Damage assessment and on-the-road frame repair

(c) m.s.gerritsen 1999

Problems with a bicycle frame can usually be attributed to two sources: metal fatigue or a gross overload by an accident. 
Fatigue is caused by  cyclic loads over time. The metal start to crack, the crack grows, and the moment will come when the remaining cross-section has become to small to resist the applied load. With this kind of failure you can discern two areas in the broken section: the crack, probably rusty or polished by the continuous movement, and the fresh fracture. 
Fatigue cracks commonly start from local high stress points such as a hole drilled in a tube (the brake cable through the top tube is a likely candidate) , from a sharp corner such as a square leverboss on a thin down tube, or where there is a large wall thickness variation as with some lug joints.
It is a good idea to check your frame occasionally, especially if you hear funny noises or suspect that your frame has become more flexible. Clean the bike, and inspect the paint for cracks or rusty lines. Concentrate on areas where something happens in the tubes, such as in the joints, around holes, starting from brazed on parts, etc.

Repairs, steel frame
If you find a crack you can have it welded or brazed, but such a repair is not stronger than the original construction, and won't last very long.  The addition of a reinforcement is to be recommended. 

Brazing is a technique where two metals are joined by a third metal with a lower melting point than the parts to be connected.  In frame building commonly used materials are silver based alloys and bronze varieties. In the latter case you sometimes encounter the term bronze welding.  This technique is still used in car body repair, so it makes more sense to head for a body shop than to try to find a bicycle dealer.

The stages in the repair procedure are as follows

  • Strip the frame, and remove all paint, dirt and grease with file, wire brush, emery cloth and degreaser.
  • Make a suitable reinforcement  and hammer it into shape. If a tube has broken completely in two, you can insert a piece of tubing: cut the ends of the reinforcementube at an angle to avoid sharp corners. 
  • Clean and  sand all parts down to the bare metal, add brazing flux, and braze the crack. Let the frame cool in still air. Never use forced cooling during or after brazing bicycle tubing. 
Welding is an other possibility for frame repair, although welding a brazed section is not to be recommended, as braze material contaminates the weld. TIG-welding is the most elegant, but not easily found in non industrial areas. With TIG welding an electric arc is struck between the work piece and a Tungsten electrode, and a filler metal is added by hand. Argon (there is a large bottle connected to the machine) flows from the torch over the work piece and shields the hot metal from the oxygen in the air. A low tech approach is welding with oxy-acetylene. This isn't very well suited to Reynolds 531 but permissible with 25cromo4 . Cro-moly tubing was developed in the thirties for welded airframe construction, so it should still be possible to do so 60 years later. But find yourself a welder who knows how to weld, a cheerful character with lots of white teeth but no welding goggles looks fine on your holiday snaps, but will not do your frame much good. 

Thicker tubing  (around 1.0mm) can perhaps be welded with a MIG welder, but only as a last resource. A MIG welder can be recognized by the characteristic sound ( as a buzzy two stroke), the bottle for the gas shielding and the spool of welding wire which is fed through a torch. MIG welders are very convenient for construction and production work,  and often found in garages, but not optimal for very thin work. The simplest form of electrical welding is the arc welder with consumable electrodes. This method is fine for shipbuilding or the construction of a garden fence, but will burn large holes in your bike.

Brazing is unsuitable for aluminum alloy frames. If you have a welded frame you could try to have the crack TIG welded, but for a bonded frame you have to try something else. (bonded tubing is usually bonded not for nothing: the alloy depends on a heat treatment which would be destroyed by high temperatures)

