If Monocoque Carbon Frames Were Built Like Carbon Forks and Wheels, the Frame Would Ride Better

The lightest high-end carbon forks and rims are properly built. The lightest high-end carbon frames are underbuilt. Compared to the lightest carbon forks and rims, the lightest carbon frames are underbuilt by 40% to 50%. Performance, durability, and safety are all crippled by this divergence.

It is no coincidence that the lightest carbon forks and carbon rims both have 40% to 50% more material per square centimeter of external surface area than the lightest carbon frames. This may be the most useful datapoint to show how underbuilt many carbon frames are.

Rims and forks are relatively simple structures that are much easier to model stresses of than frames, so their durability and safety limits are easier to determine. This also means that forks and rims should be proportionally lighter, relative to surface area, not heavier, than frames. However, the opposite it true.

We compare carbon frames to carbon forks in this first section. We compare carbon frames to carbon wheels in the second section.

Comparing Frames & Forks

"If monocoque carbon is so terrible, why does Seven use a monocoque carbon fork for their framesets?"

That's a good question, and it gets to the heart of the problem with modern monocoque frames. In short, if bike brands sourced frames built like forks, poor frame fatigue, weak impact resistance, and general durability issues would be less problematic. Figure 1 illustrates the discrepancy between frame and fork engineering.

The table below describes the deficiencies in frames compared to forks. In nearly every measure, forks are much heavier, more robust, and have shorter warranty commitments. The exception to this trend is the rider weight limit, which is nearly identical for frames and forks.

In the table below, we use Enve as the fork standard because no one could argue that they are not reputable. We don't use Seven's forks for this comparison because some readers might think we are biased.

Comparing Frames & Wheels

"If monocoque carbon frames are so terrible, why are carbon wheels so popular?"

The weight difference between the lightest carbon rims and the lightest aluminum rims is about 10%, as shown in the table below. The lightest carbon frames are about 40% lighter than the lightest durable titanium (or aluminum) frames. If carbon rims were 40% lighter than aluminum rims, they would weigh 250 grams; that would make for very dangerous riding.

Similar to the carbon fork comparison above, riders sometimes ask us about frame comparisons with carbon wheels. The logic is that if carbon wheels work well, carbon frames must be fine, too. However, if the lightest carbon rims weighed proportionately the same as the lightest carbon frames, the rims would weigh about 250 grams, or about 40% less. There is no way to build a carbon rim anywhere near that weight, yet, many carbon frames are built in that 40% overly light range.

Fortunately, it's straightforward to show how underbuilt carbon frames are compared to carbon rims. This is because we can compare metal (aluminum) rims to carbon equivalents and thereby make some inferences about metal (aluminum and titanium) frames compared to carbon frames.

If the lightest carbon frames were only 10% lighter than the lightest durable titanium or aluminum frames, we might have some durable carbon frames. However, the lightest titanium frames that still have some modicum of durability weigh in at about 1,180 grams or more. The lightest carbon frames are about 600 to 700 grams. That's a 40% variance, as illustrated in Figure 1. If carbon rims were 40% lighter than aluminum rims, the carbon would not hold up.

Rim Weight Comparison: Carbon Vs Aluminum

This table lists three comparisons of carbon and aluminum performance road rims. These show that lightweight carbon rims are consistently about 10% lighter than the lightest aluminum rims.

Not many rim makers offer both aluminum and carbon rims anymore. Comparisons across brands are sometimes necessary. Regardless, the results are consistent. The average weight difference of rims alone is about 10%, nearly identical to complete wheel system weight differentials.

Read more about carbon frame weight and other challenges in Seven's carbon white paper.

---

Footnotes for Figure 1

Back to Figure 1.

