Cycling UK Forum

As part of my rebuild I want to replace the small socket head bolts that fasten various bits to the frame. Some of them just ain't shiny any more!
Stainless bolts are an obvious choice but I've never been quite clear on their strength compared to the standard plated bolts.
Are stainless bolts ok for rack mounting points?
If the answer is of the form "That depends..." then please expound!

I've got s/steel sockethead bolts holding everything on my/our bikes and tandems they're completely safe IMO.Usually about 25p from your LBS.

By coincidence, somebody is selling them on the forum. sales and wanted section

SS is much stronger than 'normal' steel. Used it for years. Keeps it looks too.

Mick F. Cornwall

If you are a Screwfix customer you can buy 50 M4x16 for £2.10 and 50 M6x20 for £3.41 lots of other sizes and types available. Yes that is A2 stainless steel page 54.

Screwfix also sell a selection of socket and button M5 screws in a variety of materials.
My local branch sells over the counter to all.

roger

Are s/s stronger than normal bolts? They need not be high tensile steel after all.

SS is far stronger and tougher than mild steel. Try working with SS, it blunts drills!

Mick F. Cornwall

I doubt it. Stainless also tends to be more brittle.

And I think the previous two replies, between them, reflect the frame of mind in which I asked the question I've never been confident about "stainless" since the day I discovered that it is a catch all term for a range of materials with incredibly diverse properties which need not actrually contain any iron at all. Whew!

After diligent research and mature reflection I find in favour of the "used 'em for years" argument.
Thankyou

As a chartered mechanical engineer and Fellow of the IMechE, misconceptions about the properties of stainless steel never fail to surprise me, even among engineers. Is st st stronger than steel ? is red wine better than white wine ? It depends on what particular grades you're referring to. Reference is made to grade A2 - You might also see reference being made to grade A4 (these are two most common grades for fasteners) - these grades tell you nothing about the strength, but they do refer to the corrosion resistance (A4 is better than A2). If you have decent stst fixings they should also have a number after grade (e.g. A4-70) check head of screw or bolt (it's also amazing how many engineers don't know the difference between a screw and a bolt): This number is a measure of material tensile strength (e.g. 700 N/mm2).

Carbon steel screws come in various grades - a reasonably good grade for "structural" use would be grade 8.8 (this is marginally stronger than A4-80). I could go on, but you're probably sleeping - there are other issues with stst screws such as galling. Also stst screws rely on work hardening to enhance strength, but the work hardening uses up some of the materials ductility, which may make it appear to be brittle - check out British Stainless Steel Association if your thirst for knowledge remains unquenched.
:http://www.bssa.org.uk/topics.php?article=1

I on the other hand am a metallurgist and I tried to keep out of this but as someone flashed the technical expert card I couldnt hold back any longer.
Strength will not be your limiting factor, toughness will. The stainless will be enourmously more corrosion resistance, but if you start to consider the properties in petty detail you would have to consider their susceptability to crevice corrosion in the depths of a thread under tension. Dont bother nothing compares to the test of actual use in the specific environment.
in other words 'I have used it for years'.
Now stop and think a second. If you overload your rack etc. What do you want to fail? Your rack at £30, your frame at £? or a little 20p bolt? All of the mentioned bolts are more than strong enough and they are more than tough enough.
So the only real effect of your choice of material is the corrosion resistance. Unless you decide to get a bit heavy handed with the spanner and strip the threads in your frame with a high strength/hardness bolt. Or if you have a lot of money to spend on saving a few grams then buy Titanium.

A little knowledge can be a dangerous thing, they say.
A lot of knowledge has its risks too!

I only wanted to know if it was a safe thing to do

It's interesting to discover that the numbers on bolt heads have such a (relatively) straight forward meaning but I'm completely unable to relate the numbers to items found in my environment!
I would tend to recognise bolts marked 8.8 as "Normal" which means only that I have a fair idea of when the head will twist off...

It appears that the A2 70 bolts I have used may be a little weaker than their predecessors. The important word there is "little" and the Test Rig of Time will pronounce in due course!

Levity apart, chaps, just the fact that two men with relevant qualifications seem to be disagreeing "on sight" illustrates perfectly why I stopped trying to work it out for myself.
But, seriously, thanks for trying!

Gentlemen,these are bicycles we're discussing,allen bolts are 20p each from the LBS.

But on the other hand the most likely cause of failure by a bolt will be by fatigue, which neither the engineer or metallurgist have mentioned! Also depends on grade of stainless , because as we all know there is ferritic, austenitic and martenstic, and the only way something can ‘work harden’ is if it is plastically deformed (permanently deform) which is not something I want to happen to my equipment either during fitting or during service!

As regards the need for toughness, again, only applicable if material permanently deforms which is something we do not want - we don’t want any permanent deformation under any circumstances.

As for titanium, well, I can’t think of a metal that will gall easier than titanium - if you ever want to check to see if something is titanium then scrape it on glass and marvel at the metallic titanium deposited on the glass, now just imagine how easily it can gall.

So my advice is to use some stainless steel as they’re up to the job and they look pretty - I prefer the duller type as opposed to the really shiny ones as they match my character!