Impacting important characteristics such as ride quality and feel, a frame is often the defining feature of your favourite bike. So regardless of whether you’re getting stuck into a dream bike build, considering a new ride or are just a sucker for the technical detail, we’ve put together this handy guide to help you better understand the foundation of your bicycle.
Often referred to as the heart of a bike, a frame can be made from either carbon fiber, aluminum, titanium, steel or a combination of these materials. Each bicycle frame material offers a list of differing characteristics that will affect the cost, comfort, weight, stiffness, strength and durability.
It’s worth noting that it’s often how the chosen material is used by the engineers and manufacturers that matters most, and this is something that each brand will typically play with. Before we jump into the details, it’s worth considering the following factors when deciding what material is best for you;
- Weight: Your bike needs to be strong enough to carry you and any extra luggage you plan on carrying in the form of panniers, racks or mounts. Each material will have different strength properties, fatigue rates and impact resistance but there is always a trade-off between weight and durability. Another aspect of weight to consider is how important having a lightweight bike is to you. For example, If you’re chasing performance advantages than having a light weight bike would be higher on your list.
- The purpose of the bike: If you plan on racing then a stiff, lightweight bike is the ideal choice, narrowing the type of materials to choose from. Conversely, if you are touring or planning a riding adventure that requires long hours in the saddle and the ability to carry items, then durability is the priority and again, narrows the type of material to choose from.
- The area you ride: It may not be apparent that your postcode could influence the type of material your bike is built from, but consider a material like steel that will rust in wet, humid climates. A material like aluminium might be a better choice given the conditions while still providing similar characteristics to steel.
- How long you plan on having the bike: All materials will fatigue over time but some faster than others. Steel will rust if not taken care of but is more durable than aluminium over the long term. Carbon fibre and titanium frames have exceptionally high fatigue rates meaning they will last and last and last.
- Your budget: Budget is often the biggest factor in choosing a bike and consequently its material. As a generalisation, in order of most expensive to least expensive, titanium takes the cake, followed by carbon fibre, aluminium and steel. As always, the trade off between what you are willing to spend for what outcome is the key consideration.
To help provide more clarity on the various bike frame materials available, we’ve summarised each material type, its properties, characteristics and more importantly how they translate into ride quality.
Carbon fiber is undoubtedly the wonder material for modern day bikes regardless of the discipline. Thanks to a high stiffness to weight ratio, ability to customize frame shapes, high resistance to fatigue and the experimentation it affords manufacturers to create whatever product they desire, it’s no surprise that we are currently living in the age of carbon. While some materials are hard to work with and have limited design capacity, carbon fiber is easily mold-able and can be shaped in almost anyway to meet a manufacturer’s’ intended design. The ability to customize shapes and the way the material is used can make bike frames aerodynamic, stiff yet compliant, and lightweight.
Carbon fiber rose to prominence in the early 1990’s after being introduced to professional cycling in the late 1980’s. The material was hailed for its lightweight properties in comparison to steel frames of the time and quickly became the material of choice. Initially the high cost, so-so quality of carbon composites and manufacturing methods were a barrier to widespread use, but all of those have steadily improved over time. Move forward to the present day and carbon fiber can be integrated into practically component on a bike. In addition to its performance benefits, when built correctly it offers superior fatigue resistance results compared to other materials.
Pros of Carbon Fiber: High stiffness to weight ratio, finite control over the use of material, directional, low thermal expansion, corrosion resistance, durability, strength.
Cons of Carbon Fiber: Labor and knowledge to manufacture, failure potential if fractured
When referring to carbon fiber in bikes, it’s important to understand that the end product is actually a composite material made from the carbon fibers themselves and a resin, which acts as a glue or binding substance to hold and reinforce the fibers together. Far thinner than a strand of hair, the thickness of the carbon fibers vary greatly. These individual carbon fiber strands (filaments) are wound together in a ‘tow’, which is then typically woven into fabric-like sheets. The resin is often the weak and inflexible component of the composite and so the goal is to get the tows bonded as close to each other as possible.
Carbon fiber used in bicycles is often unidirectional and so the angle at which it is layered is of the utmost importance. Layering the fiber at specific angles will create strength and stiffness in the direction it is needed. For example, if the forces placed on the frame are opposed to the direction of the layup, it becomes strong and resistant to the force. However, if the fibers are layered at an angle where fibers can not oppose the force, it will flex. The key with layering is to create stiffness and strength where it is needed while providing flex in other locations where required – something the industry often dubs ‘compliance’. Other parts of the frame, or just cheaper carbon frames, may use ‘woven’ carbon-fiber, which provides similar characteristics in all directions that it is laid.
