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Frame Materials for the TouringCyclist

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by Sheldon "Totally Tubular" Brownrevised August 2010 by John "Plain Gauge" Allen

To stay young requires unceasing cultivationof the ability to unlearn old falsehoods.

--Robert A. Heinlein

Aluminum Carbon Fiber Fat Tubes Myths Ride Qualities Ringing,Damping, Suspension Serviceability Steel Stiffness Strength

Titanium Weight

Did you know that:Aluminum frames have a harsh ride?

Titanium frames are soft and whippy?

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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?

All of the above statements are equally false.

There is an amazing amount of folkloric "conventional wisdom" about bicycle frames andmaterials 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 desiredriding qualities, by selecting appropriate tubing diameters, wall thicknesses and framegeometry.

Stiffness, Strength and WeightStrength and stiffness are different properties that are often confused with one another. It isimportant 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 itsoriginal shape.

Different materials will flex different amounts for the same amount of force applied. This isstiffness.

Now imagine hanging a heavier weight on the bar, so heavy that it becomes permanentlydeformed. When you remove this weight, the bar does not snap back all the way to its originalshape, but remains bent to some extent. When the metal changes shape permanently, it is saidto "yield."

Different materials can withstand different amounts of force before yielding. This property isstrength.

StiffnessStiffness affects the riding qualities of a bike frame, since a frame suffers no permanentdeformation in normal riding.

Stiffness is determined by a property of the material called "elastic modulus" Elastic modulus isessentially independent of the quality or alloying elements in a given metal. All kinds of steel, forinstance have basically the same elastic modulus.

StrengthStrength relates to the crash-worthiness or general durability of a frame, but has no effect on theriding 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 ina particular brand/model of tubing.

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WeightIn addition to the strength and stiffness, there's also the question of how heavy a given volume ofthe material is. This is called "density."

Like stiffness, the density of a given metal is not significantly affected by the addition of differentalloying elements. Although your bike may have a sticker saying "Lite Steel (TM)," in fact, allsteel is equally heavy.

Here are some properties of the three common frame metals:

[Sheldon was a strong proponent of the Standard International system of measurement over theEnglish system, but inconsistent in the table below. It doesn't matter though, as the table onlymakes comparisons -- John Allen]

Material Modulus (psi) Yield Point Density(lb/ft3)

Aluminum 10 to 11 x 106 11 to 59 x 103psi (4-22 annealed.)

168.5

Steel 30 x 106 46 to 162 x 103 psi 490

Titanium 15 to 16.5 x106 40 to 120 x 103 psi 280

Note that the modulus (stiffness) and density (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 amodulus of 30, and a specific gravity of 490.

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, usingthe same tubing diameters and wall thicknesses, the aluminum frame would be only 1/3 as stiffas 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 ofbeing more easily damaged than either the steel or titanium frames.

The density values show that the aluminum frame would only weigh 1/3 what the steel frameweighs, 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 outof the three different metals to the same tubing dimensions!

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Real bicycles take the nature of the material into account in selecting the diameter and wallthickness of each piece of tubing that goes to make up the frame. Stiffness is mainly related tothe tubing diameter. Strength is mainly related to the wall thickness, though diameter also entersinto it. Weight is affected both by diameter and wall thickness.

A frame manufacturer can make trade-offs by selecting different tube diameters/wallthicknesses, allowing a frame to be made stiffer, or stronger, or lighter.

Steel vs TitaniumLook at the chart again. You'll see that identical steel vs titanium frames would be about equal instrength, 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 loadedtouring applications. To compensate, builders of titanium frames use somewhat larger diametertubes to bring the stiffness more into line with what riders like. This tends to increase the weighta bit, but by making the walls of the larger tubes a bit thinner, they can compensate to someextent, and come up with a frame that is still lighter than a normal steel frame.

Steel vs AluminumThe situation with aluminum is even more pronounced. The "identical" aluminum frame would be1/3 as stiff as steel, roughly half as strong, and 1/3 the weight. Such a frame would be quiteunsatisfactory. That's why aluminum frames generally have noticeably larger tubing diametersand thicker-walled tubing. This generally results with frames of quite adequate stiffness, stilllighter than comparable steel ones.

