jbt wrote:The main point is to determinate the good viscosity for it, and once you found it, stick with it.
The commercial viscosity written on the bottle is often (always) very far from the real one and varies a lot between 2 brands.
Many informations about
here.More; the physical viscosity varies a lot according to the temperature. The best oil would be the one which thermal stability is the best. This is known by the Viscosity index : the higher the better.
About this point, ATF and Citroen LHM (and water!) are the more stable, but their physical viscosity is very low and may not fit your fork hydraulic design. Fully synthetic oils makes possible to control this stability more easily than mineral ones.
There's not an ideal and easy to find oil for a fork or a bike. It depends a lot of the way it's driven ( tourism or tourist trophy?) and the weather conditions (Alaska or Australian bush?).
So you need to test and choose. Better if you limit your experimentations to a single brand with different commercial grades.
And, of course, much better if you test with new hydraulics in you fork!
Sir, yours is the first post in this thread that actually touches on the real issue.
It is the viscosity of the fluid that determines damping, not fluid weight, for example, Castrol Fork 5 has nearly twice the viscosity of Belray 10W. Part of the reason threads similar to this one abound in almost all motorcycle forums is the fundamental lack of understanding of the uncoupled relationship of "weight" and viscosity.
Viscosity Index is also not a truly useful measure as all it does is measure the stability of the fluid in question when subjected to temperature changes (although obviously a temperature stable fluid is an advantage).
There are two measure of viscosity that are useful and potentially useful.
The first is an "apples against apples" measure of fluid viscosity, this viscosity is measured in Centistokes and s normally conducted at a range of temperatures. I live in tropical Australia so the measurement of most interest to me is the one taken at 40 degrees Celcius.
To illustrate what I am talking about, please see the comparison at https://transmoto.com.au/comparative-oil-weights-table/ there are other sources on the internet, but when you look at the actual viscosity of the fluids compared I will be very surprised if most people do not have a "so that's why XYZ fluid didn't work for me" moment.
The second measurement is the "kinematic viscosity" which is essentially a measurement of the viscosity of the fluid in question under "stress". I've never bothered with it and include it only for technical accuracy.
I agree with you 100% that determining the correct fluid (or a mixture of fluids) is a highly individual thing. In my airhead experience, most people are quite happy with Castrol fork 5 which has a viscosity just slightly higher than the obscure Milspec Aviation fluid specified by BMW. I weigh a fair bit more than most people and prefer a firmer damping action, so for years I've used a 50/50 mix of Castrol Fork 5 and Fork 10. Please note that this does not equal the now no longer available Castrol 7.5, in fact the damping is slightly heavier than that fluid used to achieve.
The process of determining exactly what "mix" suits you and your riding is totally individual preference, along with perhaps some adjustment for local conditions. For example I live in a regional area of Australia and have always found from day one of BMW ownership that the somewhat "plush" action of stock forks just doesn't work very well here, other's mileage may of course vary.
And, if anyone is interested that ancient Milspec fluid, well I tracked down a Shell depot in Australia who had a drum of it and as it was out of date they offered it to me for nothing. They also gave me the priceless information that the approved aviation replacement for it was Dexron II - which leads me back to the adequacy or not of ATF - if you live in a country with smooth roads and you are of slight to average build, it's probably going to be fine for you.
My last comment is synthetic fluid better than mineral fluid?. In short - NO. There is more than enough anti-foaming agent and viscosity stability additives in any quality mineral fluid to allow the fluid to cope quite happily with any rational loading. Not that there is anything wrong with synthetic fluids, but if you are mixing and matching, make sure that it will mix happily with mineral fluid. I know I am starting to sound like a commercial for Castrol, but they do guarantee inter miscibility between their mineral and synthetic fluids, which is important when you can sometimes only source one or the other. I have been using Castrol fluid for 40 + years and have never had a problem with it. On the other hand, those using some other brands of fluids have had horrid outcomes. To draw on R65 experience (noting that what I am about to say is not unique to R65s), there is a large rubber bump stop at the bottom of the fork leg, people suddenly discovered that shortly after using a nameless brand of fluid (which advertised a "magic" ingredient called "seal swell") that their forks operation was greatly degraded, and in some cases were simply hydraulically locking. It turned out that the magic ingredient to seal seals was nothing more than about a tablespoon of brake fluid added to each bottle by the manufacturer - it certainly gave extended life to failing seals, but it was also, and inconveniently, fatal to rubber bump stops. Hence my decision, a long time ago, to stick to a known fluid.
Woohoo! Another oil thread! (Fork oil) Thu Apr 19, 2018 6:04 pm