The type of carbon fiber is an important consideration to the final product, but not as important as to how it’s used. Carbon fiber is either stiff or strong, but it can’t be both. As a result, when engineers are designing a frame they need to place stronger fibers to areas of the bike where strength is required, such as the headtube. At this location, the fibers need to be able to absorb high stress to avoid fracturing. Other areas of the frame need to be stiffer to optimize power transfer, like the bottom bracket for example. Here, higher end frames will use high stiffness fibers referred to as ‘High Modulus’ or ‘Ultra High Modulus’. It’s worth noting that there is no standard naming convention for carbon fiber, and so what one brand claims to be ‘Ultra High Modulus’ is unlikely to match what another brand claims.
Some high-end frames, such as Merida’s Big-Nine, claim to be made of approximately 400 individual pieces of carbon fiber. The complex process of choosing the right carbon fiber, reinforced with the best resin, the right layering technique, fiber direction and molding method will ultimately decide the performance of the bike. It’s never a single one of these factors that make for a good carbon fiber frame, and so it’s crucial to keep that in mind when brands are selling on carbon fiber thread counts and the like.
The downfall of carbon fiber is it can crack under excessive stress to an area such as impact from a crash or over tightening bolts. Once the integrity of the carbon has fractured, the material can become extremely fragile and dangerous to use. At this point, it either needs to be repaired or replaced.
How it’s made
Carbon fiber comes on a spool and looks like wool, string or other soft material. The carbon fiber is generally lined up together parallel and along with the resin combined in a unidirectional fashion to create large sheets of carbon fiber. This is then cut to size to be used to make the frame or other components.
The most common way of creating a bike frame from carbon fiber is the use of a mold and bladder. Creating the mold for the frames is a lengthy process requiring extensive research and development to ascertain the elements required to achieve the desired outcome. For example, an aerodynamic bike will require a different mold to an endurance bike, which requires different testing and analysis protocols. Each bike size made requires its own mold too.
Once the molds have been created, the carbon fiber composite is layered into the mold to manufacturers instructions on thickness, overlay, lay up, direction and type. Aside from the actual cutting of the carbon fiber sheets, this whole process requires extensive manual handling to be completed to specification, even the largest manufacturer has an army of workers constructing carbon fiber materials around the clock to keep up with demand.
Any wrinkle in the carbon from poor compaction is a potential point of failure and so a bladder, normally latex is placed inside and expanded to create smooth joins. Foam is also used to exert pressure from the inside of the mold in a similar way. It is then left to ‘cure’ (typically at heat) where it bonds together and hardens. The frame is then removed from the mold, quality checked, cleaned up, painted and becomes the frame you see on the road.
There are other less common methods of creating carbon fiber frames; ‘tube to tube’ construction which doesn’t require a mold, instead individual tubes are wrapped together to create the end product. Another option which can also be the case for aluminium, steel and titanium construction is the use of lugs which sees tubes bonded (glued) into joining parts, the BMC Impec and Colnago C range are great examples of this. And another option is what French-manufacturer Time does, which is weave the carbon around a tube and then inject high-pressure resin in with a special mold.
Each method has its pro’s and con’s, primarily centered around cost and manufacturing expertise.
Here’s a short clip of the procedure Time use when creating their bike frames. This is quite different to how most frames are made
Aluminum bike frames are perhaps the most common in the modern bicycle industry, with the material widely used for various components too. Aluminium as a material isn’t very dense so it can be formed into lightweight structures, making it perfect for bike frames. Aluminium frames are relatively cheap to manufacture, especially compared to carbon fiber frames which are said to take approximately 14x longer to produce.
Pros of aluminum: Cost and ease to manufacture, strength to weight ratio, corrosion resistance.
Cons of aluminum: Difficult to repair, fatigue life
As with carbon fiber, aluminum comes in multiple forms and is always ‘alloyed’ with a small percentage of other metals and minerals added. Beyond material choice, recent developments in manufacturing techniques have seen customization of frame design and subsequent ride quality come leaps and bounds. In addition to manipulating of tube shapes, the thickness of the tube walls themselves can be manipulated to create lightweight and stiff structures. The outcome is referred to as ‘butting’ and essentially thins the center of the tubes for weight reduction while keeping the ends strong for the welding point.
Straight gauge tubes don’t have any varied tube thicknesses providing consistent strength properties, whereas as single, double and triple butted tubes create different thicknesses that allow the frame to handle high-stress points at the end of the tubes without having additional weight through the middle. Single butted tubes will be thicker at one end where strength is only needed at a specific location, the bottom bracket junction of a seat tube for example. Double butted tubes are thicker at both ends, the downtube for example where additional strength is required at the bottom bracket junction and headtube. Triple butted tubes serve the same purpose as double butted tubes but further reduce weight in the center. The additional manufacturing required to achieve varying tube thickness increases the cost so the cheapest frames will be straight gauge, while the highest quality aluminum frames will typically feature triple butting.
