Materials and processes.

Steel and aluminium are the most commonly used cycle frame materials. In this extract from his book MIKE BURROWS explains what the numbers mean and how the tubes are joined together.

Anyone who regularly reads any of the popular cycling magazines will already have some idea of the properties of the various materials used for frame construction. Unfortunately it will be the wrong idea.

This is because the various reviews and road tests contained in these journals are inevitably written by journalists. Such articles are intended to be interesting and entertaining, which is I suppose as it should be. Sadly they are seldom written by cycle designers. And they are almost never written by engineers, most of whom would know better than to perpetuate (even in the name of entertainment) the many harmful myths that surround cycle frame design and construction.

The various materials used in frame construction do, of course, have different qualities. It is important to have a good understanding of them when designing cycles. But the actual characteristics are usually only measurable with sensitive instruments - and almost never with the posterior of a Homo Sapiens! Even more importantly, any differences that may exist in the final complete frame will be more to do with its design (and specifically the tube size) than the material chosen for its construction.

So then, here are some of the various materials and their properties that you need not bother looking for in your own bike!

Steel

Almost universal - some 95% of the world's bicycles are made from steel tube. And for very good reasons. It is cheap, easy to process, durable if well painted, and the hardest, stiffest, strongest, structural material available. It varies in tensile strength from 375 to 1800 MPa. This is a measure of its breaking point if simply ‘pulled' like a piece of string. Its modulus of elasticity though, which is the measure of its stiffness or resistance to bending, is the same for all grades - about 201,000 MPa. Anyone claiming that their super high strength tube-set will give a stiffer frame is lying.

Mild steel

Most steel frames are built for people who have no interest in their modulus of elasticity. Such frames are made from thick-wall tubing that has been rolled from mild steel strip and electrically seam-welded. This produces a strong, stiff, cheap, durable and very heavy frame. Most of these frames are produced and used in China and India - or given to Western children for Christmas.

Low carbon (Hi-ten)

Not much more expensive, but a lot better, is low carbon steel. This is often referred to as Hi-ten by the cycle industry. It comes either seam welded or cold drawn and is the basis for most of the West's ‘bread and butter' bikes. Where weight is not a major consideration it is a good choice. The money saved on the frame can be put into better components, giving you a more reliable bike.

Chrome moly (4130)

However, the moment you start talking about performance, weight will start to matter. This means a ‘low alloy' steel, usually referred to as chrome moly, or 4130, which is its aviation industry specification. Indeed, it was developed for aircraft structures, but that was in the days of biplanes. It is still very useful for cycles, and most of the world's ‘affordable' performance bikes are built from it.

High quality tubing - from 531 to Airmet

Chrome moly is not so commonly used by the many small frame-builders around the world. They spend a lot of time building a frame. The material cost is less important than their labour, which is much the same for regular ‘chromo' as it is for one of the fancy-numbered tubes.

At one time the only number a cyclist needed to know was 531. It was the universal tube for racing cyclists world-wide. Not so today. Reynolds, the British producers of 531, have another six or so. Columbus in Italy have a similar range. And there are French, Japanese and American products to choose from. They all tend to offer similar types of alloy giving progressively higher tensile strengths. This allows the wall thickness to be reduced, keeping up the frame's strength but reducing weight and stiffness! Top of the pile at the time of writing is ‘Airmet' from the USA. At around 1500 MPa tensile strength it needs very special machining and welding techniques.

Butted tubing

Virtually all of these specialised tube-sets and a lot of the chromo comes in ‘double-butted' form. That is, the last 50 mm or so at the end or ends of each tube is some 50% thicker than the centre section. This is not because the stress is higher at the ends but to allow for a possible loss of strength caused by the joining process.

Lugs and brazing

The joining of steel tubes is something that the industry understands very well. After all, it has been doing it for 120 years. Commonest process still is using lugs and brazing, using a regular brass filler. This can either be automated using various techniques or done exquisitely by hand. In the latter case a silver alloy may be used in place of the brass to reduce the heat input to the tube, so retaining more of its strength.

MIG welding

Faster, cheaper and increasingly used for volume production is MIG welding. Metal Inert Gas welding uses a filler rod of similar material to the frame. This rod is fed through the nozzle of the welding torch surrounded by a shield of inert argon gas. The arc that flows from torch to frame fuses the tubes and filler into one.

TIG welding

MIG is an ideal process for automation by robots and produces quite neat results. But not on the thinner wall tubing. For that you need TIG - Tungsten Inert Gas - where a fixed tungsten tip replaces the consumable steel filler rod. The resulting arc is very controllable and almost like an intense gas flame. Clever people can weld kitchen foil with TIG!

