School me on fork oil

Joined
Aug 11, 2016
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I recently had some unplanned excitement, in the form of blowing out a fork seal real good. After a nice weekend out, I noticed my fork seemed a little... 'wet' at the gas pump. Lost enough fork oil to soak my brake pads on the trip home, where I kept it dripping into a pan. Spent some time ordering parts, tearing down the front end, and got the forks sealed up tight. With my rebuild, I ended up using the recommended Showa SS-8. Trying to pretend I'm a diligent consumer though, I did some reading and cross-shopping. Unfortunately, I'm not sure how much I learned.

At first, it seemed simple enough. My manual called for SS-8, which is sold as 10W. Easy! But when my peanut sized brain types 'best 10w fork oil' into the internet box, I see threads discussing fairly significant sounding viscosity differences. Many references to a chart from Peter Verdone, in which the Showa SS-8 is surrounded on both sides by oils that aren't 10W. I see threads about Goldwing riders using ATF in their forks, and saying that it's even in the manual. I see threads about people using ATF in their adventure bikes and swearing by it. At this point, I just figured that the sky was falling and ran scared for the Showa fluid.

Even now that the bike has been fixed, I'm still left wondering what exactly fork oil is. Is it somehow related to ATF, or does ATF just happen to be similarly thick and slippery? Does 10W even mean anything? Other than just doing a straight OEM replacement as I have, how do you pick a decent fork oil?
Just beware that all those charts like from Peter Verdone (and there are many others) have one secret; they're ordered by viscosity at 40C (104F) and unless you are an MX ripper forks operate at ambient temperature. At 20C (78F) viscosity could be 2.4 times higher than indicated at 40C
 
Joined
Jul 22, 2010
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Just beware that all those charts like from Peter Verdone (and there are many others) have one secret; they're ordered by viscosity at 40C (104F) and unless you are an MX ripper forks operate at ambient temperature. At 20C (78F) viscosity could be 2.4 times higher than indicated at 40C
Motorcycle service manuals call out a specified oil weight/viscosity like 5, 10, 15, 20, etc and each of those grades cover a range of viscosity - just like in motor oils. Cycle manufacturers call out a specified weight knowing the typical use of the bike, so IMO no need to worry about using the called out weight unless someone is using the bike in some obscure environment where it's super cold or hot and want to change the way the forks operate in those conditions.

Side note - the viscosity at 40C (like in the Peter Verdone table) is called the ISO viscosity grade, which is defined as the viscosity in cSt at 40C. Example, ISO grade 68 has a viscosity of 68 cSt at 40C, and is also within the SAE 20 grade. The ISO viscosity grade rating is more exact than the SAE grade rating method.
 
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Joined
Feb 3, 2016
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It's also the only information available to the public. Unless you've got a kinematic viscometer handy, the 40°C viscosity is the best approximation of the viscosity at the lower temperatures typical of more casual riding.
 
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