STEPROUND UNDERSIZE

What determines a yo-yo's spin power and stability? Many believe it is the "Moment of Inertia." The heavier the outer rim, the greater the moment of inertia, leading to a more stable spin. This is a physically correct fact. That is why traditional undersize yo-yos tried to gain moment of inertia by making the rims extremely heavy.

However, this design has a trade-off. As the rim gets heavier, the total weight of the yo-yo increases, making the throw feel sluggish. During direction changes or hops, that dull inertia gets in the way. It’s a contradiction: you chose a small yo-yo for mobility, yet it became harder to move. Moreover, the moment of inertia gained through weight can never truly rival a full-size yo-yo due to the smaller diameter. This is how that unsatisfactory feeling of "heavy yet weak spin" was born.

STEPROUND UNDERSIZE approached this problem from a different angle.

To describe a yo-yo’s spin strength accurately in physics, you look at its "Angular Momentum." Angular momentum is defined as $Moment\ of\ Inertia \times Angular\ Velocity\ (RPM)$. In other words, the persistence of the spin is not determined by the moment of inertia alone; it is the product of it and the rotational speed.

Traditional undersize designs tried to brute-force the left side of this equation—the "Moment of Inertia"—through weight. STEPROUND UNDERSIZE takes a different approach: "Maximizing the moment of inertia through efficient weight distribution" while "structurally increasing the initial RPM." We tackle both sides of the multiplication simultaneously.

First, the mechanism to increase RPM: The key is the Size D bearing. Because its outer diameter is smaller than a standard Size C bearing, the string wraps around it more times. Before the same length of string fully unwinds, the yo-yo rotates more frequently. This means that even with the same throwing force, the initial rotational speed is structurally higher. This is a pure advantage in RPM gained through design alone.

Next, efficient weight distribution: By combining the lightweight Size D bearing with an M3 axle, we have significantly reduced the mass of the center section. Compared to a standard Size C bearing and M4 axle, the mass around the axis of rotation is much lighter. The crucial point here is that by lightening the center, we can allocate a higher percentage of the total weight to the outer rims without increasing the overall weight of the yo-yo. It’s not about adding weight; it’s about optimizing the distribution of the weight you already have.

The Size D bearing boosts the RPM, while the D bearing and M3 axle lighten the center to maximize the efficiency of the moment of inertia. A single design choice contributes to both factors of the equation at once. This is the core of the STEPROUND UNDERSIZE design.

This covers the approach to "securing spin power." However, strong spin alone isn't enough for an undersize; what’s required is "ease of movement."

Instead of simply pushing all weight to the extreme edge, we adopted a streamlined rim shape. This ensures a high moment of inertia while preventing the yo-yo's overall center of gravity from feeling unbalanced. It retains a "lightness" that follows the player's finger movements during direction changes. We intentionally balanced spin power with maneuverability rather than maximizing one at the total expense of the other.

Finally, the 44mm width: By designing it slightly wider than typical undersize yo-yos, we increased the volume and weight of the rim that can be placed on the outer edge. This compensates for the reduced diameter, allowing the moment of inertia to remain at a high level for an undersize model.

To summarize: Based on the high-efficiency Step-Round shape, the Size D bearing structurally increases initial RPM. The D bearing and M3 axle drastically lighten the center, optimizing weight distribution to maximize the moment of inertia. The streamlined rims balance agility with spin persistence, and the slightly wider design secures necessary rim weight.

"Efficient Weight Distribution × High RPM." Instead of forcing a spin through weight, we optimized both sides of the equation through design. This is the full picture of how we broke the traditional undersize dilemma of "heavy yet sluggish" to achieve a yo-yo that "spins fast, moves well, and lasts long."

The specs are undersize, but the performance is full-size. "Just made smaller" is the crystallization of this entire design philosophy.