I've been trying to put together a trans oil post...
To answer the second part of D's question,
"He highly recommended running an oil that had a MA -### on it."
DMG, The MA on the oil bottle means the oil has been tested and Safe to use
In a Shared Gearbox/Wetclutch application. (fourstroke crankcase/trans).
That pretty much is it.
I am gonna have to say that you won't go wrong following the Manufacturer recommendation on which gear oil you choose to use.
Every transmission oil thread I read ended up being locked...
The thread here on ATF Vs.Gear Lube has remained civil much longer than most.
I'm gonna throw a few observations out there and you decide.
If you feel you want to try something else its only a drain pan away...
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."
ATF has a a couple of points in it's favor.
It is originally engineered to work in a high temperature wet clutch gear box
(around 210-280 degrees) . 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,does not carry that good a 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 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.
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.
Thicker oils resists break down at high temperatures (for the most part).
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.
However, an area 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 (Straight cut gears) 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. According to some information I found from Mobil Oil Co.,
85Wwt gear oil has an SAE gear viscosity number.
30wt engine oil has an SAE engine viscosity number.
They have approximately the same SUS (Saybolt Universal Seconds) rating at both 40 and 100 degrees Celcius.
Remember when talking about "gear oil" they use different standards.
80W gear oil (AGMA scale) translates into a 40W (10W-40 or 20W-40) SAE motor oil.
(As a reference, 10W-40wt engine oil is equivalent to 75W-90wt gear oil and 15W-50wt engine oil is equivalent to 80W-90wt gear oil.)
If 10W-40 has the same viscosity as 80wt gear oil,
why buy the more expensive gear oil?
The oil in the transmission has two perform three functions.
Prevent metal to metal contact, remove heat, and suspend dirt. Now because a two stroke transmission does not have filter to remove the dirt, the suspended dirt will become a problem. Prevention of metal to metal contact is a combination of the oil film strength and stability.
Oil is basiscally a massive quantity of long polymer chains. The structure of these chains are such that they do not interlock.
This is lubrication, The strength of the chain film strength.
The factors affecting this capability are heat and intermolecual shear. Heat causes the oil to expand. When a fluid expands the amount of molecules in a given volume decreases.
Result: the viscosity decreases and the protection of an oil film cushion also decreases. Now if the heat becomes too high the oil becomes volatile.
The polymer chain breaks down then it gases off and you are left with the parafins and asphaultics in the oil. (The wonderful black tacky stuff).
Shear is the second factor. The more the fluid is cut by a rotaing element the more force is applied the long polymer chain. Eventually the polymer chain fatigues and breaks. (There goes the film strength).
Lastly the highest contributing factor to wear is dirt.
Dirt (can we say clutch particles) does not decrease the film strength or lubrication capability of the oil, what it does is displace the oil.
Dirt suspended in the oil takes up volume. So again in a given quantity there is less oil, ( in the long polymer chains)
A dirt particle is also much greater in size than a oil molecule and it has sharp edges. So as the gears mesh the dirt gets smashed between the gear faces the sharp edge of the dirt displaces the oil.
Now we are getting wear.
So what to do? Change the oil often! Give it a break! Get rid of the Dirt! If it is black or silver (aluminimum clutch plates) you are looking at dirt.
Think about it. The engine oil that comes in the Cyl with the gas is clean and every rotation of the crank expells some oil.
(there goes the dirt and the broken down oil molecule) and brings in fresh oil.
Not so the case with our tranny.
Will ATF do work? Yep. Will the cheapest 10w-40 from Wal-Mart work? Yep.
Will the high dollar synthetic do this? Yep.
Which one is better?
Realistically, if you never run it long enough to break down, get full of dirt and it has the correct viscocity and heat index for the task at hand, all will work fine. Some maybe better, but all will provide the needed lubrication.
Most manufacturers recommend a multi-grade engine oil,
(the heavy thinking has already been done for us).
One other thing;
Friction Modifiers: the additives that bond to the surface of everything they come in contact with (flywheel and clutch discs is what we're concerned about).
