Oil Level in Forks

Fred W · Jun 10, 2011

I recently drained, flushed (with ATF) and refilled my forks (with Motul 5W).

I know what the correct level is for my bike (103mm), and I know how to set it (remove springs, compress fork tube, measure from the top).

What I don't understand is exactly what this fork oil level does from a suspension response standpoint. The reason that I care is because before I drained the old oil I measured the height and it was significantly lower than spec and varied, 116mm on one leg and 126 on the other. And it would be good to know what the oil level does from a suspension tuning / adjustment standpoint. Maybe you can adjust the level to improve some particular aspect of the fork's performance?

Just looking through the parts that make up a fork, I can't see how the oil level would impact any of the dampeners. They are all down near the bottom of the slider and so would always be completely immersed in the oil regardless of the fill level.

The only thing that I can think of is that the air space / volume above the oil would effect the spring rate as that air gets compressed. Is that what we are doing here?

Peter Murray · Jun 10, 2011

Fred you got it .. It is the air gap that changes with oil levels. More oil in the forks will increase resistance to bottoming, by increasing air spring effect.

Bounce · Jun 10, 2011

Yep. Air compresses. Oil doesn't. Less air means you get a stiffer feel and are more apt to bottom out sooner.

Fred W · Jun 10, 2011

Bounce said:
Yep. Air compresses. Oil doesn't. Less air means you get a stiffer feel and are more apt to bottom out sooner.

Really? How is that? Wouldn't a lower oil level, provide less air-spring resistance and therefore be more apt to allow the fork to bottom out? Or are you saying that you might be bottoming out on the oil level instead of the mechanical limit of the fork?

Seems like you'd have to have way too much oil in there to hit the oil level before the mech limit. Even with the spring back in the fork the level is well below the top of the compressed fork leg.

LAF · Jun 10, 2011

All I know is when I did mine the oil levels on a 0 mile 10 was not the same leg to leg. I did not measure it but should have. I also had to call Traxxion as the head space is set to 180! Yep 180, that is 80mm less than stock! Now that is with 1.1 Omni springs and a spring buffer kit so I know that buffer displaces a bit of oil but still I called. I still cheated and set it at 150 mm. I used 10 W from Traxxion as that was recommended.

I also did a rear Pensky at that time. All I can say is I love the bike this way and have no clue how a stock FJR handles. I wanted it set up for a rider who with gear weighed 225-230 pounds. I go about 190-200 in street cloths depending on if it is hibernating time or not.

I hope one of the resident suspension guys answer this. I would thing it would affect dampening but am not positive. I went with three lines showing on the top, 10 clicks on the lower fork legs, and 12 clicks out on the top forks. it fits and rides great for me, sag was close enough and while not plush it is not bone jarring and great feel from the road. This was the set up from Traxxion for my weight and I never had to touch the rear Pensky with the 800 or 850 spring.

charismaticmegafauna · Jun 10, 2011

Fred W said:
What I don't understand is exactly what this fork oil level does from a suspension response standpoint. Maybe you can adjust the level to improve some particular aspect of the fork's performance?

The only thing that I can think of is that the air space / volume above the oil would effect the spring rate as that air gets compressed.

Is that what we are doing here?

Pretty much...

There have been motorcycle forks in the past with vented caps -- giving pure damping and spring rates (unadulterated by air-compression). Of course, that was part of the reason old bikers wore leather...

There have also been forks with large-ish air reservoirs -- giving relatively easily adustable ride height. Trouble is: air doesn't make nearly as consistant a spring as a good spring-steel unit.

There might be some resistance to fading with increased oil quantity (but, probably not important...)?

MCRIDER007 · Jun 10, 2011

Fred W said:
Bounce said:

Yep. Air compresses. Oil doesn't. Less air means you get a stiffer feel and are more apt to bottom out sooner.

Click to expand...

Really? How is that? Wouldn't a lower oil level, provide less air-spring resistance and therefore be more apt to allow the fork to bottom out? Or are you saying that you might be bottoming out on the oil level instead of the mechanical limit of the fork?

Seems like you'd have to have way too much oil in there to hit the oil level before the mech limit. Even with the spring back in the fork the level is well below the top of the compressed fork leg.

You have it figured out. The purpose of raising the oil level is to reduce the air space and increase the total resistance (spring plus compressed air) at the very end of the compression stroke, thus making the fork more resistant to bottoming. If done correctly, the compressed air can add as much as 25 percent resistance to the spring. However, if an excessive amount of oil is added to the fork, you can create hydraulic lock situation whereby the forks will bottom on the oil itself.

Bounce · Jun 10, 2011

Fred W said:
Bounce said:

Yep. Air compresses. Oil doesn't. Less air means you get a stiffer feel and are more apt to bottom out sooner.

Click to expand...

Really? How is that? Wouldn't a lower oil level, provide less air-spring resistance and therefore be more apt to allow the fork to bottom out? Or are you saying that you might be bottoming out on the oil level instead of the mechanical limit of the fork?

