Fork oil level contradiciton in shop manuel quantity vs height?

7521 Views 43 Replies 10 Participants Last post by LouG, 11 d ago

katmanblues

Discussion Starter · Feb 4, 2017

Hi,

I'm changing my fork spring and oil and while doing this I noticed that the oil amount I recuperated while draining was 470 mL for the Left fork (I'm still working on the right one installing new stoltec piston kit)

So I started by adding about 450mL of new oil, figuring that should be close to the 147mm oil level. But even after pumping the damping rod and letting it sit for 10 minute or so, the oil level is way too high, I got 128 mm and technically I still have about 20mL of oil to add.

I did the mesurement from the top of the outer tube with the outter tube fully rested on the fork end.

I'm puzzeled now...why is there such a big difference in oil quantity vs the oil level. Is it possible my oil level in the stock forks where too high? althought the quantity seems right?

I'll do the right fork and measure the height before draining it and see, but I don't know which number to trust...

thanks for your help

1 - 20 of 44 Posts

LouG

· #2 · Feb 4, 2017

Use oil level rather than quantity. Are the forks fully compressed when you check it?

katmanblues

Discussion Starter · #3 · Feb 4, 2017

Ok did my right fork and got 154mm oil level height for 400 mL of drained oil. If the quantity in the shop manuel were right, I sholuld have recuperated approximatly 458mL, so I'm out almost 60mL.

I'll stick with the height since the right «working fork» is about right.

katmanblues

Discussion Starter · #4 · Feb 4, 2017

yes they were

y-mt

· #5 · 1 mo ago

Hello. I also encountered such a situation. I am rebuilding shock absorbers again and want to prepare an answer in advance. What is more important? Oil level or quantity per milliliter?

DNFDOUG

· #6 · 1 mo ago

Level from the top is most important. Be sure to pump oil fully into your cartridges and fill back up before you suck out oil to get to the specified height.

Reactions: 1

y-mt

· #7 · 1 mo ago

DNFDOUG said:
Level from the top is most important. Be sure to pump oil fully into your cartridges and fill back up before you suck out oil to get to the specified height.

My friend, thanks for the response. Not sure I understood the response one hundred percent. I understand that it is better to measure from above, but what should I be careful about? I'm just using a translation and it's not necessarily understandable. Thank you.

2and3cylinders

· #8 · 1 mo ago

First of all it's a good idea to flush out the forks before adding new fluid. I use inexpensive ATF For the flushing fluid.

They make a little tool that screws on to the end of the damper rod so you can easily stroke it up-and-down to bleed out air but most importantly you bleed it then invert the forks and let them hang over night after the last flushing.

Besides the proper spring weight, fluid viscosity is the second most important thing to consider. I found that a good quality Semi synthetic blend that's 16 Cst @ 40C works well in our forks. If you search online you'll find tables for dozens and dozens of different fork fluids that give their specifications. Fluid weight like 5, 7 or 10 vary significantly among manufacturers. That's why Cst must be used.

To function properly Fork damping only needs about 10 oz. The fluid height above that provides the Air Gap that uses air as a spring, which can vary by taste. I find a fluid hight higher than spec works well but don't go by me because I changed my fork damping, with the adjustable rebound in each leg and compression in the left leg.

Damping, fluid, air gap, spring weight, preload, steering geometry, tires and their pressure, pilot weight and skill level, all together must be "balanced".

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LouG

· #9 · 1 mo ago

y-mt said:
My friend, thanks for the response. Not sure I understood the response one hundred percent. I understand that it is better to measure from above, but what should I be careful about? I'm just using a translation and it's not necessarily understandable. Thank you.

Do you have a fork level gauge?

y-mt

· #10 · 1 mo ago

LouG said:
Do you have a fork level gauge?

I have like this

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Reactions: 1

y-mt

· #11 · 1 mo ago

2and3cylinders said:
First of all it's a good idea to flush out the forks before adding new fluid. I use inexpensive ATF For the flushing fluid.

They make a little tool that screws on to the end of the damper rod so you can easily stroke it up-and-down to bleed out air but most importantly you bleed it then invert the forks and let them hang over night after the last flushing.

