Fork oil for a cheapskate?

[richard612]

I'm running Dexron III as a fork-oil and I like it a lot. But I'm about to bump up the spring rates and I'll be wanting something a little thicker.

Are there any readily-available hydraulic oils that will work well in this application and provide a little bit higher viscosity? Power steering fluid? One of Wal-Mart's two tractor hydraulic oils?

 

[Cujet]

Mobil 1 ATF is a little thicker than standard Dextron III and will hold viscosity better. I found that it was actually a little thick for my needs.

That said, the best way to adjust damping is not through viscosity but through adjustment of the damper itself. If you have a shim stack that can be changed that is the correct way, and cheap too. If the forks just have calibrated orifices the job gets a little harder and calls for some ingenuity. or the purchase of updated damper internals.

Chris

 

[tom slick]

for the minor expense of fork oil i think i'd use the real thing. when you go out and really push the bike, and your cheapo ATF foams up and your damping goes to crap you'll wish you weren't a cheapskate. it's not that expensive and it's not like you have to change it often.
what are you putting it in?

 

[Cujet]

I forgot to mention that Mobil 1 ATF is a favorite oil for many professional motocross, dirt bike and street bike suspension tuners. It has all of the desirable properties of a good suspension fluid.

Chris

 

[userfriendly]

Richard612;
For a MX bike start off with an ISO 22 hydraulic fluid with the highest flashpoint you can find at a reasonable cost, say $2.50 a litre max. That will put you right in the middle. Don't worry about viscosity indexes. That is the money trap. You can blend your own ISO suspension fluids with the help of an ISO blending chart and three or four pails of hydraulic fluid. I keep ISO 10,22,32,and 46 around for suspension fluid mixing. Turbine oils are pretty good and cheap too. You may find that an ISO 32 like an ATF is a little thick and slow for washboard and the whoops. I would say that 90% of MX bikes work the best with ISO viscosities between 17.5 and 28 which puts the adjusters somewhere in the middle. For landing big jumps, try a blend of 32 and 46. I think if you are going up in suspension fluid viscosity or cranking up your adjusters, you may need a little heavier springs.

 

[richard612]

The two bikes in question are a KLR650 and a ZR-7. The KLR soars like an eagle and lands like a piano, so there won't be any jumping. The factory oil for both bikes is listed as a 10W20.

Spectro 15W in the KLR is a touch on the thick side for my liking (45.6 cSt @ 40C), and Dexron III in the ZR-7 is almost too soft.

[ June 12, 2003, 01:50 AM: Message edited by: richard612 ]

 

[userfriendly]

Richard,
I tried an ISO 46 in my XR650R and had to set my adjusters wide open. It made the bike a board all right, but added confidence when landing front wheel first. I'm back to a 50/50 mix of ISO 22 and 32 with the clickers right in the middle. 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.
Before changing suspension fluids, make sure the sag is set and the spring rate is proper for your riding weight and style.
The reason I do not like high VI/low flash point fluids is because when they heat up in a suspension they boil and seperate leaving a thicker fluid behind. You will know if your fluid is doing this if the suspension pumps up and becomes stiff after a long hard blast.
My rule of thumb is for cold performance look for high VIs, for hot performance look for high flash point, heat stable fluids.