Slipper Clutch Explained!
A Slipper Clutch is a specially designed clutch, developed for the performance motorcycles (track & street-legal). It serves the same purpose for the motorcycles as a typical clutch would do i.e. engagement and disengagement of engine’s power to the rear wheels (via gearbox) at rider’s will. But, the main purpose to use a slipper clutch in place of a normal clutch is to mitigate the effects of Engine Braking when riders decelerate as they enter corners. And this helps them to rip some really fast lap times and gives them confidence to kill corners faster and faster and brake as late as possible. This makes it a popular track accessory.
Marc Marquez (93) and Andrea Dovizioso (04)
As I am an engineer, I would like to reveal some technical stuff too. In a high compression engine, the crankshaft has to fight the trapped charge inside the combustion chamber during the compression stroke to push the piston back up, and this produces resistance in the form of Engine Braking, or “back torque”. As the rider decelerates and downshifts, the effect of engine braking becomes more pronounced; this happens because of the rpm difference between the engine and the rear wheel.
Usually, it’s found that while you enter a corner and downshift to a taller gear, the rear wheel rolls at a higher rpm than the engine, and due to the resistance offered by the engine (because of engine braking), the wheel hops and at worst locks up!
There can be other consequences too, like, the forces jerking on the chain/belt, when it tries to turn the engine, upset the suspension and place undue stress on the tires which have to withstand the resistance posed by the engine and grip the tarmac. Moreover, due to rpm difference between engine and rear wheel, the wheel tends to turn the engine, which can cause the engine to over-rev by pushing it through its REDLINE, risking damage or seizure.
Engine Braking is more of a problem in a 4-stroke engine than a 2-stroke engine.
The reason: with only two stokes per cycle and no valves, the two-stroke engine have a much lower compression so the wheel runs relatively freely on deceleration. While, in a 4-stroke engine, there are four strokes in one cycle, and a closed inlet valve during compression stroke increases the internal pressure and thus the power requirement to push the piston back up increases. Moreover, the fewer the cylinders, the greater the engine braking. That’s why many performance motorcycle manufacturers are preferring 4-cylinder arrangement (Inline & V-four) over single cylinder and V-twin.
Now let’s come back to our question, simply put, a slipper clutch serves like a simple friction clutch when the throttle is open and as you let off the throttle, and when the rear wheel tries to go quicker than the engine, the clutch will allow a certain amount of slippage in the opposite direction, letting the rear wheel to spin relatively freely from the engine drag. This slip is very crucial in Slipper Clutch design, it should be just enough to try and equalize the revs with the wheel speed, so that when the throttle is opened again, your motorcycle is ready to shoot and fly off.
COMPONENTS OF A SLIPPER CLUTCH
Engineers are always inquisitive to know the working of a system, so here’s a brief take on Working of Slipper Clutch. Slipper clutches are of various types and designs, but the most common and popular is the “Ramp Slipper Clutch”. This works more or less on the principle of a freewheel. In a Ramp Slipper Clutch, there are two sections, one section has a set of discs (friction and steel) and the other has a hub and dog assembly. Both the hub and the dog have a series of little ramps with roller balls embedded in hub-ramps. The dog is in permanent mesh with the clutch housing, which is linked to the engine flywheel. On the other hand, the hub is splined to the transmission/gearbox input shaft alike the steel discs in the friction clutch section of the Slipper clutch assembly.
HUB AND DOG ASSEMBLY
This means that the Dog serves as the engine side and Hub as the wheel side. During normal operation, both the sections are kept intact by the springs so that they work as one unit. But, as the rider enters a corner, downshifts and decelerates, the engine side resists the pressure from the wheel side and it pushes up the ramps causing the two opposing sides to twist against each other. This forces the clutch plates apart, so the clutch ‘slips’. As the clutch starts to slip, pressure eases so the engine side starts to drop off the ramps, and the rider ends up with the clutch sitting in a point of equilibrium, where it’s just slipping. A variation of this arrangement is a Sprag Clutch.
From the above it’s clear, why it’s so popular. Until late 1980s, slipper clutch was the most under-rated track accessory.
Like Formula 1, most of the technologies found in performance motorcycles, cascade through motorsports.
Before the advent of 4-stroke engines in MotoGP, 2-stroke motorcycles just did well. MV Agusta, the manufacturer with highest number of championships in its bag, won most of them with 2-strokers. But, as they tried to turn to 4-strokers, they started to struggle, yeah, the problem was Engine Braking. It was then, the Slipper clutch came into limelight. Now, it has been around for years in Superbike racing and some homologation bikes like – Kawasaki ZX-10R, Yamaha R6, R1, Aprilia RSVs, Ducati’s Desmosedicis, 1199s, 848 and so on. The fact which really excites us is that now these manufacturers have started availing this compelling accessory in quarter-litre segment motorcycles too, Kawasaki Ninja 300 and KTM Duke 390 are a few known to be equipped with.
In Superbike racing, this is what you have to do “Brake as late as possible. Turn, at the apex. Accelerate. Stay on the bike and FIGHT, FIGHT, FIGHT!”
Comments