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Final Drive fluid capacity

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Twisties
Is there any informations as to the overall percentage of hexhead final drives that have failed?(Wondering how many are not failing and why they're not failing?)

 

No hard info. I think most BMWST members believe it is in the low % range. Maybe 1-2%.

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kdiver

The increase in void volume of 18% will in fact make no difference in reducing the pressure increase. Once the oil is placed in the final drive and the plug is installed, this sets the initial conditions. All subsequent pressure increases are directly related to the temperature increase. If you pulled the excess oil out without unsealing the system and after the unit has reached the operating temperature (which you can not do) then the reduction in fluid volume would result in the desired pressure decrease.

The only way the pressure build up can be eliminated is to vent the final drive cavity or by preventing any temperature increase (which would be very difficult).

One early suggestion to install a vent line should work, maybe using a banjo bolt connection on a modified plug, with a filtered vent line attached.

Edited by kdiver

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Twisties
The increase in void volume of 18% will in fact make no difference in reducing the pressure increase. Once the oil is placed in the final drive and the plug is installed, this sets the initial conditions. All subsequent pressure increases are directly related to the temperature increase. If you pulled the excess oil out without unsealing the system and after the unit has reached the operating temperature (which you can not do) then the reduction in fluid volume would result in the desired pressure decrease.

The only way the pressure build up can be eliminated is to vent the final drive cavity or by preventing any temperature increase (which would be very difficult).

One early suggestion to install a vent line should work, maybe using a banjo bolt connection on a modified plug, with a filtered vent line attached.

 

You are probably right. If the pressure build-up is due to heating of enclosed air, then you are correct. I was thinking it was due to oil vapor formation and heating, but I suppose that is unlikely to be the major source of pressure.

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JayW
The increase in void volume of 18% will in fact make no difference in reducing the pressure increase. Once the oil is placed in the final drive and the plug is installed, this sets the initial conditions. All subsequent pressure increases are directly related to the temperature increase. If you pulled the excess oil out without unsealing the system and after the unit has reached the operating temperature (which you can not do) then the reduction in fluid volume would result in the desired pressure decrease.

The only way the pressure build up can be eliminated is to vent the final drive cavity or by preventing any temperature increase (which would be very difficult).

One early suggestion to install a vent line should work, maybe using a banjo bolt connection on a modified plug, with a filtered vent line attached.

 

It seems to me that a higher air/fluid ratio would decrease the relative pressure increase for a given temperature change due to the increased cushioning effect of the greater volume of compressible air.

 

Jay

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Twisties
The increase in void volume of 18% will in fact make no difference in reducing the pressure increase. Once the oil is placed in the final drive and the plug is installed, this sets the initial conditions. All subsequent pressure increases are directly related to the temperature increase. If you pulled the excess oil out without unsealing the system and after the unit has reached the operating temperature (which you can not do) then the reduction in fluid volume would result in the desired pressure decrease.

The only way the pressure build up can be eliminated is to vent the final drive cavity or by preventing any temperature increase (which would be very difficult).

One early suggestion to install a vent line should work, maybe using a banjo bolt connection on a modified plug, with a filtered vent line attached.

 

It seems to me that a higher air/fluid ratio would decrease the relative pressure increase for a given temperature change due to the increased cushioning effect of the greater volume of compressible air.

 

Jay

 

If the greater source of pressure build-up was due to expansion of the fluid, compressing the air. I suspect there is far more pressure build-up from the enclosed air being heated. But really, I don't know what goes inside a device like that.

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kdiver

I started wondering about car and truck differential axles, They should have the same issues as the final drive, so to did a search: venting differential axles.

 

The results of this search are very interesting:

1. the increase in temperature causes a pressure build up

2. the differential is vented to PREVENT SEAL LEAKS

3. one company had a problem with a non-vented axle. solution: vent mod

 

Also, as the bearing begins to fail, the increased friction dramatically increases the operating temperature. This causes a greater pressure increase which leads to seal leak.

 

 

 

 

 

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RSR

Time to throw in my two bits worth on this subject.

The flash point of gear oil is 220C so vapour pressure should not be an issue.

Air is compressable oil is not so less oil and more air should equal lower pressure, don't belive me try squeezing a sealed water bottle thats full now dump some out and try again.

The heating and cooling cycles of a vented system cause it to draw in contaminants the small volume of the BMW final drive would not tolerate this well.

Running a vent line from the final drive to the transmission might solve the pressure problem and prevent contamination.

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kdiver

I agree with everything you said, except that" less oil and more air should equal lower pressure". Your example of the water bottle explains that when a confined gas is compressed (the volume is decreased by squeezing the bottle) the pressure will increase, which is what is happening in the water bottle. The difference in the final drive is that the container (ie casing of the final drive) is not changing in shape and therefore remains constant. When the air inside the casing (at any fixed quantity) is heated the molecules movement increases causing the measured pressure to increase.

pressure 2 = pressure 1 times temperature 2 divided by temperature 1.

 

This is why I first joined this thread, the hope that pressure increase will be reduced with a reduction in gear oil is still causing confusion for me.

 

Your thoughts on venting seem to be a VERY good solution. Do you know if the BMW transmission is vented? It is operating in the same manner as the final drive and would involve the same pressure/temperature relationship.

 

 

 

 

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Albion

One thing is absolutely for sure, too much oil will cause a seals failure.

 

This happened to mine after I asked a dealer to change the oil first time, and his young trainee did this 'simple' job...

 

Folks, there are good reasons why we believe in self help!

 

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mneblett
Do you know if the BMW transmission is vented?
Yes.

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Mister Tee
Do you know if the BMW transmission is vented?
Yes.

 

So is it vented?

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BMWED

Parts diagram shows a breather.

