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.
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
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 approachIf 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.
Crash damageIt 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 damageFrame 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.
Straightening the derailleur hangerSteel 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.