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Hi, Im changing my AT1mx fork oil and have looked on the factory manuals for amount but they give two amounts, 160cc in the spec and 120cc on the actual forks strip page ! Any help on which is the right amount ? and instead of their suggested 10/30 motor oil, what would best suit the forks ? thanks
I don't remember who posted this originally, but apologies to that person for not being able to credit them. But I cut and pasted it into my long-standing file on good-to-know stuff from the site, here:
I mentioned earlier that fork oils don't follow any real "standard" so you can't compare weights among brands or even among types of suspension oil within the same brand (fork vs shock oil)
A 20wt SAE motor oil is defined as an oil having a viscosity of between 5.60 and 9.29 cST (centistokes) at a temp of 100C and between 61.2 and 74.8 cST at 40C
A 30wt SAE motor oil is defined as an oil having a viscosity of between 9.3 and 12.49 cST (centistokes) at a temp of 100C and between 90 and 110 cST at 40C
Spectro 20wt fork oil is rated at 8.9 cST@100C and 65 cST@40C... right in the ballpark for a 20wt motor oil at both temps... but closer to the 30wt at high temp and at the mid- low end of the 20wt range at low temp.
For comparison, Silkolene 15wt RSF is rated at 19.5 cST@100C and 92.95 cST@40C. That's like 30wt at low temp and 50wt at high temp! And they label it 15w.
Last thing... and I know I've already said more than anyone really cares about... is the difference in the oils is the "viscosity index" or VI.
A standard straight weight motor oil has a defined VI of about 96.
The Spectro 20wt fork oil has a VI of about 111.... which is why at low temp it's at the mid-low range of the 20wt scale and at high temp it's almost in the 30wt range... it's almost a 20w-30.
The Silkolene oil has a VI of around 233 and as we saw earlier is something like a 30 - 50 motor oil.
The higher the VI, the less the oil thins out with temperature... VI is one of the factors that makes it hard to compare suspension oils (where VI varies a lot) to motor oils (where VI is somewhat standardized).
In any case, the Spectro 20w fork oil characteristics are a decent match to the fork oil Yamaha originally recommended... especially for street use.
Bottom line: Find a fork oil weight and brand that works and stick with it.
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