Is Carbon Tough Enough?
Does carbon fibre really have a valid place in rugged mountain biking, where rocks and crashes post a constant threat, or should it remain the reserve of road and XC race applications? Raoul Luescher offers his expert opinion.
In the last few years carbon has moved from a rarely seen material in mountain biking, to being the material of choice. Carbon is now not only found in simple parts like handlebars and seatposts but is used extensively in frames and also wheels—even for downhill use.
So what has changed, why is this material that was thought to be too fragile even for cross-country use now used extensively for downhill bikes and parts? Is it all marketing driven hype aimed at getting us to buy the latest and greatest or are there real engineering and performance gains to be had?
There’s no doubt that carbon gained a bad reputation from poorly designed and manufactured parts. Weight reduction was often the main goal with some earlier carbon products and the manufacturing techniques at the time frequently lead to inconsistencies and faults.
Of late carbon has ‘come of age’ in the bike world as the processes used have gained maturity. In the last three years the consistency and quality of manufacture has improved significantly.
Engineers are now able to design products with greater certainty, knowing that they’ll perform as expected. The parts are stronger and, most importantly, more reliable. With this the designed-in safety margins can also be reduced to provide lighter parts that highlight the performance gap to other materials. Parts that are designed primarily for strength can be made even stronger without suffering a major weight penalty.
So what are these changes and how does the average mountain biker know what to look for? The significant change has been in the consistency of ‘compaction’. Compaction is how the pressure is applied to force the individual fibres close together. Better compaction of the laminate has a major effect on the laminate properties, especially its compressive strength as it improves the fibre alignment and reduces porosity.
Good compaction also helps to keep the fibres where they are meant to be and there’s less chance of wrinkles developing during manufacture. Once a load is applied, a wrinkle will ‘straighten out’ before it is capable of carrying the load as intended. This forms a significant structural deficiency as it’s left to the far weaker resin to take the load (see the ‘Laminate Wrinkles’ illustration). Just as loose spokes in a wheel reduce the strength and stiffness of the wheel, wrinkles in the fibre have a significant impact on a carbon part’s strength and stiffness.
Straight, well-compacted laminate will be able to take higher loads than one that has wrinkled fibres and porosity. Cutting up the parts and viewing with a microscope is the best way to assess this, but that’s not going to help if you’re trying to gauge the structural integrity of a new bike. Ultrasound and sometimes X-ray can be used for a non-destructive inspection but few have access to these tools or knowledge of how to use them.
A more realistic option is to remove the fork or bottom bracket and do a visual inspection inside the frame; this can give you an indication of the manufacturing quality. Frames that use older and less refined manufacturing techniques tend to have large resin-rich areas and variable wall thickness (ripples, lumps and folds that you can feel inside the tubes). This variability not only makes the frame or component heavier; it also impacts the strength and stiffness, particularly when subjected to compressive loads like you typically see in the head tube area on a frame. With use it’s common to see random cracking at the wrinkles in these areas after a high load because the resin rich areas fail.
In contrast an item that is manufactured with superior methods will be smooth and uniform both inside and out. It will have far more uniform compaction and fibre placement, resulting in a stronger and stiffer product. It will also be more fatigue resistant and handle higher loads without failure.
These cut-away frames show the differences between older manufacturing techniques and more modern carbon construction. On the older frame (top) you can see the fibre wrinkles, resin rich areas and variable thickness that results from uneven compaction. The newer frame (bottom) is noticeably smoother and more consistent.
Modulus has Meaning
So the manufacturing process has improved but what about the carbon? Brands talk a lot about the carbon used in their parts being better. While some if this is purely spin, it’s also true that the material technology has improved and it continues to do so. The fibres get better and more readily available, and the resins also improve. Stronger and sometimes stiffer fibres are now far more common.
Strength is the ability to resist force, typically a straight pulling load; like a piece of rope being pulled for example. The stiffness, which is often referred to as ‘modulus’, is the elasticity or stretchiness of the material; a rubber band for example has a low modulus. These two properties combine so you can have a strong and elastic material like a bungy jumping rope or a weak and stiff material like a Kit Kat chocolate bar.
Previously the most commonly used fibre was Toray T300 or the equivalent from other fibre manufacturers. Today these have been replaced by Toray T700, T800 and even T1000, or similar newer fibres from different brands. As you can see in the fibre properties table, these newer fibres are significantly stronger.
A stronger fibre means that less is required to carry the same load as before, so the part is stronger for the same weight or lighter for the same strength. The increased stiffness of the fibres also allows parts to be stiffer for the same weight, which can also increase performance.
However, thinner, lighter and stiffer – whilst great on the road – does not always mean better for MTB use. With hard day-to-day trail riding in rocky terrain or downhill oriented applications, strength and reliability becomes the primary target. You want to be confident that the parts will not break when landing a big jump or drop. The fast growing ‘gravity enduro’ category combines DH-style riding with the need for efficient pedalling; in this setting the performance gains from high strength yet lightweight become pretty obvious. Stiffness can also be tuned to provide more traction and comfort by changing the fibre type and laminate in different regions.