1. "If the best and lightest carbon forks and rims are reasonably durable, carbon frames are significantly underbuilt." If riders, suppliers, and engineers agree that Enve carbon forks are reasonably durable and safe, and not stupidly overbuilt, then carbon frames must be severely underbuilt. The external surface area of a carbon frame is about 2.8 times more than a fork. In crude terms, a frame should weigh at least 2.8 times that of a fork. That's 1,200 grams. (425 g x 2.8 = 1,200 g)
2. Reasonably durable carbon rims: We can approach frame weight correlation from a slightly different direction, with nearly identical results as the fork correlation. Aluminum rims exist and they are, on average 10% heavier than their carbon counterparts. Therefore, a carbon frame should be (could be) about 10% lighter than an aluminum frame. That puts the lightest carbon frames at about 1,060 grams, or 80% heavier than the industry's actual lightest frames.
3. Benchmark fork: For this comparison, we use Enve's In-Route Carbon Road fork. There is no more reputable and proven fork on the market. If a supplier could build a fork lighter, they would. But they can't do that and maintain safety and a 5-year warranty.
4. Fork weight not possible: If a fork were built as light as the lightest carbon frame (the Specialized Aethos at 585 grams), the fork would weigh 208 grams. The math is 585 grams for the lightest frame divided by 1,200 grams for a robust carbon frame. This is 49%. A 425-gram fork times 49% equals 208 grams.
5. Carbon frame at 1,200 grams: A frame has about 280% more external surface area than a fork. If the lightest durable fork weigh 425 grams, a frame of equal durability would weigh about 1,200 grams or 280% more than the fork. This 1,200 grams is lower than it should be because a frame is a lot more complicated than a fork, as enumerated above in the fork table.
6. 700-gram carbon frame: Most production performance carbon frame suppliers offer a road frame in the 700-gram range.
7. 585-gram carbon frame: the lightest production performance carbon frame is the Specialized Aethos road bike. The claimed weight is 585 g without any of the hardware.
8. 55%: If a frame is built with the same mission as a fork, it is likely to weigh at least 1,200 grams, as explained in footnotes 1 and 5. If a baseline carbon frame weighs 1,200 g and superlight carbon frames weigh about 700 g, that is a 45% variance. Meaning, a 700-gram frame is underbuilt by about 45%.
9. 46%: As explained in footnotes 1 and 5, if 1,200 g is a reasonable baseline frame weight and the lightest frame made is 585 g (Specialized Aethos), then that frame is 54% underbuilt.
10. Aluminum rim at 426 grams: This is the baseline for comparing the weight of a carbon and an aluminum rim. This rim weight is also the baseline point for comparing a lightweight performance metal frame. To reach the 426 g weight, we take three of the lightest and reputable aluminum rims and average their weights. Details in the rim table above. (The rim is a hook bead, not for a tubular.)
11. Carbon rim at 383 grams: We take three of the lightest and most reputable carbon rims and average their weights. Details in the rim table above. (The rim is a hook or hookless bead, not for a tubular.)
12. Titanium frame weight 1,180g (2.6 lbs): This is about the lightest titanium (and aluminum) frame weight that can still be durable and perform well. So, we use this as the benchmark for comparing metal rim weight to carbon rim weight. Because the weight difference between aluminum and carbon rims is 10%, it is reasonable to set a carbon frame baseline at 10% below a metal frame. This 10% is actually greater than it should be, as detailed in the "small elements" and "complexity of structure" rows in the frame & fork table above.
13. 90%: The lightest carbon rims are 10% lighter than the lightest aluminum rims. This difference is an appropriate starting point for determining a projected carbon frame weight relative to a metal frame's weight. If a carbon frame is built with the same durability and toughness mission as a carbon rim, the frame would likely weigh 1,060 grams or more. If a baseline carbon frame weighs 1,060 g and superlight carbon frames weigh about 700 g, that is a 34% variance, therefore, compared to the benchmark carbon rim, a 700-gram frame is underbuilt by at least 32%.
14. Carbon frame weight projected: Based on metal and carbon rims, the lightest carbon road frames should be about 1,060 grams, or 10% lighter than the lightest durable metal frames. Obviously, that's not the case. How is it that a carbon frame can be proportionately 32% lighter than the lightest carbon rim? It can't; carbon frames are underbuilt.
15. Rim weight not possible: If a rim were built as light as the lightest carbon frame (the Specialized Aethos at 585 grams), the rim would weigh 212 grams. Projecting rim weight from a 700-gram frame, the rim would weigh 252 grams. Obviously, these are not even close to being possible. If a supplier could build a lighter carbon rim, they would. But they can't do that and maintain safety and a 5 year warranty, just as building a carbon frame much lighter than 1,100 grams is not wise (if durability is important).
16. 59% (66%): 59% is the weight differential of a 700-gram carbon frame compared to the projected aluminum/titanium frame weight of 1,180 grams. 66% is the weight differential of a 700-gram carbon frame compared to the projected carbon frame weight of 1,060 grams.
17. 50% (55%): 50% is the weight differential of a 585-gram carbon frame compared to the projected aluminum/titanium frame weight of 1,180 grams. 55% is the weight differential of a 585-gram carbon frame compared to the projected metal frame weight of 1,180 grams.

Footnotes for frame weight limits

Reference to the table above.

• Specialized: 300 lbs.
• Trek: 300 lbs.
• Cannondale: 300 lbs.
• Enve (frames): 300 lbs.
• Giant: 275 lbs.
• Pinarello: 265 lbs.
• Canyon: 264 lbs.
• Colnago: ~250 lbs.
• Cervelo: 220 lbs.