The front forks perform two functions: support and damping. The support is what holds the bike up. This function is performed by a combination of spring tension and air pressure. Springs provide a specific amount of support, and air pressure can be added to increase the preload. Discussions of preload and suspension sag are outside the scope of this article. You will commonly see the spring on a car as a large coiled spring. On the motorcycle, the spring is inside the fork.
The second function is damping. Putting a wheel on the end of the spring isn't the best way to absorb shock. The whole point of suspension is to keep the wheel on the ground when bumps are encountered - having the wheel on the end of a spring means it will jump up and down and spend half its life up in the air. To prevent this, a shock damping (or absorption) system is employed. On a car, you'll see this as a shock absorber. Some cars have a shock/spring device called a McPherson strut, where the spring is wound around the outside of the shock. On our motorcycles, this is performed using a damper.
The damper is essentially a rod connected to the bottom of the fork, with a sealed disc at the top. The disc has small holes, or orifices, drilled in it. The area of the shock in which this disc travels up and down is filled with oil. As the shock is compressed, oil is forced through the small holes. When the shock goes over a bump and extends again, oil again is forced through the small holes. Because this limits the rate at which the shock can compress and extend, it damps the resonant frequency of the spring, and prevents the wheel from bouncing up and down.
- Fork function
So now we know it needs to be changed regularly. But how do we go about that? It's important that the exact right amount of oil is inside each fork. Too little, and the orifices can cavitate, causing insufficient damping. Too much, and the pressure in the shock will get too high, and can blow out your shock seals. Filling and checking the oil usually means opening the top of the fork tube, which is a difficult job requiring a special tool, as there is tremendous pressure on that spring. You can see more detail on this in our article How to rebuild your front forks.
For GL1500's with schraeder valves in the tops of the forks, it makes it a simple job, as the fork cap does not have to be removed. Fortunately, most GL1500's have a schraeder valve in their forks.
1. Start off by removing the fork bridge. Pull it up out of the grommets at the bottom, then disengage the tabs at the top and remove it.
2. Each fork cap schraeder valve a plastic protective cap. Unscrew the cap.
3. Once unscrewed, the schraeder valve is visible. Depress the valve to release all pressure from each fork. This is best done while the bike is on the center stand, to avoid spraying oil.
4. Remove each inner cover. Start at the inner edge as shown.
5. Turn the handlebars all the way away from the side being done, and pull its tabs free.
6. Once you have got to the top, turn the handlebars all the way the other way, and pull the top tab free.
7. Next we need to get access to the fork drains. Start by removing the fender covers. Pull them free of their grommets from the rear, then disengage the tabs at the front before pulling it free. Do not rotate it outward until the tabs are disengaged, or you will break the tabs!
8. Using a hex wrench, unscrew the top hex bolt holding the wheel cover in place.
9. Remove the other two hex bolts at the bottom of the wheel cover.
10. Note that the bottom hex bolts have collars - make sure you don't lose them (sometimes they stay inside the wheel cover when the bolt is removed).
11. Pull the cover out slightly, then to the rear to remove it from the wheel.
12. The hose bracket may come apart - if it does, save the bracket with the removed bolts. Repeat the removal procedure on the other side.
13. When the bike is on the center stand, it still has a bit of weight on the front wheel. We need to remove that weight. I use a trolley jack, placed at the very front of the engine.
14. Only a very slight amount of lift is needed to lift the front wheel - just a few pounds. You just need the front wheel to barely leave the ground. Alternatively, you can get a patient friend to sit on the passenger seat, which will tip the bike back onto its rear wheel instead of the front wheel.
15. With the wheel in the air, we will now remove the schraeder valve. You may be able to do it with a regular 10mm socket, but a deep socket works better.
16. Remove the valve from the fork cap.
17. Make sure you don't lose the rubber washer on the valve.
18. Set a drain pan under the right fork, and turn the wheel all the way to the right, so that oil draining from the fork does not get onto the brake disc.
19. This is the fork drain bolt - a 10mm bolt.
20. Remove the drain bolt.
21. Careful, the oil will shoot out at first - be ready with the drain pan! Allow the oil to drain until it has completely stopped. It may take several minutes.
22. Make sure you retain the copper washer on the drain bolt. Clean the drain bolt and washer as required.
23. Get your new fork oil ready. I have used both 10 weight and 15 weight fork oil in my GL1500, and find that 15 weight is a bit too stiff for me. The lower the weight, the easier it is for the oil to flow through the orifices, which means the less resistance to bumps - and softer feeling suspension. I find that this 10 weight Bel-Ray fork oil works very well for me in my GL1500.
24. You will need a high-capacity syringe to fill the forks. I use this two-ounce Monoject syringe, actually intended for...catheterization. Excuse me while I shudder...
I cut the tip off enough to provide a decent size to fit into the hole in the top of the fork cap left by the schraeder valve.
