Scooter Variator and Clutch

A special feature of the motor scooter is its final drive via the variator. This is an ingeniously simple continuously variable transmission, made up of just a few components, which transmits the engine power to the rear wheel. The variator is ideal for smaller engines and is a weight- and cost-saving alternative to the gearbox and shaft or chain drive used on the majority of motorcycles. The variator was first installed on a scooter in the late 1950s by the German manufacturer DKW, in the "DKW Hobby", a scooter with a 75cc two-stroke engine that powered it to an impressive top speed of nearly 60 km/h (40 mph).

When it comes to the maintenance and tuning of your scooter, you'll find that the variator is pretty much at the top of the agenda. This is because, on the one hand, its components are subject to a certain degree of wear and tear, and on the other hand, a badly tuned variator can cause sluggish engine performance. 

To understand the workings of your variator, think of the transmission of a bicycle with several chain rings and sprockets (such as a mountain bike), which most of us are familiar with. When you move off, you use a small sprocket at the front and a large sprocket at the back. Then, as your speed increases and you have to overcome less resistance (e.g. on a downhill stretch), you shift the chain to a larger sprocket at the front and a smaller one at the back.

The variator works in a similar way except that it uses a drive belt instead of a chain, is continuously variable and is controlled automatically via centrifugal force adjustment, depending on the speed of rotation. 

The drive belt runs in the gap between two V-shaped pulley halves, which are a variable distance apart on the crankshaft. The front inner pulley half also acts as the housing for the variator roller centrifugal weights, which run along precisely calculated curved paths. 

A contra spring presses the rear pulley halves against each other. When you move off, the drive belt runs close to the shaft at the front and at the outer rim of the rear pulley. When you open the throttle, the rotational speed of the variator increases and the centrifugal force pushes the roller weights outwards. They in turn press the movable half of the pulley outward, so the gap between the two pulleys becomes smaller, and the drive belt is forced upwards to a larger radius, i.e. it moves outwards.

As the drive belt does not have much stretch, it presses the springs apart at the other end and moves inwards. The end position of the belt is the exact opposite to its starting position, so you now have a high (speed-increasing) gear ratio instead of a low (speed-reducing) ratio. Of course, the variator scooter must also be able to idle. An automatic centrifugal clutch has the job of disconnecting the rear wheel from the engine at low speed, and connecting it as soon as you twist the throttle and the engine speed increases to a certain level. For this purpose, there is a clutch bell housing on the drive of the rear wheel. At the rear of the variator, spring-loaded centrifugal weights, which are fitted with friction linings, rotate in this housing. 

To access the variator, you will first need to remove the variator cover. Start by cleaning the outside of the cover, and also check which components you need to remove in order to access the variator. It's possible that the rear brake line is fixed to the bottom of the cover, or the kick starter is in the way. On some models of scooter, as in our example, you will also need to remove the intake pipe of the fan cooling system or the airbox.

Any fairings which prevent you taking off the variator cover will, of course, need to be removed.

In some cases, the rear drive shaft is also mounted in the cover and fixed with a nut, which will first need to be undone. On the larger variator cover, you will also find a separate smaller cover that needs to be removed. To remove the nut, you will need to hold the variator with a special tool to stop it turning.

Once you're sure that there are no other components preventing removal of the variator cover, undo the retaining screws in diagonally opposite sequence working from the outside inwards. Note the positions of the individual screws if they are of different lengths (write it down!) and make sure you don't lose any of the washers. 

You should now be able to take the cover off. If not, check to see where the sticking point is. You may have forgotten a screw – whatever you do, don't use force. Only if you are totally sure that you haven't overlooked any screws is it OK to resort to the rubber mallet to loosen a very tightly fitting variator cover. 

Once you have successfully removed the cover, make sure that any adapter sleeves remain in place and can't get lost. 

If the rear drive shaft projects into the cover, you will generally find there is a loose bushing somewhere, which you also mustn't lose. Clean the inside of the cover thoroughly, removing all dust and dirt. If there is oil in the variator housing, it means you have a leaky engine gasket or drive gasket, which must definitely be replaced. You are now looking at the variator.

To fit a new drive belt or new variator rollers, start by undoing the retaining nut of the front pulley halves on the crankshaft journal. While doing this you will need to hold the variator still with a special locking tool. 

If the front pulley is toothed, you can buy a locking tool that fits your scooter. If there are holes or stury fins on the face of the pulley, you can grip these with a suitable holder. 

Skilful DIY mechanics can make their own locking tool or holder out of flat steel. If you block the pulley by holding the cooling fins, be very careful to avoid damaging or breaking the fins.

