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Old 11-19-2007, 11:05 AM   #1
meat popsicle OP
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Ever wonder about fork oil?

Yes, this is an “oil thread”, but it is not the same’ole Amsoil thread so don’t glaze over just yet or open the Peanut Gallery.

fork oil - also know as suspension fluid
I decided to do the maintenance on my forks and I was in the process of selecting fork oil. My shock will be farmed out so I focus on the front end below, but the info should apply to shock oils. As I dug around for information the more I read the more I began to wonder. After quite a bit of digging I didn’t find what I was looking for on ADV or KTMTalk, so I spent some time on the Web trying to figure them out.

It seems there are some misunderstandings about fork oils. They were likely started by the industry themselves, using official standards that do not apply well to fork oils (SAE lubrication oil weights like 2.5wt, 5wt, 10wt). Not only that, but many fork oil manufacturers don’t bother providing the correct standards that best characterize what are primarily hydraulic oils (SUS/VI and ISO-VG). Then stir in some internet gossip, footloose and fancy-free chat, and straight-up shootin’ from the hip bullshit… and you can end up with a few folks at best throwing up their hands in disgust or at worst parading around with empty PBR bottles like they found The Holy Grail.

Now I am not claiming I know everything about fork oils; I was just trying to understand them so that I could choose decent fork oil. It seemed to me this stuff wasn’t well known on ADV or KTMTalk so I thought it might help someone out if I posted it. Is this information critical? Well I think riders have been enjoying their bikes without it, so in that sense no it’s not. But it could help you optimize the operation of your suspension, and at the very least give you reliable performance indicators for the different brands of oil. If you have something to add or correct please post it.


(gratuitous fork pic)

Looking for info
So at first I dug around on ADV and KTMTalk. From my limited survey of fork oil threads related to my bike I came up with these:

Can someone explain fork oil? Confused on what to run, especially for sand.... – has a discussion of fork oil weights
LC4 fork oil level? – an ATFer chimes in, and some interesting and reasonable rebuttals
Fork Maintainence, complete or routine? – more ATF
WP fork expert needed – more ATF
640Adv'03 Fork Oil Change? – hmmm, check KTMTalk
atf instead of fork oil, will it hurt? – more ATF, with a link to:
Mobil 1 ATF Fork Fluid, Other brands with less friction?? – yet more ATF

Well I didn’t find a good comparison of fork oils like I have found on brake fluids (such as THIS). I did not even find a big discussion of fork oil, but I did find conflicting opinions and a lot of ATF. This made me curious so I started looking around for more information.

More searching…
So I went fishing on the Net. Without having the correct search terms it took awhile but I finally came across this: The Secret World of Fork Oil… I think it is a good introduction to fork oils, but more importantly it gave me some good search terms.

Particularly the term “viscosity” and the tests used to measure it. The big take-home point I have to offer is this: even though there is a “stamp of approval” (i.e. SAE) doesn’t mean the viscosity scale is appropriate for fork oil. Is it completely wrong? No, but my reading led me to believe SAE weights gloss over important details on fork oils. The article then went on to introduce me to a scale used for hydraulic fluids, which is more appropriate for fork oils. It also said why the SAE scale is used on fork oils: you already know it and are comfortable with it.

Digging deeper
With the terms from that article I was able to find a bunch of information, good or otherwise, on fork oils and hydraulic fluids in general. Here is some of it:

Let me just start with what I think is one of best links, which includes a summary chart of the various oil brand’s viscosities and grades! Peter Verdone Designs (but remember nothing is perfect… )

Here is a very good article on the effects of oil “weight” and height by Jon Stoodley (with great info on suspensions in general)

And a bit of info on viscosity from: Maxima Oils, a discussion of fork oil “weights” in a
chat forum, and of course the ever present ATF proponents in another another chat forum.


(more wanton fork art)

are we there yet?
What did I learn from all that digging? I can’t say for sure , but at least I know the SAE rating on fork oils is not the greatest scale and can be misused. Verdone said (in his link above) there are different SAE tests and something marked 5wt can actually be more viscous than another oil marked 7.5wt, etc. Yes, given the right circumstances you can be your own grandpa… so don’t laugh this off. No, stop laughing... anyways, I now believe hydraulic fluid measurements and related scales/grades are more reliable indicators of how specific oils will behave in your forks.

