Just make the whole thing out of carbon...

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IF there’s one material everyone wants it’s carbon-fibre. Thousands of us deck out our machines with carbon fairings, huggers and even stick-on bits to stop the key scratching the dash. Some people go the whole hog and build entire bikes from the stuff.

There’s no doubt that if you want to make a component as strong and light as possible, then carbon is the material to use. But ask an expert if that’s because it’s stronger than steel, aluminium or even titanium and you’ll probably get an answer you won’t understand. That’s because it is, and it isn’t…

Like any material, carbon-fibre’s properties come from its basic structure. The carbon itself consists of very thin strands of monofilament molecules, long molecules attached end-to-end. These are formed in a highly complex (for which, read expensive) process which involves heating the carbon to around 4000°C, bathing it in a variety of chemicals then forcing it through tiny holes. The final strands are just six microns – or six thousandths of a millimetre – in diameter, compared with 25 microns for glass fibres.

The strands are then woven into a cloth which can either be very thin (as fine silk) or much thicker depending on what you want it for. But the type of weave is also important, as it affects the strength of the finished product. For example, it can be made very strong in one direction while relatively weak in another. Conventional materials like steel or aluminum are equally strong in all directions.

Other materials can be woven into the carbon matting to produce different effects. Adding some aramid (better known by its commercial name of Kevlar) makes the carbon-fibre more bendy and helps it stand up to abrasion. In some military aircraft, aluminium strands are woven in as this makes the material radar-absorbent. Or, more relevant to bikes, invisible to Gatsos!

There are three methods of producing a component from carbon-fibre. One is to ” wet lay ” it in much the same way as glass-fibre items are made. First the resin which binds the fibres together is painted on the inside of a mould, the carbon-fibre matting is laid on top and further resin is dabbed on. This is time-consuming as it takes around 24 hours for the resin to set, but the results make it all worthwhile. The excess resin makes the finish very deep and lustrous, giving the carbon a gorgeous three-dimensional look.

The second method is to use a carbon-fibre mat which is pre-impregnated with resin. This is laid into the mould, then put into an autoclave (a high-pressure oven) where it’s sucked into shape and heated to about 120°C to activate the resin. The finish isn’t as good as wet-laid carbon, but the process only takes a few hours.

A recent development from the world of Formula One car racing is resin infusion. The carbon-fibre mat is laid into the mould dry, then the air is drawn out of one end while resin is bled into the matting from the other. This way, paint can be applied to the mould so the component comes out coloured (carbon-fibre is notoriously difficult to paint). This method is particularly good for complex components.

So if carbon-fibre is so good, why isn’t every part of your machine made from it? A handful of specialist firms have built machines – such as the £27,500 Carbo Tech Ducati above – with carbon frames, swingarms and the rest. But it’s very expensive and very tricky to get right.

The first problem is deciding what shape the components should be. The most common mistake is to use carbon-fibre in a design intended for steel. The two materials are so different that at best the part won’t be making use of carbon-fibre’s advantages, and very often it will actually be weaker.

A good example of this can be seen in the fashion of making push-bikes with carbon-fibre frames – even though the material ends up weak and flexible when it is rolled into tubes. Titanium is a far better option, it’s very light and makes very strong frames.

An exception is the weird-looking racing cycle designed by Lotus for Chris Boardman, which used carbon-fibre as the sheet material it really is, forgetting the usual preconceptions of what a bike should look like.

Another complication with carbon-fibre is that you have to design the material itself before you create the component – by deciding what its directional strength should be (what weave to use), how much resin to use (as an approximate rule, more resin means reduced stiffness), whether to weave in another material, what moulding process to use and so on.

Even the type of resin is important. Some types are affected by ultra-violet light, and after a few years of exposure to bright sunshine the carbon-fibre layers start to come apart. This can be disastrous. Imagine what would happen if your wheels started de-laminating because you’d been riding in summer! The good news is that most carbon-fibre is very durable, so you don’t have to worry about your bike collapsing. But the type of carbon used still has to be taken into account. Only then can the component itself be designed, and all ideas about how steel, aluminium and so on work have to be thrown out of the window. Carbon-fibre works best as thin-walled, large box-type structures and you need as much as possible of the final shape to be finished in the mould as it’s very difficult to machine afterwards.

Carbon can actually be TOO stiff for some uses. For example, motorcycle frames have a certain amount of flex deliberately designed into them. It’s this that gives riders much of their feedback, and it also reduces some handling problems. That’s why aluminium is so popular. Engineers know exactly how it will perform under the stresses and strains of life on the road or track so they can design-in the amount of flex they need right from the start.

With carbon-fibre, it’s more of a black art. Even at Formula One or GP level, practical experience is just as important as the latest computer predictions. Designing-in flex with carbon-fibre is extremely difficult, so much so that there are only a handful of specialist firms who can do it. There are a few carbon-framed bikes out there, but even GP machines like Honda’s NSR500 use aluminium frames.

A final word of warning – carbon-fibre is lethal after a crash! When it’s shattered a cloud of microscopic particles is emitted. These are tiny shards of carbon which can work their way into the lungs, causing a lot of damage over a period of time. And once larger shards penetrate the skin, they work their way in deeper and deeper. It has been known for shards to go in one side of a hand or finger then emerge the other side a few weeks later!

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MCN Staff

By MCN Staff