Choosing gear Oil

[Barry]

The forums been quiet of late so maybe even an oil thread of sorts would be welcome.

I’ve just changed the transmission oils in my bike after 6000 miles and they all came out nice and clean with hardly any black paste at all on the magnetic drain plugs.  Choosing a replacement gear oil should have been easy as one mineral 80W90 ought to be the same viscosity as the next and mostly they are. I commute a short distance every day and worry that none of my oils get hot enough. If you have ever tried rotating the rear wheel by hand before going on a ride when it's very cold and then done the same when you get back with all the transmission oils warmed up there is a very significant difference. Cold oil drag must absorb some power and use fuel. So when choosing oil I tend towards something a little thinner if possible but for the transmission I wanted to stick with 80W90.

The oil manufacturers web site will usually tell you the viscosity at 40 Deg C and at 100 Deg C.  But how about me setting off for work tomorrow at something like 5 Deg C or even 0 Deg C what will be the viscosity then ?  No manufacturer tells you up front although they could if they wanted to as anyone with knowledge of Tribology could work it out and oil companies employ lots of Triboligists. I didn’t know how to do the calculation but I at least knew it could be done.

I went in search of the formulas and stumbled across something very much better - a web site that does it all for you and then plots the graphs.

http://widman.biz/English/Calculators/Graph.html

It turns out that if you know the usual published information (viscosity at 100 Deg C and 40 Deg C) you can calculate the viscosity index and then from that you can calculate the viscosity at any other temperature you choose.

I plotted four different makes of 80W90 GL5 gear oil I have already used or can easily buy. The Shell Spirax I currently use is by a small margin the thickest so perhaps not the best choice. The Shell, Castrol and Morris oils are otherwise much the same. The Total TM multigrade Transmission oil is significantly thinner at low temperatures while still being near enough the same as the others at 50 C and above. The Total oil is actually marketed as being thinner than usual at low temperatures. It does this by having a higher viscosity index than the other oils. At zero Deg C the Shell oil is nearly twice as thick as the Total and even at 20 Deg C it’s still 53% thicker. Using the Total oil should reduce oil drag on my short winter commutes so that’s what I bought. Synthetic gear oils may be better still but I haven’t checked those yet.

Various calculations and graphs can be plotted on this fantastic web site which seems to be written by a man (obviously a Triboligist) who runs a lubrication company in Bolivia.

I have also plotted graphs for Fork oil which are notoriously difficult to compare between brands and you can compare engine oils as well.

Here’s the graph for the gear oils.

[Graeme]

Thanks Barry; saved. I'll use it as soon as I change the oil. (I'm not sure what's in it at the moment, the loacl mechanic filled it after the head & rings were reco'd)

[tvrla]

Interesting data!

I've never heard of a tribologist before - could you explain?

I think one of the reasons sysnthetic is so much better for the transmission is the lower cold viscosity. Besides the drag robbing horsepower and fuel, it also doesn't lubricate as well. It doesn't get into small orifaces and doesn't pump as easily. In the case of our transmissions, it doesn't make it to the splash trough in the roof and back into the front output shaft bearing. As you know, that bearing is the main reason for most tear-downs.

[Barry]

I've never heard of a tribologist before - could you explain?  

To quote Wiki: Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear.

I did a short course on Tribology once many many moons ago not that I remember much about it. Still retain the interest in oil though.  

I'd like to try synthetic. If there is one place it would pay dividends it's in the transmission where studies have shown small but real reductions in power losses and hence fuel use. Not to mention longer gearbox life.

Only thing stopping me is thoughts of incontinent 33 year old seals.

[tvrla]

Several of my bikes have synthetic gear oil and they don't weep (or spurt, dribble, or anything else). My most recent is the 83LS that got synthetic a couple years ago when I got it. It's never lost a drop.

Thanks for the definition! I could have looked it up myself but thought it was some sort of limey slang term.

Yeah, tribology is interesting stuff, huh? When you really start taking a close look at our universe and the interactions between things, turns out it's not as simple as it seemed! One thing I remember about the topic is how molecules are constantly swapping between the two parts in contact.