When manufacturers launch new bike models, they often repeatedly emphasize the increased stiffness, weight, comfort or aerodynamics of the new model. Among these parameters, stiffness is the most valued, which is the key factor in making a bike as efficient as possible in terms of force transmission. However, there are many details hidden under the general concept of stiffness.
Among the many selling points mentioned by manufacturers to sell their products, "increased stiffness" always appears. This is actually a parameter that many people talk about but few people understand what it really means. However, just like the weight of the bicycle, many people overestimate the role of increased stiffness, and stiffness is not necessarily better.
What is stiffness
Before starting to analyze the meaning of this parameter, the first thing to do is to clarify what stiffness is. According to the definition commonly used in engineering, stiffness is the ability of a structural element (in the case of a bicycle, the frame) to resist deformation caused by the application of external forces.
That is why when we talk about stiffness, most people immediately think of the force we apply to the pedals and the degree to which the frame deforms laterally with each pedal stroke.
However, this is only one of the forces that affect the frame, and other forces, such as the effects of centrifugal force when cornering, the reaction to bumps in the road, and other irregularities encountered on the road, are often not taken into account.
Engineers who develop bicycles must take all these aspects into account in order to achieve not only extremely strong rigidity output, but also the correct impact absorption capacity, while making the entire vehicle as light as possible.
Therefore, when talking about the rigidity of the frame, we must evaluate it in different areas of the frame, so you will find that this parameter is more interesting in each area than in the other.
In simple terms, achieving the highest possible rigidity is not a problem. Just pile up the materials, especially the raw materials with high tensile strength, and you can get a stronger structure. In addition to the material, the cross-section of the tube is also important. The larger the cross-section, the greater the rigidity. But the side effect is that the weight will inevitably increase.
The arrangement of the carbon fiber is of course also very important. The current mainstream method relies on different arrangements of unidirectional fibers, which are characterized by high rigidity in the direction of fiber arrangement, but low rigidity in the direction perpendicular to the fiber. This makes it possible for engineers to achieve corresponding uses in different areas of the frame depending on how each fiber cloth is positioned.
To achieve a perfect match, very complex calculations are required. Fortunately, engineers in the 21st century can do this mainly on high-performance computers using finite element analysis software (FEA), which engineers can use to generate hundreds of virtual frames and simulate their response to different forces.
The frame design goal is to achieve balance and add stiffness only where it is needed. To do this, we define several types of stiffness in bicycle frames.
Stiff pedaling - lateral stiffness
First, the indicator we usually consider the most is lateral stiffness, which manufacturers measure in their laboratories by applying loads to the bottom bracket to simulate the forces applied by pedaling. This stiffness mainly measures the degree to which the bottom bracket area deforms every time our legs step on the cranks. Interestingly, the lateral stiffness must be as high as possible, because by minimizing lateral displacement, the vector force we generate can be the maximum force transmitted to the rear wheel.
In addition, the rear triangle must be stiff enough not to deform when the chain transmits force to the rear.
This is the parameter we reviewers try to assess how easily a bike can accelerate quickly, and this is particularly important for climbers and sprinters who demand a bike that can keep pushing hard when launching an attack or facing a large group sprint. However, at cruising speeds, most frames on the market can be said to be very efficient, as the power output is more even and not much different from what we ordinary enthusiasts can output.
To achieve the goal of high lateral bottom bracket stiffness, manufacturers have been working to choose wider bottom brackets and 30 mm cranksets. The chainstays are also often very thick, especially on their sides, but not too much to avoid compatibility with the rear wheel. Both the bottom bracket and the chainstays are often asymmetrical to balance the response to the different forces generated on the drive side and non-drive side. Different carbon fiber arrangements will also maximize this effect as much as possible.
Precision handling - torsional stiffness
A more important but less mentioned parameter is torsional stiffness. This defines how much the frame twists under different forces. This twist affects the alignment of the front and rear wheels, and therefore has a significant impact on the handling of the bike, especially when cornering.
When cornering at high speed, the bike exerts centripetal force on the inside of the curve, which creates centrifugal force, which tends to move us out of the line. Due to the differences in the construction of the fork and rear triangle, the forces on the front and rear wheels are not the same, which causes a certain misalignment of the wheels in the moving line.
But there is another aspect to consider. Lateral and torsional rigidity must be balanced to achieve the best performance without any compromises. On the other hand, too much lateral rigidity on the front and rear axles can make the bike difficult to ride when the road surface is not perfect, because it will rebound after each road impact. So there are more aspects to consider.
Smooth forward movement - vertical rigidity
If the goal of the previous points is to achieve the highest possible rigidity, then in the vertical plane it is exactly the opposite: rigid enough to avoid the bouncing effect, but at the same time, enough deformation to resolve road irregularities.
This is a very difficult parameter to adjust, because it is affected by the weight of the rider, and the design of the bicycle takes into account all kinds of riders. Of course, we can now use big data analysis of riders to infer the average size of riders of a certain size, allowing engineers to adjust this parameter more accurately.
Generally speaking, as with lateral stiffness, the cross-section of the frame tube and the arrangement of the carbon fiber will have a relatively obvious impact on vertical stiffness, and the adjustment of vertical stiffness also strives to achieve the perfect balance between vibration absorption and force transmission efficiency without affecting lateral stiffness.
Vertical stiffness usually affects aerodynamics, because aerodynamic tubes increase the vertical cross-section of the tubes, which increases vertical stiffness and becomes smaller in the horizontal cross-section, affecting lateral stiffness, which is exactly the opposite of what the frame is looking for.
The solution to this problem usually relies on a truncated virtual tail tube and increasing the horizontal cross-sectional area of the tube, but this not only affects weight but also aerodynamic performance.
What if the bike is too stiff or too soft?
As we said at the beginning, if the absolute value of stiffness is very important, it is very easy to make a bike extremely stiff using modern materials. However, few of us can last more than an hour on such a car, not only because the changes in the road surface will quickly shatter our arms and backs, but also because the reaction to any small operation is too fast, forcing us to stay tense all the time.
In fact, at a certain period in history, we once had such cars. Although they felt extremely wonderful when you first stepped on them, especially when accelerating, you soon found that such cars were not practical in the real world. As the kilometers passed, they hurt us more than they gained, or on every downhill curve, they gave us nothing but too much confidence in cornering. I believe everyone still remembers the all-aluminum alloy competition cars that were popular in the early 21st century. Those were truly "two wheels and a pole, nothing else to do".
At the other extreme, we used to regard them as "cotton cars". Probably the kind of cars that require continuous high-intensity output to maintain the cruising speed, and you will feel that more than half of your power has disappeared, not to mention the response like an old man when you accelerate.
Such a bike will also awaken people's nightmare memories when cornering and navigating. I believe that riders who own mid-range steel-frame road bikes can understand what I mean. Although we can call such a bike "elegant in shape and luxurious in texture", everyone understands.
After the above introduction, we roughly know that in most cases, higher rigidity will definitely be more popular, but it must be rigorously tested in every area of the frame and finally achieve a perfect balance of various indicators. In short, the overall rigidity of the frame has increased significantly over the years. On the latest generation of models, you may only need to step on it a few times, while on a model that may be the same model from ten years ago, you may need to step on it a lot more. Through such a comparison, you can very intuitively feel the huge impact of the development of frame design knowledge, the enrichment of design tools, and the improvement of material quality on bicycle performance.
