Fork oil

[Motosportz]

I, for one, am not a huge fan of tuning suspension performance with oil viscosity. Viscosity changes have a nonlinear effect on damping, with a greater effect at low speeds (small orifice sizes) and lesser at high speeds. You also have essentially no control over the shape of the damping curve, which is often just as much of a problem as the overall damping level.

It is far more effective to improve suspension performance with valving changes.

Totally agree

 

[Fast1]

Comercial Brands and their properties

The oil in a bike's fork or shock not only cools and lubricates the system, but is the heart of all the damping control available for the springs. All phases of the damping involve the viscosity of the oil used in the system. Oil is such a critical suspension tuning decision that is the first priority to get worked out after spring/sag choice. No matter what all of your friends say about what the greatest high speed valves are or wild shim stacks, you should agonize over oil choice. In general, Use the least viscous oil possible that produces good slow speed damping performance with the damping adjustment screws out 1 full turn from full closed. This ensures that a fair amount of fine tuning will be available for track and weather conditions.
I suggest using either Silkolene Pro RSF (PDS)(ester) or Red Line (PDS)(polyol ester) synthetic suspension oils for front and rear suspension systems. They both have very high VI numbers and have enough of a viscosity selection to produce any mix you may need. You should, however, stay with one brand whichever way you go. I have decided to use Red Line exclusively. They range from extremely thin to extremely thick, in five different viscosities. They make the oil in nearby Benicia, CA and the oils come in really pretty colors.
http://www.peterverdone.com/wiki/index.php?title=Suspension_Fluid#Viscosity_Index

 

[Theo]

I already read that pdf file; they wrote that "On a lovely Sunday morning, the ambient temperature may be 21C (70F). Motorcycle forks will run in the 26C (78F) temp range, rear shocks will run in the 65C (150F) range and rear reservoirs will be around 43C (110F). While motorcycle rear shocks require very high VIs (over 300) to function well over such a huge temperature range, motorcycle forks and bicycles do not. Anything over 100VI will be serviceable for them. "
I've read this opinion on another site, too.
They also wrote: "Rumours of Silkolene causing problems in Marzocchi forks exist. Be careful using this mix.", but there are varoius Marzocchi models, maybe with our Shiver 45 there aren't problems.
I've bought Motul Fork Oil Expert 10W (semi-synthetic). If it doesn't work well, I can take out some oil and add some 5W to create an approximate 7.5 W. I've bought a 10W because the Motul 7.5W's declared to be less viscous than the recommended 7.5 Agip.
Anyway, Kyle Tarry and CJBROWN said that it would be difficult to know one from another of the oils I listed and I'm not an high level rider who notices little differences.

 

[Fast1]

Once you change fork oil on the Marzocchi forks, have appropriate tools and a precise way to measure fork air gap you can do the complete change in less than an hour so changing or replacing oil with another weight is simple.

Does your 610 have compression adjusters? If not, I would highly recommend to install them. I noticed that the factory shim stacks were all gunked up with heavy grease which may be worth the effort to clean up if retaining them.

As for feeling/knowing any difference in forks oils. From my experience with the 630 I've noticed a night and day difference when completing purging the factory stock weight oil and going to RedLine 5w with a 100mm air gap for my riding.

You may know this but you can start with a larger air gap (say 120mm) and then ride your normal terrain to determine if the forks bottom out or not. It is very easy to add additional oil thru the bleeder valve to eliminate bottoming. Just test trial before buttoning up the forks so you know the oil volume/air gap ratio and then you can add appropriate volume if necessary.


edit,, I see that you have the SM version and my posts pertain to the TE and offroad woods riding. However the compression adjusters would be a huge advantage on the SM forks and may mitigate the need for heavier weight oil to stiffen up the fork action.

 

[Theo]

Does your 610 have compression adjusters?

No, it hasn't. I'd like to have them, but I don't feel the need so much, so, considering that they're not very cheap, never mind.

I noticed that the factory shim stacks were all gunked up with heavy grease which may be worth the effort to clean up if retaining them.

I didn't know this; thank you for the tip. How could I clean them? Could I use kerosene?

 

[Theo]

I, for one, am not a huge fan of tuning suspension performance with oil viscosity. Viscosity changes have a nonlinear effect on damping, with a greater effect at low speeds (small orifice sizes) and lesser at high speeds. You also have essentially no control over the shape of the damping curve, which is often just as much of a problem as the overall damping level.

It is far more effective to improve suspension performance with valving changes.

I run 5W oil in my forks, but they have been revalved appropriately.

Totally agree

If the damping isn't right with a 5W, adjust the clickers or revalve.

It seems that you're right.
I spoke with a man who has a workshop which only deals in motorcycles suspensions. I don't want to mention his name or the pilots he worked with, but I can tell you that he's very skilled.
He said that, contrary to what people thought years ago, when the Shiver 45 were made, the best choice is to use 2.5W or 5W and to damp the stanchions stroke with a proper valves setting. The reason is that a low viscosity oil flows more fluently.