Hydraulic forks of a Trike not only hold the front wheel in place but also act as impact absorbers (springs) and bounce dampeners (shocks). As you will see, the typical assembly is actually a tube within a tube.
The fork tube is attached to the frame while the slider (tube) is attached to the wheel. As the wheel hits a bump the slider is forced upward compressing the spring and absorbing the force. The spring recovers and the slider returns to its normal position. To stop the wheel from compressing too far and to prevent any bouncing as the slider recovers, a hydraulic system kicks in. As the slider pushes upward, oil is forced through small holes in the damper valve and is allowed to enter the fork tube. As pressure is released on the slider, the spring forces the slider down and oil is drawn back into the lower slider chamber. The fork seal does three jobs, Firstly scraping oil from the fork tube, Secondly it prevents oil from leaking out between the slider and fork tube, and finally, it prevents any muck from entering the assembly.
Normally, maintenance involves changing the fork oil every year or so and replacing the fork seal when leaking is spotted. Because of the difference in design, not all steps are relevant to all Hydraulic fork systems, so the following is a generic step by step guide of how to change fork seals.
1) Open the drain plug from one of the forks, place a bowl under it and allow the oil to drain out. Close the drain plug and clean out the other fork.
2) Loosen the brake caliper bolts and axle nut, also loosen but do not remove the fork cap bolt if fitted. If you have an air pressurized system, bleed the air pressure off first!
3) Place a jack or other support under the engine or front portion of the frame and lift the front wheel from the ground.
4) Remove the mudguard, brake calipers, speedometer cable and wheel. Some trikes may have their headlight brackets attached to the forks so their removal is also required. CAUTION: support the hydraulic brake caliper so its weight isn’t on the flexible brake pipe otherwise damage could occur.
5) Loosen the yoke clamp bolts, and any other relevant fittings and slide the fork down and out with a twisting motion
6) Carefully remove the top tube cap.
CAUTION: there may be some pressure due to the internal spring! Remove the spring. Some models have a top bolt holding the spring, if so remove it also, carefully.
7) Remove the spring and tip out any remaining oil. Remove the seal dust cover if fitted.
On the very bottom of the fork you will find a bolt that holds in the damper rod. Remove the damper rod securing bolt and slide out the fork tube.
9) There will be some kind of clip found where the fork tube and slider meet, just atop the fork seal. Remove the circlip using a pair of circlip pliers.
10) If the fork seal is still in place at this point, work it out with a pair of needlenose
pliers.
11) Inspect the fork tube for pit marks. If you see many, the fork may still leak even with a new seal or it may wear out the seal at an accelerated rate.
12) Apply fork oil to the inside of the fork seal and slide the new seal over the fork tube. Replace the fork tube, and other goodies(washers etc.), in the slider. Reattach the damper rod with its bolt.
13) The hard part is now at hand! The seal must be pushed back into place with an even pressure all around the fork tube. The only way to accomplish this is to slide a pipe or other tube over the fork tube that is about the same size as the new seal. With an even push, press down on the pipe and seat the fork seal.
CAUTION: if you are not careful, you can damage the seal! Most seals have a small spring that holds the internal rubber lip against the fork tube, make sure it is unharmed!
14) Reassemble the one fork (dont forget the circlip) and before placing on the top cap, refill with oil as per the manual.
15) Repeat with the second fork if necessary
16) Reassemble the front end of your trike
