adjust fork oil levels after spring upgrade!

[delineator]

long story short, you want less oil! (about 15-20mm less for sonic springs)

long story:

so, I replaced my front fork springs with 0.80 sonic springs a little while back and loved the way it made the bike handle and kept it from diving, but it seemed like it was a bit jarring going over bumps, so I emailed sonic springs.

turns out, you need to put LESS oil in the forks after putting in new springs. the reason for this is that the newer springs and spacers displace a larger amount of oil than the stock springs (they are thicker, longer springs and the spacers are thicker).  what you want is the air pocket remaining once its all assembled to be the right volume. 

haynes says for stock to fill to 99mm down from the top of the tube with the forks fully compressed and no spring or spacer in there.  you really want more like 115 to 120 down from the top though because of the additional volume that the new springs etc will take up. 

so i took out the oil to bring the level down to were it should be. MUCH IMPROVED!!  still handles great in corners and under heavy braking, but now absorbs the bumps and so on.

also, the method of measuring out a certain ml of oil and dumping that in is not reliable at all, because of the variable amount of residual oil left in the forks. measure down from the top.

thanks to rich at sonic springs for helping me get this figured out and fixed! great customer service.
d

[centuryghost]

I/m about to fill my forks installed with .90 sonics with new fluid after I put a new clutch in my Jeep. What weight of oil did you use? You measure the level with the forks fully compressed?

[gsJack]

Progressive specs call for 5.5" (about 140mm) from fork top to oil level for their GS500 springs with a 3/4" spacer.  Measured with forks compressed.  Bottoming out on oil is not that much better than bottoming out metal to metal with the oem GS Slinky type springs.  

[centuryghost]

I'm trying to wrap my head around this; how do you keep your fork springs fully compressed and also be able to measure the level?

[luffy]

Remove the two top caps and springs, then what I recommend is to cut tubing to a length of x mm (140 or whatever is needed) and attach to a syringe. Put a little more oil into the forks then what you need, drop the tube down the fork tube and suck... what you will have left in is the 140mm or x amount of fluid. They will stay compressed as there is no spring and no air/oil pressure making them expand. Also make sure you cycle the fork before coming to the "final" level.

[Trwhouse]

I'm trying to wrap my head around this; how do you keep your fork springs fully compressed and also be able to measure the level?

Hi Century,
That's easy.
He said it wrong -- he meant "FORKS fully compressed, without the springs."
That means you remove the springs, slide the inner fork tubes all the way into the fork lowers (that's the fully-compressed part) and then you measure the top of the fork tube down to the top of the fork oil inside the tubes.
I do it this way:
I have a set or Progressive Suspension fork springs in my 1991 GS500E.
I remove the springs, turn the fork legs upside down to drain the oil, clean it all out, then let them dry.
Then I push the tubes down in the lower legs, and pour in fresh oil. I have a piece of aluminum I pre-marked with the correct fork oil height per the Progressive instructions, and I fill to that point. When a bit of oil shows up on the bottom of the measuring stick, I'm there. It's essentially a dipstick.
When finished, extend the inner tubes out of the fork lower legs, then slowly slide the clean springs back into the fork assemblies. Go too fast and you'll squirt some of the oil out and you don't want to have to redo the measurements, do you?
I hope this helps.
Best wishes,
Trwhouse

[centuryghost]

I'm trying to wrap my head around this; how do you keep your fork springs fully compressed and also be able to measure the level?

Hi Century,
That's easy.
He said it wrong -- he meant "FORKS fully compressed, without the springs."
That means you remove the springs, slide the inner fork tubes all the way into the fork lowers (that's the fully-compressed part) and then you measure the top of the fork tube down to the top of the fork oil inside the tubes.
I do it this way:
I have a set or Progressive Suspension fork springs in my 1991 GS500E.
I remove the springs, turn the fork legs upside down to drain the oil, clean it all out, then let them dry.
Then I push the tubes down in the lower legs, and pour in fresh oil. I have a piece of aluminum I pre-marked with the correct fork oil height per the Prgressive instructions, and I fill to that point. When a bit of oil shows up on the bottom of the measuring stick, I'm there. It's essentially a dipstick.
When finished, extend the inner tubes out of the fork lower legs, then slowly slide the clean springs back into the fork assemblies. Go too fast and you'll squirt some of the oil out and you don't want to have to redo the measurements, do you?
I hope this helps.
Best wishes,
Trwhouse

Now I get it!

Thanks Trwhouse 

[delineator]

wow, lots of discussion, cool.

i think the progressives want less oil because they are progressive and get stiffer with more compression anyway.

if doing straight rate springs, like sonic or racetech, more oil means less air which means the air will act as the progressive part near full compression.  but too much oil in eather case means that the "air progressive spring" will kick in too soon = very stiff, no travel!

sonic recommended 120mm to me via email, changing the level from stock was not stated in their instructions I received, but needs to be done!
good luck!

[delineator]

oh, and i used 10 weight oil, same as stock spec i believe. what to keep the forks as responsive as possible. loading up with 15 or 20 weight would just slow down the forks, not sure why that would be good, unless you are trying to compensate for inadequate springs.

[ineedanap]

loading up with 15 or 20 weight would just slow down the forks, not sure why that would be good, unless you are trying to compensate for inadequate springs.

The object of the game isn't to make your forks move as fast as possible.  If that was the case, you'd take all the oil out.  

You want the right amount of compression and rebound.  The object is to keep the tires planted and preventing the bike from having too much uncontrolled movement.  Playing with oil viscosity allows you to do that.  

For performance riding the forks are way too "responsive".  Not having enough compression and rebound can cause lots of wierd handling problems, like not wanting to hold a turn, chewing up tires, or running wide on the brakes or gas.   Most people find 20wt at minimum to be decent.