Materials on two wheels
Whether leading the peloton, riding to work or holding up Sunday drivers, there is a bike frame material to suit your need. Simon Frost looks at the history and cutting edge of materials in bicycle manufacture.
‘Strong, light, cheap – pick two’. In 2006, years after coining this maxim now famous in the cycling world, the mountain biking pioneer Keith Bontrager wrote that it remained ‘as accurate as Newton’s laws’. 10 years later, most articles on bike materials still begin with it.
There are materials to suit every cyclist’s need and budget, be they an urban commuter, a retiree unwinding on country roads or a pro racer, scaling mountains and circling velodromes at speed. But until the mid-1980s, one material dominated bike manufacture.
In some respects, steel is to bikes what vinyl is to music – the classic format that some purists swear will never be surpassed, and one that has recently experienced a popular renaissance. By Bontrager’s three-pronged equation, steel is certainly strong and, compared with more extravagant materials, such as carbon fibre and titanium, cheap. But its density – 7,750–8,050kg/m3 – makes it difficult to produce a light frame relative to those same materials.
‘In the bike world, everything was made of steel 30 years ago or thereabouts,’ Jim Walker, Director of Enigma Bicycle Works, UK, told Materials World. ‘But it became unfashionable with the advent of aluminium and, latterly, carbon fibre. But steel is coming back, partly because modern steels are far removed from the steels of 30 years ago. It is now possible to build a steel frame that will give similar performance to carbon fibre but with that lovely steel ride quality, because steel rides like nothing else – it has a life. It is difficult to articulate these things, but it has a certain “zing”.’
Lugged 531 to chromoly 4130
From the late 19th Century up until the 1970s, steel frames were made using lugs – external steel sleeves that joined the tubes, which were then brazed into place with silver or brass. The tube materials used in frames of this ilk included Reynolds 531, a manganese–molybdenum medium-carbon steel introduced by the UK manufacturer in 1935.
The relatively low temperatures used in brazing had little impact on the strength of the tubes, but developments in metallurgy led to steels that could hold up to the higher temperatures of tungsten inert gas and metal inert gas welding, with butted frames displacing lugged construction in all but specialist and amateur construction. While lugged construction requires less skill in hand-building, and tubes are easier to replace, welding the tubes instead reduces the weight and, more significantly, expense of lugs – complex geometries of steel that are investment cast or pressed over a mandrel and welded at the seams.
Today, the most commonly used steel alloy in frame tubing is 4130, known as ‘chromoly’ for its alloying elements of chromium and molybdenum. Its balance of weldability, strength, ductility and toughness and relatively low density make it an attractive frame material. Its iron content of over 97% is supplemented by around 0.3% carbon, 0.5% manganese, 0.2% molybdenum, 1% chromium, 0.3% silicon and hundredths of a percent of both phosphorus and sulphur. With a density of 7,850kg/m3, it is at the lighter end of the steel spectrum, but offers a formidable ultimate tensile strength of 760MPa, yielding at 590MPa.
‘Probably about 15% of the bikes we make at Enigma are now steel, but a few years ago that would have been more like 1%, so there is quite a difference to a few years ago,’ said Walker.
One likely reason for steel’s recent revival is the increase in commuter riding and wide uptake of cycling as a hobby – the UK Department for Transport’s Road Use Statistics 2016 report notes that the average annual distance cycled per person in the UK has increased 26% since 1996.
Today, people ask more of a bike than they did 20 years ago, and steel offers a longevity that aluminium and carbon fibre cannot match – a steel frame, if cared for, can truly last a lifetime. The shock absorption that its dense makeup creates offers a level of comfort that aluminium cannot compete with, its notoriously unforgiving ride quality offsetting benefits of light weight and high stiffness. And one need only watch a racing accident to see how brittle carbon fibre remains – when it fails, it is immediate and catastrophic.
But if Enigma produces 15% of its bikes from steel, then what is its speciality? The answer is the most exotic of metal frame materials.
Titanium – the magic carpet ride
‘We make classic bikes. That’s what we do,’ Walker says. Occupying the higher end of the market alongside carbon fibre, titanium bikes are all round tubes and smooth riding, sharing many characteristics with steel – as Walker notes, ‘There is very little to choose between steel and titanium in terms of ride quality.’
However, when it comes to Bontrager’s trilemma, titanium replaces steel’s cheapness with lightness. ‘Because titanium is lighter than steel, you can build more performance into it for a given weight,’ Walker explains. Titanium’s niche is in the ‘sportive endurance’ category – bikes that are durable enough for regular demanding rides, but light enough to take over hills with greater ease, without sacrificing the comfort of steel.
‘Like steel, it has a wonderful inbuilt ability to absorb road shock, so it gives a very smooth ride. People call titanium the “magic carpet ride”, and that’s what you get’, he says. ‘The other chief benefit is titanium’s resistance to corrosion. You don’t have to paint it – the raw metal is fine without any protective coating at all.’
