Discussion on shifting performance of commercial vehicle transmission

The design and performance evaluation of commercial vehicle transmissions have evolved significantly as the market for these vehicles has changed over time. In the past, commercial vehicles were primarily used by a narrow group of professional drivers, and there was little overlap with passenger car users. At that time, durability was the main concern, and the primary performance indicators for transmissions focused on lifespan and structural strength. However, in recent years, with the expansion of the domestic auto market, the gap between passenger and commercial vehicle drivers has narrowed, leading to a growing demand for comfort in commercial vehicles. This shift has placed greater emphasis on smooth gear shifting and overall driving experience.

Today, many engineers focus on improving transmission shifting performance by enhancing synchronizer components. For example, replacing copper rings with steel-based molybdenum rings, or even carbon particle or carbon fiber rings, increases friction and improves shifting capacity. Similarly, upgrading from single-cone to double- or triple-cone synchronizers increases the friction surface area, which theoretically enhances shifting performance. While these improvements are effective in theory, real-world testing often falls short due to overlooked factors such as static shifting force and the influence of peripheral systems.

Static shifting force refers to the resistance a transmission must overcome when not in motion. It is a key component of the overall dynamic shifting force. If the static force is too high, even an advanced synchronizer may not significantly improve the shifting feel. This resistance comes from various sources, including friction between the shifting shaft and housing, spring forces from the synchronizer and shifting shaft, and resistance between gears and other components.

To enhance shifting smoothness, some designs incorporate arc transitions to avoid sharp corners, reducing the risk of sticking during shifts. A friction-reducing pad is also added between the self-locking steel ball and spring, allowing rolling friction instead of sliding. Mechanisms like the Schaeffler system can further improve this process.

Synchronizer gear holders and toothed sleeves are often manufactured using gear shaping or broaching techniques to ensure smooth engagement. In lighter transmissions, powder metallurgy is sometimes used to create self-lubricating parts, reducing friction and improving performance.

Research shows that optimizing the shifting stroke of the transmission is a highly effective way to improve shifting performance. For example, reducing the shift stroke from 13mm to 10mm while maintaining the same engagement length allows for an increased lever ratio, which makes shifting easier and more precise. This adjustment helps achieve a smoother and more responsive gear change, especially in commercial vehicles where comfort is becoming increasingly important.

The inertia of the driven disc plays a critical role in shifting performance. When the speed difference is fixed, higher inertia means the synchronizer must work harder, increasing both shifting force and time. As shown in the comparison table, a 7% increase in driven disc inertia leads to a 4% increase in the inertia the synchronizer must overcome and the time required for shifting. This highlights the importance of selecting the right clutch disc to optimize shifting efficiency.

Improving shifting performance requires a holistic approach that considers not only the transmission itself but also the entire drivetrain system. Factors like cable efficiency, bracket rigidity, and synchronizer design all contribute to the overall driving experience. By addressing these elements together, manufacturers can achieve a more efficient, comfortable, and reliable transmission system that meets the evolving needs of modern commercial vehicle users.