So what did I look for in fork oil? Start with the manufacturer’s recommendation, or the recommendation of the suspension shop that reworked your forks. They will likely list an SAE oil weight - fine - the oil manufacturers that do not use that scale will note the SAE weight equivalent of their oils. The recommendation is important, because it will give you an idea of what viscosity will work with your fork’s valve setup. With that in mind one of the links above said to look for fork oil that has a Viscosity Index of at least 150. On that scale the higher the number the less an oil's viscosity will change with temperature. This is good because it means the your forks will not change how they react to terrain as they heat up or cool down. For example, HERE is the spec sheet of the fork oil I bought.

There might be more to look for but I will have to leave it to folks more knowledgeable than me. For example, Verdone (linked above) states that the ISO-VG grade is a more reliable measure of oil viscosity, but as you already know recommendations do not include an ISO-VG. There might be other details such as shear stability and such but at this point my eyes started to glaze over and I decided maybe it is good enough to rely on the expertise of a good synthetic oil manufacturer. I hope that helps someone out there.
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Old 11-19-2007, 11:51 AM   #2
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wow. good stuff! gonna have to spend some time here later...
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Old 11-19-2007, 12:40 PM   #3
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Eh? My eyes, my eyes...are glazing....over...help

Wow you have done some amazing research, but I'm worn out reading
all that info. Now if I can only figure out what to use in my forks.
I guess it is a try it out, and see if you like the change.
Anyhow thanks for taking the time to check this out for us lazy ADV'rs
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Old 11-19-2007, 01:00 PM   #4
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Quote:
Originally Posted by meat popsicle
... The big take-home point I have to offer is this: even though there is a “stamp of approval” (i.e. SAE) doesn’t mean the viscosity scale is appropriate for fork oil. Is it completely wrong? No, but my reading led me to believe SAE weights gloss over important details on fork oils...
Fork oil manufacturers do a dis-service to everyone by using their make-believe "weights". There's no such thing as an SAE 5wt or SAE 7.5wt. SAE's weighting system is meant to apply to motor oils (crankcase oils), and there's nothing thinner than 20 weight in the SAE classification system.

The fork oil manufacturers should dump their phony weight system, and the outdated SUS viscosity system too, and just stick to kinematic viscosity @ 100 deg C. What's wrong with that? It's simple, easy, and unambiguous. Give us kinematic viscosity with the viscosity index. That describes the fluid well enough.
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Old 11-19-2007, 02:25 PM   #5
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I like using Amsoil's excellent synthetic #5 light "Shock Therapy Suspension Fluid" that is said to be for shocks and forks;
https://www.amsoil.com/storefront/stl.aspx

# 10 medium- https://www.amsoil.com/storefront/stm.aspx
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Old 11-19-2007, 04:40 PM   #6
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Quote:
Originally Posted by invader
I like using Amsoil's excellent synthetic #5 light "Shock Therapy Suspension Fluid" that is said to be for shocks and forks;
https://www.amsoil.com/storefront/stl.aspx

# 10 medium- https://www.amsoil.com/storefront/stm.aspx

And there's the problem with oil marketing.

Go look at the Viscosity numbers and the "5wt" Amsoil has way less viscosity than most other 5wt oils, and has the same viscosity as most 2.5wt oils. It does have a better VI (viscosity vs. temperature) rating than other 5wt oils, but it is about the same as other 2.5wt oils.

From looking at the chart at the bottom of the page that Meat linked here, it's easier to make a high VI oil in a lower weight, so Amsoil lies about the weight in order to show good VI numbers.
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Old 11-19-2007, 08:22 PM   #7
meat popsicle OP
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Quote:
Originally Posted by Luke
...

From looking at the chart at the bottom of the page that Meat linked here, it's easier to make a high VI oil in a lower weight, so Amsoil lies about the weight in order to show good VI numbers.
busted...

