Ride UK Tech Columns

#81 Living in a Material World...

(This article first appeared in Ride UK issue No.81 and is reproduced here by kind permission of Ride UK.)

“Supertherm or Ox platinum? T45 or Reynolds 853? Sanko or Tange? What’s it all about?”

Yep, several people have been foolish enough to ask me to write a tech column on all the “fancy” tubes that companies are increasingly using to make frames. Obviously this is unlikely to be an exciting story with dumb security guards and drunken antics but I will do my best to prevent it being completely unreadable.
First job though, go and put the kettle on. You will need it later and you don’t want to have to wait for it to boil. From here to the bit you need the kettle for should be just about right to let it boil, only boil as much water as you will need for a cup of tea. If everyone only ever boiled enough water for the job we could close two power-stations….
OK lets go.
Most of us are familiar with 4130, we look for a frame that is made of “100% 4130” as a minimum. But what is 4130?
4130 (and all this other fancy new tube) is over 95% Iron. Iron is a metallic element (Fe), you can’t break it down any further without getting into nuclear physics type stuff. Stick it on the hob as long as you want, it wont separate or curdle, hit it with a big stick, call it names, it wont have any effect. Iron is iron is iron!
But iron on its own is what “we in the trade” like to call “piss weak”, sorry to get so technical but its unavoidable. Raw elemental iron is about a tenth the strength of 4130. But 4130 is 95% iron! Obviously those other few percent of something-else are pretty damn important.

Do you remember in chemistry at school all that stuff about reactions and compounds? Drip some hydrochloric acid onto some marble chips and write down all the reactions taking place? Yeah well that has ABSOLUTELY NO RELEVANCE TO THIS so don’t let it distract you.

Kettle should be about to boil so go and make a cup of tea.

Back? OK.
What you have in front of you is my makeshift analogy for this month. When you made the tea it didn’t fizz and bubble with chemical reactions did it? (it shouldn’t have anyway) no gas was emitted and it didn’t glow like in the movies? That’s because it is simply a mixture. You dunked the teabag and small particles of tea leaf moved out of the bag into the tea they are just floating about in there. The same with the milk, you watched it swirl round and mix in with the water and tea-leaf-bits. Even the sugar (if you take it) although dissolved is not chemically bonded to anything it has just collapsed into tiny tiny bits that are floating about in there. It’s still 99% water but it tastes immeasurably better. This is the same as our 4130. Other metals like chromium and molybdenum as well as other elements are mixed in with the iron but they aren’t chemically bonded together.
You can drink the tea now.

Metals are crystaline. So the iron in our tube is composed of tiny little crystals all jammed together. The bonds within these crystals are immensely strong, but the bonds from one crystal to the next are relatively weak. So stick a bit of iron in a vice and hit it with a hammer and it will bend fairly easily because the crystals can just pull apart.
Steel is iron with a little carbon mixed in with it. The carbon gets in the gaps between the crystals of iron and glues then together. Now if you hit a lump of Steel rather than iron you will have a much much harder job to damage it. Remember nothing is chemically bonded together, the carbon has NOT formed any molecular bonds with the iron.
Man has been making steel a long time now and we have got very good at it, tiny differences in the amount of carbon can make big differences to the strength of the steel, but we have taken simple carbon steel about as far as it will go.
To get more out of the Steel we add other metals. Just a little bit, one or two percent, takes us to a whole new level. These are alloy steels and that’s the family that 4130 falls into. These “alloying” elements do a similar job to the carbon, filling the gaps between crystals of iron with smaller crystals of molybdenum or chromium, still with carbon filling even smaller gaps. The smaller the gaps get the closer the crystals can pack together and the stronger it gets.

But we don’t make alloy steels by carefully placing a crystal of iron next to a crystal of molybdenum and sprinkling on some carbon, instead we melt everything down in a big pot and stir it up. We dig all the ingredients up out of the ground (or recycle them from old cars etc) and they are “dirty”. Back to our tea analogy it’s like someone dunked a biscuit in there. No matter how hard you try some of the biscuit comes off in the tea and ruins it. You can fish the big bits out with a teaspoon maybe, but you can never get it all. Maybe the biscuit was worth the slight damage to the tea, or maybe you can make another cup but this is where the tea-analogy falls down.

The Steel is always going to have a little bit of contamination in there. Stuff like Sulphur and Phosphorus gets in there. In the same way that a tiny percentage of carbon increases the strength of the material a huge amount by gluing the crystals together these contaminants even in tiny amounts can ruin the material. Getting rid of these last few percent of contaminants is difficult and expensive.  

So we just mix up the right ingredients in the right proportions and bob’s your uncle 4130?
Well actually, no. Once you have all the ingredients together there is a whole range of things you can do with it that will effect the strength a huge amount too. How it is made into a tube is a big factor. Some cheaper tube is rolled up from flat strip and welded all down a seam the whole length of the tube, while the best stuff is “drawn” from a forged blank (seamless). We can also use heat to change the shape and size of the crystals, the permutations of heat and quenching are almost endless. And this is where things get complicated…
When we talk about “strength” we aren’t being very specific. There are different types of strength. A rubber band is pretty strong but it has very little stiffness so it wouldn’t be any use to us. A CD is pretty strong but its easy to bend, it takes a lot to break it but not much to permanently bend it. A stick of chalk is pretty strong (for its weight) but when it breaks it does so suddenly and shatters.

