|To stay young requires unceasing cultivation|
|of the ability to unlearn old falsehoods.|
|--Robert A. Heinlein|
All of the above statements are equally false.
- Aluminum frames have a harsh ride?
- Titanium frames are soft and whippy?
- Steel frames go soft with age, but they have a nicer ride quality?
- England's Queen Elizabeth is a kingpin of the international drug trade?
There is an amazing amount of folkloric "conventional wisdom" about bicycle frames and materials that is widely disseminated, but has no basis in fact.
The reality is that you can make a good bike frame out of any of these metals, with any desired riding qualities, by selecting appropriate tubing diamters, wall thicknesses and frame geometry.
Strength and stiffness are different properties that are often confused with one another. It is important to understand the difference, if you want to understand differences in frame materials.
Imagine you clamp one end of a metal bar in a vise, and you hang a weight on the free end, causing the bar to flex temporarily. When you remove the weight, the bar snaps back to its original shape.
Different materials will flex different amounts for the same amount of force applied. This is stiffness.
Now imagine hanging a heavier weight on the bar, so heavy that it becomes premanently deformed. When you remove this weight, the bar does not snap back all the way to its original shape, but remains bent to some extent. When the metal changes shape permanently, it is said to "yield."
Different materials can withstand different amounts of force before yielding. This property is strength.
Stiffness affects the riding qualities of a bike frame, since a frame suffers no permanent deformation in normal riding.
Stiffness is determined by a property of the material called "elastic modulus" Elastic modulus is essentially independent of the quality or alloying elements in a given metal. All kinds of steel, for instance have basically the same elastic modulus.
Strength relates to the crash-worthiness or general durability of a frame, but has no effect on the riding properties.
Strength is determined by a property of the material called "yield strength."
Yield strength is very much affected by the quality, heat treatment and alloying elements used in a particular brand/model of tubing.
In addition to the strength and stiffness, there's also the question of how heavy a given volume of the material is. This is called "specific gravity."
Like stiffness, the specific gravity of a given metal is not significantly affected by the addition of different alloying elements. Although your bike may have a sticker saying "Lite Steel (TM)," in fact, all steel is equally heavy.
Here are some properties of the three common frame metals:
Note that the modulus (stiffness) and specific gravity (weight) are pretty much independent of the quality, heat treatment, or alloying agents of the materials. For instance, all steels, from the "gas-pipe" used in department-store bikes to the exotic alloys used in multi-thousand dollar bikes have a modulus of 30, and a specific gravity of 490.
Material Modulus Yield Point Specific Gravity Aluminum 10-11 11-59 (4-22 annealed.) 168.5 Steel 30 46-162 490 Titanium 15-16.5 40-120 280
Anybody that tells you that a particular brand of steel (or aluminum, or titanium) is "lighter" or "stiffer" than another brand or model is blowing smoke.
There are, however, real differences in yield strength among different qualities of tubing.
This modulus value shows that if you were to build identical frames from the 3 materials, using the same tubing diameters and wall thicknesses, the aluminum frame would be only 1/3 as stiff as a steel one, and the titanium frame only half as stiff.
The yield values show that the aluminum frame would be very much weaker, in the sense of being more easily damaged than either the steel or titanium frames.
The specific gravity values show that the aluminum frame would only weigh 1/3 what the steel frame weighs, while the titanium frame would be roughly half the weight of the steel one.
These generalities, however, are basically meaningless, because you wouldn't build frames out of the three different metals to the same tubing dimensions!
Real bicycles take the nature of the material into account in selecting the diameter and wall-thickness of each piece of tubing that goes to make up the frame. Stiffness is mainly related to the tubing diameter. Strength is mainly related to the wall thickness, though diameter also enters into it. Weight is affected both by diameter and wall thickness.
A frame manufacturer can make trade-offs by selecting different tube diameters/wall thicknesses, allowing a frame to be made stiffer, or stronger, or lighter.
Look at the chart again. You'll see that identical steel vs titanium frames would be about equal in strength, but that the titanium frame would be about half the weight and half the stiffness.
Such a frame would likely have a whippy feel due to the reduced stiffness, especially in loaded touring applications. To compensate, builders of titanium frames use somewhat larger diameter tubes to bring the stiffness more into line with what riders like. This tends to increase the weight a bit, but by making the walls of the larger tubes a bit thinner, they can compensate to some extent, and come up with a frame that is still lighter than a normal steel frame.
The situation with aluminum is even more pronounced. the "identical" aluminum frame would be 1/3 as stiff as steel, roughly half as strong, and 1/3 the weight. Such a frame would be quite unsatisfactory. That's why aluminum frames generally have noticeably larger tubing diameters and thicker-walled tubing. This generally results with frames of quite adequate stiffness, still lighter than comparable steel ones.
The advantages of larger tubing diameter can, theoretically be applied to steel construction, but there's a practical limit. You could build a steel frame with 2 inch diameter tubing, and it would be stiffer than anything available--indeed, stiffer than anybody needs. By making the walls of the tubes thin enough, you could make it very, very light as well.
Why don't manufacturers do this? Two reasons.
The thinner the walls of the tubing, the harder it is to make a good joint. This is the reason for butted tubing, where the walls get thicker near the ends, where the tubes come together with other tubes.
In addition, if the walls get too thin, the tubes become too easy to dent, and connection points for bottle cages, cable stops, shifter bosses and the like have inadequate support.
