Fork Oil

[Constant Mesh]

One reason given for changing fork oil is that over time it becomes dirty and contaminated and why not put some new oil in.

Yet shocks also contain oil which likely gets dirty and contaminated but they continue to work fine with no oil change. And, of course, the huge population of shocks/struts on autos which contain oil. That oil is not changed and those shocks perform well for many, many thousand of miles.

What's so different about forks and their oil? Are their seals more exposed to the elements?

Is it because you can change the oil so golly gee let's perform some maintenance?

 

[HaulinAshe]

You had it right the first time. Forks have large seals and are much more exposed to the elements. Also the friction area of the tubes is exponentially greater than the typical rear shock, so much more worn material is generated.

But probably the most common reason that rear shocks don't get serviced as often as front forks, is because your hands are more sensitive than your ass.

 

[jestal]

I think a lot of fork and shock oil gets changed prematurely. Everyone comments on the way the old fork oil looks (dirty and nasty) but the appearance of the oil has little to nothing to do with the actual ability of the oil to function in the shock. It does make one feel much better to pour out that nasty stuff and put in fresh and then the placebo affect can take over.

There is a LOT of surface area on the fork tubes that travels past the seals so there is far more opportunity for dirt and contaminants to get into the oil on forks compared to much smaller diameter shock rods.

I would also guess that most fork oil changes involve changes of other sorts, too. At least an upgrade in fork oil quality. Not a simple drain and fill with exactly the same oil. An A vs. B ride is clouded with changes in oil viscosity, oil brand, friction modifiers in the different shock oils, etc. so it is not just a simple camparison of new vs. old oil. A lot of the glowing testamonials to the effect of clean fork oil may inadvertently include the effect of viscosity changes, etc.

The automotive shocks and struts are generally designed with steel tubes and ferrous wear surfaces. Lots of performance shocks are designed with aluminum tubes which are light but not as wear friendly so they will tend to "dirty" the shock oil quicker and cause it to look pretty nasty. Most struts have bellows protecting the shock rod from the majority of the dirt and filth. Also, the seals on shock rods are much more substantial than fork seals. Due to the large surface contact area the fork seals encompass (large diameter tubes) they must be fairly low tension so as to not cause a high degree of stiction. Small diameter shock rods can accomodate high tension and multiple seals will acceptable levels of stiction since the contact area of the seal(s) is relatively small. The low tension seals on forks will pass more of the dirt past them than higher tension shock and strut seals.

 

[TruWrecks]

I went to a suspension tuning class from Fluid Suspension. The instructor showed us a sample of fork oil from one of the race bikes he maintains. The oil from one season had enough metal in it you could move it with a magnet. He said it would be about the same as a street bike with about 20,000 miles on it. I'm not sure if 20,000 miles would be that bad, but it sure does keep the seals in better shape if the oil has fewer contaminants in it. The more junk, the faster the wear. The instructor highly recommended using 5W synthetic to get the best life out of the seals, or at least a synthetic blend that matches the thickness of the factory spec fork oil.

 

[Warchild]

I just happen to be in the middle of changing fork fluid on the FJR.... did the left fork last night, and the right one tonight.

I do an annual change personally, and that is absolutely not too frequent when you ride long and hard in the High Desert.... this fluid was crystal-clear when I put it in ~ 14K miles ago...... :blink:

 

[ionbeam]

Several of the fork tuning business mention the care they take to keep everything clean because the metering passages are so small. As the suspension works up and down the aperture of the metered passages will alter from no flow, tiny flow to full flow. If debris of any appreciable size gets transported to the metering circuits it can occlude fluid flow.

When I changed my fork oil, one leg was uniformly foul and had a silver sheen to the oil. The other leg had many small globules of ???? in it. The foul fluid contained fine metal particles as well as abrasive silicates. The fluid in the other leg was free of debris where it was clear and the globules appeared to have seal material, metal particles and perhaps moisture in it. While I did pass a couple of slides through a microscope I didn't investigate any farther.

 

[Fred W]

ZOMBI THREAD ALERT!!!

I'm resurrecting this old thread because I was considering starting a new one on this very same topic after some good posts made by MCRIDER007 here and here, but I wanted to focus in on just the fork oil part of the suspension equation. After thinking about the information as presented in these threads, and with a little bit of prior knowledge about the behavior of suspension fluids, the obvious question is:

What is a good (or the best?) fork oil to use when servicing the FJR forks?

Most of us are aware that fork oil varies widely, both in the actual thickness (viscosity) as compared to it's stated "weight" rating on the label, but also in how much it thins out as it gets hotter.

