suspension

[yorkie_chris]

So he assumed 650 had cartridge forks

In forks like that oil weight is control over rebound damping, there is little can be done for the compression unless you fit emulators or cartridges. Yes it is noticeable between say 12.5 and 15w.

[embee]

At the risk of a Hitlerism, be aware that the nominal grading of fork oil is pretty hit and miss. Different makes and "models" of oil have wide ranges of actual real life viscosity even within one grade rating.

Check out the big table of oils here .

My SV was due for a fork oil change, last oil was when I fitted the k-tech 0.85 linear springs and they supplied a Motul Factory line 10W. Following the linked table info, I've just put in some Silkolene Pro RSF 7.5W as a trial (high VI so viscosity doesn't change as much with temperature). Now you may all say 7.5W is far too thin for the SV with those springs, but according to the table it compares to a lot of 10W oils. I've only done a few tens of miles on it but it's an interesting result, maybe slightly more floaty than some might like but it copes with pot-holes and sunken ironworks in a very pleasant manner. I think the RSF 10W would be a bit harsh for me, so might do a mix next time.

Moral - it's not a simple as it looks.

[yorkie_chris]

I'm not convinced fork oil gets so hot that VI is a big factor. .85 is pretty high spring rate, I don't think there would be anywhere near enough rebound.

Originally Posted by yorkie_chris

I'm not convinced fork oil gets so hot that VI is a big factor.

it does get cold.

[embee]

That was the object of the exercise really, to see what difference I could feel from the oil.

The viscosity at 40C for the 7.5W RSF (37cSt) is higher than the Motul Medium 10W (33.7cSt) it had in before, but significantly it has a very much higher VI (322 vs 115), so by that reckoning they should be similar at normal summer running temps but the Silkolene RSF shouldn't get the sticky/stiff/lumpy feeling when it's cold that I certainly felt with the Motul.

Time will tell, it certainly feels more supple over bumps and holes, more "comfy" really, I'll see how I get on with it. Having said that, new oil always feels better.