The Confusion that is Fork Oil

raesewell · Apr 7, 2017

I got in touch with the FJR Club workshop here in the UK and they sent me the Motorex Racing Fork Oil 2.5W which has a viscosity of 14.6

and it feels just the way it did with the Yamaha original oil. The FJR Club workshop did some extensive research and found this to be the closest to the Yamaha 01.

Of course if you want your suspension a bit stiffer or even looser you now have the information to make an informed choice. If you go for an oil within 5 or 6 Centistokes of the original it can be dialled in or out with the adjustment, much over that and the fork internals will struggle to cope. The tube diameter and the size of the valve will only work within certain parameters. So you can go up or down in weight a bit to suit your needs, but not too much. As I found out.

raesewell · Apr 7, 2017

Update: I had an email back from Yamaha telling me to ring the Technical Dept. Which I did, the young man on the phone said he was not allowed to send me a Technical Data sheet but was happy to give me the viscosity over the phone.
Yamaha M1 = 18.7 Centistokes @ 40 C
Yamaha 01 = 15.3 Centistokes @ 40 C

So there we have it an answer at last.

Fred W · Apr 7, 2017

Great info. Thanks for sharing that.

If you go for an oil within 5 or 6 Centistokes of the original it can be dialled in or out with the adjustment, much over that and the fork internals will struggle to cope. The tube diameter and the size of the valve will only work within certain parameters. So you can go up or down in weight a bit to suit your needs, but not too much. As I found out.

This is exactly right. It is the stock valves that require you to use a thin suspension fluid. Not so much for the slow speed (adjustable) damping, but rather for the non adjustable high speed damping that is fixed and set internally by springs and reeds. Remember that you really do not want very much high speed damping to have the most responsive suspension and allow the wheel to follow irregularities in the road. It is the low speed damping that you want more of, but not at the expense of increasing high speed stiffness.

Going with replacement aftermarket valves (or cartridge) can lessen the need for such a thin suspension fluid by increasing the number and size of the high speed orifices. If you have upgraded your fork valves you really NEED to ask the manufacturer what viscosity (in cSt) they recommend with their valves. Just make sure that they understand that the bike will be used on real world streets, not a nice smooth race track.

RossKean · Apr 7, 2017

Good information. With the very high dependence of viscosity on temperature, I doubt that you would notice any real difference between them. A few degrees temperature change would make more change than the reported difference. I think it is likely sufficient to aim for anything in the general range based upon the published charts that were linked in earlier replies.

raesewell · Apr 7, 2017

Trust me the difference between an oil in the 15 - 25 range is like night and day to the oil I bought at 47 viscosity. The bike was unridable, I was out for about 20 minutes and couldn't wait to get home.

The viscosity is taken at 40 degrees centigrade, so unless you live in a really hot place I cannot see the fork oil rising significantly above that temperature.

RossKean · Apr 7, 2017

47 centiStokes is not what I would consider to be in the same general range. Agree that 47 is thick at 40 °C - much thicker at 20 °C!!

raesewell · Apr 7, 2017

RossKean said:
47 centiStokes is not what I would consider to be in the same general range. Agree that 47 is thick at 40 °C - much thicker at 20 °C!!

That it's not as I found out. So as I said anything in the 15 to 25 range depending on your preference for a bit softer or a bit harder would be fine.

At least we now have the data to make an informed choice.

Fred W · Apr 7, 2017

Yes, oil viscosity changes with temperature, which is why they specify it at two temperatures, 40C and 100C. And while the oils are a lot thinner at 100, you would never get it anywhere near that hot in a motorcycle fork, and I'm fairly certain the viscosity change with temperature would not be a linear function.

Ideally they would spec fork oil at 0 or 10C and 40C, as that is the general area we are actually using it at, but the standard has been set (probably for some other application of hydraulic fluid) and that is how it is done.

MajBach · Apr 24, 2017

raesewell said:
RossKean said:

47 centiStokes is not what I would consider to be in the same general range. Agree that 47 is thick at 40 °C - much thicker at 20 °C!!

Click to expand...

That it's not as I found out. So as I said anything in the 15 to 25 range depending on your preference for a bit softer or a bit harder would be fine.

At least we now have the data to make an informed choice.