Quote:
Originally Posted by H2oskier
Wow you have done some amazing research, but I'm worn out reading
all that info. Now if I can only figure out what to use in my forks.
I guess it is a try it out, and see if you like the change.
Anyhow thanks for taking the time to check this out for us lazy ADV'rs
I was told that I am a chatty fuker, so let me offer you the:

Three-step program:
1) start with the fork manufacturer's SAE recommendation
2) get something with a Viscosity Index of around 150 or better (scale goes above 400)
3) enjoy

Quote:
Originally Posted by strongbad
Fork oil manufacturers do a dis-service to everyone by using their make-believe "weights". There's no such thing as an SAE 5wt or SAE 7.5wt. SAE's weighting system is meant to apply to motor oils (crankcase oils), and there's nothing thinner than 20 weight in the SAE classification system.

The fork oil manufacturers should dump their phony weight system, and the outdated SUS viscosity system too, and just stick to kinematic viscosity @ 100 deg C. What's wrong with that? It's simple, easy, and unambiguous. Give us kinematic viscosity with the viscosity index. That describes the fluid well enough.
Yep. One of the viscosity measurements (cSt at 100) for an easy marketing number, and then VI for detail. What about Shear Strength?
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Old 11-20-2007, 12:05 AM   #8
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Sooooo does this all mean I shouldn't use the 10 wt hydraulic fluid I got from Tractor Supply?
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Old 11-20-2007, 12:59 AM   #9
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Hey Meat,
Very nice work in lifting the veil.
You have been busy....

For WP suspensions these guys have shims and WP suspension fluid in bulk so it is less $$$.
I used them to good effect for parts and fluids.

http://www.mx-tech.com/
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Old 11-20-2007, 01:30 AM   #10
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Know what? I'm thoroughly con-fused! So, if I have a problem with the front end diving, use heavier oil? If I want the front to absorb bumps quicker, lighter oil? I DON'T KNOW!!!! Will some one essplain this to me in lame-man's terms? Here's the scenario. 2003 F650 DAKAR, pretty soft front-end and I'm not a real aggressive rider (I'm pretty old and don't heal very well), I'd like the front to be a bit stiffer. Any suggestions?
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Old 11-20-2007, 04:45 AM   #11
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Quote:
Originally Posted by smokinjoe
Know what? I'm thoroughly con-fused! So, if I have a problem with the front end diving, use heavier oil? If I want the front to absorb bumps quicker, lighter oil? I DON'T KNOW!!!! Will some one essplain this to me in lame-man's terms? Here's the scenario. 2003 F650 DAKAR, pretty soft front-end and I'm not a real aggressive rider (I'm pretty old and don't heal very well), I'd like the front to be a bit stiffer. Any suggestions?

You need heavier springs. If your spring rates are not right no amount of oil or change of viscosity will help.

I always use M1 in my forks and adjust the valving and oil height around the oil. Changing fork oils just throws another variable into the suspension tuning puzzle.

Sag is also another easy adjusment to make on your suspension and can make all the difference in the world in how your bike handles..

I use a suspension shop to do the initial setup on my suspensions this gives you a good baseline to work from. Then it is just a matter of keeping fresh oil in the forks and setting your oil level depending on how much bottoming resistance you want.
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Old 11-20-2007, 08:32 AM   #12
meat popsicle OP
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Quote:
Originally Posted by slideways
You need heavier springs. If your spring rates are not right no amount of oil or change of viscosity will help.

...

...

I use a suspension shop to do the initial setup on my suspensions this gives you a good baseline to work from. Then it is just a matter of keeping fresh oil in the forks and setting your oil level depending on how much bottoming resistance you want.
I agree with some of slideways' points, particularly that the springs are the starting point, based on my reading. Once the springs are appropriate for your weight then you should start with the recommended fork oil weight and height. After that you can experiment with different heights, varying the fluid, and weight (a tiny bit). If you still don't like it then maybe you are a good candidate for a revalve job, which should come with a new fork oil weight recommendation.

Quote:
Originally Posted by smokinjoe
Know what? I'm thoroughly con-fused! So, if I have a problem with the front end diving, use heavier oil? If I want the front to absorb bumps quicker, lighter oil? I DON'T KNOW!!!! Will some one essplain this to me in lame-man's terms? Here's the scenario. 2003 F650 DAKAR, pretty soft front-end and I'm not a real aggressive rider (I'm pretty old and don't heal very well), I'd like the front to be a bit stiffer. Any suggestions?
slideaways gave you some good feedback and I tried to add to that above. If you have further questions we might be able to help more but start with stiffer springs.

