ATF is nominally 10w, FYI. 15 is about the max viscosity I'd put in to a fork
N.
Brings up a good question, Nils. Do you suggest using ATF or spending the extra money for fork oil?
Here's my take on this. YMMV.
First the short and sweet:
For an unmodified damper rod fork, I think that ATF is the best choice for damping fluid. If the fork isn't performing well, then first you need to address the springs and sag (by replacing or adjusting preload), and secondarily you should drain and replace the ATF, and then adjust the level of ATF to increase or decrease the volume of the air pocket that is being compressed as the fork travels. That's my opinion, short and sweet, for a straight-up damper rod fork. ATF is great stuff.
IF and only IF you have exhausted the potential of those two primary variables (spring rate and air gap), then varying the nominal 'weight' of the oil is a valuable tuning technique, but it is fraught with potential for inconsistency.
Now we can geek out a tad:
Fork oil 'weight' as stated by the manufacturer is actually a fairly arbitrary number, set by the manufacturer, and they vary WILDLY. A 'measured' value for a fork oil's viscosity is what is needed, but we can't always get it, except from third parties. ATF on the other hand is in fact a very precisely created thing: It has to be to be able to claim compatibility with the manufacturers' trademarks/specs (Dexron/Mercon, etc). And it is complicated, containing lubricants, detergents, friction modifiers, seal conditioners, and a half dozen other things I can't pronounce. Its viscosity is measured in centistokes at a given temperature of 40c, resulting in its "ISO" value. ATF's ISO is 34, no more, no less. So how do we use different (non-ATF) oils to tune the damping without it being a shot in the dark?
Using ISO 34 as our baseline for tuning, we can choose other oils to increase or decrease viscosity in relation to that number. We can do this by referring to some data that has been compiled that includes the measured ISO for different weights and brands of fork oil and find ones with the values that we seek, higher or lower than 34.
Here's a great link:
http://www.peterverdone.com/wiki/index.php?title=Suspension_FluidAnd here's a takeaway if you don't feel like geeking out and digesting it: Redline and Silkolene fork oil are the most consistent, most linear in their respective stated 'weights'. IOW, you can get predictable results going up or down in weight relative to the baseline value of ATF by using their products in increments above or below 10W.
LAST THOUGHT:
When it comes to forks that use cartridges, or emulators, or shock absorbers, this is a totally different ballgame, although the understanding of ISO is also very useful in choosing suspension fluids for tuning those too.
OK, who is still awake? :-)
