Fork oil vs shock fluid? How do they differ?

[bulwnkl]

I see that shock fluids are generally lower viscosity than fork oils, and that shock fluids often have higher viscosity indices. That's all I know. Are they designed/built particularly differently in terms of how they achieve the radically different viscosity indices? What about in terms of anti-wear properties, friction modifiers/reducers, and so on?

How about this: For a 'street bike,' have you experienced a noticeable difference in fork feel based upon whether you used a low-VI fork oil vs. a high-VI shock oil in your forks?

 

[12V71]

I don't know much about viscosity indices, but from 6 years of racing motocross, I know shock oil is designed to withstand higher temps and pressures than what forks generate. You need to have the correct oil to make modern cartridge forks perform correctly.

 

[SuperSalad]

I'm no expert on shocks, but I do know the fluids pretty well. Here's what I at least think are the facts.

Rear shocks generally have a bigger temperature swing going up around 150F at times. Forks usually stay below 100F in mild ambient temperatures. So rear shock oils need a higher VI to avoid excessive fade as they heat up. Because of the relatively excessive heat, the rear shock fluid needs to have high oxidative stability to prolong fluid life.

There is more contact area and higher side loads on forks then on a rear shock so anti-wear and frictional properties are a bigger priority in fork oils then shock oil. A high VI certainly doesn't hurt but it isn't at critical as it is in a shock oil.

Anti-foam is crucial for both fluids since foam destroys the damping ability of the fluid.

I'm sure there are other things, but that is all I can think of at the moment. There is an article up on my site about it that goes more in depth on the subject if you want to look into it.

Peterverdone is also a really good source for explanations of suspension tech.

 

[bulwnkl]

Thanks to both of you!

I'm asking these things in part because I just replaced the shock on my m/c, and took the forks off for a rebuild and fluid change. I probably _should_ have replaced the fluid a year ago, but I didn't. I have Gold Valves in the (damper rod) forks, and was running Redline fluid in them from the time I put those in. However, my schedule got messed up just before the work had to be done, so then I couldn't source the Redline quickly/easily. So I got 'RaceTech' fluid instead. Since RaceTech's fluid is re-bottled Spectro, and Spectro was locally available, my 'RaceTech' fluid has Spectro's name on the bottles.

I found it interesting that the Spectro 15W fluid has very nearly the lowest VI available in any fork oil, not just in the viscosity range I'm looking for. So I got to wondering about what else might be different between the various suspension fluids, besides VI. I was curious as to what I might 'give up' with a super-high-VI fluid like a Redline mix, the Silkolene Pro RSF, or Bel-Ray HVI, as opposed to the super-low-VI Spectro/RaceTech stuff when I'm using them in forks.

It kinda sounds like I'll either give up 'only money,' or perhaps I'll give up AW and/or friction reduction.

Thoughts?

 

[BigCahuna]

FWIW, I helped a friend change the fork and shock fluid on his old Honda Goldwing. Right there in the service manual, it said to use Automatic trans fluid. When I changed my fork fluid in the Harley, I bought Harley's fork oil, and it looked and smelled just like auto-trans fluid. So don't read too much into the marketing hype about fork oils.,,,,

 

[bulwnkl]

ATF is the wrong viscosity for my application. I use a fluid of around 45-48 cSt @ 40C. Which, BTW, is the usual ambient temp here for the next few months.

 

[userfriendly]

I wouldn't chase high VI for fair weather operation. Sleds, ice racing, indoor winter MX, then yes.
When super high VI fluids like Boeing LGF heat up, they gas-off and pressurize shocks and forks, making them ride like a brick.
I go the other way and look for a high flash point at the proper ISO grade for the ambient and application.
With a pail of ISO 68 and ISO 10 hydraulic fluid, you can blend anything in between using a chart or calculator.
If you need something thicker than ISO 68 for a road bike, maybe try a stiffer suspension spring instead.
MX forks 'n shocks usually play between ISO 10 & 46.
ATF Dexron VI is just about perfect at ISO 27 and can be thinned with ISO 10 hydraulic or R&O fluid, or thickened with a 68.

 

[sunruh]

mobil 1 atf can be (and has been) used with great results

it all depends on the feel that you want

 

[userfriendly]

Here are the pds on sunruh's Mobil ATFs. The ones not listed have low flash points.

M1 Syn ISO 36.3 flash 220C
M1 dex6 ISO 29.5 flash 220C
Delvac TES295 38 flash 236C

 

[bulwnkl]

Once again: ATF is the wrong viscosity here, and in any event my question was not whether I can use ATF in my forks.

The Spectro is in, the bike is back together, and I've had it out for a couple very short test rides. It's 110+ outside these days, so 'pleasure riding' really isn't in the cards.

The forks once again feel like they did when I first put the Gold Valves and new oil in. It seems I'll find out for myself over time what happens to the forks when I use low-VI 'fork oil' vs. high-VI 'shock fluid.'