Manufacturing processes and market trends continue to literally shape the bicycle frame.
While not as common as it used to be, the process of butting is still used in the
manufacture of bicycle frames. Meanwhile, steel, the long-running workhorse, is being
replaced more and more by aluminum—its hardy cousin that grows less expensive every year.
So what do you look for in a frame? Is next year's frame necessarily better than this
year's?
Weight
Striving to shave precious grams from frame designs, manufacturers have employed all
sorts of exotic metals and methods. Essentially, though, what you pay for is inversely
proportional to the weight of your bike. The more you pay, the less it weighs.
Geometry
In theory, aggressive angles lead to aggressive ride characteristics. Relaxed angles
lead to more casual ride characteristics. Which is best for you? The answer really depends
on how much time you spend in the saddle. If you ride a lot and aren't interested in
attacking the road or trail, go for a relaxed geometry of about 70 or 71 degrees on the
head tube. More aggressive bikes have a head-tube angle of 72 or 73 degrees.
Plain-Gauge Tubing
Even with advances in materials, manufacturing processes and design, the best frame
tubing for the buck is plain-gauge. These are tubes that don't rely on butting (see below)
or oversizing or exotic blends, but are straight and strong and easy to manufacture. As a
consequence they are cheaper. Those who are "serious" about cycling may point out that
plain-gauge tubes weigh more than butted tubes. This is true, but the difference is
sometimes only a matter of three or four pounds. If you're just out enjoying the town or
trail and not attacking mountains, then this weight difference is of no consequence.
Butting
The goal of any good bike manufacturer is to put the material where you need it. And you
need the material where the bike frame undergoes the most stress—at each end of the various
tubes. This process is known as butting.
Internal Butting—Looking at the tube, you won't notice butting because it's hidden
within the tube. So how do you know if the bike is butted? Bike manufacturers will
certainly tell you, as it's a big selling point.
External Butting—The older, more expensive way is to add material onto the outside
of the tube. This is rarely done anymore. However, you sometimes will see an extended weld.
(See below.)
There are two methods used to butt a frame tube.
Double Butting—As the tube is shaped, extra material is allowed internally at each
end of the tube. By increasing these areas of the tube, the overall tube wall thickness can
be reduced, thus saving weight.
Triple Butting—To save even more weight, the double butting process is refined by
stepping down the material at the ends of the tube. This means the butting starts out in
the standard, double-butted manner but then is thinned before stepping down again to the
normal tube wall thickness. In a cutaway, the inside of the tube looks like three terraced
rice paddies on a hillside.
Welding
There are essentially 3 ways to join frame tubes:
- Weld them using the same material as the tube (TIG welding).
- Braze the tubes together using silver or brass.
- Use lugs to join the tubes.
Each method has its proponents, yet nearly all but the very high-end bikes use the TIG
welding method. This approach is relatively inexpensive and creates a good, solid weld.
However, look closely at a bike's welds. You'll see that quality bikes offer a thick, even
weld that goes around the entire tube. On department store bikes the welds are thin and
spotty, dabbed down generally on the top, bottom and sides, but leaving open areas in
between.
Extended Welds—One inexpensive way of adding material to the end of a tube is to
simply add welding material. Generally, this is an elliptical circle or a double line
extending from the joint to about an inch or so down the tube where it fades out. What's
the problem with this method? The heat used in this process can actually weaken the tube.
After welding, manufacturers will again heat-treat the entire tube—baking it,
essentially—to bring the metal back up to par. While effective, this is a less substantial
method than actually building the butting while the tube is being drawn out.
