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Frame Materials

Weight, strength, safety, and frame materials


BrazingIt's been said that most people care about three things: sex, money, and food; and that cyclists care about four more things: price, number of gears, peer approval, and weight. This is about weight, and the related topic of frame material.

Weight has been overemphasized by the media, and manufacturers have responded with frames and components that live on the brink of failure. If you haven't heard of them or seen the photos of snapped forks and handlebars, you're just out of the loop, because they're out there. Manufacturers are building to weight, and as a result, the number of failures and recalls in "high-end" frames and forks and components has skyrocketed. That's not safe. 

When you talk about how safe a bike or part is, you're really talking about how it responds to different forces and influences that wreck it, how quickly it gets wrecked, and how much warning it gives you before it gets wrecked to the point where it'll cause you to crash. In the big picture, this is all a matter of strength. But there are different kinds of strength, and they all matter.

Impact strength is how much impact a part can take before it fails. It matters, but any impact that tests a bike or part's impact strength will send you to the ground long before the part is damaged, and once you're down, who cares about the part?

Fatigue strength is how a material responds to repeated stress, usually flex. It is important in a bicycle because there's always flex happening, as you push on the pedals and pull on the bars. Aluminum has the worst fatigue strength of any common frame material, but aluminum bicycle frames can be made with oversized tubing to eliminate most of the flex. High quality steel has the best fatigue strength of any frame material. It has a stress threshold below which it can survive an infinite number of flexes.

Toughness is about cracks; namely, a material's ability to stop a nick from growing into a crack, and a crack from growing into a failure. Steel creams aluminum, titanium, and carbon fiber in this area, too. That's why hammers, nails, rebar, and bridges are steel.

Tensile strength (UTS, for Ultimate Tensile Strength) is the most commonly cited gauge of strength. It is measured by playing tug-o-war with a skinny cylinder of material (with specific dimensions) and measuring the force required to rip it in half.

Tensile strength of tubing is important, but its importance is greatly, and I mean hugelyexagerrated in discussions about bicycle frame strength. Bicycle frames don't fail in tension. Frames and parts almost always fail due to fatigue, notch-sensitivity, lack of toughness, or impact. Window glass has a tensile strength five times higher than chrome-moly steel, but tension isn't what kills it.

Tensile strength affects safety, but any of the materials used in bicycles has plenty of it. There are more important factors than tensile strength.

Failure mode is one of those "more important factors." It is how suddenly failure occurs after the first crack, hole, or gouge. Nobody talks about it, but it's really, really, reallyimportant.

Materials that fail fast are said to fail "catastrophically." Of all materials used in bikes, none fails more catastrophically than carbon fiber, and none fails more slowly than steel. You want your bike stuff to respond to trauma by bending and denting, not shattering and snapping. Metals tend to do that. And once that's covered, you want plenty of time and lots of warning between the onset of failure (a crack, for instance) and total material separation. Steel is the first place winner here, too. Reparability is desirable, too, and steel wins that one, also.

Another quality to consider in a frame material is how well it ages; the degree to which it stays strong as it gets old, and environmental stress in the form of ozone, ultraviolet radiation, salt air, and temperature extremes affect it.

In this regard, metals are far superior to rubber, plastic, and carbon fiber. The resins used to hold the layers of carbon fiber together degrade with exposure to ultraviolet.

Steel critics cite rust as a weak point of steel, and even the word rust conjurs up images of broken chains and hole-y buckets. But rust and corrosion ("rust" being steel-specific) are protective responses to environmental conditions, and once a layer has built up, they become a protective layer against further corrosion. Super thin tubes are more vulnerable to rust than are thicker ones, and that's a good argument for avoiding 0.35mm walls in steel bicycle frame tubes. But if rust were the tube-killer the carb-al-ti folks would have you believe, there wouldn't be hundreds of thousands of 30-year old and routinely neglected steel-framed bicycle still roaming the planet. Even so, it is best to prevent corrosion in super-thin-walled steel tubes by spraying them with any number of anti-rust sprays readily available. FrameSaver is one. Boeshield T9 is another. LPS makes some good anti-rust sprays. If you like the old ways, use linseed oil. The point is, rust is a problem only in your head.

