K-TECH springs - which oil?

So I passed on the prog offer and picked up some K-Techs (0.85 / 12 stone in the old money).

Having searched, I noticed that there seems to be some debate re oil choice (weight, viscosity). I've got a feeling that 7.5 or 10w would be close c/w standard air gap retained, just wondering if anyone with the same springs in situ has had a result with either set up... ta.

clarification...oil choice c/w the K-Techs, obviously.

I'm leaning towards a 7.5 weight with standard gap.

[yorkie_chris]

What brand of oil

[embee]

I'm the same weight as you and have 0.85 k-tech linear springs in my curvey SKS with stock damping internals.

I use Silkolene Pro RSF 7.5W, which I would describe as "comfortable" rather than "sporty". It suits my riding, you may prefer something slightly firmer. It doesn't have to feel rock hard to handle reasonably well.

The now infamous table of viscosities (see the white table at the bottom) shows the 7.5W Pro RSF viscosity as 37cSt at 40C, which is quite typical of many 10W oils of other brands (hence YC's comment). High numbers for the viscosity index (VI) around 300 mean the viscosity doesn't change as much with temperature, which is why I use the Pro RSF.

RSF 10W might be rather firm, but you could always mix 7.5 and 10. Consider what the numbers might mean for a different brand/type of oil.

Something in the ballpark of 35cSt at 40C would be a good start, bear in mind a lower VI would mean it would be thicker at lower temp (i.e. typical working temp in UK climate) so stiffer than RSF 7.5W when cold.

Thanks YC for the prompt and Embee for the linky, you are spot on - it doesn't have to be rock hard to handle well, especially on our roads. In fact quality sporty suspension (TTX/Showa BP etc) can surprise with its compliance on a dodgy B road, that is the mark of quality damping.

Anyhow, I've actually got some Morris Fork oil in 5 and 10w, some folk do rate it highly and its nice that its as British as fork oil gets, but without any spec to hand its guesswork. Will see if I can find a Morris pdf.

Embee with set up as described (7.5 w) you are using the standard air gap, correct? My idea was to get the right oil and avoid the guesswork of altering the air gap.

EDIT MORRIS LINKY http://www.morrislubricants.co.uk/Do...%20Iss%201.pdf

Morris 5w @ 40c = 20 Cst
PrO rsf 5w @ 40c = 27 Cst
Pro RSF 7.5w @ 40c = 37 Cst
Pros RSF 10w @ 40c = 47 Cst
Morris 10w @40c = 50 Cst

SO if you mix the Morris to a 7.5w you end up with a figure similar to the RSF 7.5 @40c. .

But I note the rsf VI is significantly higher than Morris 5w (300 v >140)

That, I suspect, is the difference between cheap and expensive fork oil. The redline 2.5w has a massive VI...

[embee]

Yes, std air gap (from memory 106mm, may be wrong on that).

Mixing oil unfortunately isn't a perfectly linear business, but you're not likely to be far out if you assume a simple proportional relationship, so 50/50 would be about right I suspect.

With a lower VI it's going to be thicker than the Pro RSF when cold, so aiming at 50/50 would probably be in the right general direction I suspect.

Since you already have the oil, why not just give it a try. Ultimately it's going to be suck it and see, it might turn out just right for you. If not then you know where to go next, at least with changing fork oil it doesn't break the bank.

yup, like you said, may as well give it a go.

Will report back in due course...

I've got some silkolene Pro RSF 15wt oil waiting to go into my forks for when i get the spacers made up for the debrix emulators which are also waiting to go in. the bike already has 8.5 K-tech springs in it.

However i'm a bit concerned that the 15wt will be a bit of an overkill, is 10wt in the Silkolene stuff the preferred weight for the pointy or is 15 going to be ok?

[yorkie_chris]

Try RSF10, you are better with slightly fast rebound than too slow.