Once you have loosened and removed the nut, you will be able to remove the front pulley. If there is a kick start drive wheel sitting on the shaft behind the nut, make a note of its position. 

You will now be able to access the drive belt. The belt must not be cracked, brittle, frayed or have any broken teeth. It must not be oily and must have a certain minimum width (check with your local dealership to find out what the wear limit is). A large quantity of rubber debris in the housing caused by abrasion may indicate that the belt is not running correctly in the variator (investigate the cause!) – or may just be due to the fact that maintenance is long overdue. Premature wear to the drive belt may be due to an incorrectly installed or worn pulley. 

If the pulley halves are grooved, they must be replaced (see above). If they have turned blue due to heat, this means they have warped or were incorrectly installed. If the drive belt does not yet need replacing, wipe it clean with brake cleaner and mark the correct running direction on it before continuing. 

To check the variator rollers, or to replace them, pull the front inner pulley half with the variator housing off the shaft as a unit. 

The housing may be fixed to the pulley or loose – so hold the unit very firmly together as you remove it to make sure the components don't all fall apart, and to keep the variator weights in place.

Now dismantle the housing of the variator rollers – make an exact note of the position of the individual parts. Clean them with brake cleaner. 

Check the variator rollers for wear and tear – if they are worn, display flat spots or angular wear, or are irregular in diameter, replace them as a set.

When assembling the variator housing, the variator rollers and tracks have to be either lightly greased or installed dry (ask at your local dealership). 

If the variator housing is fitted with an O-ring, this will need to be replaced with a new one. When fitting the unit on the shaft, take care to ensure that the variator rollers stay in position in the housing, otherwise you will have to remove it again and reposition the rollers.

Press the rear pulley halves apart, so that the belt can slide deep between the two halves and has more space at the front. 

Now install the front outer pulley half of the variator with all its components – lightly grease the shaft before fitting the bushing. Make sure the drive belt is positioned evenly between the pulley halves and do not jam it. 

Before putting on the nut, double-check that all components are in their original position, and apply a little threadlock adhesive to the nut. 

Use the locking tool again and a torque wrench to tighten and torque the nut in accordance with manufacturer specifications. If it makes it easier, get someone to hold the locking tool for you again while you work! Check again that the pulleys of the variator are running straight in relation to the sealing surface of the housing when you rotate the variator. 

If they are wobbling, check that you have mounted everything correctly! Tauten the drive belt by pulling it slightly out from between the rear pulley halves.

Remove the clutch bell housing from the shaft so that you can check its inner running surface and the linings of the centrifugal weights. Check with your local dealership about wear limits – linings that are less than 2 mm thick or have worn unevenly need to be replaced immediately.

If you are changing the clutch linings and springs, it is better to pull the entire rear pulley unit, complete with clutch, off the shaft, as it has to be screwed on and this is made more difficult by a spring inside. To do this, first remove the drive belt. To undo the central nut on the shaft, hold the clutch bell housing firmly with a special tool that grips in the openings of the bell housing, or hold the exterior of the bell with a strap wrench. Once again, it may be helpful to get someone to hold the gripping tool in place while you undo the lock nut. 

If the nut is located on the outside, you can undo it before removing the variator cover – in which case, you will already have carried out this step, as in our example. When you have unscrewed the nut, remove the clutch bell housing and, as mentioned in the previous section, check for signs of wear and tear (grooves). If the clutch linings are worn or the springs of the centrifugal weights have become stretched, you will need to pull the pulley unit, complete with clutch, off the shaft, as described in the previous section. This unit is held together by a large central nut. 

To loosen this nut, grip the clutch with a suitable tool, such as a metal strap wrench, and use a special wrench of the correct size to loosen the nut – water pump pliers are not suitable! 

This spindle also helps to control the spring during reassembly so that it is easier to put on the nut.

Once you have detached the clutch from the pulley, you can replace the springs and the linings. Always use new circlips when changing the linings and make sure that they are securely seated. 

Care of the clutch bearings

The bushing of the pulley unit generally contains needle bearings – it is crucial not to allow any dirt to get into the bearings and to make sure that they are running smoothly and easily. If necessary, clean by spraying out with PROCYCLE Brake Cleaner and re-grease. Check the bearings for leaks – because any grease landing on the drive belt may cause it to slip.

Installing the clutch

To install the clutch, proceed in reverse order. Tighten the outer central nut using a 3/8", 19 – 110 Nm torque wrench – check with your local dealership for the correct torque. Before closing the variator housing again, double-check that all the components are correctly installed and then re-mount all external components back in their original position.