Large diameter thin-wall tubing.The advantages of larger tubing diameter can, theoretically, be applied to steel construction, butthere's a practical limit. You could build a steel frame with 2-inch diameter tubing, and it wouldbe stiffer than anything available--indeed, stiffer than anybody needs. By making the walls of thetubes 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 onereason for butted tubing, where the walls get thicker near the ends, where the tubes cometogether with other tubes.

In addition, if the walls get too thin, the tubes become too easy to dent, and connectionpoints for bottle cages, cable stops, shifter bosses and the like have inadequate support.

Stiffness and ride qualityFrame stiffness (or the lack of it) doesn't have as much effect on ride quality as many peoplewould lead you to believe. Let's look at it from a couple of different directions:

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Torsional/lateral stiffnessThis is mainly related to the stresses generated by the forces you create frompedaling. Any frame will flex around the bottom bracket a bit in response to pedalingloads. 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 framesare 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 isfairly 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.

Lateral stiffness can be an issue particularly when there's a touring load on the rearrack. A frame that is too flexy in the top tube, head tube and seatstays will feel "whippy"and may be prone to dangerous oscillations at high speeds. Some of this flex is in theluggage rack itself, but there can be enough flex in the frame to aggravate thiscondition.

Vertical stiffness(Since this article deals with frames, the issue at hand is road shock transmitted fromthe rear tire to the saddle. Ride qualities experienced at the handlebars are to a largeextent determined by the fork, as well as geometry, and flex in other bolt-on parts, butare unrelated to the choice of frame material.)

Much of the commonplace B.S. that is talked about different frame materials relates toimagined differences in vertical stiffness. It will be said that one frame has a comfy rideand absorbs road shocks, while another is alleged to be harsh and make you feelevery crack in the pavement. Virtually all of these "differences" are either the imaginaryresult of the placebo effect, or are caused by something other than the frame materialchoice.

Bumps are transmitted from the rear tire patch, through the tire, the wheel, theseatstays, the seatpost, the saddle frame, and the saddle top. All these parts deflect toa 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 saddleitself. If you have a lot of seatpost sticking out of a small frame, there's noticeable flexin the seatpost. The shock-absorbing qualities of good-quality wheels arenegligible...and now we get to the seat stays. The seat stays (the only part of thissystem that is actually part of the frame) are loaded in pure, in-line compression. In thisdirection, they are so stiff, even the lightest and thinnest ones, that they can contributenothing worth mentioning to shock absorbency.

The only place that frame flex can be reasonably supposed to contribute anything at allto "suspension" is that, if you have a long exposed seatpost that doesn't run too deepinto the seat tube, the bottom end of the seatpost may cause the top of the seat tube tobow very slightly. Even this compliance is only a fraction of the flex of the exposedlength of the seatpost.

The frame feature that does have some effect on road shock at the rump is the design

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of the rear triangle. This is one of the reasons that touring bikes tend to have longchainstays -- they put the rider forward of the rear wheel. Short chainstays give a harshride for the same reason that you bounce more in the back of a bus than in themiddle...if you're right on top of the wheel, all of the jolt goes straight up.

Ringing, Damping and Suspension[This section added by John Allen]

All bicycles have resonant frequencies at which they ring (oscillate) in different ways.You can hear a metal tube ring if you tap it with a fingernail, but resonances below theaudible range most affect the feel of a bicycle. For example, the "speed wobble"already mentioned is the frame's ringing in torsion as the head tube tilts relative to therear triangle, excited by the front wheel's turning slightly to the right and left. This isgenerally worse with a taller frame, because the front triangle is less stiff in torsion, andthe resonant frequency is lower. Resonances also can affect the feel when going overbumps. Two frames which are equally stiff may feel different if they resonate at differentfrequencies.

Damping is the tendency of ringing to die out. All metal frames have very low damping-- they ring long enough to produce a clear tone. A carbon-fiber frame will give a dullsound if tapped, because carbon fiber has more damping than metal. This may affectthe feel to some degree, though much less at the low frequencies which affect framefeel.

Rubber, leather and flesh are highly damped -- and so the greatest damping in abicycle/rider system by far is in the tires, the saddle and the rider's body, unless thebicycle has suspension.

