Understanding fork oils.

[John Orchard]

I've been studying-up on fork oils lately, thought others might be interested.

Firstly, fork oil is hydraulic oil, it does the same job in both circumstances but is under less stress in a pair of motorcycle forks.

Hydraulic oil thickness is measured by flowing the stuff through a measured size orifice at two different temperatures, this measurement is called centistokes, cSt for short, cSt40 is with oil at 40 degrees and cSt100 is with the oil at 100 degrees.  The lower the number, the thinner the oil.

The conventional way of describing a fork oil thickness is by an SAE measurement like 5wt or 10wt, this is a carry-over from describing motor-oil weights, these descriptions were used so it's easier for the common man.

Please note, the given SAE figures do NOT guarantee that the 5wt in one brand is the same as the 5wt in another brand, see this chart http://mahonkin.com/~milktree/motorcycles/oil-weight-script/oil-weight.pl This chart gives you the cSt40 & cSt100 values for most fork oils on the market, use it when deciding what oil weight you need.

You'd get confused if you were running Silkolene 5wt and went to PJ1 2.5wt thinking it would be thinner when in fact it is thicker!

When deciding on an oil for your front forks, the cSt40 measurement is what you are concerned about because the oil in front forks does not get very hot, it is not under high stresses plus the fork is sitting out the front of the bike in a cooling air-stream.

When deciding on an oil for the rear shocks, take note of the cSt100 measurement, rear shock oil is under higher heat building stresses, plus it/they are behind a hot engine.

Hydraulic oil is much cheaper than motorcycle fork oil, I have been using it for years without noticing any wear or performance differences from 'motorcycle fork oil'.

Hydraulic oil is sold under its ISO thickness rating 'cSt40', the below list is the general lighter thickness hydraulic oils and their rough SAE equivalent ...

AW20 = 2.5wt
AW32 = 5wt
AW42 = 10wt
AW46 = 15wt
AW68 = 20wt
AW100 = 30wt

So if you go and buy hydraulic oil at least you know exactly what cSt40 weight you are getting.  I would imagine that you'd have to contact the manufacturers of the oil to confirm a cSt100 figure.

Wait for the experts to tune-me-up now  ;-)

[Nathan S]

That list of the correlation between cSt40 and SAE "weights" has made a lot of things clear!
In particular, I've never been able to work out why 15W feels so much like 10W, and why Restakor gave unexpected results when you played around with different weight oils.

Thank you!

[XC83]

Thanks John

I never considered Hydraulic fluid - where would you purchase it from?

Here is a link to a site with a lot of info on the topic. Scroll down a little for a list of fork oils compared by Cst.
I try and stick to one brand now, so you know where you are on the soft - hard scale.

http://www.peterverdone.com/wiki/index.php?title=Suspension_Fluid

[John Orchard]

I priced 1 litre of Belray 30wt at the local bike shop ... $30, I just called a local hydraulic workshop ... $35 for 5 litres!!

[Moto]

What about the "Seal Swell" additive in branded fork oil. Always made me laugh.
I work in mining and when you see what hydraulic oil has to put up with there I am sure that bike suspension is hardly a challenge. I have been using it for years.
Same goes for transmission oils.

[shelpi]

Ive got a small oil testing kit does ppm, viscosity, filtering etc one of the tests is test tubes and ball bearings measures time taken of the weight (bearing) through the oil, its been good for checking the weight of oils and at temperatures etc

[John Orchard]

Just went to the local oil supplier, took my own 4 litre container, cost me $30 for 4 litres of ISO100 (30wt), $7.50 a litre  :-D

They also had full synthetic engine oil 10w40 & 5w30 for $8.25 a litre! Plus heaps more!  Anyone in Melb northern suburbs can contact me and I'll give you the details.

[Slakewell]

I use mostly Automatic Transmission Fluid in my forks and then just change the forks to work with it. This way I can just get some at supercheap for like $2.00 per litre. It works out around 12.5wt and when you own Husky's this important as they need topping up often. Thanks for the insight John. 

[John Orchard]

I just did some checks on a few various spec sheets for ATF ...

BRAND              cSt40         cSt100
Amsoil              36.00           7.50
Castrol             37.00           7.30
Mobil                35.00           7.60
Pennzoil           34.35           7.23
Valvoline          36.10           7.70

Seems like ATF is more consistent in it's viscosity than MC fork oils. So that puts ATF at around 7.5wt.

[Tomas]

Not sure about using motor oils in your forks and shocks. They have additives to stop carbon build up etc and that may not be compactible with  rubber seals and orings  inside your shocks/forks. Hydraulic oil should be ok. Worth trying I guess 

[Simo63]

I just did some checks on a few various spec sheets for ATF ...

BRAND              cSt40         cSt100
Amsoil              36.00           7.50
Castrol             37.00           7.30
Mobil                35.00           7.60
Pennzoil           34.35           7.23
Valvoline          36.10           7.70

Seems like ATF is more consistent in it's viscosity than MC fork oils. So that puts ATF at around 7.5wt.

Sounds good. ATF is a brilliant oil, dsigned to work hot or cold and also designed to work with steel, friction plate material, rubber seals and gasket material.  One of the most versatile oils around I reckon.

Like Slakey, I use it in my forks and have never had a problem.  At around 7.5wt it's perfect for me and I tune/adjust the forks to suit the oil.

[Tim754]

Agricultural 42aw, 46aw, 68aw or 32aw etc  hydraulic (seal friendly) multipurpose oil around  $80/100 for 20Lt.. and that is the name brands...... 500ml bottles of fork oils are a big rip off con!
Auto trans oils need a little bit more research to find a weight you want and a little bit of study too on what other additives have been ,well added , still mega value. Remember to reseal all drums completely as some can be Hygroscopic* , meaning they can adsorb and retain water like brake fluids.

Still if you want to pay $68.95 for a litre of some you beaut, wizz bang , shit hot name bragging/posing rights fork oil ,be my guest.

Hygroscopic*  is the correct terminology not  Hydroscopic

[80-85 husky]

ATF makes an excellent two stroke gearbox oil...use the "F" type old ford borg warner....usually 8 bucks a litre

[oldyzman]

Certainly good info John, I hear Motor oil foams up. probably ok for dirt track i have had it in my yz for 6 years - no probs. I also have used a mix of diff oil and gearbox oil in another YZ125E successfully.
Brett

[John Orchard]

Penrite do an ISO 15 which is basically around a 2 wt, good for rear shocks, in-particular Ohlins.

http://www.penriteoil.com.au/pis_pdfs/0MB%2015%20SUSPENSION%20FLUID%20JULY%202013.pdf