School me on fork oil

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10
Location
Nevada
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?
 
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2,932
Sorry I can't help with a MC! Frankly I'm glad I just deal with mountain bike forks. They can be difficult enough.
 
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4,856
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OK
As long as you aren't looking for a specific performance, it really doesn't seem to matter. I've used Supertech ATF to rebuild dirt bike forks and it felt identical to the factory fluid. In the past I've used Repsol 10w and more recently Motul 15W.

It. all. feels. the. same.
 
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939
Location
PR CA, USA
ATF I was told is basically 10 weight with extra additives. Forks are super simple and unless you are a pro rider it shouldn’t cause any issues ever. I used King off road shock oil in my last fork rebuild. It’s a 5wt. $30 a gallon. Couldn’t tell a difference. I mostly ride trails and dirt roads. I use it for my truck but a gallon would last a lifetime for forks only.
 
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937
Location
Colorado, USA
I use ATF as well, there is a fork oil weight chart somewhere on the internet and if you look at the viscosity ATF tests out at 10 wt. Valvoline Max Life ATF is synthetic and is in the forks of my ZRX 1200 after the full fork rebuild including bushings this summer.

If I recall ATF did a better job than typical standard fork oil with respect to maintaining viscosity when it heated up.
 
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2,187
Location
Arizona
Look for a post or few from Molakule on this topic. He's commented before.

Apart from that, remember that for fork oil you're looking at (or should be) the 40C / 100F viscosity, not the 100C / 212F viscosity. Fork oil runs cool, even on motocross bikes. Shock oil runs hot, so you'd want to look at the 100C / 212F viscosity for that, but not for fork oil.

I don't claim that ATF won't perform the basic function of being liquid in your fork, but it's not built to be a fork oil. This is where Molekule's comments are pertinent. MotoTribologist may have also commented on this. I can't recall just now.
 
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2,260
Location
Seattle-ish, WA
To answer your questions, fork oil is hydraulic fluid like ATF. One is supplemented for it's operating environment, the other for its unique environment. Since fork oil is a smaller market, less economies of scale. ATF does the job cheaper for those who understand. Same reason it gets used as cutting fluid on drill presses, etc. ATF can work in a fork (and has worked in all of mine for decades). Unless you have some very specific tuning requirements, ATF is usually the easiest thing to dump in and works just fine. Whether it's an HD with some pvc pipe preload on it, or a sport bike with some air adjust, ATF works great IME.
 
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2,086
Location
missouri
Dex 6 is thinner and had a very high VI,, so it will not thin as much when it gets hot. Improved a older Kawasaki that was very stiff the first few miles.
 
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838
Location
NJ, USA
Suspension-Fluid-Chart2.png


I prefer this layout to a comparison chart with regard to the data provided, but I definitely have some extreme bias 😉

The 10W is only a general reference point as other's have said. If you want the "best", I'd go with the one that is closest to the recommended fluid at 40°C and has the highest VI. There's other aspects that might make one better than another, but the viscosity and viscosity stability over temperature ranges is the prevailing performance consideration for suspension fluids.
 
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2,154
Location
Cedar Park, TX
please understand that fork oil really cannot heat up that much.
too much surface area for the amount of oil with too little action
now a shock can boil water but not the forks
and this is serious mx ama pro - on tv - not a street bike and there is only 1 "the doctor" ie VR

yes i have used Mobil 1 ATF and it was interesting...not bad at all
there is as much to "feel" of the forks when it comes to volume of oil as it does the weight of the oil used.
just went in the garage and put my fingers on a L of Bel Ray high performance 2.5W ... which is at the top of the chart above ;)
 
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5,622
Location
the canyons
Of our sportbikes that have had the fork oil changed, one that I've raced had it's spring rate and damping adjusted specifically for me, and the use it has been subjected to. The other was using better internals, and just optimizing the suspension for better but more general all-around performance.

Both have Redline fluids of specific viscosity for the application in them. They both work fantastic.

