soon wears off, and it is harmless to skin or if you get it on your fingers and in your mouth, but it tastes horrible.

It is a lot better then WD40.

 

[Yellow Saddle]

whilst handlebar bolts are "critical" in the sense of holding the handlebar bolts on, bear in mind they're screwed into soft alloy, so the crappyest of steels will be far stronger than the allow it's screwed into. Thus, any reasonable stainless bolts will be fine.

No, this is not how it works in the real world. On a bolt, the first thread root (see diagram) takes most of the strain and thereafter the next two. Further down the bolt there is just about no strain. This means that the tensile stress from the torqued bolt is concentrated on a sharp edge or a stress riser, as it is called. Though cyclical stresses where the bolt is stretched and relaxed in cycles (as you pump on the handlebar), this stress riser develops into a crack and propagates through the bolt.

If you need an analogy, think if a packet of crisps - just the packet. It is made from some sort of plastic that is quite strong until it gets a little tear in it and then it just rips. Steel is the same in a way. You'll always see that when a bolt breaks, it breaks at the first root. There are ways to reduce the stress there and this could be based on the type of materials used (high tensile steel works better than stainless or mild steel) or, you could reduce the shaft diameter of the blank piece of bolt between the head and the first thread. This would allow the strain to concentrate there and stretch a smooth piece of steel without any stress risers, giving it better durability. This is in effect what a double butted spoke does.

This isn't an issue with other bolts on the bike that aren't subject to cyclical stresses, such as the cable pinch bolts or water bottle cage bolt, for instance.

Next time you go ride and pull hard on the handlebars, give this some thought. It may sink in if you are a meter or so away from tarmac leaning hard over the bars.

 

[Profpointy]

No, this is not how it works in the real world. On a bolt, the first thread root (see diagram) takes most of the strain and thereafter the next two. Further down the bolt there is just about no strain. This means that the tensile stress from the torqued bolt is concentrated on a sharp edge or a stress riser, as it is called. Though cyclical stresses where the bolt is stretched and relaxed in cycles (as you pump on the handlebar), this stress riser develops into a crack and propagates through the bolt.

If you need an analogy, think if a packet of crisps - just the packet. It is made from some sort of plastic that is quite strong until it gets a little tear in it and then it just rips. Steel is the same in a way. You'll always see that when a bolt breaks, it breaks at the first root. There are ways to reduce the stress there and this could be based on the type of materials used (high tensile steel works better than stainless or mild steel) or, you could reduce the shaft diameter of the blank piece of bolt between the head and the first thread. This would allow the strain to concentrate there and stretch a smooth piece of steel without any stress risers, giving it better durability. This is in effect what a double butted spoke does.

This isn't an issue with other bolts on the bike that aren't subject to cyclical stresses, such as the cable pinch bolts or water bottle cage bolt, for instance.

Next time you go ride and pull hard on the handlebars, give this some thought. It may sink in if you are a meter or so away from tarmac leaning hard over the bars.

View attachment 90470

So if I understand your explanation (quite possibly I don't) - you are saying that because load is cyclical, the bolt needs to be "stronger" than the thing it's screwed into (the flimsy alloy bit)?

Have I understood this right ? And if so, are you sure?

 

[Pale Rider]

Having seen some of the fixings made of cheese on cheap Chinese bikes, I don't think stainless or otherwise is important so long as the bolt is of good quality.

 

[e-rider]

I am sure the OP meant rust inside the hex cavity. Other than plugging it with a blob of grease, nothing will stop it. It a) traps water and b) has very little plating inside.

yes, drop some oil in there, and then again after every few wet rides - it does stop it 100%

 

[Yellow Saddle]

So if I understand your explanation (quite possibly I don't) - you are saying that because load is cyclical, the bolt needs to be "stronger" than the thing it's screwed into (the flimsy alloy bit)?

Have I understood this right ? And if so, are you sure?