A filler is still usually used to fill the joint, in the same way as brazing. This is the preferred process for high volume production of Chromo frames. It even has devotees in the world of handbuilding. Most of the modern high strength alloys can be ‘tigged' successfully. But long term fatigue may not be as good as traditional lugged construction.

Fillet brazing

Slowest but nicest is fillet brazing, using an oxy-acetylene torch and a bronze-based filler rod. A large fillet is built up, rather than a lug, so providing some stress distribution at the joint. The fillet then has to be filed, sanded and polished smooth - a very slow process. But the appearance is oh so elegant. Look at a Chas Roberts some time ....

Aluminium Alloy

Although only a small percentage in global terms, aluminium frames are a big and growing part of the performance scene. Aluminium has a lower tensile strength than steel, typically 225-750 MPa, and a considerably lower modulus of about 54,000 MPa. It is much more prone to fatigue if badly used. But it is a third the weight of steel. This is why it is what most aeroplanes are made of today - although not in tubular form.

All structural aluminium is alloyed with small percentages of other elements, in the same way as the high grade steels. It is designated into groups that share a similar main additive. They run from the 1000 series which is pure, to 8000 which has lithium as the main addition. Higher numbers are not necessarily better and grades of one series can be stronger or weaker than those of other series.

6061 and variants

The commonest grade for the cycle industry has been 6061. This is magnesium and silicon based, is quite strong at 420 MPa, can be cold formed, has good corrosion resistance and welds well. However, it requires specialised heat treatment afterwards, or it will crack at the joint. This involves heating the whole frame to 450°C for one hour, quenching in cold water to fully anneal the welds, then reheating to 140°C for 30 minutes to achieve the famous T6 status. For that is what T6 is - a condition, not an alloy as such. Various aluminium heat-treatable grades can be T6.

My own employer Giant uses a very interesting variation officially designated 6013 but known as Cu92. It has a high percentage of copper, normally used only in the 2000 series. This gives a very useful increase in strength but does not affect its other qualities - other than price! The 2000 series themselves can be high strength but are non-weldable, and prone to corrosion if unprotected.

7005 and 7020

The other big favourite, especially with the mass market producers, is 7005 or 7020. This is a stronger material than 6061. It shares its formability and generally good corrosion resistance, but does not have such good welding characteristics. It does not lose as much strength as 6061. But even if heat treated, it can never return to full strength. What happens is called stress corrosion. This is cracking caused by the constant flexing that any frame experiences, coupled with galvanic corrosion between the ‘grain' boundaries of the weld. The galvanic corrosion is caused by impurities in the form of dissolved metal salts present in everyday water.

The best way to make use of the high strength of 7005 or 7020 is to produce an extreme form of butting. This reduces the weight but leaves enough metal for a safe weld. This, I suspect, is the reason for the unusual square tubes with machined recesses used on the stylish Pace Research bikes.

5082

The third of the main structural series alloys is 5082 which is very popular with French manufacturers. It has high strength (around 375 MPa tensile), good formability, and the best corrosion resistance and weld quality of all alloys. Hence its main use is boat building.

Its drawback is that, unlike the other alloys, it is not a heat treatable grade. So, whilst the heat of the welding process will lower its temper, it cannot be reversed. The only way it can be hardened is by “cold working”, which is what happens as it is drawn through the final die in the tube-forming process. As in the 7005 grade, extreme butting would be a good idea.

2014 and 7075

If you decide to bond your frame together rather than weld it, you have a couple more grades of aluminium to choose from. 2014, which is one of the commonest aviation grades (about 470 MPa tensile), has low corrosion resistance and low formability but machines well. 7075, which is the strongest grade of aluminium commercially available at around 565 MPa tensile, has similar qualities.

8000

The one we are waiting for, but which is a bit slow in arriving, is the 8000 series. The main alloying element is lithium, which is used in the unusually high percentage of 25%. It is of such low density that not only is this the strongest alloy yet, but also the lightest. It even has a higher modulus of stiffness. But it is barely in commercial production and not in tubular form at all.

Jointing techniques

The bulk of aluminium frames are TIG welded in a very similar way to regular Chromo, but with a much longer fillet at the joint. This can either look smooth and elegant as on Cannondale and Klein frames, or a bit ‘functional' as on most others. This is not because of a difference in the welding but down to an individual with a file and emery cloth spending a lot of time on each frame.

Adhesive bonding is still very popular, with Bador in France and Trek in the USA being the largest users.

Aluminium can be brazed but I am unaware of anyone currently producing frames in this way. However, at Giant we add some of the more delicate fittings this way.