Servicing forks - ATF listed in manual, what is equivalent in fork oil?

[mangocrazy]

I've currently got the front end of my venerable VFR in pieces, with the steering stem off at the machinists. While it's apart I figured it was time to change the fork oil, as it's been in there for close to 10 years. The forks are a bit posh for the VFR - they're RC30 cartridge jobbies with separate rebound and compression adjustment and chunky 43mm legs. On asking t'Internet what fork oil I should be using, the resounding answer is ATF (automatic transmission fluid). This was a regular thing back in the day for damper rod forks, Honda seemed to specify ATF for nearly all of their bikes in the 80s and early 90s.

However I'd like to stick some proper fork oil in there, but am not sure what viscosity to use. As far as I can tell, most ATF back in the day used to be equivalent to 10w oil. Can anyone confirm or deny this? I'll be giving Darren at MCT a ring in the morning to ask an expert, but wondered what the general consensus of opinion was.

[Le_Fromage_Grande]

Cartridge forks are usually 5W or 10W, ATF is about 15W and far too thick for cartridge forks. RC30s are a 1988ish bike, you could definitely buy proper fork oil in 1988, I was buying it for MX bikes, probably from where Darren worked at the time.

[DefTrap]

I've used ATF and 10wt fork oil and my really-average-fairly-non-demanding riding style really couldn't tell much difference.

[mangocrazy]

From reading around it would appear (anecdotally) that 80s ATF was roughly equivalent to 10w oil, but nowadays the viscosity of ATF can vary quite widely. A lot of people really rate ATF for old damper rod forks, but for cartridge forks I'll stick to fork oil. It's just a question of what viscosity for the RC30 forks. For all modern USD forks, 5w is recommended, except for KTM who have to be awkward and specify 4w (really!).

[Le_Fromage_Grande]

Why don't you look it up in an RC30 manual, there must be one online somewhere

[Le_Fromage_Grande]

3 minutes on Google says 5W

[mangocrazy]

Why don't you look it up in an RC30 manual, there must be one online somewhere

Every manual I've googled says ATF, but quantity of fluid differs. The oil I drained from the first fork leg is noticeably more viscous than 5w.

[mangocrazy]

Darren at MCT reckons that 7.5 would be about right. I've stripped and cleaned one fork leg and replaced the fluid with 10w oil and in comparison to the other unmolested fork leg there is significantly more resistance when bouncing on it, and more resistance than you'd expect just from filling with fresh oil.

So it's back to the shop for some 7.5w Putoline, then...

[Demannu]

You'll not get away with it that easy sunshine, viscosity is where it's at now!
Not all oils are created equal, or even similar!
Fyi, ATF is roughly 32 CTS at 40c btw. Plenty of comparison charts out there to help

[mangocrazy]

A very interesting link, thanks. I tried following the link on the YT page to the suspension fluids equivalence chart and it went to a 404 - page not found. So I went to their web site, dug around a bit and found the chart here:

https://motoiservices.com/en/suspension ... nce-chart/

When I looked at the chart I was struck by the difference in viscosity between Putoline HPX R 7.5w and 10w fork oils. The 7.5w has a cSt of 32.1 and a VI of 151, whereas the 10w has a cSt of 50.1 and a VI of 151. I'm sticking with Putoline as it's a good quality oil and the local bike shop (only a mile or so away) stocks their complete range.

I think it's time for some rough and ready viscosity testing...

[Le_Fromage_Grande]

Darren at MCT reckons that 7.5 would be about right. I've stripped and cleaned one fork leg and replaced the fluid with 10w oil and in comparison to the other unmolested fork leg there is significantly more resistance when bouncing on it, and more resistance than you'd expect just from filling with fresh oil.

So it's back to the shop for some 7.5w Putoline, then...

Drain half out and replace with 5W - this will give you 7.5W (maybe)

[mangocrazy]

Drain half out and replace with 5W - this will give you 7.5W (maybe)

Yes, that's an option. I've got some Putoline 5w on the shelf so it's doable. First need to find out how Putoline 10w compares to what's currently in the forks.

[mangocrazy]

OK, so cancel all that - I've been barking up the wrong tree (and not for the first time). What I should have done was to compare the oil that came out of the forks with what I have in various bottles of suspension fluid. I also should have listened a bit more closely to what Darren said to me.

Basically I'm now 99% certain that the correct viscosity of oil for these forks (and what has been in them for the last 5-10 years) is 5w. Not 10w, not 7.5w but 5w. I did a simple drip test onto a near vertical surface to see how long it took different viscosities to cover a set distance. The old fork oil took 1 minute and 3 seconds, 5w oil took 1 minute and 20 seconds and the 10w oil probably still hasn't reached the bottom. Realistically, oil that's been in forks for years should have lost some viscosity and will flow quicker, so those rough and ready figures make sense to me.

After seeing those results, something Darren said came back to me. He'd said that if the shim stacks had been re-valved then it was almost certain that 5w oil had been used, as that's what most modern cartridge forks are based on. The last time my forks had been apart was when they'd been to John at Revs Racing to have the forks re-sprung and re-valved. So it was a pound to a penny that John had put 5w oil in them.

So forget all about ATF equivalence bollocks, 5w is the stuff to use in these forks.