Comparing Bike Frame Materials
Feb 24, 2019The most defining component of any bike is the frame. While the frame geometry is the biggest determining factor in what the bike is best suited for, the frame material comes in a comfortable second place. Even with similar geometry, a bike built of one material will have different abilities and ride qualities compared to a similar bike made from a different material.
Which material is best? As in many things, the correct answer is “it depends”. There are no magic bullets when it comes to cycling; if something were always better for every application, it would be the only option available.
Worth noting: none of these comparisons are absolute. How a frame is made matters just as much as, or more than, what material it’s made from. Nevertheless, frame material still has a significant effect on a bike. For example, It’s nearly impossible to make a steel frame lighter than one made of carbon fiber, or an aluminum frame as strong as titanium.
In this article, we’ll examine and compare the four most common frame materials: steel, aluminum, carbon fiber, and titanium.
Pros: strong, easy to repair, compliant, cheap
Cons: heavy, limited range of tube shaping
First, a clarification: we’ll be talking about chromoly steel. Hi-ten (high-tensile) steel is a cheaper material used on the kinds of bikes you find at discount stores. If care enough about your bike to read an article comparing frame material, don’t get this.
Compared to hi-ten steel, chromoly is much stronger, which allows frame builders to use less of it and still make a frame of considerable strength. Hi-ten steel isn’t much heavier by volume, but frames made from hi-ten steel are heavier due to needing more material to achieve the same strength, and even then, chromoly frames are typically stronger.
Of the four materials examined here, steel has the longest history and is still in widespread use. For many applications, chief among them touring, cyclists prefer steel to any other material due to its high strength, ease of repair, smooth ride quality, and low price point.
However, steel (even chromoly) is the heaviest frame material, isn’t particularly stiff, and has limitations on the ability to shape tubing to give different qualities to different parts of the frame. Butted tubing refers to tubing that has a different thickness inside the tube at different points (the outer diameter will typically remain constant), which can make the frame more flexible, whereas straight gauge tubing will be stiffer, but beyond that, steel has few options when it comes to changing the ride quality.
That said, the ride quality of a steel frame is usually considered one of its advantages, especially for long-distance touring. Steel is strong-but-flexible material, which gives it the ability to soak up vibrations on rougher surfaces. The result is a ride that’s frequently describes as plush, smooth, and comfortable. This isn’t great if you’re trying to win a race, but if you are, you’re probably not riding a heavy steel frame anyway.
Pros: Lightweight, stiff, relatively inexpensive
Cons: Harsh ride, limited durability
Aluminum first started becoming popular in the 1970s and 1980s. Previously, almost all bikes were made of steel. Aluminum frames offered the advantage of being lighter than steel frames by about a pound.
While aluminum has a higher strength-to-weight ratio than steel, it has a lower strength-to-volume ratio, which means more aluminum must be used to make a frame as strong as steel. In addition, aluminum frames will generally use much larger tubing to make the frame stiffer and prevent the softer metal from bending.
In the end, aluminum frames are generally stiffer than steel ones. This is great if you’re trying to win a sprint, but many cyclists find the stiffness of an aluminum frame “harsh”, as opposed to the plush, smooth ride steel provides.
Despite high stiffness, aluminum has low strength and no fatigue limit, meaning the strength of the frame begins to slowly deteriorate the moment it bears any weight. While a quality aluminum frame can last a decade or more, the ride quality will change over time, and an aluminum frame is unlikely to last a lifetime. This also makes aluminum frames an unlikely choice for bikes that will carry heavy loads, but some manufacturers have made burly aluminum frames that hold up well for touring.
Aluminum remains a popular choice for both road and mountain bike riding due to its light weight, stiffness, and relatively low expense. It may not be the best choice for all-day comfort, life-long longevity, or especially heavy loads, though some aluminum frames are built with these goals in mind.
Pros: lightest weight, aerodynamic, customizable for different uses
Cons: expensive, impossible to repair
The material most often used by professional cyclists, carbon fiber is renowned for its especially light weight and, in many cases, aerodynamic profile. As opposed to other materials, carbon fiber is (surprise!) a fibrous material, made from weaving microscopic fibers into a material that’s both stronger and stiffer than steel.
In addition to these properties, carbon fibers can be woven into different patterns that promote either stiffness or compliability, as desired, in different parts of the frame, and even in different directions (eg. stiff torsionally, but flexible laterally). The strength and stiffness of a carbon tube is determined not only by the architectural shape of the tube, but by the weave pattern of the fibers on a microscopic level. As such, carbon tubes can be made into almost any shape imaginable without sacrificing strength, allowing for the frame to be made into especially aerodynamic shapes, if desired.