Friction modifiers are typically long chain organic acids that bond in a polar fashion to a metallic surface. The organic chain side of the molecule stands off of the surface and holds a layer of oil. These additives protect when the oil wedge is lost by keeping that tiny last bit of oil in place.
Extreme pressure additives contain phosphorus, sulfur and chlorine. The way these work is that when the oil film is over loaded and there is actual metal to metal contact, the additives weld themselves to the high spots (asperities) that are hitting together. The additive welded to the metal is quite weak and breaks off easily.
This causes wear, but it makes the surface more smooth and ?broken in?.
This once again is a simplification of a pretty complex topic, but the more discussion and understanding the better.
95% of the time the tight clearances are not where you will find the wear, down to the point of failure. The wear in a bikes gear box will mostly be found in the areas that call for thicker oil to soften the impact, (Dogs,Shift forks and drum).
I would think that thicker oil is a better choice. A bikes tranny does not have any real hot spots that cause seizures or bad scaring past the point of normal wear for the most part.
(I know... There are always exceptions, like the rash of crf450-x Trannies on bikes converted for street use afew years back)
Here, Film strength is the key. And film strength must take into account the total package and not simply one aspect, such as viscosity. Now you could have a 60wt that was chock full of additives, and may well be a good protector, but there are also things like corrosion protection, foaming, heat/oxidation resistance, etc..that also must be considered. Not to mention that it is a fact that the thicker the fluid, the less energy your dyno will see. If you are racing at the top level of you field this could well be of great importance.
Clutch baskets and shifter forks take lots of abuse. They are pretty much considered consumables. And yes, perhaps a thicker fluid with a good additive package would allow them to remain alive a bit longer, but how much longer? And at what
expense? If we saw lots of gear failures from such a fluid, I would be the first to say stay away, but we don't.
Sure there is probably a better fluid to use other than ATF, but so often folks who do chose to buy the high-dollar boutique oils for $10-12 a qt. fail to change it out as often as they should.
And no matter what fluid is used in a 2t wetclutch, the contamination that collects needs to be changed out fast.
ATF might provide better clutch action and shifting...
...at the price of increased wear and tear on the gears.
ATF has additives for clutch packs in automatic transmissions that do also help clutch action in dirt bikes.
Its is thinner than any gear lube I know so it can help a notchy bike shift better.
I haven't found it has the gear cushion additives that gear lube does
The design and action of our transmissions is a little different than an automotive automatic.
A cars Automatic Transmissions gear sets are made with very little clearance specs and have almost zero lash. Also each gear set has its own set of clutches (called clutch packs).
If I understand them correctly, when the auto trans shifts from one gear to the next it basicly releases one set of clutches (or band) and applys another, there is no sliding of gears or impact, in the transition of power from one gear set to another.
This is why you almost never hear of your daily driver automatic transmission breaking gears, and a automatic transmission rebuild kit is mainly a bunch of clutches and seals (not gears).
On a Motorcycle the gears have alot of clearance and lash, they bang around back and forth and basically beat on each other every time you blip the throttle. A bikes transmission gears will wear no matter what kind of oil you use. It might not wear to the point of popping out of gear but if you visually inspect them after some use you will see signs of wear (unlike a automatic transmission gear which will show little to no signs of wear for the same amount of time used).
What I'm basicly saying is a automobiles automatic transmission is built with very little clearance (very tightly) and this is why it can get away with such thin fluids as ATF. On the other hand a motorcycles transmission is built with much wider tolerance, thats why its a good idea to run a oil with enough viscosity to help shield the gears from impact & metal to metal clashing.
In the '70's a lot of racers used ATF to help with shifting and clutch action but also knew they were going to get increased wear and tear on the gears. This was an acceptable tradeoff.
So the answer to which is better solely depends on how you ask it.
Whats better for the gears? Gear lube hands down.
Which provides the better clutch action (and possibly shifting depending on the bike), the answer is ATF.
Suffice it to say that those of us that do not use ATF are as convinced that is not good; as those that are using it are convinced it is as good...
... and realize neither side will likely sway the other. Everyone needs to make their own decision and be happy with it..
Oy-VA...
Tuck\o/
