Link and swingarm pivot maintenance to get the most out of your linkage monosus!

The monosus (single suspension), which has a link in the middle of the suspension, has an advantage over the double suspension, which directly connects the swing arm to the frame, in that it allows a greater degree of freedom in suspension design and the pursuit of the ideal. However, in order to get the most out of this feature, it is important to reduce friction loss in each moving part. If the links are full of resistance, even a high performance rear shock will not perform to its full potential, so proper maintenance is necessary.

The feature of the link type suspension is that it is soft at the beginning and can be hardened as the suspension contracts.

The rear shock tends to get dirty all the time due to splashing water and splashed chain oil. The only thing you can see on the links are the bolts and nuts, but you have to understand that the collars and bearings also work in a harsh environment and need to be maintained. If water on the damper rod rusts the plating or gravel on the rod damages the oil seal, the oil inside the damper will leak and the damper itself will become useless.

The Kawasaki GPX250R's link-type monosuspension consists of a central rocker arm attached to the frame and left and right tie rods attached to the swingarm. When the wheel strokes, the swingarm pulls up the tie rods, which in turn push up the rear shock connected to the right side of the rocker arm.

As you know, there are two types of rear shock fitted to the swingarm: twin shocks, which are more common on naked and traditional models, and monoshocks, which are more common on sports models. Since the majority of supersport models, especially racers, use monoshocks, you probably have a strong impression that monoshocks have higher performance.

With twin shocks, where the rear shock is installed between the swingarm and the frame, the displacement between the wheel and the rear shock is simply proportional, from the most extended to the most retracted position. On the other hand, the mainstream monoshock, link-type suspension, incorporates a link mechanism between the rear shock and the swingarm. ThisBy changing the distance between the frame and the rear shock, and between the rear shock and the swing arm, the linkage mechanism creates a situation where the amount of stroke of the rear wheel and the amount of stroke of the rear shock are not in a simple linear proportional relationship.

A simple linear, which may be hard to understand, is called a progressive characteristic.Progressive means that the stroke of the rear shock increases in proportion to the stroke of the wheel.If the spring constant is constant, then the strength required to contract the spring by 1 mm is the same whether the spring is in its most extended or most compressed position. If the spring constant is constant, then whether the spring is at its most extended or at its most compressed position, the strength required to contract the spring by 1mm is the same. In the case of twin shocks, the stroke of the rear shock is simply proportional to the wheel, so a softer spring with a smaller spring constant will give a softer ride initially, but will remain soft until full stroke. On the other hand, if you use hard spring with large spring constant, you will feel hard ride and push up even when you drive straight road.

On the other hand, the progressive characteristic of the link-type monosus allows the rear shock to move less when the amount of wheel stroke is small, such as when driving on a straight road, and uses the area where the reaction force of the rear shock spring is small for a comfortable ride. In the area where G forces are applied to the car body and the wheel strokes are large, the shock uses the area with large spring reaction force to stabilize the car body. The heavier the car and the more power the engine has, the more load the rear shocks will have to carry, so the springs need to have a high spring rate to support the load. However, in the small stroke range, the rear shock can be stroked so that the reaction force of the spring does not become too large, thus ensuring a comfortable ride.

The more fulcrums in the link, the more friction loss.

The grease on the surface of the collar, which is inserted into the three connecting points of the rocker arm, has lost its oil content and stuck like a viscosity after being left for almost 15 years. It's still good because it was stored indoors, but outdoors, the hard chrome plating on the collar surface can also rust.

I sprayed some parts cleaner and scrubbed with an abrasive sponge, and it was a relief that there was no rust on the plating. It depends on the model, but if the needle roller bearing is press-fitted from both sides of the rocker arm, only the middle part of the collar where the left and right bearings do not touch may rust and get stuck on the bearing.

Fortunately, a collar like the one in the image above, which looks corroded, was polished with a sponge and the plating was still alive as shown below. If the plating film is corroded to the extent that it is gouged, the collar should be replaced with a new one. Assembling it with plenty of grease because the parts are not available may result in damaging the bearing.