Changing the fork oil at all, is better than most bikes get. As it is often a neglected maintenance procedure.
 
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937
Location
Colorado, USA
Remember for decades upon decades ATF was recommended as fork oil for all the major brands. Fork internals with respect to lubrication and flow properties still have the same requirements as they did back then. Just like motorcycle only engine oil versus non- motorcycle oil, the myth has been debunked.

-Definitely fork oil versus shock oil, totally different beasts so yes do not confuse them or apply ATF to that circumstance, IMO.

That's a good chart above however it's not the one I was referencing. The one I saw had Red Line and M1 ATF in 10 weight territory with as good of specs as most 10 weight fork oil in terms of actual viscosity and viscosity retention with respect to higher temperatures.
 

ActualDinosaur

Thread starter
Messages
10
Location
Nevada
Thanks for the responses, I'm appreciating all the people reporting good experiences with ATF! It truly wouldn't have crossed my mind at all, if it weren't for the Goldwing owners. Everyone seems to agree that there is minimal or no change to feel... which makes a lot of sense if ATF is 10W (I had no clue 🙃). I guess at that point, it's just an additives game. I trust that the ATF additives, at the very least, are not harmful to many / most forks. I've seen quite a few reports of it being in manuals of older bikes. Is the additive set for fork oil drastically different? I'm not even sure if that information is out there for me to find. I'd love to know how similar they really are. Wonder how long a transmission would last with fork oil...

I snipped a few quotes to keep length down, but this still ended up a little large!

Hydraulic oil like ISO32 is good fork oil too.
This sounds less weird than ATF, as someone who's used neither.

I don't claim that ATF won't perform the basic function of being liquid in your fork, but it's not built to be a fork oil. This is where Molekule's comments are pertinent. MotoTribologist may have also commented on this. I can't recall just now.
Thanks for pointing me to a few reads! As far as my particular case goes, I'm inclined to agree with you. ATF isn't in my manual, and fork oil says fork right on it! I like the peace of mind, even if it's unnecessary. Recommendations from forums and manufacturers suggest that ATF works well. However, if I don't have any around and I'm buying oil anyway, the cost difference is pretty minimal.

ATF does the job cheaper for those who understand. Same reason it gets used as cutting fluid on drill presses, etc.
I have an acquaintance who swears by ATF for machining jobs. I'm the opposite of a machinist, but I keep a little squirt bottle of mineral oil handy. For whatever small drilling or tapping projects I tackle myself, I think just about anything would do!

This is precisely what I saw referenced. Thanks for linking!

I prefer this layout to a comparison chart with regard to the data provided, but I definitely have some extreme bias 😉

The 10W is only a general reference point as other's have said. If you want the "best", I'd go with the one that is closest to the recommended fluid at 40°C and has the highest VI. There's other aspects that might make one better than another, but the viscosity and viscosity stability over temperature ranges is the prevailing performance consideration for suspension fluids.
I can definitely appreciate some nice formatting, that's quite easy on the eyes. Makes sense that just finding an oil with a good, stable viscosity would likely be what's 'best'.

Changing the fork oil at all, is better than most bikes get. As it is often a neglected maintenance procedure.
Absolutely true! There were certainly more miles than I'd like to admit on that oil.

Remember for decades upon decades ATF was recommended as fork oil for all the major brands. Fork internals with respect to lubrication and flow properties still have the same requirements as they did back then. Just like motorcycle only engine oil versus non- motorcycle oil, the myth has been debunked.
This is what got me. I was quite surprised hearing that ATF was listed as standard. My bikes have clearly all been too new, as it's never been in my manuals! As far as engine oils go, I have some experience using Rotella in previous bikes. Affordable and worked great for me, but I've been sticking to motorcycle specific oils on this machine. No real reason why, other than the Castrol 4t being pretty competitively priced around here.
 
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14
Yep the manual for my 1985 VF1000F2 says ATF for the forks, I will be doing them soon as the last person in there fitted the seals with a bent spoon, so they leak like crazy
 
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