February 29 2012 by Jason Fogelson
I'm no mechanic. That's been proven to me many times, in most unpleasant ways. But, I still think that it's important to understand how my motorcycle works. That way, I can recognize small problems before they become catastrophes, and I can work with my mechanic to make things work properly. This week, I decided to look deeply into my motorcycle's clutch to unravel its mysteries.
The job of the clutch is to temporarily disconnect the engine from the transmission. This disconnection is essential to safe operation of a modern motorcycle. Otherwise, you'd have to turn off the engine at every stop, and you'd never be able to change gears. Your car has a clutch (or more), even if it has an automatic transmission. Even your electric drill probably has a clutch. On a motorcycle, we pull in the clutch lever in order to disengage the transmission, then slowly let the clutch lever out in order to engage the transmission gears with the engine -- and the bike moves under power. But what's happening inside the clutch when we pull and release that lever?
As you probably know, I own a 1993 Harley-Davidson Sportster named "Manny." My bike is equipped with a 5-speed manual transmission and a hand clutch. Sportsters and Big Twins have what's known as a "wet" clutch, which is just what it sounds like -- the "wet" part is transmission fluid in a Big Twin or engine oil in a Sportster. Some bikes, most famously Ducatis, come with a dry clutch -- but they still operate in much the same way.
When the clutch lever is pulled, the transmission is disengaged from the engine. Inside the clutch, there are a series of friction plates. They're round, with a hole in the center, and they fit over a clutch hub. Pulling the clutch lever releases the pressure that keeps the friction plates touching tight against each other, allowing the flywheel to spin freely. Release the lever, and the friction plates catch on each other, and engage the engine to the transmission. It is necessary to have a clutch in order to change gears -- you disengage the transmission from the engine for a moment, switch the gear alignment, then reengage, and you're riding in a different gear. The friction plates help you to negotiate the disengagement and reengagement smoothly. Next time you ride, try to feel what's happening inside your clutch as you engage and release the lever. You should be able to find the friction point -- where the plates begin to engage -- and the engagement point -- where the transmission is fully married to the engine. The smoother you can facilitate this engagement and disengagement, the smoother your ride will be.
On a Harley-Davidson, the clutch is released mechanically, with a cable and springs. On some bikes (and some cars), the clutch is hydraulic, and operates via fluid pressure. There are plusses and minuses to each form of clutch. In general, the mechanical clutch requires more hand strength; the hydraulic clutch is more complex and requires more maintenance. Adjusting the mechanical clutch on your Harley-Davidson is a very simple operation requiring just a couple of wrenches. You don't have to crack the engine case or even get your hands very dirty -- you simply adjust the tension of your clutch by tightening and loosening the adjustment nuts on the cable. Keep your cable well-lubed and adjusted, treat your clutch politely and you may never need to do any further clutch-related work than that.
This is hardly a treatise on clutches. It's just a quick introduction. If you want to dive deeper, you can explore your bike's factory service manual (you did buy one of those for your bike, didn't you?). You may never apply a wrench to your bike, but at least when your mechanic talks to you about your friction plates and clutch hub, you'll have some idea of which part of the motorcycle he's referencing.