Due to the customizability of carbon fiber, down to the microscopic level, carbon frames can be made as strong, stiff, or compliant as desired. While steel is typically the strongest material, a carbon frame can be made equally strong. Similarly, a carbon frame can be made as stiff as, or stiffer than, an aluminum frame. Most often, carbon frames attempt to strike a balance between lightweight, strength, stiffness, compliance, and aerodynamics, but frames that specialize in one of these fields are also common.
Carbon fiber is an expensive material, and while it can be made very strong, it’s difficult to fully repair. While many manufacturers advise against it, a cracked carbon frame can be repaired, but you’ll have to mail it to a specialty repair shop, costing you heavily both in terms of money and time.
Pros: Strength, smooth ride, light weight, no weaknesses
Cons: Expensive!
Sometimes considered the crème de la crème of frame materials, titanium has almost no drawbacks.
Professional racing cyclists don’t use titanium, since carbon fiber can be made lighter and more aerodynamic, and therefore faster. Titanium isn’t far behind in any category though, and has all the strength and reliability of steel. A quality titanium frame will last a lifetime without ever feeling outdated.
While titanium can’t be customized for specific applications like carbon fiber, it’s common to see butted tubing (like steel) or oversize tubing (like aluminum) used to give the frame specific properties for specific purposes. However, titanium’s greatest advantage is its lack of disadvantages. A titanium frame is versatile enough to win a criterium, then strap on 80 pounds of gear and ride from coast-to-coast.
All this comes at a price: Titanium frames aren’t cheap! Only a select few manufacturers make titanium frames at all, and essentially every bike with a titanium frame will be high-end or custom.
Ratings
OK, so carbon is light, steel is strong, but how much lighter and stronger are they? Let’s take a quantitative approach.
For our purposes, we’ll grade every frame material on a 0-10 scale based on criteria: strength, weight, stiffness/compliance, aerodynamics, and reparability. In each category, the best material will get a 10 (even if it’s still not perfect). Every other material will get a score proportionately accurate to a 10.
For example, if steel is the strongest and titanium is 90% as strong, steel will get a score of 10 and titanium a 9. If carbon is the lightest and steel weighs twice as much, carbon will get a 10 and steel will get a 5.
While no two frames are identical, we’re assuming a “typical” frame of middling quality for each material. Let the throwdown begin!
Strength
First, a clarification: strength and stiffness are two different properties. Stiffness means how much force it takes to bend a material at all, even if it returns to its normal shape. Strength means how much effort it takes to bend a material permanently.
In particular, steel is known for high strength, but low stiffness, meaning it can be easily flexed, but it’s hard to permanently bend. Aluminum, by contrast, is the opposite. It’s hard to bend at all, but once you do, it often stays bent.
Strength is the property that’ll most determine the longevity of your bike. If you want to keep your bike a long time, invest in a strong frame. Strength is also important to people who intend to carry a heavy load on their bike, whether that’s camping gear for a long trip, or simply themselves.
It’s worth noting that there are two main types of strength - impact and fatigue. A material with a high fatigue strength will stand up to lots of “ordinary” use before failing, while high impact strength means a bike can take stronger catastrophic damage before failure. These aren’t always correlated; for example, carbon has a high fatigue limit but lower impact strength, while aluminum has reasonably high impact strength but essentially no fatigue limit.
These scores are a subjective attempt to combine both types of strength. A frame of any material can be beefed up until it’s stronger than a typical frame of any other material. Take these scores as a general rule, not an unbreakable law.
Steel: 10
Titanium: 9
Carbon: 7
Aluminum: 5
Weight
This one’s fairly straightforward. How much does it weigh?
Like all of these categories, we’re going with a “typical” median value. Some steel frames can be surprisingly light, and other frames might be made bulkier than normal, whether for the purpose of improving strength, or perhaps simply in the hands of a less skilled frame builder.
The frame of a bike is only a small portion of its weight, and while one material might be twice as heavy as another, it’ll still only add 1-2 kg, maximum, to the weight of the bike. Steel frames, the heaviest material, weigh less than twice as much as comparably strong carbon frames.
It’s also common to see the main frame and the fork of a bike made out of two different materials. In particular, the fork of many bikes is carbon, regardless of the material of the rest of the frame.
Carbon: 10
Aluminum: 8
Titanium: 8
Steel: 6
Stiffness/Compliance
Stiffness and compliance are inverses of one another, like light and heavy, hot and cold. Stiffness is great if you’re trying to win a sprint, but compliance is much better for all-day comfort.
Aluminum and some carbon frames are stiff enough to minimize any energy loss in the transfer from pedals to tires, but are so poor at damping vibration you could probably tell if a coin under your tire was heads or tails. Steel, titanium, and some carbon frames are excellent at flexing, just barely, under stress and can soak up all the micro-bumps on a rough stretch of pavement, but the tradeoff is energy loss when hammering on the pedals.
