DIY! How to Change Front Fork Oil and its Effectiveness Taught by Professional Mechanic

How to

Suspensions that are important to receive shocks from the road surface and when turning in the corners are achieved by two functions: springs and dampers. The fork oil is an indispensable element for dampers that absorb shock.

If the amount of fork oil reduce or the viscosity is not maintained, the damper performance will be reduced and you will feel uncomfortable when stroking the suspension or cornering. Therefore, it is ideal to change the front fork oil every 10,000 km of travel or once a year to maintain a sporty ride.

1. Front fork oil is important for stabilizing the body posture.


Place the jack under the engine to stabilize the vehicle and pull out the front fork after removing the tire and fender. You can check the point rust of the inner tube and oil leakage from the oil seal firmly if it is a single fork.

Regardless of the size of the engine displacement, upright or inverted, most of the front fork of the motorcycle has a front fork oil (although you may use grease instead of oil like HONDA MONKEY in the 50cc era). When the front fork stretches and contracts due to uneven road surface and acceleration, the damper suppresses the movement,and the resistance when the fork oil passes through a narrow passage called an orifice is the damping force.

As the fork moves through the damper with the stroke of the fork, the resistance adds heat to the oil, and as it passes through the narrow gap, the force is applied to the fork, and it is whisked by the expansion and contraction. As with engine oil, the application of temperature and mechanical stress causes the performance of the fork oil to deteriorate and the viscosity decreases.

If the viscosity decreases, the resistance when moving in the damper becomes small and the damping force decreases, and the contracted fork will stretch quickly, making it comfortable to ride without fluff and stiffness. Fork oil also needs to be changed regularly because the fork stretches faster and the timing is difficult than expected when the brake lever is released before the corner.

By the way, the oil is also contained in the rear shock and it will deteriorate, but this can not be disassembled and replaced at the individual level, so overhaul and replacement will be commissioned by a professional mechanic.


2. If you pull out the old front fork oil, check its condition.


Since the top cap of the ape can not be removed without a separate front fork, loosen the inner tube by fixing it with a vise, etc. When sandwiching with a vise, use a soft counterweight to prevent damage to the inner tube. If there is no vise, pull down the front fork halfway, tighten the under bracket once and loosen the cap.


When putting old oil into the waste oil, let's check the dirt and smell. When the sludge is mixed with the oil discharged without transparency, the inside is cleaned with parts cleaner and kerosene to remove the dirt. After that put the front fork upside down to dry the inside.

When changing the oil on the front fork, remove the fork assembly from the body. As a prerequisite, it is important to remove the front tire so that it is stable under the frame and the lower part of the engine by supporting it with a jack. Once the front fork is single item, remove the top cap and drain the old oil.

Among the upright forks, an orthodox damper-rod style is used to push the cap across the waist, as the shortened fork spring can stretch out as soon as the cap is removed. When discharging the fork oil to the waste oil receiver, also pay attention to the turbidity and smell of the oil.

Depending on the brand of oil, the degraded front fork oil has a peculiar smell like a damaged vegetable, so you can make some judgment by the smell. The oil itself is less viscous due to its deterioration, but since muddy sludge is sometimes precipitated at the bottom of the oil, it is recommended that you spray the oil with a fork or inject a small amount of kerosene or gasoline into the oil when the oil is mixed.

Also, when the oil seal of the front fork is damaged and the oil is leaking, the replacement work is performed together with the fork oil change. This oil seal change will be introduced on another occasion.



・Depending on the brand of oil, the degraded front fork oil has a peculiar smell like a damaged vegetable.
・If the fork oil is leaking, replace the oil seal.


3. Adjust the injection amount of the new front fork oil correctly.

▲ The new fork oil has a sense of transparency and, of course, does not have a strange smell. This product uses OEM Kawasaki oil, but if the viscosity is the same, the manufacturer does not matter. Although the oil capacity of the ape is enough in one liter because it is less than 200cc on one side, in a large vehicle there is also a model that is more than 500cc on one side so check in the service manual in advance.
The fork oil is poured into the inner tube so as not to spill. In order to make the fork oil spread across the narrow passage of the damper, the inner tube contracts while the upper edge is closed by hand, and the operation is repeated several times in the state of contracting and closing by hand while pulling up.

When injecting a new fork oil, the viscosity and volume of the oil are important. The fork oil has different viscosities, such as #5, #10, #15, the larger the number, the higher the viscosity, and the higher the viscosity oil, the more damping force. The high damping force means that the fork is not easy to contracts and stretch, and when traveling hard, it may increase the oil viscosity.

However, since there is a side effect that the ride comfort deteriorates when the damping force is increased modestly, it is better to make it the same viscosity as OEM if the town ride is main. If the oil you have used so far has decreased in viscosity due to deterioration, you should be able to feel a firm damping force by replacing it with new oil of OEM viscosity.

In the case of HONDA APE of the sample model, the viscosity of the fork oil is #10, the oil capacity is 174cc±2.5 cc, the oil surface is 131mm from the upper edge in the state of all bending of the fork single. The oil capacity and oil surface are important along with the viscosity because the air at the top of the oil also plays an important role for the front fork.

If the oil capacity is small and the air chamber is large, the rebound of the air spring is weak, if the capacity is large, the air chamber decreases and the rebound of the air spring increases. it is important to have a good understanding of what is going on.

Whether it is appropriate to determine the injection amount using the amount of oil or the oil surface, it does not matter if the oil inside the fork is completely missing, but if there is some residual oil, it is more accurate to determine the injection surface. When inserting the fork spring into the inner tube, if the winding condition is different at both ends of the spring, in the case of the ape, the assembly is made by the one with the gap jammed down.

When replaced with a new fork oil, the speed of pushing and extending the front fork is reduced, the movement of the suspension is calm, and it is also possible to enjoy a sporty riding because it is easy to step on when the brake is applied strongly.



・The larger the viscosity number, the higher the damping force of the fork oil.
・When the damping force is increased, the ride comfort deteriorates.


Check the oil surface by measuring the distance from the end of the inner tube to the oil by inserting a gold ruler with the front fork shortened. Since the oil surface of the ape is 131mm, it is correct if the adhesion mark of the oil is in the position of 19mm when inserting 150mm gold ruler. If less than the oil level, add the front fork oil, if it is high, remove it.


The fork spring is a dual-rate type with different winding density up and down. It depends on the model whether the side where the density is clogged comes up or goes down. In the case of the ape, the dense side is lower. In the case of dual-rate spring, since the spring rate is lower on the dense side, it will contract first.


Tighten the cap while pressing so as not to lose the tension of the spring. The o-ring on the cap is designed to prevent the air in the inner tube from escaping when the fork contracts. If the o-ring is broken due to deterioration, the air spring will not be able to keep the airtightness in the fork, so replace it.


After setting the front fork on the body, attach the axle shaft and front fender before setting the tire to make sure that the left and right forks are properly parallel.

The performance of the fork oil should be replaced regularly, paying attention to the viscosity and volume, which will inevitably deteriorate.

Fork oil that travels through the inner tube as the front fork strokes. The performance degradation due to aging can be refreshed by the periodic replacement.

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