No easy job
Its difficulty in manufacture makes titanium a time consuming, highly skilled and, inevitably, expensive material to work with. ‘We use aerospace-grade titanium – 3AL2.5v, an alloy containing three parts aluminium, 2.5 vanadium and the rest pure titanium. We also use 6AL4v, which is an even harder alloy, and much more expensive, but it makes for a stiffer frame that gives more performance,’ Walker explains.
These grades of titanium are incredibly tough on tools, as Jim’s son, Joe, Enigma’s Master Frame Builder knows well. ‘We use an abrasive zirconium belt to sand down the tubes, and we go through a lot of those belts. It’s tough stuff – the 3AL2.5v and, especially, the 6AL4v – so we spend a fortune on belts,’ he says.
The other key difficulty is in its welding. In Enigma’s early days, a decade ago, the younger Walker spent countless hours perfecting weld sequences before they were suitably pleased with the results to produce a titanium bike. ‘If you’re putting a lot of heat in a certain place then it’s going to pull the tube over to that side – you have to balance where you are welding the material to make sure the heat is evenly distributed, or the frame won’t be straight.’
And there are other considerations that do not apply to steel welding. ‘You have to keep everything meticulously clean. Once a segment has been cut, before we can weld it, we put it into a degreaser, then wash it in hot water to remove any oils, and then acetone the tube to remove any final dirt or grease,’ he explains.
‘Titanium also has to be welded in an atmosphere without oxygen, so we use a back purge system to create a flow of argon gas throughout the inside of the frame, and the gas is pumped through the torch on the outside as well.’
After the tubes are tack-welded together in a jig, Joe creates a careful two-pass weld. ‘We do an autogenous weld first, using a pulse at about 40 amps that fuses the two sections together, without the use of a filler rod. After that, you make a second pass with a filler wire, which creates a beautiful fishtail bead effect, which people look for when buying a titanium bike – they want to see how good the welds are.
‘Titanium is much more difficult to weld than steels, but it welds much more attractively when you know what you’re doing. With steel, you hide the weld with paint, but with titanium it’s important that the welds look nice, and you only get one shot – if it’s a poor weld, you can’t make it any better’.
As (the elder) Walker notes, carbon fibre frames are visually distinctive, a world away from titanium. ‘They really slap you around the face – although the basic diamond remains the same, being a monocoque made in a mould, carbon fibre frames can be adapted to all kinds of shapes.’ Its high stiffness-to-weight ratio and formability into highly aerodynamic shapes make the distinctive carbon fibre the material of choice for racing bikes.
But lightness is also desirable in a type of bike that has experienced a meteoric rise in the past 10 years, in line with urban cycling rates – TFL’s Travel in London 8 Report notes that from 2004–2015, daily average cycle journeys in the capital increased by 76%. The fold-up bike is a product of this culture, allowing commuters to combine public transport with cycling. One can assume it is carried more often than any kind of bike, so it has to be light.
The UK motorsport firm Prodrive announced in September 2016 that it would begin production of a folding bicycle weighing just 6.7kg before the year is out. The Hummingbird, which is being manufactured by Prodrive Composites in Milton Keynes, is significantly lighter than even the lightest Brompton, which weighs in at 9.8kg. Most folding bikes weigh 12kg or more.
‘To create the world’s lightest folding bike, we knew we would have to use composite materials,’ Matt Bradney, Director of Business Development at Prodrive Composites told Materials World. ‘This has not been done successfully before because joints and latches are structural weakness and to make them safe you would cancel out the weight savings of carbon fibre. The Hummingbird circumvents this problem with its unique folding mechanism, which compliments the use of carbon fibre.’
The pivot of the folding mechanism’s swing arm is concentric to the bottom bracket shell, in an area reinforced by the set axle and bearing housing, making it a strong structural pivot point and removing the chain tensioner, which means the chain cannot come off.
The main frame, forks, handlebars, seat post, saddle and crank are all formed of high-performance, bi-directional pre-impregnated carbon fibre, with high-grade aluminium for the brakes and levers, plastics for the bearings, and stainless and carbon steels for the chain, cables and other hardware.
‘Carbon fibre is amazing because it allows you to create naturally strong shapes compared with other frame materials such as aluminium and steel, which are limited by tube shapes and require welding, creating weakness. Magnesium would be a really good frame material, but there are few companies producing it in tube form,’ said Bradney. Like titanium, carbon fibre’s sacrifice is economical.
‘New materials will change the cycling industry, from better alloys to new strains of composites. The driving factor of bicycle innovation is materials technology,’ said Bradney, and no talk of new materials would be complete without a mention of the ‘G’ word. ‘We hope in future to produce graphene-infused carbon fibre frames, which would drastically reduce the weight.’
Graphene or no, the dictum will likely ring true for years to come. Strong, light, cheap – which two would you pick?