The clutch is a mechanism which can engages and disengages, and is used for transmitting rotation. When the clutch is engaged, movement is transmitted through the clutch from the crankshaft to the clutch bell, clutch housing, or clutch drum. When the clutch is disengaged, movement is not transmitted through the clutch, and the crank is able to spin freely without load. A disengaged clutch allows the engine to remain running while the moped is at a stop.
There are several kinds of clutches found on mopeds, including variator clutches, disc clutches, and centrifugal clutches.
The centrifugal clutch is fairly common, and is found on the Puch E50 engine. In a centrifugal clutch, clutch shoes are held close to the crankshaft by springs, away from the clutch bell. As the crankshaft spins, its motion is directly transferred to the clutch shoes, which spin inside of the clutch housing, but without touching it. As the RPMs increase, and the rotation becomes more and more rapid, the weight of the shoes, along with centrifugal force begins to push the shoes away from the crankshaft, towards the clutch housing. Eventually, the clutch shoes begin to touch the clutch housing, and cause it to rotate as well. However, initially, this light contact will not cause the clutch housing to rotate at the same rate as the crankshaft and clutch shoes. This intermediate stage of clutch engagement is called "clutch slip." When centrifugal force pushes the shoes out far enough that they contact the clutch housing with sufficient friction to cause it to rotate at the same rate as the crankshaft, the clutch has engaged fully.
In the Puch E50 engine, part of the clutch housing is toothed, and these teeth mesh with a final drive gear, which is located on the same shaft as the front sprocket. The number of teeth on the clutch housing and on the final drive gear determines the "internal gearing" of the Puch E50 engine.
However, if the RPMs drop to a point where centrifugal force can no longer hold the shoes engaged against the clutch housing, the springs will push the shoes towards the crankshaft again, and the clutch will disengage to allow the engine to idle.
When starting an engine with a centrifugal clutch, such as the E50, a device is needed to force the clutch to engage, so that rotation may be transferred from an operator controlled mechanism (like the pedals) to the crankshaft to facilitate starting. In the E50, a plate sits just on top of the clutch shoes. Normally, it does nothing. However, when the clutch engagement lever is pulled, it pushes the plate towards the clutch bell, which has grooves into which flanges of the clutch plate fit. When the clutch plate is pressed into position, the clutch housing is forced to rotate. On the inside of this plate is also a thin cork lining. As the clutch plate is pressed into its grooves, this cork lining presses against the clutch shoes, and forces the clutch shoes, and by extension, the crankshaft, to rotate with the clutch housing, and the operation of the pedals.
Check out Moped TV (click on the on demand button to browse old episodes) to see a video about removing a clutch and installing after-market clutch springs.
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-Clutch bell, drum, or housing