Not quite. Because the load is cyclical, the bolt has to be "stronger". I use quotation marks because the strength is quite specific but that is a long story. I don't say it has to be stronger than the aluminium or whatever it is screwed into. It is just that the failure always happens in the bolt, not the nut. The latter is almost always beefier than the bolt and thus undergoes less strain (stretch). A single overload will usually strip the threads in the softest of the two materials, as you probably know. However, we are not talking about overload failure but about stress cracks. These can develop at loads way below yield.

 

[Profpointy]

Not quite. Because the load is cyclical, the bolt has to be "stronger". I use quotation marks because the strength is quite specific but that is a long story. I don't say it has to be stronger than the aluminium or whatever it is screwed into. It is just that the failure always happens in the bolt, not the nut. The latter is almost always beefier than the bolt and thus undergoes less strain (stretch). A single overload will usually strip the threads in the softest of the two materials, as you probably know. However, we are not talking about overload failure but about stress cracks. These can develop at loads way below yield.

Well every day's a school day on here :-)
I too was using "stronger" in quotes for same reason.

I can see the point you're making. I guess with a bit of give in one or other material, all the threads within the nut will share the load, but the weak point is the neck or topmost un-attached thread. That said, do you think it's an issue with any reasonable steel (into alloy) here; steel being less prone to fatigue than alloy. It maybe that fatigue is a different thing that what you're describing, but I assumed it was similar because of cyclic load issue. Obviously crap bolts made of cheese would be the wrong thing even as a cup-hook. A2 stainless - though not as strong as 10;8 or whatever (in some sense of strength) - would these be sensible here? I read somewhere that stainless was sometimes prone to cracks - but then, stainless covers various recipes anyway.

I'm not an engineer (which is probably obvious) but do have some understanding, and keen to learn.

 

[Yellow Saddle]

Well every day's a school day on here :-)
I too was using "stronger" in quotes for same reason.

I can see the point you're making. I guess with a bit of give in one or other material, all the threads within the nut will share the load, but the weak point is the neck or topmost un-attached thread. That said, do you think it's an issue with any reasonable steel (into alloy) here; steel being less prone to fatigue than alloy. It maybe that fatigue is a different thing that what you're describing, but I assumed it was similar because of cyclic load issue. Obviously crap bolts made of cheese would be the wrong thing even as a cup-hook. A2 stainless - though not as strong as 10;8 or whatever (in some sense of strength) - would these be sensible here? I read somewhere that stainless was sometimes prone to cracks - but then, stainless covers various recipes anyway.

I'm not an engineer (which is probably obvious) but do have some understanding, and keen to learn.

The load in threads, whether we're talking nuts or bolts, are generally only taken up by the first three or so threads. There are formulas engineers use which are a function of the thread pitch. Beyond a certain number of threads there is no advantage in increasing the engagement depth. But as I say, if you overload a bolt in a nut, either one of them will usually strip completely i.e. all engaged threads will strip off. If the bolt neck is the weakest point, that shears off before the thread strips and is quite common on mild steel bolts rusted into place. Whether or not this happens depends on the depth of the engagement and the type of steel. Most bolts are made from steel alloy of some sorts. Even mild steel isn't pure iron, it has some carbon in it and although carbon isn't a metal, when carbon is added to iron we still call it alloy. Obviously mixing in other metals such as chrome, vanadium or manganese would also create a steel alloy.

All steels are prone to fatigue. However, if the particular steel alloy has a higher tensile strength, fatigue would set in either later or under higher stress conditions or both. A fatigue crack and a stress crack isn't the same thing. A stress crack starts at a point (called stress riser or raiser depending on your nationality) and travels forward. A windscreen that cracks long after a stone nicked it is a typical example of a stress crack. Windscreens, being highly stressed structures because they are bent into shape and the outside part is under tension and the inside of a curve under compression, easily fail from stress cracks. A little stone nicks it and if it is nicked at a stress area, all the stress is concentrated on a small nick and splits the glass open. The split ends at a point which again sees a concentration of forces and splits further. It is a continuous and seamless process. Call it a traveling stress crack if you like. It is a single event.

Fatigue is different. From continuous cyclical strain, the metal crystalises in such a way that adjacent crystals along the stress line no longer have any atomic attraction to each other and they separate. If you search here on CC for "fatigue crack" with me as the author, you'll come across a photo posted by nickyboy of his cracked Campagnolo wheel. We discussed fatigue cracks there.

