Fork oil weight?, height?


5 replies to this topic
  • yz-dwg

Posted July 11, 2008 - 09:47 AM

#1

Hey fellow WRer's, I am going to replace my stock fork seals and oil on my '06 450, I weigh 180 and am a "C" rider at best, I only race for fun but try to ride hard on the trails. By my research I figure 3.5 or 5wt is about stock, I dont plan to revalve or spring but hear that I may get some better performance with different oil weights and height's, any body have any advice or imput? I am doing it myself and am looking for some cheap magic, what say you?, thanks in advance for your responces:thumbsup:

  • WR450FGreg

Posted July 11, 2008 - 02:22 PM

#2

.
Hard to give accurate recommendations, it's totally rider-specific....

As a "rule of thumb" lighter weight oils will reduce damping effect (make the suspension move quicker through it's up and down strokes).
Heavier oil will slow this down.

Raising the oil level will maintain the same action through most of the stroke but increase resistance as the forks reach bottoming out point. In other words stiffen up the last bit of the stroke.

Your Owners Manual should detail the recommended limit of oil level in the forks.

You probably know all that, but that's my two cents worth anyway!
Good luck with it.

I'm doing a similar thing with my own '08 forks today.
(I was going to also add some pre-load spacers but I might just try the raised oil level first. I'm getting old and need to slow down!)
I'll be raising my oil level to about 110mm as a trial.

Greg

  • jandmsuburban7

Posted July 14, 2008 - 12:52 PM

#3

Hi, buy the best racing fork oil and dont put in yamaha fork oil. My own experience says it doesnt hold up and hasnt got the quality. Use 5 wt or 7 wt. I am 195 and I use about 25 cc over what is called for. This helps dramatically when I hit a big jump or bump or cow (just kiddn). If you were bottoming before, then add more fluid, if not, the fluid you'll put in is a 100% better than the stock!! If this didnt satisfy you there is plenty more if you search back on fork fluid. Jason

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  • grayracer513

Posted July 14, 2008 - 02:29 PM

#4

As a "rule of thumb" lighter weight oils will reduce damping effect (make the suspension move quicker through it's up and down strokes).
Heavier oil will slow this down.

Not true, at least not entirely. The viscosity of the fluid does little to affect the damping characteristics of the fork. They are designed to be like this so that the damping stays relatively constant throughout the operating temperature range. The difference between a cold 7wt and a heated 7wt is about 5 times the difference between a 7wt and a 5 or 10wt.

Because, fork valving operates based on pressure differentials across the valve, viscosity does not play a big role in the damping performance. If the fluid is thicker, the valve simply opens farther to achieve the desired pressure difference. Where viscosity comes into play is anywhere that the oil is forced through a fixed orifice of some type. The hydraulic bottoming stops in the 48mm fork used on WR's since '05 is an example of this.

Raising the oil level will maintain the same action through most of the stroke but increase resistance as the forks reach bottoming out point. In other words stiffen up the last bit of the stroke.

True. Changing the oil volume inversely changes the volume of captive air in the fork, and thus the "stiffness" of the air "spring". Another way of looking at it is that a higher oil level raises the compression ratio of the fork. The KYB single chamber fork tends to feel harsh with oil levels higher than 105-100mm (90mm is higher than 100). Lowering the level, then is desirable, but you can run into trouble with bottoming, or if you get it down around 130 and ride hard, foaming problems. The right spring rates are very important.

With all this in mind, you can try a couple of things. If you are having bottoming problems, and your spring rates check out as correct, then you may try raising the oil level to see if it fixes it. If it becomes overall harsh, then you might instead try a heavier fluid, such as a 7wt, at a lower level such as that you were formerly running.

  • WR450FGreg

Posted July 14, 2008 - 02:46 PM

#5

Not true, at least not entirely. The viscosity of the fluid does little to affect the damping characteristics of the fork.....


I hear what you're saying grayracer513, and I've been told similar from someone else in the past, but I still have trouble getting my head around it!

I did an experiment years ago with a pair of RD350LC forks. I pulled them out of the bike, put 5 weight in one and 25 weight in the other. One was definately damped more than the other (heavier effect on compression/rebound? I can't recall which now) when pumped up and down on the garage floor.
These were pretty primative forks, so clearly i need to move into the present with my suspension theories!

Thank you for the "re-direction" !!! :thumbsup:
I'm happy to receive it.

Greg

  • grayracer513

Posted July 14, 2008 - 02:55 PM

#6

I hear what you're saying grayracer513, and I've been told similar from someone else in the past, but I still have trouble getting my head around it!

I did an experiment years ago with a pair of RD350LC forks. I pulled them out of the bike, put 5 weight in one and 25 weight in the other. One was definately damped more than the other (heavier effect on compression/rebound? I can't recall which now) when pumped up and down on the garage floor.
These were pretty primative forks, ...

Yeah. I should have said "modern" fork valving when I said that. The old stuff like the RD was different.




 
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