Mixing fork oils

Am I right to assume that mixing forks oils is ok and straightforward?

If I mix equal quantities 2.5wt (Stendec) and 10wt (Castrol motorbike fork oil) I will get a 5wt oil?

doesn’t that give you 6.25wt ?

You are very right, Sir. I am a muppet.

So it is straightforward, just not for dunces like me.

I believe it is not so straightforward as that and I wouldn’t mix different manufacturers oil incase they are incomplatable

Mixing fork oils possible but it’s not linear. I had this conversation with silkolene many years ago. The make 7.5 is was one third 5 and two thirds 10 (or it could have been the other way around)

Yup, likewise, I can’t remember exactly how complicated it is, but I know it’s not as simple as it looks.

Yeah, I thought it was half of one and half of another to get oil weight in the middle of the two, but apparently I was wrong. There’s some weird ratio to do the mix, but I’m sure someone will be along shortly with the proper answer.

What are your starting oils and what weight are you after?

I have a 2.5wt and a 10wt, different brands, and I would like a 5wt.

Shame it is not just that simple!

The other thing is, no two companies have the same scale of weights.

Not mixed them but I believe Rockoil says on container that can mix, so maybe their website could give a formula. I`d query that not all mfr weights are different though, surely its a standard system ??

This is best estimated by the rule of mixtures
Fractionofa+fractionofb=C

so if you mix equal quantities of 2.5 and 7.5 oil its
(0.5 x 2.5) + (0.5 x 7.5) = (1.25 + 3.75)=5

I wouldn’t be worried mixing one type of oil with another.

That’s just a complicated way of saying “add them together and divide by 2”. Which doesn’t neccesarily work with oil viscosities. Though it’ll give something close enough most likely.

And it’s not standard system. There is one but it doesn’t relate to weight. That’s just a name companies give instead of light, medium and heavy.

Yes thats because I used 50 50 as a simple example, it does work with oil viscosities. It is used commonly used to ESTIMATE viscocities in the oil and polymer industries.

How about telling the man how to do it right then rather than just an estimate 😕

Here’s a silkolene mix chart, based off the cST at 40 degrees for various of their oils… And what you notice quite quickly is that there’s a dissociation between the cST and the stated oil weight, it’s nonlinear, which unfortunately buggers everything up if you want an exact result.

But then as’s been said, no 2 10W fork oils will perform identically anyway, it’s all over the place even without getting into quality control. So, personally I wouldn’t stress about it too much but if you want to get exactly what the manufacturer wants that basically means buying exactly what the manufacturer tells you to do, or finding out the actual kinematic viscosity numbers at the desired temperature and then finding an equivalent product.

Either that or just accept “fairly close”

Given that you have two different brands, working out the ratio to combine them to match a nominal weight, possibly of a third brand is pretty meaningless. Trial and error is as good a method as any at this point.

because the difference is less than a gnats todger. At room temp the actual calc of the above proportions yields a viscocity of 4.87 with most fork oils which I guarantee is less than the error in measuring the volume during mixing in the shed you won’t tell the difference.

How about keeping your gob shut unless you know what you are talking about? 😕

Any oils listed 2.5, 5,7,15 wt are all referring to the SAE standard for measuring oil viscocity.

worth a read!

Onzadog this just backs up my calc exactly the guy gives the silkolene chart as a reference

from silkolene chart to make 25 from their 13.6 (2.5 wt) and 26.7 (5wt)
They say mix 13% of 13.6 and 87% of the 26.7

Which is (0.13×13.6)+(0.87×26.7) =1.768+23.229=24.997

I rest my case.

or from the red line mix chart to make (picked at random) to make 17 from 9.8 and 18.4 they reccomend using 16% of the 9.8 and 84% of the 18.4

Soooooooo

(0.16×9.8)+(0.84×18.4) = 1.568+15.456=17.024

I rub my case in.

But looking down the table shows that not all oils claiming the same weight are not that similar in viscosity

Yes I accept your point, I think thats down to manufacturers using different temps to measure their viscocity, and possibly the reporter using differnet temps to measure them at. But the majority of the reported weights correlate broadly with the measured viscocity, there are a few that are wildly out there, but they seem to be brands I’ve never heard of!!!! So I’m happy to revise my comment based on this..

alexonabike – Member

I have a 2.5wt and a 10wt, different brands, and I would like a 5wt.

Shame it is not just that simple!

Assuming these two brands are mixable then using the rule of mixtures gives

0.66×2.5+0.34×10=5.05
0.67×2.5+0.33×10=4.98

There’s interesting, if dated, info on Peter Verdone’s website. Fox oil and Silkolene are near identical on CsT ratings. I’ve just changed mine and assessing the fork on my local red run (the potholed roads now left in my part of Edinburgh) I can say the action is improved. But then Fox had not put in the correct amount so that will account for some of the difference.

“W” ratings are clearly applied differently by different manufacturers but CsT ratings are quoted at defined temperatures of 40 and 100 degrees so are directly comparable. Finding the CsT rating of an oil may not be so easy of course.

With 12 clicks on a Fox fork there is a bit of latitude on oil viscosity I would have thought.

I used that list to buy some 10 wt fox oil. I zoomed in on a pic and then looked down the list until I saw the silkolene stuff that matched the viscosity. It’s even the same colour so have to assume it’s the same stuff rebranded. Glad they list the proper spec on the bottle along with the claimed weight.

Onzadog
I used that list to buy some 10 wt fox oil. I zoomed in on a pic and then looked down the list until I saw the silkolene stuff that matched the viscosity. It’s even the same colour so have to assume it’s the same stuff rebranded. Glad they list the proper spec on the bottle along with the claimed weight.

Yeah it’s useful, my only issue would be with that data is that he is (or was?) selling a service that relies on his data proving how inaccurate manufacturers specs are.

weight

wt, as in 10wt, 20wt, is not short fo weight.

its short for “winter”

ie: visocity of the oil when cold

Olly, that’s interesting. Sure that works for suspension fluid being 10wt in the same was as engine oil being 10W40 for example? Not sure why winter viscosity of suspension fluid would be the critical measure.

Couldn’t find any reference to that either. I’m not trying to be contrary, if I’m wrong, I’m interested to see where.

I think by default suspension fluid just uses the standard automotive viscocity ratings, (SAEJ300 doc) if you want a ref to W or Wt referring to winter look at wikepedia “motor oils”

I did and I saw reference to W being winter but every mention of wt I saw was weight. I also saw oil rated at 60 with no winter component but refered to as 60 wt.

I did and I saw reference to W being winter but every mention of wt I saw was weight. I also saw oil rated at 60 with no winter component but refered to as 60 wt.

There is some confusion but I think this clears up the meanings

SAE J300 has established eleven viscosity grades, of which six are considered Winter-grades and given a W designation. The 11 viscosity grades are 0W, 5W, 10W, 15W, 20W, 25W, 20, 30, 40, 50, and 60. These numbers are often referred to as the ‘weight’ of a motor oil.

So based on that I think you have something Onzadog, W refers to winter and I’ll bet WT is shorthand for weight. In science and eng we use “wt” as shorthand for weight…

So maybe Olly’s definition above needs revising to W is short for winter… (in fact all the refs I can find to Winter only mention W and not Wt). i have SAEJ300 (or whatever it is) at work, I’ll dig it out tomorrow and get to the bottom of this..

Cheers toys. Whatever the answer, it’s not going to make any difference to my lift but that doesn’t stop me wanting to know now the question has been raised. 😀

Like you say, in engineering, I’ve only ever known wt mean weight.