Understanding Quickshifters and Slipper Clutches

Forget about brushing up on your cornering skills. If you really want to go fast then focus on changing up and down the gears swiftly and cleanly

Before we go further, it's important to grasp how a motorcycle's 'drivetrain' (engine and gearbox) works. Very early machines worked on direct drive - ie the engine was directly connected to the rear wheel - so you had to switch off the engine when you hit a junction or to change gear. But then came the clutch - a mechanism that separates the running engine from the rear wheel. The clutch transmits power from the engine to the rear wheel via the gearbox, giving the rider control over the connection between crankshaft and transmission. Use the clutch lever to disengage the clutch and the engine will run freely without turning the rear wheel; disconnect the drive and, by removing the load from the gearbox, the clutch will also allow different gear ratios to be selected.

So how does a clutch work? Imagine a circular friction plate connected to the crankshaft, and a steel plate connected to the gearbox shaft. The friction plate rotates with the crankshaft and as it's pushed against the steel plate, the friction plate turns the steel plate, thus transfering drive from the engine to the gearbox shaft. Slacken off the pressure between the two plates to create a slight gap and 'slip' will occur so the engine will run freely without rotating the gearbox shaft.

Clutch springs push the plates together to transmit drive, and as you pull the clutch lever in you slacken the pressure from the springs, causing the clutch to 'slip'. The gearbox shaft is connected to the rear wheel via a drive mechanism, usually chain or belt. 

Rather than using one gearbox plate and one crankshaft plate, modern bikes get multiple plates enclosed in a casing where the gearshaft's steel plates are sandwiched between the crankshaft's friction plates - with the plates either running in oil (wet clutch) or er, not (dry clutch). Reckon it's weird that a mechanism ruled by friction runs in oil? Well, oil cools the plates and lubricates bushes and bearings, as well as quietening that rattling noise which punters might find disconcerting on a production bike. Race bikes usually run with dry clutches as they tend to be more compact: because a wet clutch does not produce as much friction, it needs to be larger or contain more plates.

Problem is, by the time the rider has pulled in the clutch, rolled off the throttle, changed gear and rolled the throttle back on not only a fuel injected bike will they have lost time but the bike will pitch and dive, upsetting handling, suspension and geometry. Even clutchless change-ups take on average 300 milliseconds for a skilled rider to perform - yep, that's a lot.

Enter quickshifters. These gadgets kill the engine momentarily, taking the load off the gearbox to let you shift up the gears without backing off the throttle. The secret lies in interrupting the engine faster than is humanly possible to operate the throttle so time spent shifting gear plummets typically to between 15 and 50 milliseconds - that's quick. As well as saving time (see chart below), it won't unsettle the suspension and the bike will be more stable, especially in a corner. Oh, and a constant throttle is also more fuel efficient, albeit marginally!

The quickshifter comprises a mecanical sensor that triggers an electronic interrupt device to the bike's ignition. On a bike fitted with old-style carbs, this box is hardwired into the ignition coil wires to interrupt the ignition's electrical supply, while on the device interrupts the ignition via the wiring harness at the ECU. Translogic has developed a system for fuel injected bikes that interrupts the signal from the ECU to the injector (hence cutting the fuel supply, see Fuel's Gold, September issue), again via the wiring harness at the ECU.

The mecanical sensor also varies. It is either positional - ie you adjust it to the positioning of the gear lever, and as the lever's position moves it sends a trigger to the electronic interrupt box. Or it's a pressure sensor - ie shifting the lever without shutting the throttle puts pressure on the gearbox, and a pre-determined amount of pressure triggers the control box. A positional system tends to fall out of adjustment and if you need to replace the gear lever at the track (ie after a crash) you have to re-adjust the entire system, hence GP and WSB teams opt for the pressure trigger: it's more convenient to install and is less likely to fall out of adjustment, but it puts pressure on the gearbox and because it works with greater tolerances, it isn't as consistent.

Guide to Shifters and Clutches

Quickshifters have a bad name in the real world, and Translogic's David Bruton says it is because there are many reasons for them to go wrong - so they do. They fall out of adjustment and can suffer from bad manufacturing, bad cabling, a bike's dodgy wiring and bad installation. But just as bikes are getting more refined and reliable, quickshifters too are becoming more sophisticated. Kliktronic's Keith Holland says: "Whereas quickshifters used to work via a straightforward single shot timer circuit, today manufacturers use chips that are programmed to do more sophisticated tasks."

Indeed, quickshifters were developed as a racing tool and when used on the road at low revs, a lightning-fast kill time makes for a clonky up-change. Although kill time is adjustable, as long as it's fixed for every gear it's always a compromise. This is less of a problem on race bikes as they use a narrow rev band compared to the wider operating range of road bikes. So Translogic developed a quickshifter with speed sensors (Intellishift) that adjusts the duration of the interrupt according to the engine's revs, giving smooth upshifts throughout the powerband.

