Fork Oil Wt?


11 replies to this topic
  • brandonizr

Posted November 06, 2007 - 04:51 PM

#1

01 YZF426, looking to see what weight fork oil people are running for desert riding? I'm currently using stock valving with 5wt but my suspension guy recommended 3wt since i'm strictly desert, any thoughts? 6 ft 185lb rider
Thanks

  • grayracer513

Posted November 06, 2007 - 05:22 PM

#2

The "suspension guys" I've talked to have pointed out that oil weight isn't really that important, since forks are built to work on pressure differentials across the valving, rather than flow rates, expressly so they won't be affected by changes in fluid viscosity due to heat.

What will make more difference on your single chamber KYB fork is adjusting the oil level down a little to make the fork a bit cushier. Don't overdo this, or foaming will result. Try it at 110 mm and see what happens.

Another cute little piece of info regarding the SC Kayabas is that the same clicker that regulates rebound damping also serves as a bleed off for the mid valve. If the fork seems harsh on the small hits and edges, try a little less rebound damping.

  • 2grimjim

Posted November 07, 2007 - 09:33 AM

#3

The "suspension guys" I've talked to have pointed out that oil weight isn't really that important, since forks are built to work on pressure differentials across the valving, rather than flow rates, expressly so they won't be affected by changes in fluid viscosity due to heat.

What will make more difference on your single chamber KYB fork is adjusting the oil level down a little to make the fork a bit cushier. Don't overdo this, or foaming will result. Try it at 110 mm and see what happens.

Another cute little piece of info regarding the SC Kayabas is that the same clicker that regulates rebound damping also serves as a bleed off for the mid valve. If the fork seems harsh on the small hits and edges, try a little less rebound damping.


I have to respectfully dissagree with you suspension guys. Changing oil viscosity can have a profound affect on how the suspension damps. You're of the age that I'm certain you have had plenty of experience with the ancient 'damper tube' forks. I know my old '75 Honda rode like a cement truck when I put 30wt fork oil in it.

Viscosity by definition is a fluids resistance to flow, regardless of pressure differential. All things being equal a more viscous fluid will flow at a lower rate than a less viscous fluid. There is no magic, even in modern forks that compensates for difference in viscosity. That doesn't mean that a fork could not be designed (or maybe there is one, but it's not on any modern MX bikes) to compensate for viscosity change due to temp changes.

I can understand why some suspension tuners would advise against fiddling with fluid viscosity though. If someone has a problem with not enough high-speed damping/bottoming resistance but the low-speed is fine than switching to a heavier fluid would only complicate things.

  • grayracer513

Posted November 07, 2007 - 10:00 AM

#4

First, a 1975 anything is a long way from current fork technology. They worked on completely different principals, for the most part.

Second, the difference between 3 and 5 wt oil is very slight compared with that of a comparison of older, vastly heavier 15, 20, and 30 wt oils.

The "magic" in modern fork valving, or your car transmission, or your engine's oil pump, or in any number of other places, is that the valving opens farther to allow oil to flow through it rapidly enough to hold at the desired pressure. It's not even new, on a technological scale, even though it's relatively new to suspension hydraulics.

  • 2grimjim

Posted November 07, 2007 - 10:36 AM

#5

I understand what you are saying about the varible opening of the valve stack in modern forks but the flexing shim design functionally is no different than a spring loaded ball; the opening size is proportional to the pressure differential. The opening size is dependant on force applied (force counteracting the spring) and nothing else. The thought behind the variable opening is that a higher viscosity fluid will generate a greater difference in pressure (a fallacy) given and equal application of force a corrosponding larger opening of the shims. Simplified logic would lead one to believe that this would negate the effects of a more viscious fluid but that is simply not true. Given EQUAL input forces the presure differential (at the wheel and valve stack) is going to be constant, regardless of the fluid viscosity. What's missing is the taking into account of the time period that the force of impact is dissapated. The more viscous fluid will flow at a lower rate (time) because the force applied is constant. This will translate to less force being absorbed by the spring and more force perceived by the rider.

The small difference between 3wt and 5wt oil may not make much difference but it can be used as a fine (vs. coarse) tuning tool in some applications.

The physics behind a damper tube fork and a modern fork are unchanged. The only change is how the phases of fluid transfer are managed more precisely.

  • creeker

Posted November 07, 2007 - 10:47 AM

#6

01 YZF426, looking to see what weight fork oil people are running for desert riding? I'm currently using stock valving with 5wt but my suspension guy recommended 3wt since i'm strictly desert, any thoughts? 6 ft 185lb rider
Thanks

Well... my response is nowhere near as profound as the others, but I use 5wt Honda fork oil in my 00' 426 with the 110 mm method Gray mentioned above and I am just a little bigger than you. Although, I don't ride desert.

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  • grayracer513

Posted November 07, 2007 - 01:20 PM

#7

The more viscous fluid will flow at a lower rate (time) because the force applied is constant.

This is only true in the event that the pressure on the fluid cannot be relieved by the controlling valve work, or when the flow is high enough that the input and output passage diameters become the limiting factors. The applied force that originated the fork movement (the upward impact at the wheel) will not change that much, but if the fluid in the damping unit does not flow at a high enough rate, the force applied to the fluid within the fork, i.e., the fluid pressure, will increase until the valving responds by opening the control passage farther. Only when the ability of the valve or the connected passageways to accommodate the increased flow has been exceed will the rate of flow change.

The fork on a current YZ450 will perform more or less exactly the same when dead cold at 70 degrees as it does at the end of the day, when fluid temperatures may approach 150 degrees or more within the cartridge. That temperature change takes the fluid from a cold viscosity of 16.2 in the case of Maxima 5wt fork fluid to a hot viscosity of about 5.5. That is a net 294% change in viscosity that basically goes totally unnoticed by most riders. The change from 5 to 3 wt is a 19% change.

  • 2grimjim

Posted November 07, 2007 - 01:46 PM

#8

cold viscosity of 16.2 in the case of Maxima 5wt fork fluid to a hot viscosity of about 5.5.


Just curious, where did you get the vis, data from?

I suppose we could discuss this for days.........

Suspension tuning, unlike engine tuning, is someting that is highly subjective albeit there are some fundementals that apply to everyone.

I think that's something we may both agree on.

  • grayracer513

Posted November 07, 2007 - 01:54 PM

#9

The viscosity data for Maxima products is on their web site.

We can agree that you may like your suspensions or engines set up differently that I do, but the principals of physics on which they operate won't change because of anyone's preferences.

  • brandonizr

Posted November 08, 2007 - 09:20 AM

#10

That temperature change takes the fluid from a cold viscosity of 16.2 in the case of Maxima 5wt fork fluid to a hot viscosity of about 5.5. That is a net 294% change in viscosity that basically goes totally unnoticed by most riders. The change from 5 to 3 wt is a 19% change.


Very interesting, that puts a lot into perspective
My suspension guy ended up going with 5wt and just made some other adjustments. Thanks for all the input:ride:

  • grayracer513

Posted November 08, 2007 - 09:48 AM

#11

For clarity, I should have noted that the viscosity figures in my post were in centistokes (cSt), and not SAE numbers.

  • Wiz636

Posted November 08, 2007 - 03:07 PM

#12

For clarity, I should have noted that the viscosity figures in my post were in centistokes (cSt), and not SAE numbers.


I was wondering about that...:worthy: :worthy:





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