As well as butting, aluminium frames can be manipulated by a process known as hydroforming. Hydroforming is a way of shaping metals through the use of a mold and fluid. The aluminum tubing is placed into a mold that is a specific shape. Pumps then inject fluid at extremely high pressures causing the aluminium to press into the mold and take the intended shape. This technique is commonly used to optimize tube shapes for additional stiffness without requiring extra material to be used as reinforcement.
Manipulating frame design can achieve a lightweight bike, that is stiff and compliant at the same time. With a low density, at the same thickness it’s not as strong as steel, but is much lighter and is more resistant to corrosion.
The downside of aluminum is that is will fatigue over time quicker than carbon fiber, steel and titanium. If designed and treated properly, this could potentially be a lifetime, but it’s worth consideration if you are purchasing or building a ‘forever’ bike.
Choosing an aluminum frame is the most cost effective solution for those seeking performance on a budget.
How it’s made
Once the tubes have been manipulated and butted (or not) they need to be joined together, this is commonly done via welding.
TIG welding is the most common process and gives manufacturers a chance to showcase their expertise. TIG welds use the same material as the frame and the aim is to create a smooth, thick, even weld around the diameter of the connecting tubes. Poor workmanship will see the weld be varied with uneven thickness and have gaps around the diameter of the tubes. The top manufacturers in the world now use robotic welders for absolute consistency.
Ah steel, the no fuss workhorse of bike frame materials. Steel was the universal choice of racing bike frames until aluminium appeared in the 1970’s and 1980’s and carbon fiber took over in the 1990’s.
There are two distinct types of steel used in the bicycle industry. The first is high tensile, or otherwise known as ‘Hi-Ten’, this is a cheaper grade steel commonly found in cheaper bikes, especially those from department stores. It offers an incredibility poor strength-weight ratio and so manufacturers will typically use this material in order to hit low price points. By comparison, higher-end steel bikes are likely to use chromoly, or CroMo for short (generally short for chrome molybdenum) steel, which as an alloyed steel offers superior strength properties to Hi-Ten and so can be butted and made thinner/lighter.
Steel is inexpensive, exceptionally durable, highly resistant to fatigue, easily repaired and easy to work with. Unlike carbon fiber and aluminium, damage to a steel frame is typically easily repaired. Somewhat surprisingly despite its strength, steel offers good levels of compliance thanks to its elastic properties. The downside of steel is that it is prone to oxidization (rust) and carries a weight penalty over other materials.
Touring and adventure bikes are commonly made from steel thanks to the exceptional durability and strength on offer. This enables riders to carry large amounts of luggage without compromising the bikes performance. Steel also features heavily on entry level and recreational bikes where weight isn’t such an important factor.
Pros of steel: Cost and ease to manufacture, strength, durability.
Cons of steel: Weight, corrosion resistance.
How it’s made
Joining steel tubes together involves similar welding processes to aluminum frames, but further options exist including ‘Brazing’ and the use of lugs. Brazing is similar to TIG welding but uses a ‘filler’ material that is melted and used to join the tubes. The filler material is commonly silver or brass but could be a myriad of other alloys too. Lugs act as connectors at junction points of the frame, creating a sleeve for the tubes to slide into. Prior to constructing the frame, the ends of the steel tubes are precisely cut to fit perfectly into the lugs and are then brazed together. The use of lugs creates an immensely strong joint, and being an older manufacturing technique carries a side perk of giving a bike a ‘classic’ look.
Steel frames are perhaps the easiest frames to repair due to the availability of the equipment and supplies, as well as the material’s resistance to repeated heat. For this reason, steel is a great option for commuters, recreational cyclists and touring riders that need a highly durable bike.
Titanium shares many properties with steel but is lighter, more resistant to corrosion and is more durable. The downside is that it is much more expensive and labor intensive. It takes significant expertise to produce a high-quality titanium bike frame. Like aluminium and steel, the titanium used in bicycles is an ‘alloy’, and will typically feature a small percentage of aluminium and Vanadium in its composition.
Titanium never really had its time to shine given it became a usable material for bike frames around the same time as aluminum and carbon fiber, both more affordable and easier to work with. Nevertheless, titanium has a better stiffness to weight ratio than steel, offers similar compliance to carbon fiber, and is virtually indestructible. Almost all titanium frame manufacturers offer a lifetime warranty against manufacturing defects as a result.
Pros of titanium: Strength, durability, corrosion resistance, ride quality, weight
Cons of titanium: Cost of material, difficult to manufacture
How it’s made
Titanium closely follows how an aluminum or steel frame is made. Once the tubes are formed and/or butted, they are then commonly TIG welded together. The welding process is different from steel and alloy as titanium reacts poorly to oxygen. Argon gas is typically pumped in to purge out the oxygen during the welding process as a result. Some manufacturers go as far to create completely oxygen-free welding chambers, filled with argon.
In addition to the material being labor intensive and more expensive in its raw state, titanium is far tougher on tooling (harder to cut, manipulate) too. As such, it’s typically only offered by smaller, boutique and custom bike builders.