For that reason, our score here is a subjective one. Which frame material is the best of both worlds? Because of its high adaptability, carbon can be made stiff where it counts (like the down tube and chain stays), but compliant where it matters most (like the seat tube and seat stays). Other materials can attempt to do the same by changing the diameter, shape, or butting (progressive thickness) of the tubes, but there’s only so much you can do with a continuous material like metal.
Since this is primarily a site for touring bikes, we’re favoring compliance over stiffness, but we’re giving the highest ratings to the materials that allow for the most fine-tuning.
Carbon: 10
Titanium: 9
Steel: 8
Aluminum: 6
Aerodynamics
Increasingly, it’s understood that aerodynamics is a bigger factor than weight when it comes to the speed and efficiency of a bike. Similar to weight, the frame itself is a small factor in the total aerodynamic profile of a bike; the rider plays a much bigger role. In the case of touring bikes, the size and placement of panniers also plays a much bigger role in aerodynamics than the frame itself.
Carbon is the winner here, since it can be easily molded into different shapes without greatly sacrificing strength. Since titanium and aluminum can make up for lower strength by adding more material without adding too much weight, they come in ahead of steel.
Carbon: 10
Titanium: 8
Aluminum: 7
Steel: 6
Reparability
Frame failures are rare, but they can happen, usually as a result of a crash. An ounce of prevention is worth a pound of cure (see Strength), but it’s nice to know that if something happens to your bike, you can get it fixed and continue on your way.
Steel is by far the easiest to repair. Almost any mechanic with a welding torch can get a steel frame back to rideable quality. Titanium and aluminum need more specialized tools to weld, and you’ll likely need to send your bike to a specialty mechanic, but it can be done.
A metal frame is more likely to bend, rather than break entirely. And that means it can be bent back. A frame that’s been noticeably bent should be replaced, but bending the frame back into place might be good enough to ride on for a few days. The lower the fatigue limit (like aluminum), the less safe this is, and the more likely the frame is to bend again. Primarily for that reason, we’re ranking titanium slightly ahead of aluminum, even though they’re equally difficult to permanently fix.
Carbon, by contrast, doesn’t bend; it cracks. More familiarity with the material has resulted in carbon frames getting much stronger over the past several years, but there’s no changing the fact that once a carbon frame cracks, it shouldn’t be ridden until fixed. Also, while a cracked frame can be repaired, carbon remains effectively irreparable in the field.
We're cutting all the scores in half for this one. While it's important, it's something that rarely, if ever, comes up.
Steel: 5
Titanium: 3
Aluminum: 2
Carbon: 1
Final Score
Carbon: 38
Titanium: 37
Steel: 35
Aluminum: 28
Of course, these scores aren’t meant to be taken too seriously. Half of the categories were fairly subjective, and one rider might consider one category much more important than another, whereas we gave equal weight to most categories. There’s no bike, nor frame material, that’s best for everyone.
We also didn't take cost into account for the purpose of this score system. It shouldn't be surprising to see the most expensive materials at the top, but it's worth pointing out the most expensive wasn't the winner, and the cheapest material scored nearly as high as anything else.
Considering the slant we at PackJournal have for touring, it may be surprising to see carbon scoring highest. Again, we threw out cost as a factor, and that alone would most likely move steel to first place on the list. “Steel is real” still rings true, and often gives you the best bang for your buck.
That said, carbon utility bikes have come a long way. Once thought of as delicate and only suitable for barely supporting a 60 kg professional cyclist, there are now carbon frames capable of standing up to all the punishment of a ‘round-the-world tour. Throw in all the other benefits that come with a carbon frame, and it proves to be one of the best materials available, provided it’s optimized for its intended use. There aren’t many complete touring bikes with carbon frames on the market, but it’s a growing trend. Some of the best touring bikes available are carbon frames, and it’ll be exciting to see what the future holds for this material.
Unsurprisingly, titanium scores highly, but one might expect titanium to be the runaway winner. Jack of all trades, master of none, titanium’s greatest advantage is its lack of disadvantages, but its greatest disadvantage is its lack of advantages. For any specific purpose, there’s always something better, but for all purposes, it’s hard to beat titanium (except for the price).
Aluminum, while low on our list, shouldn’t be thrown out entirely. Worth saying again, how a frame is made matters as much as, or more than, its material. Aluminum is still an especially popular material for mountain bikes, where the cushion provided by suspension and wide tires essentially mitigates aluminum’s greatest weakness: its “harsh” ride and low fatigue limit. On a mountain bike, the suspension and tires will take the punishment instead of the frame.
In conclusion, think about what’s the most important quality for your bike to have. Speed? Unbreakable strength? Comfort? Longevity? What’s your budget? Answering those questions will help you pick a frame material that’s best for you.
