Carbon mountain bike: Recognising and repairing damage

Even if it is often too expensive for complete bike manufacturers to send a damaged frame to the producer for repair - who is usually based in the Far East - the expensive carbon fibre dress does not have to go to the press immediately. One thing is clear: hobby mechanics should keep their hands off carbon fibre mats and resin. Carbon fibre repairs are a job for experts. Such professional repairers usually come from the world of motor racing. Companies that manufacture carbon products for high-class motorsport, for example, usually have the expertise. These experts can repair cracks and even entire holes in carbon frames. For such repairs, the correct mixing ratio of resin and hardener and the ideal bond with the carbon fibre fabric are important. In the case of damage, there is basically a choice between two repair methods: the application of simple bandages - not very attractive visually and usually associated with large variations in stiffness. For the second and technically better solution, the individual fibre mats must first be ground off at the damaged area. Only then can new layers be applied. After painting, there is hardly anything left to see of the repair. On the following pages, we will show you what to look out for when handling carbon fibre.

HOW TO RECOGNISE DAMAGE TO CARBON

As a rule, carbon parts are prone to breakage. Nevertheless, carbon frames should be checked regularly - especially after crashes.

1. preventive medical check-up

Various testing institutes or manufacturers such as Canyon (pictured) offer non-destructive testing. Weak points can be recognised using computer tomography or radioscopy. However, examination with an electronic microscope or classic tapping is often the better method. Defective laminate sounds duller than intact laminate.

2. too much torque

One of the most common types of damage is a crushed seatpost. The thin-walled carbon seatposts can often only be tightened to a torque of 4-5 Newton metres. Even a professional needs a torque spanner for this. The seatposts do not break immediately, but only after repeated tightening or overloading.

3. safe signs

After every heavy fall, you should check your frame for cracks. Cracks like the one in our picture can be superficial, but they can also indicate a damaged carbon structure. The latter can lead to a sudden breakage and serious accidents later on under heavy loads. You should contact the manufacturer or a carbon specialist with your frame now at the latest.

THE CARBON REPAIR

Even a hole the size of a golf ball is not a death sentence for your frame. Before requesting a crash replacement from the manufacturer, it is worth enquiring about a repair. Jochen Stecher from carbon specialist Jostec shows how carbon repair works. Attention: Carbon repair is only for experts!

REPAIR ADDRESSES FOR CARBON*
* Lists are not exhaustive; sorted alphabetically

• Bräutigam GmbH, Daimlerstr. 13, D-71691 Freiberg am Neckar, www.braeutigam-gmbh.eu
• Carbon-Klinik, Seiboldstr. 37, D-86911 Riederau am Ammersee, www.carbon-klinik.de
• CarboRep, carbon frame repair, Nürnberger Str. 90, D-91052 Erlangen, www.carborep.de
• Jostec, Schmitten 1, D-88279 Amtzell, www.jostec.com
• MAW Mariotto Art Works, Römerweg 111, CH-4574 Lüsslingen, www.maworks.ch
• Poly Tube Cycles, Braunstr. 12, D-41836 Hückelhoven, www.polytube-cycles.de
• Triple-M, Wies 7, A-4720 Kallham, www.triple-m.net

Repairs to laminated threads, bearings or bottle cage eyelets (no frame tubes or large cracks):

• Koehn GmbH (Reset Racing), Göttinger Chaussee 12-14, D-30453 Hannover, Tel. +49-(0)511-473204-40, www.reset-racing.de

Testing and inspection institutes:

• Carl Messtechnik, Thyssenstraße 183a, D-46535 Dinslaken, www.carbon-bike-check.com
• Velotech.de, Gustav-Heusinger-Str. 21, D-97424 Schweinfurt, www.velotech.de
• Zedler - Institute for Bicycle Technology and Safety, Teinacher Straße 51, D-71634 Ludwigsburg, www.zedler.de

TREAT CARBON CORRECTLY

Carbon is extremely robust and sensitive at the same time. Simply banging on the screws is not an option. Here are the most important tips for handling carbon fibre.

Screw handles on carbon handlebars should always be tightened to the correct torque. If there are two clamping rings, tighten the outer one with a higher torque than the inner one.

Carbon assembly paste belongs in the clamping areas of the handlebars, stem and seat post. It reduces the tightening torques and increases the friction between the individual components.