This suspension oil viscosity chart may not be fully up to date, but it does give us something to work off of. We can see in the chart that the stock Yamaha 01 (Kayaba) suspension oil has an initial viscosity of 14.57 cSt at 40C (100 F), and it thins out to 3.45 cSt when it heats up to 100C (212 F) as might happen during very heavy use.

Most owners just grab something off the shelf that says "5W" on the label and hope for the best. I've heard that each of the shops that do suspension work each have a recommendation for a particular oil to use, but that is probably more related to what they can get at a good price in bulk quantities than any actual superiority of the fluid.

It would seem logical to conclude that the best fluid to use would be one that most closely replicates the original fluid's viscosity, or maybe even just a tad thinner, at 40C, but that also doesn't thin out as much as the Yamaha 01 as it heats up, so it will give more consistent damping that does not fade as much under heavy use.

Again referring back to the chart it would appear that the oils that most closely meet those criteria are:

cSt @ 40 / 100 C

Yamaha 01 (Kayaba) 14.57 / 3.45

Torco RSF Light (Purple) 14.50 / 4.70

Maxima Racing Shock Fluid (Light, 3wt) 14.0 / 5.20

Silkolene Pro RSF (2.5wt) 14.0 / 5.80

The last one seeming to be the "best" choice, and is readily available, although at a $20 a liter premium street price.

Comments? Other recommendations?

 

[Bounce]

Been using Silkolene (synth) since my first fork service.

 

[Fred W]

Been using Silkolene (synth) since my first fork service.

Which one? They make several different synthetic ones.

 

[RossKean]

I have been using Bel-Ray 5 wt. No particular reason but it seems to work for me. I haven't tried anything lighter. The Bel-Ray is readily available and reasonably inexpensive. Forks are GenII, upgraded (Hyperpro) springs.

Should I try something else next time?

Ross

 

[MCRIDER007]

Most owners just grab something off the shelf that says "5W" on the label and hope for the best. I've heard that each of the shops that do suspension work each have a recommendation for a particular oil to use, but that is probably more related to what they can get at a good price in bulk quantities than any actual superiority of the fluid.

It would seem logical to conclude that the best fluid to use would be one that most closely replicates the original fluid's viscosity, or maybe even just a tad thinner, at 40C, but that also doesn't thin out as much as the Yamaha 01 as it heats up, so it will give more consistent damping that does not fade as much under heavy use.

I have never tried to determine how much fork oil heats up under normal street use but Peter Verdone says the temperature only goes up by about 10F (easy to test, go for a ride and fell the temperature of the lower fork leg)....so I don't think there is much of a viscosity change from riding, just from the outside temperature itself (which can vary the viscosity by up to 25 percent). The rear shock is a totally different story, that oil can go past 250F and I was told by a long time track rider that the shock's damping can start to go away by the end of a 20 minute session. I have noticed a significant change is a shock's performance on a cold 40F day, very stiff at the beginning of the ride, but about normal after 20-30 minutes whereas the fork's oil never does seem to warm up. The bottom line is that using a oil that does not thin out is very important for a shock but pretty much a non-issue for the forks, at least in a normal street riding environment.

Most cartridge forks are valved for oil around 16cst@40C but GP Suspension designed their systems using oil that was 22cst@40F and I think Traxxion does the same with their AK20s. Suspension shops go through a lot of fork oil and buy their fork oil in 55 gallon drums and the heavier oils happen to be the viscosity that are available in bulk sizes. Its premium oil and works great in their products and whatever viscosity they use should continue to be used.

I recently used Golden Spectro in my forks for the first time and was very impressed with how smooth the forks seemed to be with the new oil, much more than I have ever previously noticed with Belray, Honda SS7, Motorex, and Silkolene.

 

[builderwill]

I used the Honda Proline HP 5 wt last time I changed the FJR's. Not too bad at 14/quart. Could only find it in quarts and I believe you needed just over 1 quart. Does fork oil have a shelf life in an opened container?

I have been using Bel-Ray 5 wt. No particular reason but it seems to work for me. I haven't tried anything lighter. The Bel-Ray is readily available and reasonably inexpensive. Forks are GenII, upgraded (Hyperpro) springs.
Should I try something else next time?

Ross

I'm using this on my zx14 forks right now. Very available and cheap and I need right at one liter so none left over.

 

[MCRIDER007]

I have been using Bel-Ray 5 wt. No particular reason but it seems to work for me. I haven't tried anything lighter. The Bel-Ray is readily available and reasonably inexpensive. Forks are GenII, upgraded (Hyperpro) springs.
Should I try something else next time?

My viscosity chart has slightly different numbers than Fred's and shows Yamaha 01 at 15.6cST@40C and Bel-Ray 5 wt at 17.1.........which is pretty close. If you mostly ride in temperatures below 75F, you might find a small improvement by using a lighter weight oil.

