An Introduction to Bicycle Frame Construction
Here are the basics you should know about the common construction methods behind bicycle frames.
is referred to any long shape such as a tube or spoke that has more material on one end than the other. Double butting means the ends are one thickness, say 2.0 mm and the middle is thinner, say 1.6 mm, such as a spoke. Triple butting means the first end, middle and other end are each different thicknesses.
Because different parts of the tube are under more or less force from various loading conditions (potholes, sprinting, normal riding ect.) tubes and spokes can be butted to thicken members where more force is, resulting in lesser stress. This creates frames that are equally or often strong than unbutted frames but much lighter.
TIG weld usually. Welded connections are very strong when properly done. Signs of a good weld include a very uniform flow and the smaller amount of weld material the better. Welding uses very high heats and can often leave minor residual stresses in the material from the differential cooling of the joint and surrounding tube. This can weaken the joint but generally isn't a problem.
Fillet Brazing is when tubes are joined by a large amount of brass or brass silver mix brazed around the joint. When properly done it makes the joint look as if it was one peices with a slow curve from the verticle peice to the horizontal. Fillet brazed joints are very strong, but heavier than welded. However, because of the smooth transition the fatigue life is extraordinary and longer than either of the other methods for joining tubes. Fillet brazed frames are generally touring frames that will see a long rough life but take skill and time to do properly. They are often more expensive than lugs for that reason.
Lugged frames were the frame of choice until the innovations and wide spread use of TIG welding in the 1990s. Lugs are generally cast from a mold as small peices of metal that allow tubes to be fit into them and then brazed with silver or brass to complete the joint. Lugs can be beautifuly crafted and have ornate cut-outs referred to as pantographs. Signs of a good lugged frame are thin, sculpted lugs that have been filled down by hand to reduce stress concentrations in the transition from lug to tube. A simple lug that is point and filled very thin at it's end is the sign of a quality frame builder. Pantographed lugs, or lugs with cut outs are also a sign of quality.
Some of the best lugged frames came from Italy (Colnago is my favorite) and Japan but many custom builders in the US have become reknowned including Richard Sachs, argueably the best. Quality lugs are brazed with silver that is underneath the entire lug with no gaps left between lug and tube. This creates a stronger and longer lasting joint.
A process where a hot peice of extruded or cast metal is repeatadly smashed with huge force into shape.
Forging gets the metal on the outside of the item stiffer and stronger than the metal deeper inside. This is especially useful for items under bending and torsional stresses which are highest at the outer most parts of the cross section. By forging something you are effectively making the material at the outside more resistant to the higher stresses it will see.