But downshift and you'll still have to pull the clutch lever in and shut the throttle - again, it's down to how the drive system works. While the engine drives the wheel under acceleration, as the bike decelerates it's the rear wheel that drags the engine round, so interrupting the ignition has no affect on the gearbox load. And this is the root cause to rear-wheel hopping, locking and over-revving as you brake or downshift from high revs on a four-stroke.

While the likes of Rossi could mash down the gears on their 500cc GP two-strokes and fly into corners without a twitch from the bike, it's a different story on the new generation four-stroke MotoGP bikes. Here's why.

On a high compression engine, the crankshaft must fight the trapped compression inside the combustion chamber during the combustion stroke to push the piston back up, and this produces resistance in the form of engine braking, or 'back torque'. As you decelerate and downshift, resistance can become too much for the rear tyre to cope with (especially under hard braking or in the wet when traction is poor), so the wheel hops and at worst locks up. Also, the forces jerking on the chain as it tries to turn the engine upset the suspension and place undue stress on the tyres, which must fight resistance from the engine as well as grip the tarmac. Then, by driving the crankshaft the rear wheel can cause the engine to over-rev by pushing it through its redline, risking damage.

With only two strokes per combustion cycle and no valves, strokers have a much lower compression so the wheel runs relatively freely on deceleration. Instead, as the inlet valve of the vastly more complex four-stroke shuts it increases internal pressure (compression) in the chamber, and the fewer the cylinders, the greater the engine braking and the bigger the problem (splitting the compression into four cylinders tones it down - a V-twin has two great lumps of compression so resistance is stronger, with engine braking most pronounced on a single). At 13,000rpm it takes around 13bhp to push the piston back up the barrel of a two-stroke to turn the engine over. On a four-stroke, this figure can be as high as 45bhp.

On the track, four-stroke riders will spend time and energy blipping the throttle to equalise engine revs to wheel speed to prevent locking the wheel during deceleration, and drag (or 'slip') the clutch to stop wheel chatter through the corner, while compression damping is often used to stop the rear wheel from jerking around, resulting in harsh suspension. Alternatively, they fit a slipper clutch.

Slipper clutches have been around for years in Superbike racing and some homologation bikes came with rudimentary slipper clutches: Kawasaki's ZX-7RR, Suzuki's GSX-R750 SPR, Honda's RC30 and 45, Aprilia RSV Mille R and Ducati's 748R. Now, they've started to find their way onto the latest production bikes - Kawasaki's ZX-6R, Suzuki's SV1000 and Benelli's Tornado all have slipper clutches, while next year the likes of the Kawasaki ZX-10R, Yamaha R6 and R1 are all rumoured (or confirmed) to get one, as well as some of the latest supermotos and dirt bikes.

Quite simply, a slipper clutch provides full friction when the throttle is open, but shut the throttle and when the rear wheel tries to go quicker than the engine, the clutch will allow a certain amount of slip in the opposite direction, letting the back wheel spin relatively free from engine drag, like on a two-stroke. The amount of slip is crucial: there must be enough drag to equalise revs with wheel speed for a smooth drive when the throttle opens again, but not enough drag to overload the rear tyre.

There are different types of slipper clutches, the most common being the 'Ramp' (or 'Cam') slipper clutch and it works like this: In normal drive mode (ie when the engine turns over the chain), the two sides of the clutch are held together tightly by springs so all the drive train components are linked solidly to the primary gear on the crankshaft. But as you decelerate and the chain starts pushing the clutch over, a series of little ramps inside the clutch come into play. As the engine side resists pressure from the wheel side, 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 you end up with the clutch sitting in a point of equilibrium, where it's just slipping. A variation on this mechanical arrangement is the 'sprag' clutch that runs on sprag bearings instead of ramps.

The most advanced slipper clutch is the electronically-controlled Active Clutch developed in 2002 by Yamaha for its M1 MotoGP bike. It's controlled electronically by the engine's ECU, using the on-board sensors (ie throttle position, gear position, front/rear wheel speed) to determine the amount of slip the clutch must provide under all conditions. The ECU then slips it accordingly. Variations on the system are rumoured to be used by other teams.

But the guys at Translogic and Techtronics reckon downshifters that electronically match engine revs to wheel speed via the ECU are a better bet than expensive slipper clutches that are still proving to be unreliable. Not much is being revealed yet, but expect to find them in racing circles and even production soon.

Sounds good, but the next big thing to hit production could be the launch control button that propels the MotoGP bikes off the grid. Could be vital to win the traffic light GP...