Carbon fibre 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 fibres to areas of the bike where strength is required, such as the headtube. At this location, the fibres need to be able to absorb high stress to avoid fracturing. Other areas of the frame need to be stiffer to optimise power transfer, like the bottom bracket for example.
Here, higher end frames will use high stiffness fibres referred to as 'High Modulus' or 'Ultra High Modulus'. It's worth noting that there is no standard naming convention for carbon fibre, 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 individual pieces of carbon fibre. The complex process of choosing the right carbon fibre, reinforced with the best resin, the right layering technique, fibre direction and moulding method will ultimately decide the performance of the bike.
It's never a single one of these factors that make for a good carbon fibre frame, and so it's crucial to keep that in mind when brands are selling on carbon fibre thread counts and the like.
The downfall of carbon fibre 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. Carbon fibre comes on a spool and looks like wool, string or other soft material. The carbon fibre is generally lined up together parallel and along with the resin combined in a unidirectional fashion to create large sheets of carbon fibre.
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 fibre is the use of a mould and bladder. Creating the mould 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 mould to an endurance bike, which requires different testing and analysis protocols.
Each bike size made requires its own mould too. Once the moulds have been created, the carbon fibre composite is layered into the mould to manufacturers instructions on thickness, overlay, lay up, direction and type.
Aside from the actual cutting of the carbon fibre sheets, this whole process requires extensive manual handling to be completed to specification, even the largest manufacturer has an army of workers constructing carbon fibre 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 mould 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 mould, quality checked, cleaned up, painted and becomes the frame you see on the road.
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 mould. 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. Aluminium 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 fibre frames which are said to take approximately 14x longer to produce. Pros of aluminium: Cost and ease to manufacture, strength to weight ratio, corrosion resistance.
As with carbon fibre, aluminium 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 customisation 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 centre 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 centre. The additional manufacturing required to achieve varying tube thickness increases the cost so the cheapest frames will be straight gauge, while the highest quality aluminium 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 mould and fluid. The aluminium tubing is placed into a mould that is a specific shape. Pumps then inject fluid at extremely high pressures causing the aluminium to press into the mould and take the intended shape. This technique is commonly used to optimise tube shapes for additional stiffness without requiring extra material to be used as reinforcement.
Manufacturers of aluminum frames have evolved their designs to be very durable and with an excellent stiffness-to-weight ratio. Steel has not stood still, with tubing manufacturers becoming more sophisticated in working steel into new alloys and shapes that make it a good choice at any price level.
This alloy is generically called chrome-moly, chromoly, CrMo, Cromo, Chromo, and most quality steel frames use a variant of chrome-moly. This principle of engineering frames to use less of a stronger, stiffer material is true for all frame materials.
The materials themselves are not lighter, it is the way they are used that allows the builder to use less material to build a stronger frame. At one time nearly all high quality frames were made from Chromoly.
Steel frames are also relatively easy and cheap to repair, and the technology has been around for a long time. Which frame material is the best? This effect is ameliorated by the now common use of carbon fiber forks or suspension to soak up road shock. In order to guarantee strength, series aluminum must be very precisely thermally treated after being welded, then quenched, and then artificially aged series is more forgiving.
An aluminum frame can be made stiffer and lighter than steel because it is not nearly as dense. Titanium alloys are half as stiff as steel, but also half as dense. Aluminum forks are generally stiff and light, and can be shaped aerodynamically. They also offer excellent compliance for comfort on rough roads. Titanium Titanium also called "ti" is one of the longest lasting, strongest, and most expensive frame materials. Many cyclists and experts feel that it combines the best characteristics of all the other frame materials.
It rivals aluminum in weight, is as comfortable as steel and it has a sprightly ride and electric handling that many riders swear by. The frames feel "alive," as if each pedal stroke gets a boost from an inherent springiness in the frame.
Titanium is hard on metalworking tools, requires expensive titanium welding rod and must be joined carefully in a controlled environment.
Consequently, titanium frames are very expensive to produce, which explains their high purchase price. These designations refer to the amount of aluminum Al and vanadium V alloys used in the titanium. But both titanium alloys are excellent; they may even be combined in a frame.
Only a few companies manufacture titanium forks and they are very expensive due to the additional costs in material and construction. Also, because extra strength is needed in the fork steerer the upper tube , ti forks usually outweigh other high-tech tillers. These two considerations are why most ti frames come with carbon forks. Carbon Fiber Carbon fiber also called "carbon" and "graphite" is a relatively new material and unique because it's not a metal.
It's a fabric that's impregnated with a glue called resin that allows shaping and joining the material. Carbon frames are extremely light, stiff and durable. Its greatest advantage is that carbon can be manipulated essentially in endless ways because builders can orient the fabric strands however they want , which means it can be fine-tuned to provide just about any ride qualities desired.
It's also impervious to corrosion and can be built into beautiful shapes producing Ferrari-like looks. Like titanium, because construction is somewhat complicated and because carbon fabric and resins are costly, carbon frames are on the high end of the cost spectrum. To describe these frames manufacturers use terms such as "high modulus" and "void free," which tells you that it's high-quality carbon fiber material and stellar construction. Sometimes, these designations appear on frame "tubing" decals.
Be sure to ask if you have questions about the carbon material used in a frame. Carbon is a popular material for forks due to its natural ability to absorb shock while offering fine handling.
Although, they're not the best choice for heavy or aggressive riders, there are even all-carbon forks weighing less than a pound.
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