Broken frame (snapped, cracked, broken weld)
If you managed to break your frame on a ride, it’s unlikely
that you should continue riding from a physical perspective. The effects of
shock or concussion can set in slowly and when you least expect them to.
However, assuming you are currently pumped full of
adrenaline, have noted the correct number of limbs are still attached, and want
to get out of the woods, you need to put the bike back together well enough to
carry or push it out.
Frame broken in two
At this point, your options are to strap the two parts
together by attaching the front wheel to what’s left of the main frame triangle
or to attempt to lash the head tube back on to the other tubes.
Attaching the front wheel to the main triangle gives the
bike a measure of stability and may let you “wear” it like a backpack.
Lashing the bike back together depends on the tubes being
suitable shapes after the crash. In this case, stretch backpack straps from the
head tube to the seat tube. Alternatively use the straps or an inner tube to
lash a large stick across the break. This may provide enough strength to keep
the frame rigid enough to push the bike out.
Don’t worry about “saving” the frame at this point. Unless
it’s a custom Ti frame, the cost of rebuilding is going to exceed the cost of a
new frame. If it is a custom Ti frame, I suggest a different frame maker next
time anyway. Do whatever it takes to force the pieces back together
sufficiently to make the frame rigid enough to wheel along.
Frame broken in rear triangle (seat/chain stays)
The rear triangle (seat stays and chain stays) are the
thinnest tubes, and often are subject to a lot of flexing even in normal
riding.
If one breaks anywhere along its length – or, more likely,
pulls apart at a weld – then although the bike will handle really poorly and
your shifting may not work like it should, the bike should remain rideable as
long as you are careful.
To strengthen the broken area, find a suitable object to
either space the frame properly to hold it apart (between the seat stays where
rear rim brakes attach, between the seat and chain stay if the dropout weld
failed) or hold it aligned (along the length of the seat or chain stay). Now
wrap it thinly with inner tube, then rope or zip ties – checking wheel, chain
and derailleur clearance as you go.
Before you embark on a major fix, check that areas around
the broken area are not also cracked. Also check the opposite side. If one
section of the rear triangle has failed, it is likely that there were increased
stresses on another part of the frame that compensated.
If everything else looks solid, carefully ride the bike out,
dismounting for any technical sections.
Broken frame at a weld
Weld-related cracks may appear when you are "just riding along" but they probably started some time ago.
Even if a poor weld was the cause of the problem, there is normally a telltale thin line that forms over a period of time before the final break occurs. Regular bike washing is a great time to check frame welds for cracks.
Because tubes are thickest at the weld areas, there may be enough residual strength in the frame to lash the pieces back together.
See if the weld areas will align relatively well, and if so, lash them with inner tube, then rope (inner tube stops the rope from slipping). Remember that some tubes on the bike frame are under tension when you ride it (down tube), and others are under compression (seat tube). Your lashing has to take that into account – will the weld be trying to pull itself apart, or push itself together?
Surprisingly, you may be able to ride the bike after this fix. It will lose a lot of stability, and depending on the quality of your lashing, there’s the potential for sharp tube ends to work loose and impale you at any moment. However, on smooth surfaces, you should be able to coast along.