Transport Only use carbon frames with built-in transport locks in the dropouts. This prevents the dropouts from being scratched and the fork or rear triangle from being compressed if they are accidentally loaded. Never place sports bags or similar items on a bike that is being transported horizontally. Never transport your carbon bike with a carrier with a standard claw clasping the down tube, seat tube or top tube. The clamping forces required for safe transport can damage the tube, sometimes invisibly from the outside. Protect the bike with foam tubes. These are available either from DIY stores (pipe insulation) or from bike shops (waste from transport material). Transport the bike vertically in a car and, if possible, in a professional fastening system.

Switch off Never lean your bike with the top tube against a railing, road sign or similar. The handlebars may hit, the bike may roll away and the top tube may be damaged. Lean the rear wheel against a tree (post). The friction of the tyre prevents the bike from rolling away and tipping over. Lean the saddle and, if possible, the handlebars against a wall or a wider pillar. Here, too, the friction in the contact area provides a solid stand. Or simply lay the bike down, but not on the gear side.

Tool

Never use a mini-tool on carbon fibre parts. The screw connections can be tightened until they crackle - but then the material has already failed and you will probably have to replace the part.
Always use a torque spanner to tighten screws on carbon constructions.

Paste instead of fat: No grease in clamping areas where carbon is at least a partner. This also applies to the clamping areas of the seat post in the frame. Use special carbon assembly paste instead.

Assembly and maintenance: Do not clamp the frame in a mounting stand with a claw mechanism. This can easily crush a tube due to the leverage. Stands with a three-point mount are better, or you can clamp the bike to a metal seat post. Unlike the seat post and stem, mount the brake levers and shifters in such a way that they can still twist in the event of a fall. Only ever tighten the bolts gradually.

Precaution: If the brake lever or the shifter hits the top tube in a fall, this can damage the frame. Whenever possible, mount the handlebars high enough to prevent a collision or use a headset with impact protection (e.g. Acros Blocklock).

MYTH OR TRUTH

What is actually true? Does a film on the down tube protect against stone chips or not? We dispel the most common myths about carbon in bicycle construction.

Fast and favourable: Compared to aluminium or steel, the amount of work required for carbon frames is higher. Much more manual labour is involved in the manufacturing process. This is why a carbon frame costs up to ten times as much as an aluminium frame.

Dangerous cracks: Not every crack in the carbon fibre is a death sentence. With white paintwork in particular, these hairline cracks usually only appear on the surface. With other colours too, but they are not visible to the naked eye. Cracks in the paint are harmless to the structure. However, the handlebars in the picture have been through it.

Chainstay massacre? Chain slap poses no danger to carbon chain stays. This has been proven by researcher Franz Höchtl from the Technical University of Munich. Nevertheless, chainstay protection does no harm, as the paint and structure are damaged on the surface after thousands of impacts.

Screw loose? Carbon screws are superior to all other screw materials in terms of strength. They are also lighter than aluminium or titanium. But: do not use on handlebars, seatposts and disc brakes, as the torque required is too high.

Dust harmless? You should never inhale the fine dust produced when sawing off carbon parts (e.g. when shortening handlebars). This is because it is toxic and harmful to the respiratory tract. Therefore, always wear a face mask.

Film protects? According to a study by the Technical University of Munich, you can safely leave out the protective film on the down tube. A film only protects the paint, but not the carbon fibre structure. Fine cracks are also easier to see without the film. Rusting impossible?

Rusting impossible? Carbon cannot rust - that's true! But aluminium inserts are often fitted to the head tube, bottle cage eyelets and dropouts, which can corrode. Therefore, store the bike clean and dry after a ride in the rain.

Does carbon burn? Whether you use a lighter or a Bunsen burner, carbon only burns very reluctantly. Frames and parts made of carbon fibre are therefore not easily flammable under any circumstances.

Interview with Dirk Zedler, expert for bicycle technology and safety.

After 20 years of experience with carbon damage: What breaks most often?
Frames are very frequently affected, but the cause is crucial. Firstly, there are defects due to weak construction or poor workmanship. The second cause is overloading, for example due to a fall or previous damage due to incorrect handling.

How does carbon compare to aluminium and steel?
In my experience, steel and aluminium frames are much more susceptible to so-called fatigue fractures. As far as the service life under normal conditions is concerned, I am completely relaxed about high-quality carbon. It becomes critical when the bike falls over. Incidents in which aluminium or steel tubes are only dented lead to a crack in carbon.

What are classic handling errors?
Far too often, carbon fibre is broken. Assembly requires care. Another classic: carbon frames clamped onto the roof rack. The wrong combination of parts also causes damage. The days when you could assemble a bike with a multi-tool are over with carbon parts. Clamping in particular requires expert handling and special tools.