If designed and manufactured correctly, carbon composite is the highest strength material for many of these products. Carbon composite can be more than twice as strong in tension compared to high grade alloy steel and more than five times the strength of aluminium alloys. Fatigue and corrosion resistance mean it can also last a long time. This is why carbon is the material of choice for highly loaded aircraft structures, Formula 1 race cars and other high-performance items. It also affords a lot of flexibility, allowing designers to create more complex shapes that can provide better tyre clearance, improved standover height and optimised shock or pivot locations. All this adds to the ride quality and explains why carbon is currently the material of choice for mountain biking too.
It can’t all be good, what are the downsides? Due to the fibre only being strong and stiff along its length, a carbon part needs to have fibres running in a range of directions to be able to sustain loads from different angles. Design-wise it’s much more complex, and this adds to the cost; the fibre type, angle, thickness and placement in the laminate can change the properties of a part significantly.
The resin selection can also affect how it performs with environmental exposure such as temperature and also the impact resistance. Toughened resins are available that reduce the spread of damage. New self-healing resin technology is being developed, where the material repairs itself after damage, however it will be some time before this technology is used in bicycles. High temperature capacity is generally not a problem for MTB use, unlike the road clincher rims that have seen many rim-brake overheating failures.
Impact resistance is usually considered the biggest downside of carbon in mountain bike applications. This type of damage can occur but it typically affects thin-walled lightweight carbon parts that aren’t designed to handle impacts. It’s similar to an aluminium drink can; it’ll hold your weight if you stand on the top but you can crush the sides easily in your hand. With carbon composite, if the fibres aren’t placed to take a particular load the part will not be strong in that direction.
The fear of impact damage has come about from weight-optimised designs and particularly from road cycling, where it’s all about maximising stiffness and minimising weight—not surviving impacts. Road frames typically have large diameter tube shapes with very thin walls, sometimes down to 0.8mm. Thin laminates, although they can meet the design loads, just don’t handle impacts well. There is a minimum thickness required depending on a range of factors before impacts can be tolerated with carbon.
Other users of carbon composite such as aerospace and Formula 1, design and test for impact survivability. Companies such as Boeing and Airbus are not going to put planes in the sky that will fail catastrophically after a bird strike. A well designed and manufactured carbon part can have superior impact resistance to other materials whilst still being lighter.
Additionally, even if the carbon laminate is damaged from an impact it can often be repaired. Advanced composite repair as used in the aircraft industry has proven itself to be reliable for over 40 years in the skies. Combined with non-destructive damage assessment such as ultrasound scans, carbon parts can be quickly and safely repaired, and it’s often very cost effective compared to replacement.
While the stigma that carbon holds will undoubtedly take a long time to shirk off, the material really has come of age. It can offer greater strength, stiffness and in some instances higher impact resistance, all whilst being lighter than traditional materials. Carbon is no longer just for XC racers or weight weenies; when designed and manufactured properly, it’s a superior material for just about any mountain biking application.
Impact Testing Example:
This test was conducted at Luescher Teknik to look at the impact resistance of an alloy DH/all-mountain rim compared to a carbon equivalent. For this we secured a section of rim in a holder and dropped a mass from a set height. The weight was set on a slide, so the impact was controlled and at the same point on both rims. A rounded nose was used to simulate a rock strike and the impact energy was increased in steps to monitor the damage growth. This was done without a tyre, as it made for a more direct and repeatable test of the rim strength.
The lower energy impacts deformed the alloy rim to the point where it wouldn’t hold air as a tubeless setup but the carbon rim remained undamaged. The final damage was terminal in both cases but the carbon rim retained its shape and would still have been rideable to get you home.
Note that the carbon rim was 25% lighter and 6mm wider internally than the alloy sample. It’s a good example of how a well-designed composite part can resist severe abuse better than more traditional materials.
The testing of automotive wheels by the Geelong based company ‘Carbon Revolution’ has shown similar results. While rims are initially more expensive, they can last a lot longer and save wheel rebuild costs.
Alloy DH RimHere’s the final impact damage sustained on the alloy downhill rim. It has caved in completely and the sidewall has cracked—you’d probably be walking home with this level of damage.
Carbon Trail RimThe carbon rim has delaminated around the impact zone however it has maintained its shape and would still be rideable.
• A video comparing steel driveshafts to carbon: www.youtube.com/watch?v=Ai2HmvAXcU0
• Santa Cruz Bicycles testing aluminium versus carbon: www.youtube.com/watch?v=xreZdUBqpJs
• Red Bull F1 crash test: www.youtube.com/watch?v=7wFeBRXvdik
• Mercedes F1: www.youtube.com/watch?v=7d6ttJqOzyw
• Boeing wing test: www.youtube.com/watch?v=sA9Kato1CxA
• Carbon revolution wheel technology: www.carbonrev.com/technology/#nav_anchor
Raoul Luescher has worked with composite materials for over 20 years. He has been employed by Boeing Aerospace, contracted to Defence and the Australian Institute of Sport. As the director of Luescher Teknik, Raoul consults on design, manufacture and quality for a range of composite material applications, primarily in the sporting field but also to the defence, mining and automotive industries.
www.luescherteknik.com.au & www.carbonbikerepair.com.au