So when choosing a steel for a bike frame we want a mixture of strengths. We want “ultimate tensile strength” (UTS) this is the strength against breaking completely.
We want a high “Yield Strength”, which is the load to permanently deform (or bend) it.
We want “fatigue strength” or resistance to repeated loading and unloading.
And we want good “elongation” which is how much it can bend before it breaks.

Unfortunately these properties are often linked. Using heat treatments we can fine tune the properties but as the UTS goes up the material gets more brittle, so the elongation goes down and the UTS and Yield strength get closer together and lower fatigue strength. This means that we get less warning about problems. With a super high UTS material the tube might look fine then suddenly break, not good for a bike.

“So that’s great George. I get all that but it doesn’t actually answer the question now does it?”

No. It’s a fair cop. But knowing this will make you a bit more immune to the marketing babble I hope.
Now lets look at some specific examples:-

Frame A, is a typical no name Taiwanese clone product from a pub car-park. It may say “tri-moly” or have a sticker on the downtube saying “100% chromoly” but this is pretty meaningless. It may mean that that one tube is 100% chromoly but there are a million materials that would fit that bill and most are rubbish.

Frame B, is an entry level bike from one of the proper BMX companies. The marketing blurb says it is 100% 4130. Now we are talking. It is going to be a reasonable alloy steel tube with the right proportions of Chromium and molybdenum in it. It will be considerably stronger than Frame A but how much of the various impurities are in it is anyone’s guess.

Frame C, is a rider owned company frame. Maybe it is branded 4130 or says ANSI 4130. ANSI means it conforms to the “American National Standards Institute” specifications for 4130. Their equivalent of our “British Standards”, the ANSI have laid down a tight specification for 4130 tube that it will have to have met. The specification will include maximum levels for impurities and probably a minimum set of physical properties. ANSI 4130 will be good stuff. Branded 4130 is also likely to be better than non-ANSI or unbranded tube but it depends on the brand.

Frame D is a top of the line model. The marketing blurb is screaming about the tube, maybe it’s an air-hardening tube maybe it’s not, but the tube is being pushed as something very special.

So which is stronger?

Impossible to say. The strength of a complete frame is down to a lot more than simply what it is made of.

Maybe frame A is made of tube that is half the strength of frame C but if there is twice as much of it then it will probably be just as strong and quite possibly stronger! Though maybe too heavy ever to ride.

Frame D may be made of tube that is 50% stronger than Frame C but if they have used a tube 50% thinner to save weight (remember all these tubes are 95% iron so the weight differences for a given size and thickness of tube are negligible) it wont be any stronger and might well be weaker when you take other factors into account.

So what are some of the other factors? Well the detailed design of the frame will play a big part but that’s getting a bit off topic. In terms of what this article is covering, the big issue is the welds.

Some tube makers marketing will seem to be telling you that their fancy air-hardening tube actually gets stronger with welding! Now that’s an implication that I take with a huge pinch of salt. I mean, what are the chances?

We are talking about a tube that has an ultimate tensile strength close to twice that of generic non-ANSI 4130. It might be sold as a “seamless” tube meaning it started life by being forged into a big blank by having a spike driven through the middle of a huge round slab of material. This forging was then “cold drawn”  (“cold” meaning “not as hot as it could be” still pretty hot in my book) through a series of dies over and over again to get it from a short squat donut to a long thin walled tube. This process wasn’t easy and it was expensive. All to avoid the weaknesses introduced by welding up a rolled strip….. But hang-on a second. This stuff supposedly gets stronger with welding!!!???!!!

The drawn tube has also undergone numerous special heat treatment processes; hardening, quenching and  tempering to get these amazing properties.
But when we weld it we reduce it to a puddle of hot piss, mix it with a puddle of hot piss from the tube it is joining and add in some welding rod. We then just let this cool in the atmosphere with no quenching or tempering… and it’s stronger?!?! I don’t think so.

If we go back to the marketing hype and read a little more carefully we find that “stronger” isn’t really quantified that clearly. This doesn’t mean that the tube is shite. Far from it, but it does mean that you shouldn’t believe everything you read…

Lots of the fancier frames also make a big deal out of the fact that the tube is butted. Butting is where the tube gets thicker in certain parts. A single butted tube is thicker at one end, double butted means it’s thicker at both ends. Beyond that the system breaks down, a triple butted tube could be have a thick bit in the middle for making say a pair of handlebars or it could mean that one end is thicker than the other…

Butted tube means that the tube can be thicker where the welds are and thinner in the middle to save weight, unfortunately it can also mean that the middle part is easier to dent when you drop the bike.

So what does all this mean? What conclusions can we draw?

Well reducing all mankind’s knowledge of materials to one easy to absorb essay with tea-analogy was never going to be a realsitic goal, so now you have to use your common sense. If the maker of the tubing for your frame is prepared to put their name to it, then that’s generally a good thing. Companies like Reynolds have been making excellent tube for a very long time and their reputation depends of keeping the quality up. But equally once that tube leaves their factory they don’t have any control over how it is used. If the frame builder uses tube that is simply too thin and makes an arse of the welds then the best tube in the world is still going to make a crap frame that breaks. The flip-side of this is that some companies can take very basic tube, and by careful design and good quality control, make a brilliant frame that will last a long time.
So in summary good tube is nice but it wont make up for bad design or workmanship. If a company has never made a good frame then just because they are now using “Bobs-Bonza-Butted 97ZZ” wont make it any better. Or if the frame seems too good to be true, for example because it weighs less than a fart, then it probably wont live up to the hype…. But if a company has a reputation for quality and uses some wonder material to make realistic weight savings then it could be a good bet if you can afford it….

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