In stem and seat post and seat clamp bolts, the crack forms like a windscreen crack. Spokes break in the same way, usually at the first thread just under the nipple's blank start bore.

 

[SpokeyDokey]

Can't stand rusty bolts - all my hexes* are stuffed with waterproof grease.

I know - sad!

*Even the piddly ones under my saddle.

 

[zacklaws]

Avoid stainless steel as its not ideal for anything load bearing, I stripped a head on a bolt on my seatpost that holds my saddle on and found there is plenty of bolts etc on Ebay cheap as chips and took a fancy to stainless myself as I thought it would be ideal, but just before I bought them, I noticed the seller had listed them as not suitable for any load bearing situations so I just bought some normal steel bolts and confirmed it that the info was correct. Having had a set of handle bars snap on me once and never wanting to experience it again, the last thing I would use is stainless to hold my bars on.

Can't stand rusty bolts - all my hexes* are stuffed with waterproof grease.
.

Oddly enough read about this myself somewhere today and they recommended silicone sealant as it would stay in place and could be picked out easy if required.

 

[Yellow Saddle]

Avoid stainless steel as its not ideal for anything load bearing, I stripped a head on a bolt on my seatpost that holds my saddle on and found there is plenty of bolts etc on Ebay cheap as chips and took a fancy to stainless myself as I thought it would be ideal, but just before I bought them, I noticed the seller had listed them as not suitable for any load bearing situations so I just bought some normal steel bolts and confirmed it that the info was correct. Having had a set of handle bars snap on me once and never wanting to experience it again, the last thing I would use is stainless to hold my bars on.

This only confuses the issue. All bolts are by definition load bearing. If they cannot bear a load, we cannot use them to clamp parts together.

Stainless steel bolts can bear loads and even loads as large as their steel counterparts. It is the cyclical nature of the load that makes them unsuitable for some applications on bicycles.

You say you then fitted "normal steel bolts" to your handlebars. Not knowing what "normal" is, I do hope it is not mild steel.

Anyone interested in the complexities of choosing the right bolts for the right application may want to spend some time here:

http://www.bssa.org.uk/topics.php?article=104

 

[e-rider]

Avoid stainless steel as its not ideal for anything load bearing, I stripped a head on a bolt on my seatpost that holds my saddle on and found there is plenty of bolts etc on Ebay cheap as chips and took a fancy to stainless myself as I thought it would be ideal, but just before I bought them, I noticed the seller had listed them as not suitable for any load bearing situations so I just bought some normal steel bolts and confirmed it that the info was correct. Having had a set of handle bars snap on me once and never wanting to experience it again, the last thing I would use is stainless to hold my bars on.



Oddly enough read about this myself somewhere today and they recommended silicone sealant as it would stay in place and could be picked out easy if required.

stainless steel bolts are great for mudguards and water bottle cage fittings though - they don't rust and don't carry any significant loading either

 

[Profpointy]

This only confuses the issue. All bolts are by definition load bearing. If they cannot bear a load, we cannot use them to clamp parts together.

Stainless steel bolts can bear loads and even loads as large as their steel counterparts. It is the cyclical nature of the load that makes them unsuitable for some applications on bicycles.

You say you then fitted "normal steel bolts" to your handlebars. Not knowing what "normal" is, I do hope it is not mild steel.

Anyone interested in the complexities of choosing the right bolts for the right application may want to spend some time here:

http://www.bssa.org.uk/topics.php?article=104

Great link - I love stuff like that.

 

[zacklaws]

You say you then fitted "normal steel bolts" to your handlebars. Not knowing what "normal" is, I do hope it is not mild steel.

Looking at the bolts that I bought they were "High Tensile" steel bolts. (too late last night to look for what I had bought so just summerised)

And nowhere do I say that I fitted them to my bars, it was for my seatpost. The OP wanted them for his bars, and just mentioned that I would not trust them having experienced a handlebar snapping, having bolts snap holding your bars on would have the same hair raising effect of loosing your steering followed by a spectacular crash.