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The clutch is a mechanism which is used for transmitting rotation from one part to another, such as from the engine to the drive sprocket. 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.

Most mopeds will have two clutches, a starter clutch, for starting the bike and a centrifugal clutch for making the bike go. Designs of these essential components vary widely, but all are similar in principle.

Centrifugal Clutches

An aftermarket 3-shoe centrifugal clutch for the e50 engine. The stock e50 clutch typically has only 2 shoes.

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, the weight of the shoes, along with centrifugal force begins to force the shoes 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.

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.

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.

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.

e50 Clutch alignment

When installing a new clutch, it is important to check the distance between the pressure plate and the face lining of the clutch. If the gap is between 0.012" and 0.020" your clutch will engage, if the gap is larger the clutch will not engage fully when the lever is pulled.

To measure this gap use a feeler gauge between the castellated part of the clutch drum and the pressure plate (the flat bit on the "back" of the centrifugal clutch) if the distance is greater than 0.012" and 0.020" you will need to adjust it.

First things first, this is easier to do on the bench and therefore I'm going to describe how to do it that way, it maybe possible in situ... I'm also going to assume you know how to dismantle and remove your clutch.

Before you take anything apart - check your end float The end float is the distance between the circlip nearest the bearing and the primary gear, this distance should be 0.004" and 0.008" - if the gap is larger - you'll need to change your shims.

Disc Clutches

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Variator Clutches

see Variator

Clutch Parts

See Also