ATF is probably not a bad alternative as it has plenty of anti wear additives and should be shear stable.
It’s probably higher quality than most so called dedicated fork oils.
It’s probably higher quality than most so called dedicated fork oils.
Fork oil differences besides viscosity?
- Thread starter JoelTheMole
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Thank you, this sort of info is exactly what I was looking for.
So in your experience how would these compare to something like a mid quality synthetic engine oil (maybe Mobil1) or more modern synthetic ATF (thinking something generic like maybe Castrol Transmax or similar)? Particularly as far as sheer, anti foam, and anti oxidation go?
I would imagine a quality fork oil and synthetic engine oil would be comparable in low friction and wear resistance, and a modern synthetic ATF would be lower in those but higher in anti oxidation, or am I way off base?
I think this is true in the short run and in fairly mild street conditions; anything with the same cst @40C is going to feel very similar day one on a run to the grocery store. However, forks do seem to be one of the most often neglected or at least extended interval parts of a motorcycle. How about on day 700+ hitting a road that looks like the beach at Normandy or gravel doing 70mph? I imagine you are going to see some much more significant differences there depending on what type of fork oil you used.
As far as friction goes on bigger abrupt bumps I agree that makes little difference, but on both short fast bumps and more gradual transitions I can tell a difference. While it may not apply to everyone, or even most, I ride a FJR1300ES which does not actually have swappable fork bushings, so wear reduction is quite important to me as the cost of two new fork assemblies would be half what I paid for the bike.
Regarding stiction, most of it is from the seals and the dust boot, not from the internal workings of the fork. I always grease them before installation and the stiction is minimal.
If you install them dry, you will probably have a lot of stiction.
If you install them dry, you will probably have a lot of stiction.
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Yea, I usually lightly coat the fork tubes with a silicone based grease containing PTFE, which is how I know increased stiction can make a difference. Getting a used bike with dry fork tubes and adding a little lube on the outside can make it very clear.
I know you don't want to consider viscosity, but simply put, and engine oil's low temp viscosity will be really high. You'd probably have to compare it to a "30W" fork oil at least. Besides that, an engine oil like Mobil 1 probably won't have the level of shear stability you'd want to see. A motorcycle specific engine oil should be more shear stable, but your usually looking at even thicker viscosities, then.
ATF would be a much closer comparison, and people have been using ATF for decades with reasonable success.
With regard to your comment about wear resistance being a factor for your costs. Ask the manufacturers for data. Ask them if they have coefficient of friction and wear data for "4-ball wear" and "Falex pin and vee block" tests. (Before anyone jumps down my throat about them not being real world tests, I KNOW. They are simply some basic wear tests that a lot of manufacturers might already have the data on hand; and they at least give some idea of the tribological characteristics of the fluid.)
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It isnt that I don't consider viscosities of fork oil, I do think that viscosity IS very important for fork oil, but viscosity and the difference between advertised weight on the front of the bottle vs the kinematic viscosity at 40C are just about all that dominate in every conversation on fork oil anywhere you look on the internet. You can flip through those links kschachn put up and see that very clearly.
I started this conversation because I wanted to know some of the other properties of fork oil, how they compare to other common oils, and how you can tell a "good" fork oil apart from one that is just overpriced marketing, as regulation in this field seems lackluster at best, and the whole thing just seems like the wild west filled with snake oil salesmen with likely a few legitimately well thought out products hiding in the crowd.
As far as engine oil vs fork oil "weight" goes, that seems to mean very little as it appears "weight" and cst @40c can mean a very wide range of things with fork oil. That being said this is a common reference sheet on fork oil:
Suspension Fluid - Pvdwiki
Mobil1 0w-8 is listed as 23 cst @40C, which would fall somewhere in what is commonly sold as the "2.5-5 weight" fork oil range, Mobil1 0w-16 is listed as 39.5 @ 40C which would fall right into the middle of the "10 weight" range, you would need to go as high as Mobil1 15w-50 with a cst of 125 @40c to fall into what is commonly called the "30 weight" range. So as far as fork oil viscosity compared with engine oil it does not seem as crazy far off as you might imagine, especially with some of the lighter weight modern engine oils.
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Viscosity index would seem important for fork oil unless you want damping force to change radically with temperature. That suggests to me a good synthetic oil.
What is the viscosity index of ATF compared with fork oil ?
What is the viscosity index of ATF compared with fork oil ?
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The problem is for most street motorcycles fork oil temp doesnt seem to have that wide of a range of temps. Forks are out front and have a relatively big surface area. My own testing with an IR thermometer on several different street motorcycles has shown that even as much as 10-15F above ambient is rare. Considering most street motorcycles are ridden in a fairly constrained range of ambient temps, Viscosity Index just doesn't seem that important for fork oil.
The oil in a monoshock in the rear is a different story, but that is a different conversation.
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That's pretty much right.
Shock oils, yes, hot hot hot. VI is the most important property.
Fork oils might hit maybe 90°F if it isn't already hotter outside so VI is pretty much inconsequential when you formulate it for the operating temp.
Owners of the dirt bikes could probably benefit from higher VI oils. The oil in the forks probably does get hot when riding bumpy trails.
I’m following this thread with interest. Great questions have been asked. I service a LOT of bicycle forks (and rear shocks) and I’m about to service the forks on my motorcycle. I’ve long wondered the same question and felt that suspension fluid is long on hokum and short on verifiable data.
@SuperSalad Have you ever seen fork oil get blended to twice the wear resistance of an ATF?
Also, someone should point out the significant difference and importance of foaming between common man PCMO vs hydraulic oils.
Also, someone should point out the significant difference and importance of foaming between common man PCMO vs hydraulic oils.
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Anti foam is going to be the biggest reason to go with fork oils ...... Motor oils will foam much more and you will have inconsistent feel such as the first few strokes would be very harsh followed by very weak strokes as the oil foams
We use Maxima for all our fork builds usually the 10 wt but it can vary some depending on the application
Yes fork oil is quite a bit more per fluid ounce vs ATF But when we are talking brand new stock motocross bike is about 13k
If one were to go the ATF route that would bring lots of different weights and variables Dex/Merc .
LV . ULV . Dual Clutch
I think the easy explanation is a company offers the same " fork oil " formulas in different weights to allow for changes in desired feel
Would ATF harm / help anything on a consumer street bike with a suspension travel of a few inches? ...... No
We use Maxima for all our fork builds usually the 10 wt but it can vary some depending on the application
Yes fork oil is quite a bit more per fluid ounce vs ATF But when we are talking brand new stock motocross bike is about 13k
If one were to go the ATF route that would bring lots of different weights and variables Dex/Merc .
LV . ULV . Dual Clutch
I think the easy explanation is a company offers the same " fork oil " formulas in different weights to allow for changes in desired feel
Would ATF harm / help anything on a consumer street bike with a suspension travel of a few inches? ...... No
No, I can't say I have. Fork oil wear resistance numbers on low-load sliding contact wear was pretty consistent with what you'd see in a good hydraulic oil.
Good read:
www.mototribology.com
Suspension Fluids | MotoTribology - MotoTribology | Motorcycle Lubrication Education
Suspension fluids are a topic of interest for me for two reasons. First I like the fluids for their simplicity. They are straightforward to design, yet there is a lot of opportunity to fine tune and optimize a fluid for specific suspension designs. The second reason is that no matter how much I […]
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