How to Prevent Oil Leaks and Rod Damage as it is Difficult to Notice?

The mono-shock has become the standard rear suspension system for sportbikes, as it allows more tire stroke than a traditional two-suspension system, progressive spring and damper characteristics, and mass centralization by placing the heavyweight in the center of the vehicle. However, because it is difficult to see from the outside, it is also difficult to notice any deterioration or damage. Let's take a look at the rear suspension from underneath the motorcycle once in a while.

Because it is located in the center of the vehicle, it is easily exposed to dirt while driving.

The Kawasaki GPZ400F shown here has a collective muffler and the center stand has been removed, so a double pantograph jack is used to lift the rear tire. Note that the maintenance stand (racing stand) that supports the swingarm can be used to remove the tire, but not to remove the rear shock or linkage.

When you remove the rear tire, you can clearly see that the rear shock is located on the centerline of the tire. Although the rear shock has already been removed, the stock fender covers the rear shock from the swingarm, so the tire is not exposed to rainwater. However, this is the lower part of the motorcycle, so it can get dirty easily.

The use of mono-shocks in the rear suspension was first introduced in motocrossers in the late 1970s, and was followed by on/This is a mechanism that has been increasingly adopted by all off-road vehicles. In addition to supporting the load of the rider and luggage and ensuring stability while riding, the use of mono-shocks has improved the performance of the suspension, which is essential for sport riding.

In the case of two shocks placed between the swingarm and the seat rail, there is a simple proportional relationship between the amount of stroke of the rear tire and the spring reaction force. If the spring constant, which indicates how much the spring expands or contracts under load, is constant, then the greater the stroke (the more the rear tire sinks), the greater the spring reaction force. Therefore, when driving on winding roads or circuits where the load on the suspension is high, a spring with a large spring constant, in other words, a stiff spring, is required.

However, using a stiffer spring has the effect of making the ride uncomfortable due to the high reaction force, even when the load on the suspension is small, such as in town or touring. In addition, the shock between the swingarm and the seat rail becomes a sticking point, limiting the amount of stroke.

On the other hand, mono-shock, especially mono-shock with links, can be made to have different characteristics than two shocks. Progressive characteristics that are effective in improving performance. This is called the "spring reaction force". Simply put, the reaction force of the spring is small when the stroke of the rear tire is small, and it increases when the stroke is large. The lever ratio of the linkage increases the load on the spring in relation to the stroke of the tire in a quadratic manner, resulting in a softer ride on the street and a tighter ride in sports driving without being crushed.

At the same time, the mono-shock is mounted close to the swingarm pivot, as opposed to a two-shock with the weight attached to the rear of the swingarm, which helps to centralize mass by being closer to the engine, the heaviest component of the motorcycle. With the exception of traditional and small-displacement models, where style is important, and the Super Cub, which must also meet the needs of business use. The trend towards mono-shock on sportbikes, both off-road and otherwise, is a natural progression.

A mono-shock has a simple connection between the swingarm and the seat rail, whereas a linked mono-shock has multiple levers and pivots. A linked mono-shock has multiple levers and pivots. Lubrication between the bushings and bearings is essential to ensure smooth movement of the pivots while bearing the weight of the body and rider, and cleaning and greasing are essential. If the motorcycle is a big model with heavyweight, the suspension will stroke even with insufficient lubrication, but if the motorcycle is forced to operate, the bushes and bearings will be burnt and the movement will be poor, and the shock will not be able to transmit the load change smoothly to the shocks in the small load range, which will worsen the ride quality.

Along with the links, the rear shocks themselves need to be checked and maintained. Most mono-shocks are located low down in the center of the motorcycle, where they are vulnerable to the dirt that the tires roll up while driving. Although there are fenders and mudguards to protect the rear shock from dirt, they are not waterproof or dustproof. The same is true for the two shocks, which are difficult to see from the outside of the vehicle, and can deteriorate before you even notice.

Beware of bump rubber disintegration and piston rod rust.

This shock is equipped with a remote type preload adjuster, so it is removed from the frame as a set. It also has a dial-type damping force adjuster on the lower mount.

The surface of the piston rod is covered with a thin layer of rust that may grow on the surface of the piston rod, so I polished the surface with a metal compound and sprayed the rubber seal with a lubricant. There is a little oil stain on the cap of the shock absorber, so there might be a little leak already.

The donut-shaped bump rubber set on the piston rod has completely disintegrated, so I took it out of the gap between the springs in a ragged state. This part is not supplied as a single item, so you have to remove the spring and lower mount and insert rubber feet sold at home centers, or ask a suspension specialist to overhaul it.