Defect tolerance is the least-talked about material quality, and it's risky to talk about because it's easy for a reader to get the idea that the writer must be tolerant of defects. But it is a real category, a real quality, a real thing that rocket scientists care about and bicycle makers ought to.

Defect tolerance is the ability of a material to be safe even when defective.

It matters because perfect quality control is impossible, no matter how white-coated engineers in sterile rooms you have monitoring production. Some bug or booger or bubble will work its way in, and then what? 

Then you want a material that maintains its integrity.

The least defect-tolerant material used in bikes is carbon fiber. (And most carbon fiber comes from China, as a matter of fact.) The most defect-tolerant is steel.

Shock absorption, vibration damping, and comfort get talked about a lot in bicycle media, and they warrant some mention somewhere in this catalogue, and this is the only page that makes sense for it.

Shocks get absorbed by movement, either compression or deflection or a combination, but there's a notion among bike riders that they can get absorbed without movement. Folks who ought to know better claim that carbon forks offer a "plush, shock-absorbing" ride, even though they don't compress at all, and are designed not to deflect enough to make a difference in comfort. Yet, the media and so many "experts" who work in retail shops and at the manufacturing level continue to praise their shock-absorbing qualities. If you want shock absorption, ride higher volume tires at lower pressure, and don't grip the handlebar as though though it's a jackhammer. Suspension forks absorb shocks, but only by moving. 

Vibration damping is another largely misunderstood quality whose real benefits may or may not be detectable by a human riding a bike. It is how long a material continues to vibrate once it has started to vibrate; presumably, when it hits a bump. Wind chimes don't dampen vibration well, which is why they bother your neighbors. Vibration is high-frequency flex. How a material flexes is determined by its Young's modulus, and how something like a bicycle frame or fork flexes is determined by its material and physical dimensions. But a bicycle isn't a dangling metal cylinder; it's a composite of metal, rubber, air (in the tires) and other materials, and there's a body on it. The water is so muddied that there's no way for a human to accurately isolate "vibration damping" from all the other things going on during a bike ride, and even more, to discriminate the quality of damping, and attribute it to the proper source.

Physical comfort is how comfortable you are on the bike, and although that's our favorite topic of all time, we'll sum it up in these few words: Raise the bars to take weight off your hands. Scoot your seat back an inch or more farther than the experts tell you to. Ride bigger tires at lower air pressure. Relax your body and grip. Ahhhh.


Silver lug closeupPsychological comfortjust means that if the bike fits you perfectly but you have doubts about it or issues with it, you won't be comfortable. Some women who grew up riding "girls' bikes" without a top tube can't get comfortable on a bike with a "diamond-style" frame. They feel trapped on it, and stressed. Young male whippets who are used to curling their bodies over a the smallest possible frame (say, a 56cm for a 6-footer), aren't "psychologically comfortable" on a 62cm, only because they know it's a 62.

We like steel bikes best, and lugged ones most. But the material and method mean nothing unless the bike is not only well-designed, but well-designed for you, and right for your kind of riding.

We like road bikes that allow you to fit a tire at least 32mm wide. We think all bikes should be able to fit fenders, short (sub-41cm) chainstays are dumb, and modern road gearing is too high.

Brazed-on front derailleurs are okay only if they're positioned right for the smallest chainrings you'll want to ride, and they usually aren't. So, think twice before buying a frame with a brazed-on front derailleur. Find out how small a chainring you can use.

Frame weight is 1/4 as important as bike weight, and bike weight is 1/10 as important as body weight. If you want to go fast, ride harder and more often. We're not suggesting you should want to go fast unless you're racing, but if you do, that's how you'll achieve it. You can buy a Lightspeed, but it won't make you light or give you speed.

It has been said many times that engineering matters more than materials. But it's not as simple as that. Yes, you can make a strong bridge out of styrofoam, or bike frame out of almost anything, but certain materials have inherent qualities (and don't forget defect tolerance) that plain make them more desirable. Steel has been around a long time. People who don't even ride bikes, but cock an ear when they hear tech talk and Tour de France and Lance and all...they "know" that steel is old and heavy and not good anymore. They're wrong as can be. As a strong, safe, repairable, beautiful, practical, and rugged frame material, it is still the best.