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If you’ve read the post about the motorcycle transmission, you’ll know that the clutch is responsible for temporarily disengaging the engine from the rear wheel.
But what about the clutch itself? What does it consist of?
And what’s the difference between a “wet clutch” and a “dry clutch?”
In this blog post, you’ll discover:
- The 4 main components of a motorcycle clutch
- Differences between wet and dry clutches
- How long a motorcycle clutch typically lasts
- What the “friction zone” really is
Prefer to get this information in podcast form? Listen to the 30 Minute Motorcycling Podcast episode about the clutch and clutch friction zone:
Why Do You Need to Disengage the Engine from the Rear Wheel?
The short answer is so you can change gears.
If you didn’t have a clutch, the only way to change gears would be to come to a complete stop. This would be fine for first gear, but not so for any higher gears.
Have you ever tried to pull away from a stop in fifth gear?
Clutch Operation – Hydraulics vs. Cable
Hydraulics
Like a hydraulic brake system, a hydraulic clutch uses transmission fluid, similar to a car.
And just like hydraulic brakes, the lever for the clutch isn’t as stiff when you squeeze it compared to a cable-operated clutch.
However, it also means that you need to renew and replace the transmission fluid every now and then.
Cable
Cable-operated clutches are cheap to make, and as a result, they’re the most widespread form of clutch for motorcycles.
It’s not hard to see why – compared to hydraulic clutches, cable-operated clutches are much simpler to maintain.
This means they don’t require you to buy any additional fluid for the clutch to work.
However, the clutch cable will wear out eventually, and when it does, you’re in for a clutch cable replacement job.
In a mechanical brake system, there’s no liquid whatsoever.
Instead, power is delivered through a metal cable that tensions as you squeeze the brake lever - similar to how many motorcycle clutches work.
Although not so much on modern motorcycles, these brakes are highly common on bicycles and e-bikes.
What Does a Motorcycle Clutch Consist Of?
The clutch hub
The hub (or “center”) of the clutch is locked onto the input shaft through a series of tiny grooves called “splines.”
It’s these spines that make it possible for the hub and the input shaft to rotate together and for the clutch to move freely back and forth along the shaft.
If the clutch couldn’t move freely along the input shaft, it might not disengage fully.
Do you have a clutch hub for an older bike? Then you should know that it might contain asbestos, which is toxic to inhale.
Clutch plates
Aside from a pressure plate, clutch plates are either friction or steel, which sit between the housing and the clutch hub.
The smaller the clutch, the more plates it will contain.
The two types of plates alternate between one another – i.e., if you’ve got four friction plates, you’ve got four corresponding steel plates as well.
While you’re riding your bike, the two plate types are pressed together by either a set of coil springs or by a large diaphragm spring.
As you pull the clutch lever in (e.g., shift into second gear), the pressure plate separates these two plates – either through a cable or through hydraulics.
While the clutch is disengaged, these friction plates press the clutch hub against the flywheel, which transfers the power from the engine to the transmission.
Diaphragm or coil spring and pressure plate
Coil spring
Diaphragm spring
Many motorcycles clutches have between four and six coil springs to maintain tension on the clutch plates.
Coil springs are cheaper to make, although they can cause clutch drag or slipping if the tension is uneven.
Also, the more a coil spring is compressed, the stiffer the clutch lever will be.
Another alternative is the diaphragm spring, which gets its name because it resembles a diaphragm.
Compared with the coil spring, the diaphragm isn’t as stiff when you pull in the clutch (apart from the first few compressions when the diaphragm is brand new).
Clutch housing
This part is easy to explain – it’s the part fitted on the end of the input shaft and holds all the components mentioned above together.
Wet vs. dry clutch
The most significant difference is that a wet clutch is soaked in engine oil, while a dry clutch operates in “dry” conditions – i.e. with no extra oil.
Outside of MotoGP, however (and a couple of Moto Guzzi models, most road motorcycles use the wet clutch type.
Why do road motorcycles use wet clutches?
The biggest reason is that it doesn’t produce as much friction as a dry clutch. This reduces clutch wear and tear, but also:
- Increases the clutch lifespan
- Reduces noise
How Long Does a Clutch Last?
If you look after your clutch properly, it can last up to 32,000 kilometers (20,000 miles).
On the flip side, if you don’t look after your clutch properly, it’s only good for as little as 8000 kilometers (5000 miles).
What is the “Clutch Friction Zone?”
Finally, let’s talk about the “clutch friction zone” – a term you probably heard while you were still learning how to ride.
But what is the “friction zone” from a mechanical point of view?
As you begin to release the clutch and connect the engine to the rear wheel, the clutch’s rotation must match the engine’s rotation.
If you roll on the throttle and let go of the clutch too early, the engine will receive an insufficient amount of power and you’ll stall it out.
Anyway, between “fully engaged” and “fully disengaged,” there’s a clutch zone where the rear wheel begins to get power from the engine.
Once you find this zone, you’ll experience that the bike will begin to move forward – albeit slowly.
As you probably already guessed, this is what the “friction zone” is – it’s the point where the engine will begin to transfer power to the rear wheel.