The main points to watch out for with mono-shocks mounted on the lower part of the vehicle are rusty piston rods, oil leaks from the shock absorbers, and also deterioration of the bump rubber. All of these points are also applicable to 2 shocks, but as mentioned above, it is easy to overlook because it is difficult to see the shock body.

Both damage and deterioration can be caused by the moisture, grit, and dust that the motorcycle is exposed to while being ridden and the moisture it receives from the ground during storage. The same can be said for the rear shocks, as the small grit from the rain that the front wheel rolls up can matt the paintwork on the front of the engine crankcase.

If the shock strokes with rainwater and gravel on the piston rod, it may catch and damage the oil seal of the shock absorber. If you are driving in the rain, you can expect the rainwater to act as a lubricant and scrape it off at the lip of the oil seal, but if you store the shocks in a dirty condition after driving only the water may evaporate, causing the gravel to stick to the piston rod. There are two types of scratches. It is easy to get fine scratches if you rub the paint with a dry rag when you wash the motorcycle. If you suddenly stroke a dry piston rod, you'll damage the oil seal.

It would be ideal, but not realistic, to clean the piston rods after every ride on a machine that is not designed for racing. However, rather than continuing to ride the motorcycle without checking it and then suddenly noticing an oil leak and panicking, simply checking it once in a while, cleaning it and spraying it with rubber parts lubricant should be enough to delay the development of dirty rods and rust on the plating.

As well as dirty or rusty piston rods, check for oil leaks from the shock absorber. Most mono-shocks are inverted with the absorber at the top, opposite to the upright type that is common with two-shocks (even two-shocks with a reservoir tank are inverted). The inverted type has the damper oil in the upper part of the shock, so if the oil seal at the bottom of the absorber is damaged, the oil will flow out through the piston rod.

If the oil drains out, the damper piston at the end of the piston rod will not generate any damping force even if it is activated, so the contracted spring will extend with the same momentum and the shock will not be damped. Shock absorber oil leakage is not a problem that can be repaired by an individual, so it must be repaired by a specialist or replaced with genuine parts or an aftermarket shock unit. Oil leaks due to aging or dirty damper oil can also cause performance degradation, so it is not always possible to keep using your motorcycle as long as you take care of it, but it is absolutely effective to pay attention to the condition of damper pistons to avoid oil leaks as much as possible.

On the other hand, deterioration of the bump rubber is inevitable with age. Bump rubbers are made of flexible materials such as rubber and polyurethane, which act as shock absorbers when the shock is retracted to near full stroke. In this case, it would be ideal to remove the mounts screwed to the springs and damper pistons and insert new bump rubbers, but as bump rubbers are not the only genuine parts sold, you will have to ask a suspension specialist to repair them as in the case of oil leaks.

Once the shocks are removed I want to clean and grease the links as well.

If you remove the rear shock, you should also remove the linkage and swingarm to clean and grease them as well. This motorcycle had not been maintained for more than 10 years and the grease had dried upon each part.

The rocker arm, which connects the swingarm to the rear shock and linkage, is subjected to a lot of force, and the lack of grease on the surface of the collar has left some marks on the plating. As the plating is not yet peeled off, I could reuse it by cleaning and greasing, but if the problem is worse, I need to replace the parts.

It's not a good idea to ride your motorcycle because you're afraid of damage or loss of performance from the rear shock, it's a good idea to maintain it after riding.

If you have a full cowl model with only a side stand, it can be very difficult to remove the rear shock itself. Even if this is the case, it is still a good idea to lay down on the floor or ground to check the condition of the shock and links from the back of the motorcycle. Then, if you have access to a brush or rag, wipe the shocks and piston rods clean and spray them with a rubber-to-metal lubricant. After spraying, wipe the piston rod with a clean rag after a few strokes.

If you have a motorcycle with a center stand or a vehicle with a side stand, and you have a jack that can support the engine, you can remove the shock from the vehicle and clean and lubricate the piston rod by itself. If you have a jack, you can remove the shock from the motorcycle body and clean and lubricate the piston rod.

Also, if your working environment and tools allow you to remove the rear shock, you may want to take this opportunity to clean and grease the links. You can read more about disassembling and maintaining the links in another article.

In any case, the performance of a mono-shock is certainly superior to that of a two-shock. In order to maintain this superiority, regular condition checks and maintenance should be carried out.

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