How to change your fork oil


Step-by-step tutorials on how to maintain and fix your GL1500
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How to change your fork oil

Post by WingAdmin » Sun Jul 08, 2018 8:24 pm



Changing the damping oil in your forks is an often-overlooked regular maintenance item. Neglecting the fork oil can cause it to turn to sludge, limiting lubrication and causing damage to your front suspension system.

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
Fork function

Of course, this is a very simplistic explanation of how the GL1500 fork system works, but it gives you a general idea. The fork oil is the oil which is squeezed repeatedly through the orifices every time you go over a bump. It also collects any dirt or dust that makes it past the fork seals and holds this dirt in suspension, to prevent it from damaging the sliders and bushings. Lastly, it lubricates those sliders and bushings, to prevent them from wearing too quickly.

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.

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2. Each fork cap schraeder valve a plastic protective cap. Unscrew the cap.

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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.

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4. Remove each inner cover. Start at the inner edge as shown.

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5. Turn the handlebars all the way away from the side being done, and pull its tabs free.

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6. Once you have got to the top, turn the handlebars all the way the other way, and pull the top tab free.

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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!

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8. Using a hex wrench, unscrew the top hex bolt holding the wheel cover in place.

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9. Remove the other two hex bolts at the bottom of the wheel cover.

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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).

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11. Pull the cover out slightly, then to the rear to remove it from the wheel.

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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.

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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.

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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.

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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.

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16. Remove the valve from the fork cap.

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17. Make sure you don't lose the rubber washer on the valve.

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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.

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19. This is the fork drain bolt - a 10mm bolt.

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20. Remove the drain bolt.

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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.

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22. Make sure you retain the copper washer on the drain bolt. Clean the drain bolt and washer as required.

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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.

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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.

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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.

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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. :)

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27. You will need a quality brake cleaner spray, preferably one that is safe for plastics.

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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!

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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).

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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.

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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"

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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.

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32. Affix one end of the Mity-vac to the copper tube.

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33. Insert the long end of the copper tube into the schraeder valve hole.

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34. Pump the Mity-Vac until it begins to suck air. As soon as it does, remove the tube. Repeat on the other fork.

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35. When finished, your pump container will contain the excess fork oil.

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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.

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37. Reinstall the inner covers in the reverse of the way they were removed, starting with the tab at the top.

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38. Finish at the inside bottom, and repeat on the other side.

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39. Reinstall the schraeder valve caps finger-tight.

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40. Insert the tabs of the bridge at the top, then press the bottom into its grommets.

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41. Push the wheel covers into place from behind.

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42. Insert the top bolt, careful not to cross-thread it, but do not tighten it.

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43. Insert the bottom two bolts (don't forget their collars!) and tighten them. Then tighten the top bolt. Repeat for each cover.

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44. Push the front tabs of the fender cover into place, then push the rear boss into its grommet.

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If you have any questions or problems with this procedure, please feel free to comment or ask!



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Scooter363y
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Re: How to change your fork oil

Post by Scooter363y » Sat Sep 01, 2018 5:12 am

Very good article. But I would like to point out one mistake that could spell disaster!

The drain bolt is not a 10 mm bolt! It is a much smaller size. Brobably a 6 mm bolt with a 10 mm hex head.

This will need a small amount of torque to properly set. Usually 6-9 ft/lbs of torque not 25. That much would strip the threads.

Ride safe
Scooter

Themrfixitall
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Re: How to change your fork oil

Post by Themrfixitall » Sat Sep 01, 2018 7:07 am

Very clever! May I point out however since the amount of oil in the shocks is very important that you also would take in to consideration the height of the cap. The tool used for measuring the oil amount is actually resting flat on the tube. Plus usually the oil amount given is often without the springs.
This is a clever way to change the oil in the shocks however I would recommend using the regular method for couple of reasons. For one when using this method you will not get all the old oil out, specially when it comes out in the condition like shown on the picture. I would recommend complete flush and since you would have them already off also the seals.
Also, when oil is being applied air will get in ether you like it or not. You have to get that out with pressure or vacuum.

brschillinger
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Re: How to change your fork oil

Post by brschillinger » Sat Sep 01, 2018 8:53 am

Not being the original owner of my '94 Aspencade, I don't know if the springs are original or not. Is there a way to tell without disassembling the tubes?

DannyBWebbSr
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Re: How to change your fork oil

Post by DannyBWebbSr » Sat Sep 01, 2018 9:09 pm

I recently did fork bushings and seals, but opted to completely disassemble forks due to high milage and no maintenance history. The only way to know spring type is to pull them out unless previous owner recorded change in owners book or that envelope of receipts.

tprender
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Re: How to change your fork oil

Post by tprender » Tue Sep 04, 2018 1:48 pm

I like this idea very much, thanks for the write up. I found a section of 3/8 soft copper tubing in the garage and the piece that I had was just a little to large to fit into the hole. Then found a piece of plastic tubing and marked 14 in by wrapping duct take at that mark. It took a little bit of effort to get the plastic tubing to go down all the way in, but I did it. The tubing that I had was about 5-6 ft long, so I just sucked on the other end. I was able to take out about 1 oz out of each fork. Will take it out for a ride later today to see if there is any difference. Thanks again for the idea for checking the oil level.

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WingAdmin
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Re: How to change your fork oil

Post by WingAdmin » Tue Sep 04, 2018 1:52 pm

tprender wrote:
Tue Sep 04, 2018 1:48 pm
I like this idea very much, thanks for the write up. I found a section of 3/8 soft copper tubing in the garage and the piece that I had was just a little to large to fit into the hole. Then found a piece of plastic tubing and marked 14 in by wrapping duct take at that mark. It took a little bit of effort to get the plastic tubing to go down all the way in, but I did it. The tubing that I had was about 5-6 ft long, so I just sucked on the other end. I was able to take out about 1 oz out of each fork. Will take it out for a ride later today to see if there is any difference. Thanks again for the idea for checking the oil level.
The reason I used copper pipe is twofold:

- You can bend it to set the height, which makes it right (for your bike), every time
- You won't drop it into the fork accidentally, which will require opening/disassembling the fork to retrieve it!

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Re: How to change your fork oil

Post by virgilmobile » Tue Sep 04, 2018 2:06 pm

Very nice article..If I may..
there's one more tidbit of useless information I'll add.
The dampening function is not equal both ways...The "restricting valve"or ring with the tiny holes does float in the housing.
If I remember correctly,it takes more effort to extend than compress...without the spring,just testing the slide operation.

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curtm
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Re: How to change your fork oil

Post by curtm » Tue Sep 11, 2018 8:52 am

Looks so easy until you discover no schrader valve, then its a different story

Curt

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virgilmobile
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Re: How to change your fork oil

Post by virgilmobile » Tue Sep 11, 2018 9:33 am

Just like mine..Drill and tap..insert a removable plug...Then stand on the seat to screw it back on.
I make a mess replacing mine.Remove the plug..remove the drain bolt..capture the oil ..clean up the mess...measure and inject new oil with a "Cajun injector"...(meat injector)..job done..I didn't use a Schrader valve..just a small allen bolt with a "O"ring seal...



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