Centrifugal clutcH

Principle of operation

When you tie an object to a piece of string and rotate it around a vertical axis, then there is a tension (or force) that you feel in the string, and this force is commonly referred to as the centrifugal force.  The more mass the object has or the faster you rotate it, the greater the force will be.  This force is directly proportional the mass of the object, but is also proportional to the square of the speed of rotation.

All the clutches we use on karts rely on this principle for their operation, and the easiest way to understand how it is put into practice is to examine the clutch of the Comer C50 engine as shown in the exploded view below.  The 3 clutch shoes are held together with springs, but if unhindered, they are allowed to slide radially outwards along the legs of an inverted ‘Y’ shape of the boss as indicated by the dashed arrows.

The whole of the clutch assembly is fitted to the end of the crankshaft, but note that the clutch drum itself is mounted to the crankshaft via a needle bearing i.e. it is free to rotate on its own.  As the crankshaft (engine) speed is increased, the shoes begin to move outwards against the forces exerted by the springs until their outer diameter eventually equals that of the inside of the clutch drum.  At this point the friction between the shoes and the inside of the clutch drum takes over and they then begin to rotate as an assembly, finally achieving the same speed as the engine.

Engagement speed

Until a while ago, the clutch engagement speed for all classes was specified in the rules as being a maximum of 4 000rpm.  For the Rok classes its usually 3 500rpm.  The engagement speed is engine rpm at which the kart, without a driver, will start to move when on a flat surface.  Prior to 2019, this requirement was stated in the Karting Regulations a.k.a. the SSR’s, but is now found in the Technical Regulations covering all the various classes.

Clutch components and sizes

The major clutch components for the various classes are shown below.

When checking whether the individual components are within specification, reference should be made to the relevant series and class technical specifications as, for example, certain classes allow for the number of teeth on the pinion to be changed whilst others do not.

The basic operation of all the clutches was discussed using the Comer as an example.  For the Kid- and Mini-Rok classes, if one examines the clutch disc you will notice that there are 3 slots in the disc and there is an oval hole at the end of each slot.  Because the slot is very close to the outer edge of the clutch disc, a ‘weak area’ is created which allows the portion of the disc with the shoe affixed to it, to bend slightly along the bend line indicated in the picture.  As the rotational speed of the clutch disc is increased, centrifugal force then allows each part with its shoe attached to move radially outwards and make contact with the inner wearing surface of the clutch drum, thereby engaging the clutch.

The Rotax clutch disc employs a similar principle to that described above.  Referring to an earlier picture, both the older and newer style of clutch drum are allowed to be used in the Rotax Max classes.  The newer style of drum is easily identified as it has a rib added along its outer edge which was an attempt to minimize cracking of the drum.  Despite this, they are still prone to cracks albeit to a lesser extent, so they should be checked on a regular basis.  If cracks are present, they are more easily spotted when viewing the internal wearing surface rather than the outside of the drum.

The same clutch is used across all of the Rotax classes.  On the DD2 class, although the disc and drum have the same diameters as the other classes, the disc itself is ~3mm thicker and the drum also has a different overall height.

Servicing and cleanliness

Apart from the DD2 class where it is quite a mission to gain access the clutch as it is hidden within the gearbox housing, the clutches on all other series and classes are readily accessible after removing the main chain guard over the rear sprocket, the chain, and also the small plastic chain guard that is bolted to the side of the engine (Rotax and Rok).  Before attempting to remove the nut that holds the clutch drum in place, the crankshaft needs to be locked to prevent rotation.  Special tools are available to aid this, but a screwdriver blade lodged against the teeth of the flywheel ring gear can also work when all else fails.

The contact area between the clutch and the clutch drum has to be kept dry at all times and no lubrication is allowed to be present when it is stripped for inspection.  A certain amount of lubricant weeping from the needle bearing is allowed to be present, but this obviously has to be kept to an absolute minimum as typically shown in the picture below which is an extract from the Rotax regulations.  A similar amount of weeping would be acceptable for the other series.  Note that on the Rotax clutch, there is an O-ring that must remain fitted at all times to ensure good sealing between the clutch drum and the needle bearing.

Servicing of the clutch i.e. stripping, cleaning and inspection, should be done at regular intervals regardless of the series or the class.  When doing so, remove the clutch drum and the clutch from the motor to inspect the drum and clutch for cracks.  Also check the friction surface on the clutch, and confirm that all the relevant dimensions are within specification according to the regulations.  Regularly clean and apply new grease sparingly to the needle bearings within the drum and ensure that there is not excessive grease on the inside of the drum as previously discussed.

Emile McGregor - MSA Technical Consultant