Discussion on shifting performance of commercial vehicle transmission

Commercial vehicle transmissions have undergone significant changes in design and performance evaluation as the market for these vehicles evolves. In the past, the audience for commercial vehicles was limited, and truck drivers rarely had access to passenger cars. At that time, durability was the top priority, and the main performance indicators for transmissions focused on the life span and strength of the system. However, in recent years, with the expansion of the domestic auto market, the lines between passenger and commercial vehicle drivers have blurred, leading to a growing demand for comfort in commercial vehicles. This has increased the expectations for smooth shifting performance. As a result, improving transmission shift quality while maintaining the "light car feel" has become a key trend among major manufacturers.

When it comes to transmission shifting performance, many engineers focus on synchronizer capacity and shifting force. Improvements often involve replacing copper rings with steel-based molybdenum rings, or even carbon particle or carbon fiber rings, which increase the friction coefficient and enhance shifting capability. Additionally, moving from single-cone to double-cone and then to triple-cone synchronizers increases the friction surface area, thus improving performance. These theoretical advancements have significantly boosted shifting performance, but real-world results often fall short due to overlooked factors like static shifting force and the influence of the surrounding transmission system.

Static shifting force refers to the resistance encountered when the transmission is not engaged. It plays a crucial role in the overall dynamic shifting force. If this force is too high, even an advanced synchronizer may not improve the shifting experience. Factors contributing to static force include friction between the shifting shaft and housing, spring forces from the synchronizer and shift lever, gear sleeve and tooth holder resistance, and more. To minimize this, manufacturers aim for low surface roughness (Ra ≤ 0.8 μm) and use linear bearings in high-end models to reduce sliding friction and improve performance.

Smooth transitions at the shifting steel ball location prevent sharp corners that could cause sticking. A friction-reducing pad is often added between the steel ball and spring, allowing rolling rather than sliding friction. The Schaeffler mechanism is another solution that enhances smoothness. Similarly, gear holders and sleeves are designed with precision to ensure smooth engagement, sometimes using powder metallurgy for a self-lubricating effect.

Research shows that reducing the shift stroke of the transmission can greatly improve shifting performance. For example, decreasing the stroke from 13mm to 10mm and increasing the lever ratio from 1 to 1.3 can make shifting smoother and more efficient. Therefore, minimizing the shift stroke while ensuring proper engagement length is essential in modern transmission design.

Another important factor is the driven disc assembly. The inertia of the clutch disc directly affects the synchronizer's ability to match speeds. Increasing the inertia by 7% can raise the required shifting force and time by about 4%. This highlights the importance of optimizing the entire transmission system for better performance.

The efficiency of the transmission cable and the rigidity of its bracket significantly impact shifting performance. Improving the overall system—rather than just the synchronizer—leads to a more cost-effective and comprehensive solution. As the industry moves toward greater comfort and efficiency, a holistic approach is essential for achieving superior shifting performance in commercial vehicles.