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RSR

The air volume does decrease as the oil expands so I grabbed my pocket ref and tried to figure how much.

Now I'm no engineer so this stuff is way over my head, math class was 30 years ago and I have to make a lot of assumptions so please don't complain because the numbers may be wrong Im just trying illustrate a point.

Assuming the FD temp rises from 70F to 150F and the total case volume is 500ml.

180ml of oil will expand to 188ml increasing the pressure by .35psi add in the gas expansion the pressure rises 4.65psi

220ml of oil will expand to 229ml increasing the pressure by .50psi plus gas expansion equals 5.23psi increase.

I think a .58psi decrease should solve the FD problem, Don't you? :rofl:

Now I think it is time to let this thread die a natural death RIP.

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Ken H.
Do you know if the BMW transmission is vented?
Yes.

 

So is it vented?

Yes I know that yes it is vented.

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kdiver

if we use the formula P2 = P1 x (V1/V2) x (T2/T1)

 

T1 = 70+460 = 530

T2 = 150+460 = 610

 

P1 = 14.7

 

now to include oil expansion (and air compression)

for oil fill of 220 ml expanding to 229 ml, the volume of air is 500-220=280 ml compressing to V2 of 500-229=271 ml

 

calculating P2 = 17.48 psi (increase of 17.48-14.7 = 2.78 psi)

 

now to include oil expansion for fill of 180 ml expanding to 188 ml

the V1 air = 320 ml and V2 air = 312 ml

 

calculating p2 = 17.35 psi (increase of 2.65 psi)

 

The net advantage of adding less oil seems to be 2.78-2.65 = 0.13 psi

Thanks for the help.

Edited by kdiver

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Bernie

Would it help to heat the oil to 200 F before filling the rear drive. Then as it cools, it would create a vacuum and no pressure during riding. :rofl:

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Tech1

I have worked on finding a proper volume of oil in a contained, non-vented, application. It wasn't quite as small as the final drive, but the pressure "and" temperature were reduced by finding an optimum oil level. Part of this was probably due to less windage, and less windage equals lower temperatures. We also had another application (I wish I could go into details), that we added an air volume into a part and it reduced the internal pressure in the product which has to be sealed. Most seals can take at least a few psi of pressure and we have seals designed to run at 30 psi without problems. One thing we did run into was a case where the seal would push out of the case from the pressure and heated aluminum part. We redesigned the part to retain the seal on the outside.

A point brought up earlier that does make a difference, the temperature the product was assembled at, although we find that it can very slowly equalize with enough time. If unit is sealed at 0 psi and then cooled, it is possible that air will pull in past the seal lip, then when heated again the pressure will be higher.

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hopz

So how much oil do we put in the final drive?

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marcopolo
So how much oil do we put in the final drive?

 

180 ml (down from previously specified 220 ml). 75W90 synthetic GL-5.

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Chuck U Farley

I'm new here and to BMW Motorcycle maintenance. I have an 04' R1150RT and the BMW service manual stated 250ml as the FD oil volume. Is this BMW FD oil volume reduction recommendation relevant to the 04' R1150RT FD?

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smiller
I'm new here and to BMW Motorcycle maintenance. I have an 04' R1150RT and the BMW service manual stated 250ml as the FD oil volume. Is this BMW FD oil volume reduction recommendation relevant to the 04' R1150RT FD?

No, this issue affects only the hexhead and newer K bikes. Your bike has a vent in the FD and the level isn't as critical.

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Ken H.
I'm new here and to BMW Motorcycle maintenance. I have an 04' R1150RT and the BMW service manual stated 250ml as the FD oil volume. Is this BMW FD oil volume reduction recommendation relevant to the 04' R1150RT FD?
This is the Hexheads forum - North American spec '05 R1200xx or newer. (Just to help you with what's what in future reading.)

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bmurphypdx

Having read through this entire thread, it seems to me that the reduction to 180 mls might be because BMW has realized that a "complete" drain of the fluid not likely therefore the original spec would have resulted in an overfill.

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marcopolo
Having read through this entire thread, it seems to me that the reduction to 180 mls might be because BMW has realized that a "complete" drain of the fluid not likely therefore the original spec would have resulted in an overfill.

 

I recall reading BMW's rationale in a service bulletin (not that I can readily find it) and what you suggest is not it. The specified fill quantity for an oil change has always been less than that specified for a fresh fill (as in a new final drive) even before this most recent change. Seems pretty clear that BMW understood from the get go that you'd never get all the oil out during an oil change.

Edited by marcopolo

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Firefight911

The reduction in volume created a larger air gap. This, in turn, would reduce extra pressure against the seals after the FD unit heated up.

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dirtrider

Hi Mark

 

Here is the part from the service bulletin that pertains.

 

Yes the initial fill is slightly more than the 180cc fill at change time. It also gives the reason for the reduction as “see #1 below”

 

 

 

 

“ 1. Increased temperature and high pressure lead to premature wear on the shaft sealing rings.

 

For point 1: The gear oil capacities have changed for initial filling and when changing the oil in the rear-wheel drive.

The new capacities:

For initial filling: 200 ml

For changing oil: 180 ml

(The new RSD 05/2009 contains this modification)”

 

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bmurphypdx

OK. Thanks.

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Dave_C

O.K. thanks for all the input here. i just changed my FD fluid a few days ago, put in 250ml. Changed it again today, put in 180ml. Also lubed the driveshaft spline, which probably only got done once before. I will probably start doing that every fluid change. I have a new FD(replaced under warranty at 35,600 miles) with a drain plug on the bottom. So far, so good. BTW, I have not seen anything, anywhere on how often to lube the driveshaft spline. Anyone know?

Edited by Dave_C

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