It is no exaggeration to say that the front fork oil seal is the Achilles heel of the front suspension. It prevents oil inside the suspension from leaking and foreign matter from entering from the outside, and as it is constantly rubbing against the inner tube, deterioration and wear are inevitable. If front fork oil leaks, it will contaminate the brakes and tires and also affect the performance of the suspension itself, so it is necessary to repair it as soon as possible.
Suspension becomes less pliable when the oil level decreases as well as fork oil degradation.
When the dust seal, which scrapes off dirt and other debris from the inner tube, is finely cracked and there are ring spots of fork oil on the inner tube, the oil seal below the dust seal is likely to be damaged as well.
Drain the oil in the suspension first, whether the inner tube and oil seals are sound and the fork oil change is being performed, or whether the seals are being replaced. If the fork springs are unequal pitch, make sure they are up and down when removing them.
The fork piston inserted in the inner tube is removed by loosening the bolt at the bottom of the outer tube. If the bolt and the fork piston turn together, use an impact wrench to turn them vigorously, or temporarily assemble the fork spring to fix the fork piston inside the inner tube.
Once the dust seal is shifted upward, remove the stopper ring that holds the oil seal in place. Note that even if you forget to remove this ring and pull the inner tube, you will not be able to remove the oil seal. It is not uncommon for the lip of the dust seal on the discontinued model and old motorcycles to deteriorate, allowing moisture to enter and the ring to rust bright red.
With the exception of mopeds and 50cc scooters, the suspension of most motorcycles functions through a combination of springs and dampers. When a tire is pushed up over a gap in the road surface, the suspension springs contract to absorb the shock. When the retracted suspension extends, the damper adjusts the speed to a lower level.
Whether single-rate or double-rate, the reaction force = spring constant of a metal coil spring is determined for each spring. In the case of upright front forks, the amount of fork oil and the height of the fork oil level are specified when the fork springs are inserted into the inner tube and the fork oil is injected because the stiffness of the fork varies depending on the capacity of the air chamber in the inner tube.
When the front suspension retracts, the spring is pressed down and a reaction force is generated. When the inner tube enters the outer tube, the air chamber in the inner tube is compressed to generate a reaction force like an air spring. At this time, if the oil level is higher than the specified value = more fork oil, the air chamber will be reduced, so when the front fork sinks, the pressure increases and the suspension becomes stiffer. This characteristic may be used to raise the oil level in suspension settings, but care must be taken because extreme changes may cause the air chambers to become too narrow and the stroke amount itself to decrease = the suspension will not sink.
Conversely, as the fork oil decreases, the capacity of the air chamber in the inner tube increases and the internal pressure does not rise sufficiently when the front forks retract, resulting in softer suspension.
One of the causes of front suspension softness over time is a decrease in viscosity due to the deterioration of fork oil. Suspension dampers utilize the resistance of fork oil as it passes through narrow passages, but the fork oil that flows through the suspension during constant strokes loses viscosity due to shear during sliding and increases in oil temperature. This is why periodic oil replacement is necessary.
Another cause is fork oil leakage. If the oil seal cannot remain airtight due to point rust or hardened lips on the inner tube, the fork oil in the suspension will leak out as the internal pressure may increase when the front forks contract. Not only is it dangerous if the leaked fork oil travels through the outer tube and adheres to the wheels, tires, and brakes, but the suspension becomes softer as the oil volume decreases.
You may think lightly that a small amount of fork oil leaks from the oil seal, but the service manual specifies the amount and level of fork oil of 1 mm per unit. When suspension settings are made at will, they may be changed from standard values, but otherwise oil leaks should not be left unchecked in terms of driving performance.
- Point 1: When fork oil is drained from the front suspension, the volume of the air chamber in the inner tube increases, softening the suspension.
Centering of fork pistons is important when replacing oil seals
Fix the caliper mount or axle shaft holder part of the outer tube with a vise, and pull the inner tube strongly several times. The bushing at the end of the inner tube will hit the oil seal like a sliding hammer and come out. Depending on the model, the oil seal may remain in the outer tube even after the inner tube is released. In this case, use an oil seal remover to remove the oil seal.
When replacing the oil seal or dust seal, it is important to replace the sealing washer (copper washer in the foreground) that is set on the socket bolt that holds the fork piston in place with a new one. When the socket bolt is tightened to the specified torque, the washers will adhere to prevent fork oil leaks.
When setting the oil seal on the inner tube, the top of the inner tube can be protected with thin vinyl to prevent damage to the seal lip.
Install the oil seal, with fork oil and grease applied to the lip, onto the inner tube through the taut vinyl. At this time, be careful not to turn the lip of the oil seal inside out.
When fork oil leaks occur due to point rust in the inner tube or hardening or deterioration of the oil seal itself, it is a natural maintenance practice to correct the symptoms by replacing the inner tube and oil seal. In addition, from the point of view of suspension setting, front suspension with low oil level due to oil leakage needs to be repaired appropriately.
Although the replacement of the oil seal and dust seal is presented here, the work process is the same when replacing a rusted inner tube, since the inner tube is pulled out when removing the oil seal press-fitted into the outer tube.
When assembling the front suspension, the fork piston inserted in the inner tube is secured with a socket bolt at the bottom of the outer tube, and then the dust seal is press-fitted into the outer tube. Care should be taken in centering the fork piston at this point.
The end of the fork piston has an oil lock piece that rapidly increases the hydraulic pressure of the fork oil when the suspension is at full stroke to prevent the inner tube from bottoming out. Due to the delicate positioning of the socket bolt and fork piston threads, the oil lock piece may shift slightly from the center of the outer tube. This will cause the oil lock piece to stick in the inner tube when the suspension is fully bottomed out.
To avoid this, insert the inner tube to the bottom with the fork piston temporarily attached to the outer tube and check that it strokes smoothly without interference with the oil lock piece at full bottom. By extending and retracting the inner tube while turning it, uneven contact with the oil lock piece can also be eliminated. When filling the assembled front suspension with fork oil, stroke the inner tube several times to push out any air remaining inside the damper and then adjust the oil level according to the service manual.
Fork oil leakage due to oil seal deterioration or inner tube rust is one of the typical problems that can occur on a daily basis for discontinued models or old motorcycles. While it is unpleasant and dangerous to have oil leaks contaminating many parts of the motorcycle body, it is very important to perform maintenance as soon as possible in terms of suspension performance.
- Point 1: Slight misalignment of the oil lock piece at the end of the fork piston may interfere with the inner tube and cause friction loss, so center the fork piston during assembly.