Quote:
Originally Posted by bmwktmbill
Hey Meat,
Very nice work in lifting the veil.
You have been busy....

For WP suspensions these guys have shims and WP suspension fluid.
I used them to good effect for parts and fluids.

http://www.mx-tech.com/
bill.
Verdone's chart shows Motorex (KTM's fluid mfg.) 5w to have a VI of 200, so that should be fine.

Quote:
Originally Posted by Bobmws
Sooooo does this all mean I shouldn't use the 10 wt hydraulic fluid I got from Tractor Supply?
Depends on your forks and their recommended weight. Also, what is the fluids Viscosity Index?
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Old 11-20-2007, 06:22 PM   #13
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OOOOO.....I gotta headache! More oil=slower bottoming rate? Heavier spring rate=....slower/stiffer bump absorbtion? But it will also carry the front-end higher, right? Because it's pushing it up. Which also means....less nose-dive under heavy braking? So far so good?
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Old 11-20-2007, 07:10 PM   #14
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Quote:
Originally Posted by smokinjoe
But it will also carry the front-end higher, right? Because it's pushing it up.
No. Oil doesn't push to affect general ride height; springs do. Spring rate determines how statically "stiff" a suspension feels. You need the right spring rate so that the suspension compresses about 25%-30% with you just sitting on the bike; this gets the bike at the proper ride height. (Preload can also adjust ride height but is more of a band-aid when you don't have the right spring.)

Oil weight affects damping.... how the suspension reacts to bumps. It effects the transient/dynamic behaviour, not the static. The heavier the oil, the more damped it will be to bumps. Compression damping is how much the shock will resist the compression when you first hit the bump; rebound damping resists how much the shock will resist the rebound of the spring as it releases the energy stored during the bump.

Heavier (higher wt - more viscous) fork oils will cause more damping to occur - make the suspension stiffer on bumps but will have no affect on ride height or how the bike sits when just riding along on a smooth road.

If you have suspension that is fully adjustable for both compression and rebound damping, you shouldn't stray far from the fork oil weight specified in your service manual. It is better to run the oil the fork is designed for and then use the clickers to adjust damping. But if you can't get the right damping with the clickers or your suspension isn't adjustable, then you might want to try different fork oils and/or the amount of oil in the fork (adjusable by the level of oil in the fork with the springs out). But on modern, high-quality suspensions, this is not something you normally do - you use the clickers.

- Mark
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Old 11-20-2007, 08:53 PM   #15
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Quote:
Originally Posted by smokinjoe
More oil=slower bottoming rate? Heavier spring rate=....slower/stiffer bump absorbtion?
Not quite. Too weak springs sit further through the stroke, so you get to a stiffer part of the spring sooner, making for a harsher ride than the correct stronger spring. Over-preloading weak springs means harsh initial travel and the virtually the same tendency to blow through the stroke and bottom-out. So says my local Ohlins and WP experts (IIRC this is because the damping is velocity-sensitive? that there has to be motion before the valving comes in to play, it is reactive); and so says my poor blistered hands from over-preloading and over-rebound damping my front forks before last Saturday's 3-hour enduro. Note to self: don't make take major untested modifications to races, especially long ones!

Quote:
Originally Posted by markjenn
Heavier (higher wt - more viscous) fork oils will cause more damping to occur - make the suspension stiffer on bumps but will have no affect on ride height or how the bike sits when just riding along on a smooth road.
However it will definitely affect the dynamic ride height. Too thick oil will slow the rebound such that the suspension will not rebound fast enough and will pack down over a series of bumps. This too will put the forks at a part of the stroke where they can't handle the bumps properly.

The Ohlins guru in NZ states too much rebound damping as the number one cause of crashes. He sees way too many bikes with too much rebound. A rough test is the static push: on rebound the suspension should overshoot once and settle. Not overshooting is a sign of too much rebound damping.
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