ARTEdaCORSA
Though not a recent technology, and once considered the preserve of dry clutches and big twins, slipper clutches (also known as a back-torque limiter) are now ubiquitous in high performance wet clutch applications, largely thanks to Turin based STM...
Coincidentally, leading Italian performance clutch specialist STM originally started out in business at just around the same time that the ill-fated NR500 was being parked in a dark corner of shame at Honda - having failed to prove that four-stroke motorcycles could mix it and win against the then dominant two strokes when it made its 1979 race debut.
In fact, the NR500 featured many firsts and advanced, even experimental features - such as oval pistons, a monocoque frame, inverted telescopic forks, carbon brakes, rising rate rear suspension linkages and, which is where the STM story eventually comes into this story - a slipper clutch.
 |
| Valerio Gaffuri, CEO |
STM started out as a manufacturer of components for robotics and measuring instruments, and while that gave the company an excellent foundation as a high-quality precision manufacturer, it didn't migrate towards the two-wheel industry until the 1990s, starting first of all with the production of special high-strength, lightweight components for bicycles.
 |
| Dry conversion kit for Ducati Hypermotard 950 |
Very soon after that a prototype of the company's future anti-slip clutch was developed, and with two Superbike world titles achieved after it made a debut on Ducati's race bikes, STM's pathway to future success was mapped out.
The concept of the slipper clutch is quite straightforward. A slipper clutch (also known as a back-torque limiter) has an integrated freewheel mechanism to control and reduce the effects of engine braking under deceleration. It also reduces wear and tear on the engine and transmission, thereby improving its durability. A slipper clutch prevents the rear wheel from locking up, eliminating "hopping" and improving the performance and handling - especially under deceleration and cornering.
The main purpose of a slipper clutch is to prevent engine over-revving and rear wheel hop (or chatter), especially under hard braking. It does so by partially slipping until an engine's speed matches with the vehicle's speed.
A straightforward concept it may be, but designing and manufacturing products to achieve that goal is anything but straightforward. The three most important characteristics needed in the design, materials and production processes are quality, quality and quality.
The slipper clutch consists of two bases, one with dog clutches and ramps with ball bearings, a splined hub and clutch plates. In normal operation, the dog clutches mate, driving the transmission. When a back torque comes from the transmission, the splined hub slides up the bearing ramps, disconnecting from the clutch plates and allowing a limited "slip" between input and output.
 |
| Team HRC riders Alvaro Bautista and Leon Haslam |
 |
| Special Dark Edition EVO-GP - dry conversion kit for Ducati Diavel |
It will partially disengage or "slip" when the rear wheel tries to drive the engine faster than it would run under its own power. The engine braking forces in conventional clutches will normally be transmitted back along the drive chain, causing the rear wheel to hop, chatter or lose traction. This is especially noted on larger displacement four-stroke engines, which have greater engine braking than the company's two-stroke or smaller displacement counterparts.
Slipper clutches eliminate this extra loading on the rear suspension - giving riders a more predictable ride and minimising the risk of over-revving the engine during downshifts. Slipper clutches can also prevent a catastrophic rear wheel lock-up in the case of engine seizure. Generally, the amount of force needed to disengage the clutch is adjustable to suit the application.
 |
| CLG clutch |
By transferring the technology developed, and the experience gained on the track of slipper clutch designs for mass-produced models, the era of anti-friction clutches began and STM was very quickly accepted as an innovator and leader of a nascent but important niche market.
While Honda's ill-fated NR500 (the New Racer 500 - or, as many nicknamed it at the time, the "Never Ready" 500) failed to realise Honda founder Soichiro Honda's dreams of marking the company's return to racing by "sticking it" to the two strokers, much of the then advanced technology deployed on the NR500 has passed into the mainstream.
In the case of the slipper clutch, its moment came in 2003 when STM took the critical step of applying its anti-skid technology to oil-immersed clutches for the first time and BOOM! That would see the slipper clutch concept take its place alongside many of the other advanced ideas seen on the experimental NR500.
Embracing the apparently unlikely environment of the wet clutch with anti-slip technology greatly extended STM's market opportunity, and collaboration with the Honda, Suzuki, Yamaha, Kawasaki and BMW teams has resulted in the development of cutting-edge products that combine component durability and reliability with precision and performance.
In that same year, 2003, STM also made its slipper clutch debut in the world of Motocross and Supermotard - receiving instant acclaim and immediately contributing to many victories.
In 2017, STM Italy achieved UNI EN ISO 9001:2015 standard compliance - providing its OEM, distributor, dealer and race shop customers and riders with the confidence of knowing that its products are backed by the ultimate Quality Management System certification.
Materials excellence is central to the reliability and performance that STM achieves, using Ergal (zinc-aluminium alloy), low silicon content Anticorodal alloys with outstanding mechanical properties, electrical conductivity and corrosion resistance, stainless steel and titanium; all machining is on in-house CNC mills.
 |
| STM at EICMA 2019 |
In a world of profound technological transformation, the STM vision will remain at the forefront of anti-friction technology development, anticipating and shaping the future with innovative solutions and market-leading manufacturing quality for MotoGP, WSBK, Motocross and Supermotard racers.
The company says it will also now start to leverage its competencies and reputation as it expands its aftermarket performance parts and components programmes through continuous improvement, one step at a time. KAIZEN.
STM
www.stmitaly.com
