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Discussion Fork Oil vs ATF vs Hydraulic fluid

Discussion in 'Maintenance' started by ruckusman, May 23, 2018.

  1. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    So I'm about to service the ZXR forks and I'd previously seen mention of people using Dexron III automatic transmission fluid in forks which required about a 7wt oil.

    There seem to be two oil weights floating about SAE and Non SAE which seems to be 1.5x SAE weight, so SAE 5 = non-SAE 7.5 - numbers from the penrite spec sheets.

    Why would I bother with the effort when I can just get fork oil - well ZXR specifies 5wt, though I had encountered somewhere mention of people going slightly higher with a mix of 5wt and 10wt - that's two litres at ~$25/litre.

    So I did some digging and some reading and hit some good pages.

    First promising link was this one, there is some great information on this page - https://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=466621

    It mentions both ATF and ISO graded hydraulic fluids as well as the important VI - viscosity index for heat stable fluids.
    One poster also gives some target oil viscosities for various situations - please note that he provides ISO grades in the numbers below - not SAE weights.

    Generally MX bikes need a very fast acting fluid usually in the 16-28 range, big fat enduros like your KLR and my XR around 22-32, street sport 32-46, 46+ for the big bikes. Tractor hyd fluid is usually around a 46-52, ATF 32-38, SAE 10 engine oil 46ish. Engine oils make very poor suspension fluids.

    That info is very useful for anyone with access to tractor or industrial machine hydraulic fluids in various grades for mixing and matching.

    So with some starting points on what the poster claimed were the oil weights I came across this page with the cross reference weights and other aspects - notably viscosity measurements in centistokes and viscosity index numbers - lots of specific details on how the actual measurements are made.

    http://www.machinerylubrication.com/Read/411/oil-viscosity

    OK so now I have a clue about what I'm looking for in specs.

    Easy search gives me data sheets on two ATF's Mobil 1 ATF and Valvoline Dex III available from Woolworths of all places - nice to know in a pinch I could get some ATF from Woolies if I needed SAE-10wt/ non-SAE 15 fork oil and the numbers are very good on the Valvoline.

    Mobil 1 Synthetic
    https://www.mobil.com/english-us/passenger-vehicle-lube/pds/glxxmobil-1-synthetic-atf
    Viscosity, cSt (ASTM D445)
    @ 40 ºC 36.3
    @ 100 ºC 7.4
    Viscosity Index 176

    Valvoline Dex III
    https://go.lupinsys.com/valvoline/h...92730caff909/search_api/ATF_DX_3-PI_Sheet.pdf
    Viscosity, cSt (ASTM D445)
    @ 40 ºC 34.1
    @ 100 ºC 7.1
    Viscosity Index 175

    Also two Penrite fork oils 5wt & 10wt, which as luck would have it went on special the end of last week at Supercheap so I grabbed one of each. Makes this slightly redundant for the short term, but it's info worth sharing.

    Penrite 5wt - Full Synthetic - spec sheet says covers - Fork Oil Grade 7.5 (Non SAE) Fork Oil SAE Grade 5
    https://www.penriteoil.com.au/assets/pis_pdfs/MC FORK OIL 5 (FULL SYNTHETIC).pdf
    Viscosity, cSt
    @ 40 ºC 20.0
    @ 100 ºC 5.0
    Viscosity Index 193

    Penrite 10wt - Full Synthetic - spec sheet says covers Fork oil grade 15 (Non SAE) Fork Oil SAE Grade 10
    https://www.penriteoil.com.au/assets/pis_pdfs/MC FORK OIL 10 (FULL SYNTHETIC).pdf
    Viscosity, cSt
    @ 40 ºC 37.9
    @ 100 ºC 7.5
    Viscosity Index 174

    So in conclusion a Penrite 10wt full synthetic is amazingly close to an ATF fluid - perfect substitute.
    For anyone with access to ISO graded hydraulic fluid you've also now got some good starting points for conversion to SAE weight fork oils.
     
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    Last edited: May 24, 2018
  2. Andych

    Andych Well-Known Member Premium Member

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    You just have to make sure that the Hydraulic oil is suitable for use on standard BunaN rubber seals.. some dont play nicely with these seals although they are the super expensive ones so its unlikely they would be used...
    Hydraulic oil has additives to inhibit foaming and they tolerate a very high temperature.. but really.. with the Penrite at those prices... why bother...lol
    I bought some as well when it came on special... :)
     
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  3. Linkin

    Linkin The Apprentice Premium Member Dirty Wheel Club Contributing Member

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    Thicker oil slows down the response of the valving. I went with standard 5w oil in the forks. The springs were within spec on free length.

    The compression damping was too stiff so I adjusted it, now I think I've got the springs to stiff as well.
     
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  4. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    What do you tip the scales at @Linkin? I seem to recall comment of you being a lean mean fighting machine, I'm not so lean nowadays :(
     
  5. Linkin

    Linkin The Apprentice Premium Member Dirty Wheel Club Contributing Member

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    I would be going with 5w oil and dialling up the spring preload.

    The important thing is to get the static sag front & rear the same
     
  6. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    Yeah only the Valvoline ATF spec sheet didn't make mention of seal compatibility, but I'd be awfully surprised if it was incompatible with all seal materials
     
  7. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    I'm plus 30kg in the buff - I may end up needing firmer springs - I'm definitely not average learner rider weight
     
  8. Andych

    Andych Well-Known Member Premium Member

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    Apart from @Linkin herefater known as "The Greyhound" nobody is.. lol
    I would give the weight limit on a Harley a run for its money... that is why the SRX is going to the torquey 350 cc's
     
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    Last edited: May 23, 2018
  9. minimac

    minimac Active Member Premium Member Contributing Member

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    Thanks for posting that research. I found it helpful.
     
  10. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    Just like to add - ISO Graded hydraulic fluids are available at supercheap - BIG 20 litre containers though - they've also got hydraulic fluids at places like Enzed, I was in there last week in search of something else so didn't notice bottle sizes but plenty of hydraulic fluid
     

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