Seems like you'd have to have way too much oil in there to hit the oil level before the mech limit. Even with the spring back in the fork the level is well below the top of the compressed fork leg.

i was referencing hitting the artificial level created by the oil. less air means less compression means stiffer front... to a point. where that point would be is left to someone working with oil levels instead of oil viscosity to modify fork stiffness.

although air compresses, it doesn't compress 100%. there's a practical limit to its compression in this application. when that's reached, you've "bottomed out" according to my misuse of the term. it would seem that hydraulic lock (above) is a more accurate term, wielded more effectively than mine.

i would suspect that there's also an increased risk of blown seals (but you could claim it was just ice cream).

Fred W · Jun 10, 2011

Gotcha. Thanks.

OK, so I guess the level being a bit low (as I found it) wouldn't result in a horrible situation. Which is good since I could barely tell any difference after correcting it.

I could feel a difference in damping with the fresh suspension oil and actually had to open the settings up quite a bit.

vmaxman99 · Mar 28, 2015

I had to replace the seals on my '07 this winter. I removed the seals using the press method, without taking the forks apart.

I drained, flushed, refilled, drained and flushed the oil and have had them draining for over a month after installing the new seals.

Today I refilled the forks with fresh oil. Since I had the springs still installed, I used the recommended amount of oil, 670 cc's (22.3 oz)

and ended up with an air gap of 53 mm from the top of the fork tube, fully compressed. Does this sound about right?

Thanks in advance.

Mike

Geezer · Mar 28, 2015

vmaxman99 said:
I had to replace the seals on my '07 this winter. I removed the seals using the press method, without taking the forks apart.I drained, flushed, refilled, drained and flushed the oil and have had them draining for over a month after installing the new seals.

Today I refilled the forks with fresh oil. Since I had the springs still installed, I used the recommended amount of oil, 670 cc's (22.3 oz)

and ended up with an air gap of 53 mm from the top of the fork tube, fully compressed. Does this sound about right?

Thanks in advance.

Mike

Did you pump the forks a few times to make sure you got all of the air out?

dcarver · Mar 28, 2015

Fred, if you have some spare time, read up on current dirt bike fork technology..

Fred W · Mar 28, 2015

dcarver said:
Fred, if you have some spare time, read up on current dirt bike fork technology..

That post was 4 years ago. I have learned a lot about motorcycle suspension since then.

Fred W · Mar 28, 2015

vmaxman99 said:
Since I had the springs still installed, I used the recommended amount of oil, 670 cc's (22.3 oz)and ended up with an air gap of 53 mm from the top of the fork tube, fully compressed. Does this sound about right?

To measure the air gap you need to remove the springs. With the springs out (and the forks fully extended) you should have 92mm of air space. It sounds like you may be in that ballpark. Depends how much the springs displace.

vmaxman99 · Mar 28, 2015

Fred, the forks are full of 670 cc's of oil and I really don't want to drain them out again to check for a 92 mm air space. All the internals are stock, including the springs.

Oh, btw, shouldn't it be with the forks fully compressed, not extended? Don't tell me nobody ever checked the air space after filling the forks with oil and installing the springs! Also, I've pumped them many, many times and they've been sitting upright for awhile. The oil level did go down a bit just from sitting and bubbles rising.

Mike

Fred W · Mar 29, 2015

vmaxman99 said:
Oh, btw, shouldn't it be with the forks fully compressed, not extended?

Yes, I misspoke. The fork is fully compressed with the spring out when checking.

You don't really need to drain the oil from the fork to check it. Just pull the spring out slowly, so you don't drip oil all over. then compress the fork leg and measure away.

TownsendsFJR1300 · Apr 2, 2015

I recently converted my FZ6 forks over to air assist forks (springs still used) by adding a shrader valve atop each fork leg cap. Each leg calls for 133mm's of oil below the top of the compressed fork. I did bump the oil viscosity from 5 to 7wt.

The more oil you have in the fork tube, the less space there is for the air to compact thus lessing up front end "dive" upon hard braking.

I had approx 1.25" of static (no rider) sag with the bike upright. There's a little over 5" total travel).

With the modified caps on,(correct amount of oil) and then added (after some experimenting) approx 14 PSI in each tube. It picked the nose up to a static height of approx 15mm's (vs approx 30mm's before), the front end not nearly as mushy and better yet, considerably less nose dive with hard braking.

To date, no oil leakage, nor air leakage and infinitly adjustable. They also take some of the load off of the springs which tend to sag with time and age..

Fred W · Apr 3, 2015

What is retaining your seals from being blown out?

While only 14 psi at rest the pressure will rise significantly as the fork is compressed.

08FJR4ME · Apr 3, 2015

I have those on my XR650L,

I was at a lost as to why you were changing shock oil on the new bike. But the mention of the reserected thread cleared that up.

Carry on,

Dave

Oil Level in Forks

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