Motorcycle Clutch – How They Work

Motorcycle Clutch Components Seperated

The Modern, Manual Motorcycle Clutch does a lot more than the textbook definition of this word Clutch suggest. While the definition is “to Grasp, Seize, Clench, and Hold”, we think of them as quite the opposite. Mention Clutch and most of us will describe the Slipping Action that allows us to control engagement and movement from a dead stop as well as give the pause in the transmission of power for sequential Gear Changes.

Motorcycle Clutch Fiber Disc Steel Plates Sanwich

Alternating Layers of Fiber Friction Disc and Steel Clutch Plates Clamped Together By the Pressure Plate and Springs

This article will focus on the Action, the Why, and the How the different components of the Modern and Manual Motorcycle Clutch come together to make these mechanical assemblies work. The reference to Modern is relative because the earliest motorcycle clutches were little more than a lever and roller that removed or added slack to the leather belt that connected the engine and rear wheel. The reference to Manual is needed because some smaller bikes and ATV’s use Automatic Clutches that work through increasing RPM and centrifugal forces that move outward to engage 2 friction surfaces to couple the engine and transmission.

In their natural, at-rest state, the alternating layers of fiber Friction Plates (crankshaft driven) and metal Clutch Plates (transmission input) are tightly clamped and stuck together. An outer hub/Basket (crankshaft driven) is used to maintain the Friction Disc while an inner hub/Boss is used to maintain the metal Cutch Plates that drive the transmission input shaft. The alternating sandwich of fiber disc and metal plates is clamped tightly (stuck together) by the Pressure Plate and Pressure Plate Springs. This sandwich stays tightly clamped until the Clutch Lever is pulled.

Motorcycle Clutch Assembly Held Together By Pressure Plate and Springs Motorcycle Clutch Assembled Side View

The Fiber Friction Disc are normally constructed of aluminum that is covered with friction material on both sides. These Disc have large, squared Tabs on their outer circumference and mesh with the squared slots of the outer Basket. The purpose of the slots is to allow the Disc Tabs to slide in/out of the clamped position whenever the Clutch Lever is pulled. Pulling the lever affects a separation of the Disc and Plates – thus separating Engine Power and Transmission Input.

The result of Lever Pull is a pause in power transmission to the Input Shaft of our Transmission or if the Lever is being released – a gradual application of Friction and growing transfer of power to the Input Shaft. Notice that I did not use the terms movement or driveline. Power is passed through the transmission only when in gear. The Clutch can of course be fully engaged with no movement when the transmission is in Neutral.

While most Disc are aluminum, I have owned motorcycles that use composite Disc covered with friction material. Wear and Abuse of the Clutch can often be seen on the surfaces of the Clutch Basket/outer hub. Sudden loading/hammering and shock from hard engagement (better known as popping) of the Clutch can and often does cause wear and roughness along the surfaces of the Basket where the friction disc tabs mesh with the Basket. As this surface wears or gets damaged, the Disc are slower to slide and open/close. While you can see some results of this in the photos, I’ve seen many larger capacity clutches far worse. The impact or repeated abuse? Loss of release smoothness.

Clutch Basket Showing Friction Disc Tabs Wear and Damage

Clutch Basket Damage Shows On the Friction Disc Tab Surfaces. Slower and Uneven Release and Engagement Are Symptoms

If you have a clutch that no longer feathers like it did when new and you’ve abused it, this is where to look. Replace your Basket and Disc to get back to new release performance. At least one Kawasaki MACH IV (750/H2) that I restored was so bad that the Disc tabs would get hung up on the Basket and not open and in some shifts, they would not clamp – even with the springs and plate under pressure.

Let’s talk about that Pull from the lever now – as the lever is pulled, it either pulls a cable or pushes a fluid (hydraulic) down to and at the side of the engine case. There, a Clutch Throw-Out or Pusher will convert the energy from the pulled lever to movement that separates the outer Basket from the inner Boss/hub and gives us the desired pause in energy transmission.

The methods of opening the sandwich (Throw Out) range from Poor, Good, Better, and Best.

Motorcycle Clutch Lever Throw Out Poor Design
Poor is how I’ll describe the old standby. As used in our 1973 Yamaha DT3 here and in hundreds of other models, the cable pulls a short arm. The short arm rotates a coarse Screw/Threaded insert that is moved inward to push against a long push rod. The rod is partially exposed to the worst of environments – the drive chain as it spans the distance between the left cover to the Pressure Plate on the other side of the crankcase. Both the Screw/Threaded insert and the rod receive the worst in dirt, oil, grime, and contaminants as the chain makes the sharp turn at the front sprocket. The example shown is almost new as the parts came from a bike with less than 20 miles on it since cleaning and repair. This gets my Poor rating because at 1,000 miles and when used in the mud and dirt as this bike was intended – the moving components will be much worse.

Motorcycle Clutch Lever Pull Throw Out Internal Poorly DesignedGood is the hydraulic clutch. Those push the same type of rod through the crankcase to push the Pressure Plate on the other side of the crankcase.

I rate these as Good because I’ve seen some that will use the exposed push rod configuration and others that use the enclosed push rod configurations. I give these only Good because of the potential for loss of use through fluid loss and the expense of replacement.

Motorcycle Clutch Lever Pull Throw Out Better DesignBetter is the Cam Type. This uses a vertically mounted shaft with a short arm attached to the top. When our Cable pulls the arm, it rotates the shaft which has a eccentric cam (like the throw on a crankshaft). The eccentric pushes a ball/roller and that pushes a short rod that pushes the Pressure Plate to open. I give this style Better because all moving parts are sealed inside the crankcase and are bathed/lubricated in engine oil.

Motorcycle Clutch Lever Pull Throw Out Best Roller Bearing StyleBest is the Internal Teardrop Bearing design. While I have only seen this used on one motorcycle, it’s function, feel, and performance is the Top of the Food Chain in my opinion. This model uses 3 roller bearings and two hardened metal “Teardrop” ramps. One is held in a fixed position while the other is rotated by a short arm. The entire bearing assembly is held inside the crankcase and is bathed/lubricated in engine oil. There is no rod and nothing passes through the crankcase (left to right) as most motorcycles do, all of this Teardrop Bearing has it’s parts maintained on the Right side immediately at the end of the clutch.

Best Motorcycle Clutch Lever Pull Roller Bearing System Is Bathed In Fresh Engine Oil and Lives Inside the Engine Cases

This model of clutch pusher has delivered my all-time favorite feel, ease of maintenance, and linear Pull/Release feel of all the Clutch Systems I’ve ever used. Designed and put into use back in 1969 in the Honda CB750 Four, I have wondered why others did and still pass a dirty, grit covered rod through the seal and into the crankcase. Study the photos and you just might ask yourself this same question – Why. Yes, It’s that much better!

Best Motorcycle Clutch Lever Pull Throw OutBest Motorcycle Clutch Lever Pull Throw Out Roller Bearing Type


 
 
 
 
 
 


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