Ok, so now I'm confused when before I was just...ignorant. I typically ride my FJR near load limits what with my size and gear. I finally had the forks done at GP and noticed a great improvement but, after 10,000 miles or so, they are feeling a little like they need some servicing. I have no idea what weight they used and no idea how to service my forks. I'm attending the tech day in a few weeks and changing out my fork fluid was priority one. I had been advised to just go with Motul 7.5 and to grab a few quarts on my way down. Those charts don't seem to show Motul 7.5. And even if they did, I am wondering what "centistroke" rating I should really use.

One other question, you stated early in your findings that oil weight means nothing and viscosity is what we are truly after. However, in one of those charts it lists the VI beside the "centistroke (or whatever it is) at 40C and there seems to be no correlation. For example, two oils can be roughly the same VI but their centiStoke is completely different. Why?

MCRIDER007 · Apr 24, 2017

GP Suspension used Motorex 5w fork oil to develop their damping cartridges and that oil has a viscosity of 22.6 Cst at 40C which is pretty close to a number or other premium fork oils, some called 5w and others called 7 or 7.5 wt. I have also mixed oils (Honda SS7 and Honda SS8) to achieve that viscosity with good results.

When Jay brought Motul 7.5 to tech day to install in his forks we searched the Internet to verify the actual viscosity of that oil but were unsuccessful, but he has been very happy with the results so that is an viable alternative if you can find it. Maxium 7w is the right viscosity, easy to find, and seem to be a favorite of many suspension shops.

Fred W · Apr 24, 2017

The viscosity of the suspension fluid is the centistoke rating, and is expressed at two temperature points. The one that is most germane in a fork application is the 40 degree C rating, as the forks would never get any hotter than that, if they even get that high.

The 100C viscosity is more important in an enclosed shock where the temperature can get a bit higher, though probably still not as high as 100C (212F).

The Viscosity Index column is a numerical depiction of how much the fluid changes viscosity between those two temps. In other words, how much thinning happens when you heat the oil. All oils will get thinner, the VI just shows how much. It's not particularly important in the case of a fork oil.

MajBach · Apr 24, 2017

Fred W said:
The viscosity of the suspension fluid is the centistoke rating, and is expressed at two temperature points. The one that is most germane in a fork application is the 40 degree C rating, as the forks would never get any hotter than that, if they even get that high.
The 100C viscosity is more important in an enclosed shock where the temperature can get a bit higher, though probably still not as high as 100C (212F).

The Viscosity Index column is a numerical depiction of how much the fluid changes viscosity between those two temps. In other words, how much thinning happens when you heat the oil. All oils will get thinner, the VI just shows how much. It's not particularly important in the case of a fork oil.

And what do the numbers mean beside some oil ex., Maxima 125/150?

MCRIDER007 · Apr 24, 2017

MajBach said:
Fred W said:

The viscosity of the suspension fluid is the centistoke rating, and is expressed at two temperature points. The one that is most germane in a fork application is the 40 degree C rating, as the forks would never get any hotter than that, if they even get that high.
The 100C viscosity is more important in an enclosed shock where the temperature can get a bit higher, though probably still not as high as 100C (212F).

The Viscosity Index column is a numerical depiction of how much the fluid changes viscosity between those two temps. In other words, how much thinning happens when you heat the oil. All oils will get thinner, the VI just shows how much. It's not particularly important in the case of a fork oil.

Click to expand...

And what do the numbers mean beside some oil ex., Maxima 125/150?

The 125/150 is the equivalent viscosity of Suspension Fluids which are measured by pouring a specified volume of the fluid through a funnel and counting how many seconds it takes. As such, the viscosity of such oils is much more consistent than fork oil that are simply labeled 5w, 7w, 10w. Rule of thumb is that 85/150 is a 5w, 125/150 is 7.5w, and 165/150 is 10w.

bigjohnsd · Apr 24, 2017

This post should be stickied in the technical Section

MajBach · Apr 24, 2017

MCRIDER007 said:
. When Jay brought Motul 7.5 to tech day to install in his forks we searched the Internet to verify the actual viscosity of that oil but were unsuccessful,

https://transmoto.com.au/comparative-oil-weights-table/

or this one

https://mahonkin.com/~milktree/motorcycle/fork-oil.html

Interesting discrepancy:

https://www.amazon.com/Motul-100-Synthetic-Fork-Oil/dp/B000VJM1E8/ref=sr_1_4?ie=UTF8&qid=1493070195&sr=8-4&keywords=motul+fork

https://www.amazon.com/Motul-Fork-Factory-Line-105926/dp/B01LNWGQ6A/ref=sr_1_3?ie=UTF8&qid=1493070195&sr=8-3&keywords=motul+fork

https://fortnine.ca/en/motul-shock-oil-factory-line

MajBach · Apr 24, 2017

MCRIDER007 said:
. Maxium 7w is the right viscosity, easy to find, and seem to be a favorite of many suspension shops.

