Newbie question- how to add new oil to front forks

I completely rebuilt the forks on my other 73' 850 last fall but I have never simply drained and added fresh oil. I remember the spring is attached to the fork tube cap but I am having a difficult time getting the cap and spring to come out so that I can add the 180cc (not 150) to the forks. Is the spring pushed down against the rod or damper cap enough to hold it from coming up? I just don't want to start prying and break something! Thanks for your replies!

I changed oil weights last year w/o disassembling the forks. I drained the oil from the plugs at the bottom of the legs; put a jack under the frame so I could control the front-end height, un-screwed the caps and allowed the springs to rise just enough to allow oil to be added via a funnel; refilled with the correct amount of oil. I also measured the amount of oil I removed to ensure I didn't over fill it. Takes a while to drain, pushing slightly on the front end will push it out quickly. All but maybe 5-10cc's came out.

2WheelsUp said:

I changed oil weights last year w/o disassembling the forks. I drained the oil from the plugs at the bottom of the legs; put a jack under the frame so I could control the front-end height, un-screwed the caps and allowed the springs to rise just enough to allow oil to be added via a funnel; refilled with the correct amount of oil. I also measured the amount of oil I removed to ensure I didn't over fill it. Takes a while to drain, pushing slightly on the front end will push it out quickly. All but maybe 5-10cc's came out.

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Thank you, yes that is what I discovered after posting. I went back out to the shop and fiddled around with things and finally the springs began to appear! Then I did just what you describe.

And I am sure you discovered to pour the measured amount of new oil SLOWLY !

Sometimes, when the bike is on a stand, the weight of the front wheel is carried by the fork caps. You might need to lower the bike, so the front wheel rises and the caps and springs protrude above the top fork yoke. Then you can pour the oil in more easily.

I have a modified damper system, so I have changed fork oil as much as 10 times in 2 months to experiment with different oil viscosities... After a few times, you scratch your head and start to think of better ways to do it... Here's mine

1) I have 2 bolts that are the same thread size as the drain plugs. Those bolts are hollow with their heads cut off. I attach a piece of fuel line to each of them and run the end of each line into a container to catch the old oil as it drains. I replace the drain screws with my hollow screws, grab the brake lever and pump all the fluid out of the fork dampers. It won't all come out, but most of it will. If it's absolutely filthy, you want to pull the dampers and fork sliders apart and clean the whole system. If the fluid is pretty clean (it never is) reinstall the fork slider drain screws and go to the next step

2) I put the bike on the center stand and unscrew the fork tube caps. I put some wood under the rear tire to tip the bike forward on the center stand so the now loosened fork caps pop upward on the damper rods as the bike is tipped forward. I like to get the bike to a certain angle which gives the most space in the opening of the fork tube which is right by the damper rod nuts. I stick the siphon tube in and press the plunger.

I use this siphon pictured below and pump the new fluid into the fork tubes at the widest gap below the fork caps, being careful to flow the fluid at a rate slower than it drains downward so I don't make a mess.



I remove the wood under the rear wheel when both legs are filled, tip the bike up so the fork caps drop into the fork tubes and tighten the fork caps.

Mr Tedious here....as noted the oil is almost never really clean so do your due diligence and pull the sliders off and do a complete clean. Internal spring forks will be busy leaving particles from the spring and stanchion which will wear everything out quickly. Internal springs need a teflon sleeve about them. But even that would leave some sort of particles.
Well now that I ran my mouth on that I am out to the garage to pull the front end. Stormy day perfect for that sort of activity.

o0norton0o said:

I have a modified damper system, so I have changed fork oil as much as 10 times in 2 months to experiment with different oil viscosities... After a few times, you scratch your head and start to think of better ways to do it... Here's mine

1) I have 2 bolts that are the same thread size as the drain plugs. Those bolts are hollow with their heads cut off. I attach a piece of fuel line to each of them and run the end of each line into a container to catch the old oil as it drains. I replace the drain screws with my hollow screws, grab the brake lever and pump all the fluid out of the fork dampers. It won't all come out, but most of it will. If it's absolutely filthy, you want to pull the dampers and fork sliders apart and clean the whole system. If the fluid is pretty clean (it never is) reinstall the fork slider drain screws and go to the next step

2) I put the bike on the center stand and unscrew the fork tube caps. I put some wood under the rear tire to tip the bike forward on the center stand so the now loosened fork caps pop upward on the damper rods as the bike is tipped forward. I like to get the bike to a certain angle which gives the most space in the opening of the fork tube which is right by the damper rod nuts. I stick the siphon tube in and press the plunger.

I use this siphon pictured below and pump the new fluid into the fork tubes at the widest gap below the fork caps, being careful to flow the fluid at a rate slower than it drains downward so I don't make a mess.

I remove the wood under the rear wheel when both legs are filled, tip the bike up so the fork caps drop into the fork tubes and tighten the fork caps.

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I'd like to hear your results on viscosity testing please. Thanks, BH NC USA

Blewdy Yaink said:

I'd like to hear your results on viscosity testing please. Thanks, BH NC USA

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Here's a link to a thread that shows my modification


So,... the whole premise of my modification was to have adjustable valves so that you can use a given viscosity of fork oil and use the valve adjustment to control the response. With stock norton commando dampers you control the response with the choice of fork oil viscosity given that your valve orifice size is fixed since there's no adjusters.

