Among gravel cyclists, the debate about wheel diameter choice (650b vs 700c) is often polarized and oversimplified. The choice of wheel diameter and tire size add various layers of complexity, and understanding the performance implications, especially ones tangible to the average rider, requires a deep dive into a multitude of variables: Are we evaluating performance for on-road or off-road use? Do the wheels compared share an identical overall wheel diameter, are they inflated to the same PSI, are they made from similar rubber composition, etc.?
For the sake of simplicity, we’ll narrow our scope to examining two stock spec choices we use in building Montu Kopis gravel bikes: 650 x 47B and 700 x 40C . We assume, to further simplify matters, that rubber compounds for the tires are identical or differences are negligible. In paragraphs that follow, we’ll discuss performances for both on-road and off-road terrains, as well as the significance of tire pressure in the given wheel sizes. By the end of this article, you should be equipped with a better understanding of which wheel diameter aligns best with your needs.
At the outset, it’s important note that there’s no better or worse option — they’re just different. It all depends on which wheel diameter is a better match for your riding style and preferences.
What is 650b and 700c?
The 700c wheel has established itself as the conventional choice for road and gravel bikes. 650b emerged as an attractive alternative tailored to meet the demands of different riding styles. With their ability to accommodate larger tire volumes on modern road and gravel frames, 650b wheels are inflated to lower pressures than your average 700c tire (again, all other things being equal, larger volume tires are inflated to lower pressures than smaller volume ones), this pressure characteristic in itself significantly contributes to the benefits of 650b wheels, which will be discussed further in the following sections.
650b vs 700c Performance: What are the key factors?
In comparing the performance of 650b and 700c wheels in different cycling conditions, it’s important to analyze factors such as rolling resistance, traction, weight, bouncing, and comfort.
Rolling Resistance
Rolling resistance is the energy loss occurring during tire deformation during cycling. This is an inherent characteristic of deformable bodies, such as tires, as they undergo repeated cycles of deformation and recovery. As a result, this energy loss is dissipated as heat and sound.
As mentioned, 650b tires are typically inflated to lower PSI levels, amplifying what is known in physics as hysteresis loss due to increased tire deformation. With each rotation, the tire operating at lower pressure undergoes more flexing, resulting in more energy dissipation and, thus, higher rolling resistance. This contrasts with 700c tires, often inflated to higher PSI levels, effectively reducing tire deformation under the same load (the cyclist), thus mitigating this energy loss.
Another crucial aspect in understanding rolling resistance is the concept of contact patch, which is the area of the tire in contact with the ground. With the lower tire pressure for 650b wheels, the contact patch expands. This increased contact area increases the maximum coefficient of friction, requiring more watts to maintain forward momentum. In contrast, higher pressure in 700c wheels results in a smaller contact patch, which in turn, decreases rolling resistance. That being said, there is evidence to support that higher volume tires can actually have less rolling resistance than narrower ones*.
Traction
Despite being associated with heightened rolling resistance, 650b wheels offset this downside with improved traction which is favorable in off-road cycling. The lower tire pressure of 650b wheels allows them to conform more effectively to irregularities in rougher terrains, expanding the contact area. Additionally, fitting larger tires in general increases wheel engagement with the surface, offering an advantage in rougher terrain conditions where maintaining traction is necessary. With better traction, you also benefit from increased stability and control with the 650b wheels.
Weight Penalty
Typically, the majority of 650b wheels tend to have a smaller outer diameter relative to 700c wheels, therefore resulting in a lesser weight. This reduced rotating mass of 650b translates to quicker acceleration and deceleration, improving the steering and responsiveness of the bike. And you can imagine how the combination of lighter wheels and better traction also offers advantages in climbing, making the 650b also effective in uphill conditions.
However, because of the lower weight of 650b wheels, it also translates to a lower momentum at a certain velocity. This makes it more challenging to maintain speed as compared to the heavier 700c wheels, which also has the added benefit of lower rolling resistance.
Bouncing and Comfort
Lastly, the choice between 650b and 700c also affects the ride’s comfort and bouncing characteristics. The wider and lower-pressure 650b wheels create an air cushion that improves impact absorption, and lowers the risks of punctures. This can provide a smoother, more comfortable ride on rugged terrains. Conversely, the more rigid feel provided by the higher pressure in 700c tires are ideal if you prefer more direct road feedback.
Conclusion
In summary, 650b stands out for its capability to run at lower pressures, increased traction, and superior comfort which is favorable for riding on technical off-road terrain. Meanwhile, 700C’s lower rolling resistance and higher momentum makes it advantageous for smoother tarmac or mixed-terrain rides where you know you’re not going to encounter very rough terrain, and if you enjoy a more noticeable road feel.
Remember that this comparison goes beyond the notion of “which is better or worse”, but instead offers a deeper understanding of which riding style you prefer.
If you’re still uncertain of your build and considering trying both 650b and 700c options, it’s essential to invest in a frameset with wide tire clearance. Our Montu Kopis frameset has 700 x 50C or 650B x 2.1” tire clearance, which provides the adaptability you need when switching to different wheel sizes. Whether you want to race gravel, explore networks of dirt and gravel roads, or plan multi-terrain epic rides with no certainty of what road conditions are like, the Kopis can do it all. It has the balanced ride you expect from a fast and light gravel bike, and the versatility of a bikepacking and adventure bike.