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A new back-torque limiter for high power motorcycles: functionality, design and development methodology

Following the evolving driver comfort perception the motorcycle market has been characterized in the last years by increasing development efforts in order to meet customer’s expectations. Determining factors cited by motorcycle owners for their purchasing choice are “compact size”, “quick start-up and acceleration” and “easy handling”. The overall vehicle comfort and handling is strictly related to the transmission functionality. Motorcycle transmissions have complex dynamic properties, some of which are of interest from handling and comfort points of view and other for rider safety and vehicle components durability concerns. Idling, rattling, shifting impacts, driveline resonances and other vibrational behaviours are physical phenomena which result from the interaction between the driveline components and depend on drive boundary conditions. A special phenomenon in particular is more relevant for safety concerns than for comfort implications: the chattering. This appears mostly during coasting manoeuvres, when the driver and vehicle weights are transferred to the front axle, reducing the load on the rear axle. The deceleration is normally achieved also through single or multiple down shifts, for bringing the engine to the optimal operation range and fully utilizes the breaking effect. Under this condition the unloaded rear wheel begins hopping and the motorcycle is destabilized, impeding the driver entering the corner correctly and safe. Back-torque limiters are special systems developed for high performance oriented motorcycles to mitigate the above described engine braking effect. They are designed to permit a partial clutch slip when the rear wheel tries to drive the engine faster than it would run under its own power. These systems avoid that the engine braking forces are transmitted back along the transmission causing the rear wheel to hop, chatter or lose traction. Some motorcycles currently on the market are already equipped with pure mechanical anti hopping systems. Piaggio has now patented an innovative hydraulic anti hopping system for high performance motorcycle, which is integrated directly in the gearbox and in the clutch actuation systems. This paper illustrates the system and its functionality and also the methodologies applied in the development of this new product.


Alberto Giacometti

hofer powertrain Srl
Siemens Str. 19
I 39100 Bolzano – Bozen – ITALY

Paolo Nesti (Piaggio & C. S.p.A), Marco Ghelardoni (Piaggio & C. S.p.A), Gianluca Milan (hofer powertrain Srl)

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