While I have every reason to believe that ATF is a good alternative to conventional gear lubes it has always left me with a lot of un-answered questions. I came across the following and I thought it was interesting. Even though it doesn't speak directly to the issue at hand, it does indirectly make a case for ATF in a high speed gear box, and addresses to a degree the trade off of viscosity. Just something to ponder when you are stuck in traffic.

From the book LUBRICATION FOR INDUSTRIAL FACILITIES by Heinz Bloch
This is from the section on Gear Lubrication:

The oil furnished to high speed gears has a dual purpose:
Lubrication of the teeth and bearings, and cooling. Usually only
10% to 30% of the oil is for lubrication and 70% to 90% is for
cooling.

For some reason, the high speed gear makes all the compromises
when oil viscosity for a combined lube oil system is determined.
Usually a viscosity preferred for compressor seals or bearings
is selected and gear life is probably reduced. The bearings in a
gear unit can use the lightest oils available, but gear teeth
would like a much heavier oil to increase film thickness between
the teeth.




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Rich Rohrich

"Wisdom is not a product of schooling but of the life-long attempt to acquire it."
               - Albert Einstein

If I read it right, bearings are ok with thin oil but heavily loaded and abused meshing gear teeth (read clutchless shifting) prefer an uninterrupted oil film to prevent damage - and higher viscosity oil provides said uninterrupted oil film layer better than thin oil.  Sounds like using ATF could lead to pitted gear faces and chunked  gear edges.  Sounds right?

techman

Interesting.  I wasn't able to aquire the usual Amsoil ATF used in my Husky last weekend and had to use friend's Belray stuff(85w).  The bike shifted fine, but the clutch complained bitterly.  Yesterday I went riding with the Amsoil ATF, and noticed immediately that shifting was rougher!  Had no problems with clutch fading, but could really feel the different in shifting.  I know that Huskys are known for not having the smoothest gearbox...interested in your two cents Rich.

rich,
i understand what you are saying, but how does changing ATF every 2 weeks compare (still looks, feels and smells fresh as it drains) to say 10w40 from any other brand or bel-ray gearsaver that is NOT changed as often?

i'm sure there has to be some compromises along the way unless you are changing the oil as often no matter what type/viscosity it may be.

i've seen guys drain bel-ray gearsaver that was brown.  YUCK!!!  

exactly what viscosity is 85wt that has been burnt?

I've always used the manufacturer's recomended 10w/40 in my Kawasakis, but was talking about ATF with a friend.  He commented that  ATF is an excellent detergent and would be good at suspending any particles, such as those from the wearing clutch plates.  Rich, do you know if that is accurate?

IMHO ATF has a a couple of points in it's favor.  
It is originally engineered to work in a high temperature(~280degrees) wet clutch gear box.  That is what an automatic transmission is.  So its primary design goals are: to cool the transmission, serve as a hydralic fluid in the valve body, lubricate the gears and bearings, and keep particulate suspended to prevent damage.  I feel some compromises are made in the automatic transmission application.  

One is viscosity, ATF has a viscosity in the range on ~7wt so that it can be pumped easily, flow through the valve body, and actuate the clutches and bands.  A viscosity this low is, as has been stated, not that good for film strength.  Compared to a standard transmission, there are no hard shifts in an automatic transmission that necessate the thick strong films that a standard transmission requires.  ATF is definately lacking film strength compared to any gear lube.  Granted, in a motorcycle, on dirt there is nowhere near the shock in the gear box that you would see in an automobile shifting at low speed.  And of course the Borg-Warner series of transmissions(T-5, T-56) all use ATF as a lubricant.  So?  What's the answer?  Does ATF provide "enough" film strength?  For a dirt bike?  I'll answer with an unqualifed maybe, but, gear oil is stronger.

Another big difference is in high temperature stability.  ATF "burns" at a much lower temperature then a heavy weight gear oil.  ATF is designed to work in a cooled thermally stable environment, and it performs so many functions that its additives are a compromise.  You've all seen what happens when you tow on a hill in a truck, the fluid burns and needs replacement.  Standard gear boxes, including your motorcycle's are not externally cooled and can get quite hot with the addition of a clutch and fanning fingers.  The thicker oils resists break down at high temperatures.  This is easy to answer though, if you're draining a fluid that is "burnt" then it is not heavy enough, or changed often enough.  If your ATF is coming out clean and it smells good, then you're cool.

But to get back to Rich's paste, motorcycle gear boxes are quite hardy (YZFs exceptedhttp://208.56.125.175/forums/ubb/wink.gif) and the gears themselves tend not to fail, and in those rare sorry circumstances the failure tends to be a material or design problem as opposed to a lubricant failure.  Thus the final conclusion is to run the thin ATF to minimize pumping losses and maximize horse power to the dirt.  Of course this really doesn't matter to me as I am a four stroker and untill I get a Cannondale, I have to share my engine oil with my gear box. http://208.56.125.175/forums/ubb/frown.gif

some generalities to keep in mind:

• higher relative speeds - thinner oil
• higher temp - thicker oil
• tighter tolerances - thinner oil
• greater pressure - higher film strength

[ramble mode]

Straight-cut gears, as opposed to helical or hypoid, impact tooth to tooth and shear. They would ideally like thicker oil to cushion and high film strength for the shear.  

Bearings roll and tolerance is tight. They impact and shear on a microscopic level because of imperfections and geometry. Would this suggest that the oil's main function is heat transfer? (thermal expansion makes tight tolerance = no tolerance)


I would also think that a thinner oil would have greater cooling abilities because of a thinner boundary layer, but I don't know how the thermal properties change with viscosity.
[/ramble mode]


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Kramer
member BRC, AMA
98 WR400 Dualsport!

A couple of questions: Doesn't 85wt gear oil roughly correspond to 30wt engine oil in viscosity? I know they use different rating systems. I know Mobil 1 ATF is the same viscosity as
7.5 wt fork fluid. Does fork fluid use the engine oil or gear lube rating system or something else entirely. Personally I think this is a overdone issue. Try ATF/F, Mobil 1 15W50, and 85wt-MTL and let your bike tell you the fluid and drain intervals it prefers.

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Gene K
Decatur AL
00 GG300EC

Kramer, you took the words outta my mouth.  However, the area that you briefly touched on that I think is critical is an oil's ability to resist shear.  Shear resistance and viscosity generally go hand in hand.  Motorcycle transmissions are generally shear friendly (for your above mentioned reasons), whereas hypoid gears (e.g. differentials) and, to a lesser extent, plain bearing surfaces, require higher viscosities to resist  shear. Resistance to shear is not temperature dependent; higher viscosities have been used in high temp applications due to the fact that viscosity changed excessively in common petroleum oils.  Synthetics are much more stable in that regard and do not require "high" viscosities to resist shear at elevated temps.

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Peace,
KDXIdaho


George Orwell, 1984

Although I can't contribute technically to this thread... let me just say that this is great info and I love that I am absorbing it all.  Not necessarily understanding it yet but each time a little more sinks in.

*sniff* *sniff* I love you guys...