The only significant thing that I learned from my project that I have to say about suspension is that I think most people do not understand the nature of suspension. When I first started my project I thought I would get the most comfortable feel (soft riding) and get the highest performance on smooth roads at the same time.... because I was going "high tech"!!! What I learned was that having a soft riding feel means that your suspension travels freely to absorb bumps so those bumps aren't transferred to the chassis and rider. Unfortunately that freely traveling suspension is exactly opposite to the set up that maintains the best traction when cornering on good road surfaces...

That leads to the fork in the road where you do 1 of 3 things.

1) You set the bike up for race track conditions so you can push the bike's performance. Stiffer springs and restrictive low speed damping
2) You set the bike up to have a soft feel on public roads and ride within the limits of those settings. Softer springs and less restrictive damping
3) You compromise on spring rate and damping so your steering doesn't feel completely vague on one end of the spectrum and your bike doesn't feel like you are riding a hard tail bike on the other end.

The real kernel of wisdom regarding suspension is that if you are going to compromise (#3) because your bike is a road bike that has to endure pot holes and crappy road surfaces occasionally, it doesn't pay to make an expensive modification for high performance when you aren't going to set the bike up for high performance. WTBS, I did the modification before I learned about the nature of suspension, so I do have my bike set up closer to the high performance end of the spectrum.... but of course that means my bike doesn't ride as softly as most Commandos, which was something I understood after the project was finished.

I was lucky enough to have a commando owner who lives nearby me to do a side by side test and bike swap to compare stock to modified. It was just how I describe above. His bike handled ok and was noticeably softer riding than mine. My bike felt like it steered more precisely but gave a noticeably stiffer ride. I felt more comfortable pushing my bike hard when leaned over in the corners, and his bike felt like I was riding on a cloud in comparison to mine.

I don't know what advice to give about which oil viscosity to use in a stock commando fork damper. I often recommend Jim Schmidt's parts upgrade to the stock dampers for people who don't want to spend a lot of money but want to improve the extremely crude stock parts. Parts with better manufacturing tolerances respond more consistently, so even though you can only tune your stock suspension with oil viscosity changes, the better parts narrow the variable performance so the difference between 10wt and 15wt should seem more pronounced when the assembled "better parts" are substituted....

It's easy to achieve the compromise set up (#3) with the stock dampers... so take a few days to test ride and change out fork oil viscosities. Once you are committed to a compromise set up between performance and softness of ride, you'll adjust your riding to how it feels. On the 1 to 10 scale, 5 feels like 6, but 1 feels very different than 10. If you end up at 4 or 5 or 6, you'll adjust to it...

Thank you, that's very much appreciated. I'm guessing that "tuning" a fork the Lansdowne dampers is similar in approach but more complicated in doing it.

I used landsdowne dampers before I made my modified sets. They are ok. Since they split the damping duties of compression and rebound in different legs, you could actually use different viscosities in each leg if you wanted to, or just use the damper adjusters. The rule of thumb is that rebound should be greater than compression so the rebound would get the heavier oil or tighter valve opening. If you want a better handling bike, you are giving up some ride softness. Riding straight, down a smooth road there isn't a lot of difference in handling feel, but you certainly feel the softer suspension as more of a floaty ride.

The real test of the higher performance of your damping settings are felt the most in how precise your steering feels going fast through the corners. Soft suspension feels looser with less precise steering. Higher performance suspension feels solid and steers more precisely. When you are going fast leaned over through twisties, going left, then going right, it handles precisely and predictably and inspires confidence. Softer suspension doesn't feel precise and doesn't inspire confidence.

Cornering is where you judge your high performance adjustments and if you don't want better control and more precise cornering, then you might as well just go for soft riding stock suspension and enjoy floating down the road on a cloud and take it easy in the corners...

o0norton0o said:

I used landsdowne dampers before I made my modified sets. They are ok. Since they split the damping duties of compression and rebound in different legs, you could actually use different viscosities in each leg if you wanted to, or just use the damper adjusters. The rule of thumb is that rebound should be greater than compression so the rebound would get the heavier oil or tighter valve opening. If you want a better handling bike, you are giving up some ride softness. Riding straight, down a smooth road there isn't a lot of difference in handling feel, but you certainly feel the softer suspension as more of a floaty ride.

The real test of the higher performance of your damping settings are felt the most in how precise your steering feels going fast through the corners. Soft suspension feels looser with less precise steering. Higher performance suspension feels solid and steers more precisely. When you are going fast leaned over through twisties, going left, then going right, it handles precisely and predictably and inspires confidence. Softer suspension doesn't feel precise and doesn't inspire confidence.

Cornering is where you judge your high performance adjustments and if you don't want better control and more precise cornering, then you might as well just go for soft riding stock suspension and enjoy floating down the road on a cloud and take it easy in the corners...

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You need to tell about the tubing you put on the rods and the bushings to keep the springs central I copied your post and yes it keeps the oil cleaner and fork’s quite

phippsy said:

You need to tell about the tubing you put on the rods and the bushings to keep the springs central I copied your post and yes it keeps the oil cleaner and fork’s quiet

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I used clear PETG tubing because it's very wear resistant. I bet nylon would work too, but it might be hard to find the right size tube in nylon. I got it on amazon. My dampers are modified Showa dampers so I bought metric tubing 10mm ID, 14mm OD. I was worried that it might heat up and melt so I rode around for a few weeks then inspected them and the tubes were fine. Funny that you chose that detail to notice. I had completely forgotten that I added those tubes at the end of that project. If you want to add the same thing to the stock commando dampers you would have to measure them to be sure of the fit...