Frame stiffness (or the lack of it) doesn't have as much effect on ride quality as many people would lead you to believe. Let's look at it from a couple of different directions:
Torsional/lateral stiffnessThis is mainly related to the stresses generated by the forces you create from pedaling. Any frame will flex around the bottom bracket a bit in response to pedaling loads. This flex can be felt, and many riders assume that it is consuming (wasting) pedaling effort. Actually, that's not the case, because the metals used in bicycle frames are very efficient springs, and the energy gets returned at the end of the power stroke, so little or nothing is actually lost. While there is no actual loss of efficiency from a "flexy" frame, most cyclists find the sensation unpleasant, and prefer a frame that is fairly stiff in the drive-train area. This is more of a concern for larger, heavier riders, and for those who make a habit of standing up to pedal.
Another area where lateral stiffness can be an issue particularly to the touring cyclist is the rear triangle, when there's a touring load on the rear rack. An frame that is too flexy in this area will feel "whippy" and may be prone to dangerous oscillations at high speeds. Most of this flex is usually in the luggage rack itself, but there can be enough flex in the seat stays to aggravate this condition.
Vertical stiffness(Since this article deals with frames, the issue at hand is road shock transmitted from the rear tire to the saddle. Ride qualities experienced at the handlebars are to some extent determined by the fork, as well as geometry, and flex in other bolt-on parts, but are un-related to the choice of frame material.)
Much of the commonplace B.S. that is talked about different frame materials relates to imagined differences in vertical stiffness. It will be said that one frame has a comfy ride and absorbs road shocks, while another is alleged to be harsh and make you feel every crack in the pavement. Virtually all of these "differences" are either the imaginary result of the placebo effect, or are caused by something other than the frame material choice.
Bumps are transmitted from the rear tire patch, through the tire, the wheel, the seatstays, the seatpost, the saddle frame, and the saddle top. All these parts deflect to a greater or lesser extent when you hit a bump, but not to an equal extent.
The greatest degree of flex is in the tire, probably the second greatest is the saddle itself. If you have a lot of seatpost sticking out of a small frame, there's noticeable flex in the seatpost. The shock absorbent qualities of good quality wheels are negligible...and now we get to the seat stays. The seat stays (the only part of this system that is actually part of the frame) are loaded in pure, in-line compression. In this direction, they are so stiff, even the lightest and thinnest ones, that they can contribute nothing worth mentioning to shock absorbency.
The only place that frame flex can be reasonably supposed to contribute anything at all to "suspension" is that, if you have a long exposed seatpost that doesn't run too deep into the seat tube, the bottom end of the seatpost may cause the top of the seat tube to bow very slightly. Even this compliance is only a fraction of the flex of the exposed length of the seatpost.
The frame feature that does have some effect on road shock at the rump is the design of the rear triangle. This is one of the reasons that touring bikes tend to have long chainstays--it puts the rider forward of the rear wheel. Short chainstays give a harsh ride for the same reason that you bounce more in the back of a bus than in the middle...if you're right on top of the wheel, all of the jolt goes straight up.
Where Comfort Comes FromIf you're looking for a comfortable ride, it is a mistake to focus on the particular material used to build the frame. There are differences in comfort among different bikes, but they are mainly caused by:
- Tire choice. Wider, softer tires make more difference to ride comfort than anything to do with the frame. Unfortunately, many newer sport bikes are poorly designed when it comes to tire clearance. For the last decade or more there has been a fad to build frames with very tight tire clearance, although there is no performance advantage whatsoever to such a design. Such bikes cannot accept anything but super skinny tires, and, as a result, there's no way they can ever be really comfortable. See my Article on Tires
- Saddle choice. See my Article on Saddles.
- Frame geometry. Generally, frames with longer chain stays, and less vertical seat-tube and head-tube angles are more comfortable. This doesn't make them any slower, but may reduce maneuverability (also known as twitchiness.)
- Rider positioning. See my Article on Pain and Cycling
Carbon fiber is an increasingly popular frame material, but it is fundamentally different from metal tubing as a way to construct frames. Because of the fibrous nature of this material, it has a much more pronounced "grain" than metal does. A well-designed carbon fiber frame can have the fabric aligned in such a way as to provide maximum strength in the directions of maximum stress.
Unfortunately, in bicycle applications, carbon fiber is not a fully mature technology, as tubular-construction metal frames are. Bicycles are subjected to a very wide range of different stresses from many different directions. Even with computer modeling, the loads can't be entirely predicted. Carbon fiber has great potential, but contemporary carbon fiber frames have not demonstrated the level of reliability and durability that are desired for heavy-duty touring use. In particular, a weak point tends to be the areas where metal fitments, such as fork ends, bottom bracket shells, headsets, etc connect to the carbon frame. These areas can be weakened by corrosion over time, and lead to failure.
In geometry, there's nothing as strong as a triangle. Diamond-frame bikes consist basically of two triangles. The elegance and simplicity of this design is very hard to improve upon. Billions of diamond-frame bikes have been made from tubing for over a century, and during that time, hundreds of thousands of very smart people have spent billions of hours riding along and thinking about ways to fine-tune the performance of their bikes. The tubular diamond frame has been fine tuned by an evolutionary process to the point where it is very close to perfection, given the basic design and materials. I often commute on a Mead Ranger frame built in 1916. It's a tad heavier than a more modern frame, but its general riding qualities are as nice as any bike I own.
If there is to be any major improvement in frame design, it must come either from a completely different type of construction process, such as carbon fiber, or cast magnesium; or a completely different type of design, such as a recumbent.
Any of these materials is quite sutiable for short to medium touring in industrialized countries. Titanium, while costly, is generally the most durable material choice, but aluminum and steel are excellent. Nobody's making carbon fiber touring bikes as far as I know, yet.
For extended travel in less-developed areas, steel is probably still the best choice, because in the event of damage, repairs can be made by anybody with a torch and brazing/welding know-how.
For further reading on this topic, see also Damon Rinard's frame tests at: http://sheldonbrown.com/rinard_frametest.html
|Articles by Sheldon Brown and others|