Condensed Answer: Carbon rims have the potential to be stronger than aluminum models, but the material used for the production of a rim is only one part of the equation.
The thickness, craftsmanship, architecture, and the number of spokes influence a rim’s strength too. Consequently, one cannot conclude with certainty that carbon rims are always stronger than aluminum models.
Carbon Is Strong
Carbon fiber has the following properties:
|Tensile Strength*||5.5 GPa|
|Modus of Elasticity*||520 GPa|
*Tensile Strength – the maximum stress that a material can withstand during stretching or pulling before breaking
*Modus of Elasticity – a measurement revealing a material’s resistance to elastic (non-permanent) deformation under applied stress. Or in other words, the modus of elasticity shows how stiff a material is.
Meanwhile, aluminum alloy has the following stats:
|Tensile Strength||0.31 GPa|
|Modus of Elasticity||68.9 GPa|
Carbon fiber’s tensile strength is close to 18 times higher than that of aluminum.
Carbon fiber’s modus of elasticity is 7.5 times greater than that of aluminum.
Or in simpler words, carbon is 18 times stronger and 7.5 times stiffer than aluminum.
To understand what’s yield strength, one has to become familiar with the so-called yield point.
The yield point defines the moment when the elastic behavior of the element transitions into plastic.
Before the yield point, the element will bend under pressure, but then it will return to its previous position once the external force is removed.
Past the yield point, plastic deformation occurs and prevents the recovery of the element.
Or in short, before the yield point, the element is flexible. Past it, it bends and becomes permanently deformed.
The yield strength indicates when the yield point comes.
Carbon fiber does not effectively have yield strength because it does not deform before its full tensile strength is reached.
Conversely, 6061 aluminum alloy has a yield tensile strength of 276 MPa or 0.276 GPa. When force is applied past that point, aluminum deforms permanently.
The tensile strength of carbon is 5.5 GPa. Thus, one can conclude that it will take close to 20 times more stress to permanently deform carbon than to do the same to aluminum.
Hence the saying that carbon wheels “bounce back” whereas aluminum does not. The secret is the incredibly high effective yield strength of carbon.
Carbon Rims Stay True For Longer
The extra stiffness and tensile strength of carbon are the reasons why carbon wheels stay true for longer and need a lot less truing than a set of aluminum rims.
Some manufacturers of carbon rims are so confident in the strength of their product that they offer a lifetime guarantee. If the rim malfunctions or completely fails, the user will receive a new one.
Carbon = More Aero For The Same Weight
Carbon wheels can have deeper sections than aluminum models for the same weight. Thus, the wheels end up with reduced drag without suffering from the additional rotational mass.
Another benefit of deeper rims is that they can be extra stiff – a quality that racing cyclists appreciate.
The table below is a rough weight comparison of aluminum and carbon rims.
|Aluminum rims||Weight (grams)||Carbon rims||Weight (grams)|
|Reverse Components Base Light||520g||Mavic Open Pro Carbon UST Rim||425g|
|Mavic Open Pro UST Disc Rim||430g||Lilienthal XC – 29″||400g|
|Mavic Open Pro C Rim||435g||Lilienthal AM – 29″||485g|
|DT Swiss RR 511 Road Rim||530g||Lilienthal AM – 29″||350g|
|DT Swiss TK 540 Trekking Rim||545g||Schmolke TLO 45 CL||382g|
|SUNringlé Düroc 35||465g||Mavic Open Pro Carbon UST Disc||405g|
|SUNringlé Düroc 30||440g||Mavic CXP Pro Carbon UST Rim||450g|
|Mavic Open Pro T Tubular Rim||360g||DUKE World Runner Carbon||335g|
|WTB ST TCS 27.5||598g||Mcfk UD Matt Carbon||365g|
|WTB ST TCS 29||574g||Race Face Arc 26||400g|
|Bontrager Aeolus Comp 5 TLR||775g||Race Face Arc 36||460g|
|Praxis Works AL32||490g||Praxis Works C32||424g|
|Mavic EX 1028 Disc – 27.5″ UST||580g||Race Face Arc 31||445g|
|Praxis Works AL32 – 29||518g||Stan’s NoTubes Crest CB7 29||325g|
|CHROMAG PHASE30 Rim||510||Crankbrothers Synthesis Enduro||460|
Conclusion: On average, carbon rims are about 21.4% lighter per wheel. In some cases, the total weight savings can amount to 500-600 grams which is a lot in the world of cycling where every gram matters.
Weight And Thickness Are Important
The material of a rim is not the only parameter that determines strength. For example, a thick and thus heavier aluminum rim can be much stronger than a carbon one.
However, when the weight and thickness are equal, carbon rims will more than likely survive longer without bending. Hence why carbon wheels provide more strength for the same weight and dominate competitive cycling. An aluminum wheel will have to be heavier to match the strength of its carbon rivals.
Of course, the thickness of the rim is just one of the properties determining the unit’s strength.
The design and the manufacturing quality are important too. A well-made aluminum rim will outperform a poorly produced carbon model.
Another crucial characteristic is the number of spokes that the wheel has. If all else is equal, a wheel with a greater number of spokes is heavier but also a lot stronger. Hence why touring bicycles often use the maximum number of spokes possible. Some people even go the extra length and get custom-made wheels with 40+ spokes.
Durability vs. Strength
Carbon may have greater tensile strength and elasticity, but when it comes to overall durability, it doesn’t always beat aluminum. Carbon is brittle and a small crack that goes unnoticed can lead to unexpected catastrophic failure of the wheel. Meanwhile, aluminum is tougher and has an easier time withstanding external stress. A small nick in an aluminum rim is unlikely to lead to complete disintegration.
Therefore, aluminum is a stronger/better material in the case of direct impact.
Carbon Wheels Are Expensive
Carbon is an expensive material to produce and manipulate. Consequently, carbon rims tend to cost a lot more than a set of aluminum rims. Thus, if the user has a limited budget, it’s better to buy a decent aluminum rim than a low-quality or average carbon model. It’s also recommended to stay away from “seductive” carbon rims that sound too good to be true for the price.
Summary: What You Need To Know
1. Many experienced wheel mechanics say that currently carbon is the ultimate material for rim production.
2. Carbon has exceptional stiffness and tensile strength. Consequently, carbon rims retain their shape very well and require a lot more stress to go out of true.
3. Carbon rims offer more strength for the same weight.
4. Carbon rims are harder to bend and often “bounce back” from the stress that would dent an aluminum rim.
5. The main downfall of carbon rims is that the material doesn’t deal well with impact.
6. Carbon rims are significantly more expensive than aluminum rims. Thus, they make no sense for budget bikes.
7. A high-quality aluminum rim is a better purchase than a low-quality carbon model. Thus, recreational riders have a low incentive to purchase a carbon rim.
8. The material of a rim is only one part of the equation. The rim’s thickness, build quality, architecture, and spoke count play a huge part in its strength properties. Hence why it won’t be accurate to say that carbon rims are always stronger than aluminum ones.