Not all bike frames are created equal. Since the backbone of any bike is its frame, it can be enlightening to understand how it's made and what that means for you, the cyclist. The goal of any frame is to offer extraordinary strength with minimum weight. However, frame strength is determined by many factors. Whether the frame is aluminum or chromoly is only part of the equation. The way those materials are used is equally important.
Which Material Is Right for You?
It depends. Many factors—your style of riding, your weight, your sense of adventure—all play a role in your choice of material. The following paragraphs explain the different types of material commonly used on bikes. A few bikes out there are made of exotic metals, but that's another discussion entirely.
Carbon (High-Tensile) Steel
Steel is the most commonly used material in bike frames. Carbon or high-tensile steel is a good, strong, long-lasting steel, but it isn't as light as its more high-tech brother, the steel known as chromoly.
Chromoly (Chrome Molybdenum) Steel
A workhorse of the industry, chromoly is a light, strong steel. When it is butted and shaped to take off excess weight, it can deliver a fairly light frame that will last through years of hard use. Chromoly is responsive and offers good flex while maintaining its form.
Having come a long way from the oversized tubes of old, aluminum is now less expensive and very widely used on today's bikes. It's light, strong and stiff. With proper design it can give a solid ride for climbing, or lively handling in tight situations.
Lighter than steel but just as strong, this more-expensive metal is found on high-end road or cross-country mountain bikes. It flexes so well while maintaining its shape that some very high-end bikes use the metal itself as a shock absorber.
Take a bundle of parallel continuous fibers and bind them together with glue. This creates a ply. Several plies are made up to form a laminate (just like plywood). And the laminate, if designed right, can be very tough. It's also light. So why aren't all bikes made out of carbon fiber? It tends to be brittle. The fact that metal can bend and regain its shape is what makes it last. Because of this, carbon fiber bikes are built even stronger than needed.
What to Look For in a Frame
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?
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.
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.
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.
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.
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.
What Other Factors Should I Consider?
How Long Are You Going to Keep Your Bike?
Steel will oxidize (rust) faster than aluminum. However, steel can take more stress over the long run than aluminum. Which is better? If you live in a wet climate, aluminum may be the better choice. Dry climate? You can do well with steel.
How Much Do You Weigh?
If you go much above the 170-pound mark, you'll want a bike with a higher strength. This may take an added pound of frame weight to achieve, but it's worth it in the long run. Also, steel and titanium are generally better for bigger riders due to something called elongation. They can flex more without breaking.
Is Money a Factor?
Though aluminum and titanium have come down in price, steel is still the least expensive metal. But since most cyclists like the lighter weight of aluminum or carbon fiber, manufacturers are creating bikes that are aluminum or carbon fiber and more affordable. Titanium? Still expensive.