The bandage &  first aid approach
If you have a broken leg, it stand to reason to have it set and put in a cast. The frame lends itself to the same treatment, only instead of plaster we use epoxy and glass fiber or carbon tape. If you can't find epoxy proper you could even use thin flowing epoxy glue. Epoxy for laminating purposes is very common in yacht building and surfing circles, and probably easy to find if you head for the beach. Epoxyrepair is suitable for steel-, aluminum- and composite frames, and again involves several steps. 
  • Sand the area around the crack down to the bare metal, or in the case of an composite frame till you reach the fibers.
  • Drill 2 mm stop holes at the ends of the crack in an attempt to stop further propagation , and degrease. In the case of an aluminum frame, coat the area with epoxy, and sand through the wet epoxy to get a good bonding. This is a messy job so you will need gloves to protect your skin. Wet epoxy is nasty stuff which is a known source of allergy problems.
  • Lay a suitable length of E-glass or carbon tape on a board covered with plastic sheeting, and saturate the tape with the rest of the epoxy. Use a brush or a squeegee.
  • Wind the bandage around the damaged area, and use some more layers to achieve a nice lug with smooth overlapping edges. Wrap the repair in plastic and tape and let it cure
  • Send me a picture if it gets you home ;-)
Crash damage
It is a thin line between to crash or not to crash. If you manage to pick yourself up, and are still thinking about continuing, the next question will be 'did my bike survive'? And although a loaded touring bike will drop on the panniers, and is thus reasonably protected, that in itself isn't a guarantee.
If you just fall over and the bike doesn't meet any solid objects along it's trajectory, it will be the bits sticking out which will suffer most. Parts like saddle, handlebar and levers, pedals and wheels could be in the firing line, the derailleur usually hides beneath the pannier out of harms way. If the bike crashes into something solid, the frame will get a heavy beating.
Saddle damage is usually limited to scratched corners which doesn't look nice, but isn't dangerous either. For this reason MTB saddles often have replaceable corner pieces from hard wearing Kevlar. If the saddle frame is distorted you could try to lever it back. Leave it on the seat post, so you have a firm grip at least at one end. Don't try to restraighten a bent seat post, and replace it if kinked or cracked.
Handlebars and stems are other items which you shouldn't try to bend back. If the end of the handlebar is  bent or deeply scratched it is probably safe to ride, but the closer the damage is to the handlebar clamp the greater the risk. Anything within the first 6 inches should start you shopping straight away. And push a cork in the open end of the bar if you lost the plug, always smart in case you crash again!
A bent pedal axle is immediately noticeable the first few crank revolutions back on the bike. This should be attended to as soon as possible, as the risk of getting a knee injury is large. If, when unscrewing the pedal the axle looks straight, the culprit could also be the crank. If you cannot see the deformation by eye, it is sometimes possible to restraighten the crank. Put the crank in a large vise, use a large crowbar or a heavy hide mallet. Don't do this with cheap cast cranks.
Wheels: small wobbles can be removed with the spoke key, but if the rim is severely damaged, the spoke tension will become very uneven, and longevity will be compromised. In that case you will have to consider a new rim and new spokes.
With rear wheels with old-fashioned screw-on freewheels a bent axle is very likely. Remove the wheel, spin the axle, and observe the centering of the axle in reference to the freewheel body. Replace the axle before it finally breaks, or before (not uncommon) the dropout fatigues and cracks.
Frame damage
Frame damage could be a bent or dented tube. Dents without sharp kinks are usually nothing to worry about,  a classical example is the dented top tube where the handlebar has hit the frame. Other prime spots are the down tube (where the side pull hits the frame)  or just behind the head tube, if you have a cable hanger fitted (for this reason I prefer straight cable hangers without the downward portion. If the front brake is damaged it is often possible to swap parts with the not so crucial rear brake. Or play safe and get a new one.

Whether you can continue on a bent frame depends on the extend of the damage. Maybe it is even possible to re-straighten the frame. If the frame still handles the same as before chances are that it is still true. Check the bottom- and down tube for damage (kinks, ripples, cracked paint, gaps in the lug joint) and feel with your fingertip. And is the front fork still straight?. If removal of the front wheel is suddenly more difficult, if the front wheel isn't centered anymore in between the blades, or if the axle isn't perpendicular with the fork crown (in that case the brake pads will not meet the rim squarely) something has moved. If the -steel- fork blades have deformed without sharp kinks they can be restraightend, but if the steerer tube is bent (check whether the bearings still turn smoothly, without high spots), early replacement is necessary. With more forgiving steel forks you have more leeway (i.e. miles) than with aluminum alloy, but kinks and waves are a good point for fatigue cracks to start. 
BTW: carbon frame parts do not bend: they fail by delamination which can be difficult to see and is usually impossible to repair, unless you can replace a tube.

Straightening the derailleur hanger
Steel derailleur hangers are likely to need realignment after a heavy crash. If you have an aluminum frame with a replaceable hanger, I hope you brought a spare one, as they break easily (if you didn't, you could try finding a steel derailleur hanger as fitted to cheap bikes with stamped steel dropouts). If the hanger is bent, index shifting will suffer, and you risk shifting the derailleur in the spokes the first time you look for first gear. Be very wary for this danger if your bike has been dropped on its left side.
To straighten the derailleur hanger the bicycle mechanic has special equipment, but on the road you could try the following improvisation.
  • readjust the lower limit screw on the derailleur until you can make a proper repair, then
  • remove the derailleur, it is usually possible to leave the cable in place
  • reverse the rear wheel in the dropouts, otherwise the freewheel will be in the way
  • take your front wheel, and screw it with the skewer axle to the dropout. Bend until the wheels are in parallel. If you have a thick hub, and not enough room for the skewer nut, maybe you can improvise with a 5 mm nut and a large washer. And many rear-wheel axles have the same thread as the hanger ear, so you could try screwing a borrowed rear wheel directly to the dropout