Besides the proper spring weight, fluid viscosity is the second most important thing to consider. I found that a good quality Semi synthetic blend that's 16 Cst @ 40C works well in our forks. If you search online you'll find tables for dozens and dozens of different fork fluids that give their specifications. Fluid weight like 5, 7 or 10 vary significantly among manufacturers. That's why Cst must be used.

To function properly Fork damping only needs about 10 oz. The fluid height above that provides the Air Gap that uses air as a spring, which can vary by taste. I find a fluid hight higher than spec works well but don't go by me because I changed my fork damping, with the adjustable rebound in each leg and compression in the left leg.

Damping, fluid, air gap, spring weight, preload, steering geometry, tires and their pressure, pilot weight and skill level, all together must be "balanced".

Thanks for the information. A lot of information. I'll have to process it a bit. I have the rod that screws up for the pump. I ordered all the parts that need to be replaced according to the workshop manual. I disassemble everything, clean with soap and remove grease and dry well. I've done it before but the truth is I had no indication if I did it well or not. I also have difficulty with the directions themselves, but still want to do everything myself. I would be happy if I could get advice and guidance here to do it properly. Now all I have to do is buy oil and start the work.

DNFDOUG

· #12 · 1 mo ago

I use a version of this tool that I made for myself to remove oil from the top. I measure the distance from the tip of the rod to the sleeve to set my desired height. There are many videos to demonstrate this.

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gbauto

· #13 · 1 mo ago

Just an FYI...
The hollow aluminum tube the oe forks use to adjust the compression/rebound needles work perfectly for this task.
I just make a sharpie mark for the level I want to use and then hold the assm upright and then use a syringe to suck it down to that level.

LouG

· #14 · 1 mo ago

y-mt said:
I have like this

View attachment 175022

Good, just use that. Don't worry about quantity other than to get near the final height.

Reactions: 1

2and3cylinders

· #15 · 1 mo ago

10

y-mt said:
Thanks for the information. A lot of information. I'll have to process it a bit. I have the rod that screws up for the pump. I ordered all the parts that need to be replaced according to the workshop manual. I disassemble everything, clean with soap and remove grease and dry well. I've done it before but the truth is I had no indication if I did it well or not. I also have difficulty with the directions themselves, but still want to do everything myself. I would be happy if I could get advice and guidance here to do it properly. Now all I have to do is buy oil and start the work.

What aren't you clear on?

For just a dump, flush & fill I use the ATF. If I'm taking it all apart I use Kerosene followed by a plastic/rubber safe electrical spray cleaner (brake cleaner is OK as is 91% isopropyl alcohol). With suspension, cleanliness is next to Godliness. If disassembled, I also polish by hand the chrome tubes; very laborious.

Many people get confused with the fluid air gap, spring weight and preload. Think of the former as a "soft" spring that can be easily adjusted by adding or sucking out fluid. Each spring mfgr has a "table" to calculate how many kg you need but I usually go down a notch for street riding. Preload is also easy to adjust but set it less initially while you play with rebound and compression.

Don't be afraid to experiment but whatever you do in the front must be tailored to your shock. Personally I like Penske and K-tech; the latter

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Reactions: 1

LouG

· #16 · 1 mo ago

The fork air gap is a rising rate spring. It stiffens as it's compressed.

MT09 2014 Sweden

· #17 · 1 mo ago

2and3cylinders said:
I found that a good quality Semi synthetic blend that's 16 Cst @ 40C works well in our forks.

This is close to the original oil used by Yamaha. Anyone tried a 32 mm²/s @ 40°C oil?

MT09 2014 Sweden

· #18 · 1 mo ago

2

If you let it drain over night you should be able to go by quantity?

Font Parallel Slope Circle Diagram

Font Parallel Diagram Rectangle Circle

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2and3cylinders

· #19 · 1 mo ago

MT09 2014 Sweden said:
If you let it drain over night you should be able to go by quantity?

Nope...too much residual internally.

2and3cylinders

· #20 · 1 mo ago

LouG said:
The fork air gap is a rising rate spring. It stiffens as it's compressed.

Uh, that is what I was implying by saying "soft spring". Pneumatic vs Hydraulic. Air is compressible, oil not so much.

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