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PLEASE Help Fork Oil Level

Started by whistlebritches, May 22, 2012, 05:58:25 PM

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whistlebritches

May 22, 2012, 05:58:25 PM
97 Monster 900 Showa 41mm non-adjustable forks. Both of my manuals show original amount of oil (457cc) and a depth of 79.5mm. Then the note says the amount was changed from 457cc to 487cc but does not give revised oil depth.
Anybody care to share the revised depth??. Quick math tells me that if the internal volume of the fork is a constant as the oil height decreases, then the new depth would be ~75mm. But I think I'm wrong. I'm sittin here with my laptop on my bearing press and I smell like kerosene. Please Help

bond0087

#1
May 22, 2012, 10:25:44 PM
Let me start by saying that I don't know for sure, and don't have any experience with this.

I do have a laptop that doesn't smell like kerosene on my lap rather than my bearing press, so maybe my quick math will help.

For this calculation, I'm assuming that the inner diameter of the tube that fluid is being added to is 41mm (I realize that's an incorrect assumption, but it should get us in the ballpark). I'm also assuming that there is nothing other than fluid in the tube at the top (no spring, damper rod, etc.).

D = 4.1cm
r = 2.05cm
added volume = 30cm^3

The formula for the volume of a cylinder is pi*r^2*h.  So the height of a cylinder that has a diameter of 41mm and has a volume of 30cm^3 is:

30/(3.14*2.05^2) = 2.27cm = 22.7mm

So the increase in oil level height (and decrease in the depth from the top) would be 22.7mm with those assumptions, making your new oil depth about 57mm.

A quick sanity check - 30cc's is approx. 1 fluid oz.  So your added fluid is basically a weak shot worth of fork oil (a standard shot glass is 1.5 fl. oz) .  The added height is approx. .9 inches, and by my expert guestimation, a little less than an inch in a cylinder as wide as the fork tube and a shot glass 2/3 of the way full seem like about the same amount of fluid.

Hope that's helpful, and please correct me if I'm wrong.

Slide Panda

#2
May 23, 2012, 05:47:18 AM
Catch is that the oil level may be measured with the internals in - an irregular shape. 30cc is a lot of oil to add to forks.
-Throttle's on the right, so are the brakes.  Good luck.
- '00 M900S with all the farkles
- '08 KTM 690 StupidMoto
- '07 Triumph 675 Track bike.

ducatiz

#3
May 23, 2012, 06:10:24 AM
Seems to me the height should not change.  I'd go by the height.
Check out my oil filter forensics thread!                     Offended? Click here
"Yelling out of cars, turning your speakers out the window to blast your music onto the street, setting off M-80 firecrackers, firing automatic weapons into the airâ€"these are all well and good. But none of them create a merry atmosphere of insouciance and bonhomie quite like a revving motorcycle.

Slide Panda

#4
May 23, 2012, 07:23:09 AM
Which is what he's trying to figure out - new height based on the revised volume numbers.

-Throttle's on the right, so are the brakes.  Good luck.
- '00 M900S with all the farkles
- '08 KTM 690 StupidMoto
- '07 Triumph 675 Track bike.

ducatiz

#5
May 23, 2012, 08:55:28 AM
Quote from: Slide Panda on May 23, 2012, 07:23:09 AM
Which is what he's trying to figure out - new height based on the revised volume numbers.



maybe i'm missing something.. i would just use the original height (depth) of the fluid.  that sounds about standard to me -- 3" +/-
Check out my oil filter forensics thread!                     Offended? Click here
"Yelling out of cars, turning your speakers out the window to blast your music onto the street, setting off M-80 firecrackers, firing automatic weapons into the airâ€"these are all well and good. But none of them create a merry atmosphere of insouciance and bonhomie quite like a revving motorcycle.

whistlebritches

#6
May 23, 2012, 09:06:13 AM
Yes, new height based on revised volume numbers.

ducatiz

#7
May 23, 2012, 09:08:36 AM
Quote from: whistlebritches on May 23, 2012, 09:06:13 AM
Yes, new height based on revised volume numbers.

If the new volume is due to a change in the design its possible the height would be the same, no?

Just a thought.  Most of the forks I've done in the last 15-20 years seem to be in that ballpark height.
Check out my oil filter forensics thread!                     Offended? Click here
"Yelling out of cars, turning your speakers out the window to blast your music onto the street, setting off M-80 firecrackers, firing automatic weapons into the airâ€"these are all well and good. But none of them create a merry atmosphere of insouciance and bonhomie quite like a revving motorcycle.

battlecry

#8
May 23, 2012, 10:44:55 AM
You need to make sure you got enough oil to cover the cartridge with the forks fully compressed.  I'd remove the caps and let the forks slowly collapse, then measure how much to cover the cartridges and add little more to that.  If you find you need more compression damping or anti-bottom during heavy braking, add some more to that.

Cloner

#9
May 23, 2012, 12:13:55 PM
According to Race Tech, your 41mm Showas should use 5wt oil and with the fork legs fully collapsed and the fork springs removed you should measure an oil level of 110 mm.  This is similar to the other few dozens of forks of this type I've rebuilt/reworked. 

Never go by a volumetric measurement as this assumes a completely dry fork, which is virtually impossible to get without complete disassembly/cleaning/reassembly.  The volumetric measurement should simply be used to estimate the amount of oil you'll need to service your forks.

http://racetech.com/ProductSearch/2/Ducati/Monster%20900/1995-99
Never appeal to a man's "better nature."  He may not have one.  Invoking his self-interest gives you more leverage.  R.A. Heinlein

'64 Ducati Monza 250
'67 Aermacchi/HD Sprint SS (race bike)
'00 Aprilia RSV Mille
'03 Ducati 800 SS (race bike)
'04 KTM 450 EXC
'08 Kawasaki Ninja 250 (race bike)

whistlebritches

#10
May 23, 2012, 04:03:44 PM
Thank you all for the help. Since the forks had been completely disassembled and were dry I decided to go with the volume measurement. Haynes and Ducati gave the revised # of 487cc. After priming the damper rod then collapsing the damper and the fork the 487cc filled the fork up to about 18mm from the top of the tube. I was hoping the number would be closer to the expected 70mm to 110mm. I do wonder if with a little time the air gap between the od of the inner and and the id of the outer leg will fill with oil, thus lowering the height of the oil to be more in line with what I was expecting. Any more thoughts on this situation will certainly be appreciated.
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