A suspension fork or frame is a highly-damped resonant system -- if it weren'tdamped, it would bounce up and down repeatedly after every bump. Suspension,obviously, has a major effect on the feel of a bicycle. Suspension also adds weight,affecting the feel.

Modern bicycle suspensions are mostly rather stiff, intended to protect the rider andbicycle against hard impacts, while minimizing "pogo sticking" due to pedaling out ofthe saddle. Interestingly, recent research published in Bicycle Quarterly magazineshowed that tire choice and tire pressure achieved a much greater difference incomfort than a suspension front fork in a test ride on a bumpy surface! The mainreason is that: the unsprung weight of the small part of the tire that flexes is tiny, whilethat of the wheel and fork is substantial. The rider is not rigidly connected to thebicycle, and so the sprung weight is largely that of the frame and attachedcomponents.

Where Comfort Comes FromIf you're looking for a comfortable ride, it is a mistake to focus on the particular material

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If you're looking for a comfortable ride, it is a mistake to focus on the particular materialused to build the frame. There are differences in comfort among different bikes, butthey are mainly caused by:

Tire choice. Wider, softer tires make more difference to ride comfort thananything to do with the frame. Unfortunately, many newer sport bikes are poorlydesigned when it comes to tire clearance. For the last decade or more therehas been a fad to build frames with very tight tire clearance, although there is noperformance advantage whatsoever to such a design. Such bikes cannotaccept anything but super skinny tires, and, as a result, there's no way they canever be really comfortable. See my Article on Tires.

Saddle choice. See my Article on Saddles.

Frame geometry. Generally, frames with longer chainstays, and less verticalseat-tube and head-tube angles are more comfortable. This doesn't make themany slower, but may reduce maneuverability (also known as twitchiness.)

Rider positioning and technique in riding over bumps. See my Article on Painand Cycling

Suspension, if the bicycle has it. A sprung saddle or suspension seatpost alsocan make a big difference.

Also, don't be obsessed with weight. The pound or so of difference between a cutting-edge ultralight frame and a well-constructed, heavier one (generally a steel frame) isimportant to a racer climbing a mountain pass but makes little difference to a bicycletourist -- especially not when carrying a touring load. The heavier frame may actually bemore comfortable, because it increases the ratio of sprung to unsprung weight, andbecause it also is stiffer. A steel frame with plain-gauge tubing may actually bepreferable for touring, compared with an equally-strong frame with butted tubing,because of the greater torsional stiffness and reduced tendency toward speed wobblewhen carrying a load.

Carbon FiberCarbon fiber is an increasingly popular frame material, but it is fundamentally different frommetal tubing as a way to construct frames. Because of the fibrous nature of this material, it has amuch more pronounced "grain" than metal does. A well-designed carbon fiber frame can havethe fabric aligned in such a way as to provide maximum strength in the directions of maximumstress.

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 stressesfrom many different directions. Even with computer modeling, the loads can't be entirelypredicted. Carbon fiber has great potential, but contemporary carbon fiber frames have notdemonstrated the level of reliability and durability that are desired for heavy-duty touring use. Inparticular, a weak point tends to be the areas where metal fitments, such as fork ends, bottombracket shells, headsets, etc connect to the carbon frame. These areas can be weakened bycorrosion over time, and lead to failure.

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In geometry, there's nothing as strong as a triangle. Diamond-frame bikes consist basically oftwo triangles. The elegance and simplicity of this design is very hard to improve upon. Billions ofdiamond-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 andthinking about ways to fine-tune the performance of their bikes. The tubular diamond frame hasbeen fine-tuned by an evolutionary process to the point where it is very close to perfection, giventhe basic design and materials. I often commute on a Mead Ranger frame built in 1916. It's atad heavier than a more modern frame, but its general riding qualities are as nice as any bike Iown.

If there is to be any major improvement in frame design, it must come either from a completelydifferent type of construction process, such as carbon fiber, or cast magnesium; or a completelydifferent type of design, such as a recumbent.

ServiceabilityAny of these materials is quite suitable for short to medium touring in industrialized countries.Titanium, while costly, is generally the most durable material choice, but aluminum and steel areexcellent. 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 inthe 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

Scott Nicol has a very thorough discussion of frame materials at:http://www.ibiscycles.com/tech/materials_101/

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Last Updated: 01/02/2013 17:40:15 by John Allen

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