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
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