 

[Fred W]

Good info...

So, for a reworked fork, going with the kit manufacturer's spec for oil makes the most sense, since that was what they deigned their orifices for.

But for the stock fork, something as close to stock or maybe thinner than Yamaha 01 is probably the way to go. And the ones with the larger viscosity index (smaller difference with temp) probably makes sense here in New England where one would routinely ride in temps anywhere from 100F to below freezing.

 

[MCRIDER007]

But for the stock fork, something as close to stock or maybe thinner than Yamaha 01 is probably the way to go. And the ones with the larger viscosity index (smaller difference with temp) probably makes sense here in New England where one would routinely ride in temps anywhere from 100F to below freezing.

If you are bored someday and want to do something related to motorcycles, you can use graph paper to show the viscosity of a given oil at any temperature. You plot the two points (100C and 40C) and then extend the line all the way to 0C. Your higher quality oils will have flatter curves but the oils with the least temperature variance, both in percentage and numbers, are the lighter oils. The variance between 40F and 90F is usually around 25 percent.

 

[road runner]

I have the Traxxion AK20's and they say they use Maxima 7wt. I didn't have any, but did have Bel-Ray 5wt and 10wt, so I mixed them 50/50 trying to come up with the same viscosity #'s. Looking at Fred's chart I think it's close.

 

[MCRIDER007]

I have the Traxxion AK20's and they say they use Maxima 7wt. I didn't have any, but did have Bel-Ray 5wt and 10wt, so I mixed them 50/50 trying to come up with the same viscosity #'s. Looking at Fred's chart I think it's close.

When you are using different fork oils and mixing weights to match another fork oil, you need to try to match to the viscosity at 40C because that is the temperature closest to the oil temps in the fork. You also need to make sure you are buying fork oil or suspension fluid rather than shock oil. Maxima sells 2 oils they call 7wts, one is a shock oil and the other is a 125/150 suspension fluid. Mixing Bel-Ray 5wt and 10wt will give you a viscosity that is very close to 125/150.

Suspension fluids, which come in sizes like 85/150 (5wt), 125/150 (7wt), 165/150 (10wt), and 235/150 (15wt) will have a consistent viscosity between brands because of uniform standards for measuring suspension fluid viscosity.

 

[road runner]

I have the Traxxion AK20's and they say they use Maxima 7wt. I didn't have any, but did have Bel-Ray 5wt and 10wt, so I mixed them 50/50 trying to come up with the same viscosity #'s. Looking at Fred's chart I think it's close.

When you are using different fork oils and mixing weights to match another fork oil, you need to try to match to the viscosity at 40C because that is the temperature closest to the oil temps in the fork. You also need to make sure you are buying fork oil or suspension fluid rather than shock oil. Maxima sells 2 oils they call 7wts, one is a shock oil and the other is a 125/150 suspension fluid. Mixing Bel-Ray 5wt and 10wt will give you a viscosity that is very close to 125/150.

Suspension fluids, which come in sizes like 85/150 (5wt), 125/150 (7wt), 165/150 (10wt), and 235/150 (15wt) will have a consistent viscosity between brands because of uniform standards for measuring suspension fluid viscosity.

So I guess I did OK. The Maxima 7wt fork oil is 26.7cst. @40c, and mixing the 5wt & 10wt Bel-Ray comes out to 28.95cst. @ 40c

 

[MCRIDER007]

I have the Traxxion AK20's and they say they use Maxima 7wt. I didn't have any, but did have Bel-Ray 5wt and 10wt, so I mixed them 50/50 trying to come up with the same viscosity #'s. Looking at Fred's chart I think it's close.

When you are using different fork oils and mixing weights to match another fork oil, you need to try to match to the viscosity at 40C because that is the temperature closest to the oil temps in the fork. You also need to make sure you are buying fork oil or suspension fluid rather than shock oil. Maxima sells 2 oils they call 7wts, one is a shock oil and the other is a 125/150 suspension fluid. Mixing Bel-Ray 5wt and 10wt will give you a viscosity that is very close to 125/150.

Suspension fluids, which come in sizes like 85/150 (5wt), 125/150 (7wt), 165/150 (10wt), and 235/150 (15wt) will have a consistent viscosity between brands because of uniform standards for measuring suspension fluid viscosity.

So I guess I did OK. The Maxima 7wt fork oil is 26.7cst. @40c, and mixing the 5wt & 10wt Bel-Ray comes out to 28.95cst. @ 40c

And the viscosity chart I have shows the Bel-Ray at 17.1 and 33.0 which comes out to [email protected] way you are pretty close.