Which one? What are the differences?

https://www.amazon.com/Maxima-Racing-Fork-Fluid-59901-7/dp/B0022ZUSNA

https://www.amazon.com/Maxima-58901M-Medium-Formula-Racing/dp/B000WPYTUU/ref=pd_lpo_vtph_263_tr_t_3?_encoding=UTF8&psc=1&refRID=1JB05KKC52754ZSYX2AN

Fred W · Apr 24, 2017

MCRIDER007 said:
MajBach said:

Fred W said:

The viscosity of the suspension fluid is the centistoke rating, and is expressed at two temperature points. The one that is most germane in a fork application is the 40 degree C rating, as the forks would never get any hotter than that, if they even get that high.
The 100C viscosity is more important in an enclosed shock where the temperature can get a bit higher, though probably still not as high as 100C (212F).

The Viscosity Index column is a numerical depiction of how much the fluid changes viscosity between those two temps. In other words, how much thinning happens when you heat the oil. All oils will get thinner, the VI just shows how much. It's not particularly important in the case of a fork oil.

Click to expand...

And what do the numbers mean beside some oil ex., Maxima 125/150?

Click to expand...

The 125/150 is the equivalent viscosity of Suspension Fluids which are measured by pouring a specified volume of the fluid through a funnel and counting how many seconds it takes. As such, the viscosity of such oils is much more consistent than fork oil that are simply labeled 5w, 7w, 10w. Rule of thumb is that 85/150 is a 5w, 125/150 is 7.5w, and 165/150 is 10w.

Good to know. Unfortunately it is not widely adopted, so only helps if comparing those that do use that technique. It's not like they are making it easy for us. Right?

MCRIDER007 · Apr 24, 2017

Right. When it comes to oil I don't think you can count on anything being easy....or consistent....or accurate....or truthful.

RossKean · Apr 24, 2017

MCRIDER007 said:
Right. When it comes to oil I don't think you can count on anything being easy....or consistent....or accurate....or truthful.

...or important - at least within reason! I suspect choosing ANY of the oils within a reasonable range of the "recommended" would not make a perceptible difference -

Or at least a difference that couldn't be handled with a couple clicks of adjustment. Suffice it to say the correct oil for the FJR is one chosen from the lighter end of the range.

Fred W · Apr 25, 2017

RossKean said:
MCRIDER007 said:

Right. When it comes to oil I don't think you can count on anything being easy....or consistent....or accurate....or truthful.

Click to expand...

...or important - at least within reason! I suspect choosing ANY of the oils within a reasonable range of the "recommended" would not make a perceptible difference -

See, here's the problem. You know you want something similar to the OEM stuff. People say you should use a 5 weight fork oil. You look on the shelf at your LBS and you see an assortment of 5 weight fork oils:. What you do not see is the centistoke ratings of the oils in parenthesis below

Torco RFF 5 (10.75 cSt)

Maxima Fork Oil 5 Weight (15.9 cSt)

Maxima Racing Fork Fluid 5 Weight (15.9 cSt)

Silkolene Pro RSF 5 Weight (26.7 cSt)

Castrol Synthetic 5 Weight (28 cSt)

PJ1 Fork Tuner 5 Weight (31.2 cSt)

Shell Advance 5 Weight (33.05 cSt)

The Yamaha 01 Fork oil you are seeking to replicate has a viscosity of 15.60 cSt. Using any of the oils in that list except the two Maximas is going to cause problems with the damping, yet they are all rated as "5 weight".

On the other hand Castrol Fork Oil 10W (15.0 cSt) or Motorex Racing 2.5 Weight (15.1 cSt) would work fine. but they are not "5 weight".

The Confusion that is Fork Oil

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