The advantage of the linkage type monosus is that you can add a wider range of flavor to the suspension than with twin shocks.Links increase the number of fulcrums and moving parts, which may increase friction loss.. Needle roller bearings are pressed into the bearings of the link, and grease is applied between them and the bushes and collars, so that, with sufficient lubrication, the link can be actuated in response to the stroke of the wheel to activate the rear shock.

However, if lubrication is inadequate due to lack of maintenance or age, resistance can be created in the process of transferring the impact of the tire to the rear shock. The weight of the car and the inertia of the links is much greater than the lack of lubrication, so if the load change is large, the links will be forced to move and the rear shocks will be forced to contract, but if the load change is small, the rear shocks will not be able to move easily and the shock absorption will be reduced. When we talk about suspension performance, we tend to focus on high loads, but unless you are a frequent circuit or Sunday race rider, it is more important that the suspension works well under low loads. Even on a circuit where the suspension can move a lot, it is important to have smooth movement in the low load range as the load gradually increases from the straight to the entrance of the corner.

This applies not only to the suspension but also to the carburetor and injection. If you focus on the power and setting at full throttle and neglect the area where the throttle opening is small, you may have a hard time riding when starting from a stoplight or exiting a corner. It is natural for the swingarm to move and the rear shock to contract when a large load is applied, but what is important is to remove as much friction loss as possible from the pivots, links and other moving parts so that even small changes in load can be transmitted to the rear shock without resistance.

Greasing the links is essential to allow the rear shock to do its job.

Once you've seen the devastation of the linkage, it's hard not to check the swingarm pivot. The swingarm collar can also be fat and hard to pull out due to rust in the middle, but I was lucky enough to pull it out despite the rust.

After cleaning the old grease from the needle bearings that are press-fit into the swingarm, apply a grease with good extreme pressure and water resistance. Here we use Superzoil grease. Press the grease from your fingertips against the needle of the bearing to spread it over the surface as well as the back side and press it in.

The collar of swing arm is also polished with polishing sponge to remove dirt. The needle roller bearing on the swing arm side is in contact with both ends of the swing arm and the middle part is not touched.

The vehicle in the image has been left for a long time, and the grease on the collar has completely deteriorated and the lubrication performance has been completely lost. However, looking at the rear suspension from the outside, the condition of the collar is not obvious. If you replace the rear shock only because it is not moving well in this condition, you will not be able to get the full value out of the replaced shock. Although the condition is not as bad as this, it is not uncommon to find that the rear suspension movement, especially in the low load area, has been improved and the ride comfort has been improved after just cleaning and greasing during regular maintenance. Although the improvement is just a matter of returning to the original state by greasing, there is no way not to perform the maintenance because the suspension will respond sensitively to small load changes just by removing the dirty and deteriorated grease and applying new grease.

For lubrication of the linkage area, a product with excellent water resistance and extreme pressure such as a lithium-based or urea-based grease is suitable. Molybdenum disulfide grease may be specified for some models. The bushings that have been pulled out of the link will probably be cleaned with a parts cleaner before applying grease, but the old grease that remains in the bushings and needle roller bearings that have been press-fitted into the link should also be thoroughly removed before applying new grease.

The swingarm pivot is also cleaned and greased at the same time, which will also improve the movement of the swingarm itself. Lubrication of the collars and bushes is essential for both monoshock and twin-shock bikes, as the swingarm pivot is not only the swingarm's axis of motion, but also the driving force of the drive chain.

To remove the rear shock, links and swingarm, you need to be prepared to stabilise the bike when these parts are removed. If the swingarm is removed, you will not be able to use a standard racing stand or maintenance stand, so if your bike does not have a centre stand, you will need to use a jack to support the bike from under the engine.

The maintenance of monosus links requires a lot of preparation, and the work itself is quite simple, but it is very important for the stability of the bike. Installing a high quality, high performance rear shock when the grease has deteriorated and the links are sluggish is a waste of money. Disassemble and grease the links before the rear suspension starts to feel sluggish.