25. Suck up about one ounce of fresh oil into the syringe and squirt it into the top of the fork cap. This will flush the last remnants of the old fork oil out of the fork. Allow this oil to fully drain.
26. Reinstall the drain bolt. There is no specific torque value listed for the drain bolt in the service manual, but it is a 10mm bolt, and the standard torque value for 10mm bolts not otherwise listed is 25 ft-lb. Being that this is being driven into the aluminum fork, I tend to do this by hand, tightening it until it "feels right." But...I've been doing this for many years, and my "feels right" built-in torque sensing system has been developed over many years of stripping fasteners.
27. You will need a quality brake cleaner spray, preferably one that is safe for plastics.
28. Spray the drain bolt, the wheel cover bracket, and the brake disc liberally to remove any remaining oil. It is very important that you remove the slightest hint of oil from the brake disc. Any oil on the brake disc will severely reduce - or entirely eliminate any braking ability!
29. Repeat the entire draining process on the left fork, this time turning the wheel all the way to the left before draining.
Once both forks are fully drained and the drain bolts reinstalled, begin filling the forks with fresh oil. I put 14 oz of oil in each side (7 fills of the 2 oz syringe).
30. Now we need to make sure the new oil migrates fully into the fork without any air bubbles. Reinsert the schraeder valves into the fork caps, just finger tight. This is to prevent oil from spurting out of the holes when the bike is taken off the center stand. Trust me on this, I learned the hard way! Remove the jack to lower the front wheel, and take the bike off the center stand. Compress and extend the front forks several times by rolling the bike forward and backward and suddenly squeezing the front brake to stop the bike moving. You will see the forks compressing and extending as you do this.
Next, put the bike back up on the center stand, and jack it back up so the front wheel is off the ground.
To refill the forks, Honda specifies a specific amount of fork oil. However, this assumes you have zero oil in the fork to start with, and that you have stock factory springs in the fork. If there is any oil residue remaining (there will be), or if you have aftermarket springs (many people do), adding the recommended fork oil amount will cause the fork to be overfilled. The best way to measure is by measuring the level of oil in the forks from the top of the fork tube with the cap removed - and Honda gives us this measurement. However, their measurement is with the forks completely collapsed and the springs completely removed:
1988-1994: 239mm / 9.4"
1995-2000: 194mm / 7.6"
This doesn't do us much good for a bike with springs in the forks and caps installed.
We use a simpler, easier method. Having poured 14 oz of oil into each fork, we can be fairly sure that both forks are overfilled - this is intentional.
The next step requires a small vacuum pump, like this beat-up, well-used Mityvac pump, or for those with a smaller budget, this low-cost pump will also work. We will be removing just enough oil until the correct amount remains.
31. You will need a piece of 3/8" copper tubing. This is what makes this procedure easy. Make sure the tube is completely straight, then put a 90 degree bend in one end. Make sure the bend is as sharp as possible, and is exactly 90 degrees. This is what will prevent the tube from falling into the fork - dropping the tube into the fork will require complete disassembly of the fork to retrieve it, so this is important!
The length of the tube between the bottom edge of the short end and the end of the longer section is critical. Here's the length it should be:
OEM Factory Springs
1988-1994: 408mm / 16.1"
1995-2000: 363mm / 14.3"
Progressive Springs, tight end at bottom
1988-1994: 400mm / 15.75"
1995-2000: 355mm / 14"
Progressive Springs, tight end at top
1988-1994: 406mm / 16"
1995-2000: 361mm / 14.2"
Progressive springs have one end of the spring more tightly wound than the other end. Progressive recommends the springs be installed with the tight ends at the bottom, as shown in this picture. Most Progressive spring installations are done this way.
32. Affix one end of the Mity-vac to the copper tube.
33. Insert the long end of the copper tube into the schraeder valve hole.
34. Pump the Mity-Vac until it begins to suck air. As soon as it does, remove the tube. Repeat on the other fork.
35. When finished, your pump container will contain the excess fork oil.
36. Reinstall the schraeder valve. It does not have to be too tight, as it is sealed by the rubber O-ring. If you pressurize your forks, you should do so now (use a hand-powered bicycle pump, NOT a powered compressor - this will blow out your seals). Forks typically have 1-5 psi, but for those with Progressive springs, Progressive recommends running with no added air pressure.
37. Reinstall the inner covers in the reverse of the way they were removed, starting with the tab at the top.
38. Finish at the inside bottom, and repeat on the other side.
39. Reinstall the schraeder valve caps finger-tight.
40. Insert the tabs of the bridge at the top, then press the bottom into its grommets.
41. Push the wheel covers into place from behind.
42. Insert the top bolt, careful not to cross-thread it, but do not tighten it.
43. Insert the bottom two bolts (don't forget their collars!) and tighten them. Then tighten the top bolt. Repeat for each cover.
44. Push the front tabs of the fender cover into place, then push the rear boss into its grommet.
If you have any questions or problems with this procedure, please feel free to comment or ask!
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