Fork Oil – 7.5wt vs 10wt

What happens if I use 10 instead of 7.5?

Also, can you mix two types to average them out or not? I can’t imagine oils mix brilliantly and might cause inconsistent performance.

Cheers.

Google Pete Verdone,he has lots of info on his wiki site.
Ian

10wt is slightly thicker, so the fork will be more damped. This is very unlikely to do any harm, and you may (or may not) prefer the feel. I run 10wt oil in my 55s (book says 7.5wt)

It’s a mine field if you really want to get into it. I use silkolene motorcycle fork oil in the damper leg and motorcycle engine oil in the spring leg.

Heavier oil = more damping. Simples.*

If you want something between 7.5 and 10wt, most oils weights will mix ok, especially if they are the same manufacturer.

* actually it may not be all that simple depending on what adjustments you have on your forks / shock. For the majority of forks and shocks, only low speed damping (nearly always rebound, sometimes compression) is adjustable via dials, so heavier/thicker oil will increase high speed damping, but you can adjust low speed (or what ever you have dials for) to roughly the same as with lighter oil if you want (assuming you aren’t hitting the end of your adjustment range).

My 2008 F120 normally takes 7.5. But the rebound damping has always been a bit on-off. At a recent service, Nath at Bad Ass Bikes[/url] suggested trying 10 instead. This has improved the range of rebound damping somewhat. Box intimated that Fox forks seemed quite variable in this respect: “How they are shimmed on the day”

Mixing fork oil is best estimated by the rule of mixtures
Fractionofa+fractionofb=C

so if you mix equal quantities of 2.5 and 7.5 oil its
(0.5 x 2.5) + (0.5 x 7.5) = (1.25 + 3.75)=5

Or to make 25 from 13.6 and 26.7
Mix 13% of 13.6 and 87% of the 26.7

Which is (0.13×13.6)+(0.87×26.7) =1.768+23.229=24.997

I wouldn’t be too worried mixing one type of oil with another, its just long chain polymers really, viscocisty is related to average chain length.

I forgot to mention that it made the lock-out completely fierce which is quite nice too.

Fork oil weights vary wildly from brand to brand, there’s a chart on the WWW somewhere. The proper measurement to use is centiStokes@a given temperature, and the chart lists all common brands using this measurement.

I forget the name of the eqaution needed to calculate the viscosity of two mixed fluids, but I have a program on my PC called Shell Bunker Calc which does it for me, available free from the Shell website.

TuckerUK, whilst what you say is true:you need to use the dynamic viscosity in the refutas equation to get the viscosity blending index, put this in the rule of mixtures to get the new VBI of the mixture followed by the refutas equation inverted to get the new viscosity. As he only knows the fork weight(which is derived from the dyn visc) then its a bit pointless..

If you just use the rule of mixtures then you get a pretty accurate assement of the new fork weight, given that it’ll all be measured in the shed with a syringe where the error will be as great as that from just using the rule of mixtures directly on the fork oil weights rather than their corresponding VBI..