Driveshaft/Gear Coupling/Final Drive 101

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Purveyor of Crooked Facts
Jun 13, 2005
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Skootsdale, AZ
[SIZE=14pt]Driveshaft/Gear Coupling/Final Drive 101[/SIZE]

There has been some discussion recently on lubrication and disassembly of the driveshaft, gear coupling, and final drive, or “pumpkin”. Good info was muddled in many pages of posts. So here is a new topic to get to the heart of the matter, and hopefully explain everything in a clear and useful, and of course, accurate way.

For reference, you should first be familiar with Warchild’s write-up on driveshaft spline maintenance which you can find at the following link: Spline Maintenance How-To

Most driveshaft spline maintenance discussions center around the forward part of the driveshaft where it inserts into the universal joint at the forward part of the swingarm. These forward driveshaft splines have had instances of no, or very little lube right from the factory, and also corrosion. That it why it is suggested you follow Warchild’s above article every once and a while to check the condition of these splines, and re-lube if necessary.

Also a good idea is to service the universal joint every once and a while as Bounce details at this link: Bounce's Universal Joint Service How-To

So what is at the rear of the driveshaft? There has been much discussion and debate about this. It is the purpose of this write-up to provide clear information as to what’s going on back there.

Here is a pic of the driveshaft, gear coupling, and pumpkin removed from the swingarm:


Simply, the driveshaft at it’s rear, enters the gear coupling, which connects to the pinion gear inside the pumpkin, or final drive. For reference, below is a picture of the inside of the pumpkin or final drive. It shows the ring and pinion gears. These are lubricated by the oil in the final drive. Between the pinion gear and the rear of the driveshaft is the gear coupling.



What has been the cause of much debate, is what is at the rear of the driveshaft? Well, there are splines at the rear of the driveshaft, and they then mate to splines inside the gear coupling.

Here is a photo of the driveshaft, removed from the gear coupling. Front to the right, rear to the left:


Here are close-up photos of the rear of the driveshaft splines, and the splines inside the gear coupling:



And here is a photo of the pumpkin, and gear coupling with the driveshaft removed. See the spring in the center of the gear coupling? That is there to keep the driveshaft under tension as the swingarm pivots from normal movement. Any fore and aft movement of the driveshaft caused by the swingarm’s pivoting is done in the gear coupling with aid of the spring, and not at the universal joint. The spring is actually putting some pressure to push the driveshaft out of the gear coupling.


So what holds the rear of the driveshaft into the gear coupling? Nothing but the rubber oil seal that is attached to the rear of the driveshaft. That’s it. It is due to this fact that when disassembling the pumpkin assembly from the swingarm, one must exercise caution: The driveshaft can very easily pull out of the gear coupling when you are not expecting it. (As several have reported happening) Be careful so as not to drop the pumpkin on your foot or make a mess. Conversely, it can sometimes be quite difficult to pull the driveshaft out of the gear coupling if your rubber oil seal is old and knarly. For example, below is a photo of my driveshaft and the oil seal. This was disassembled for the first time after 21/2 years and 64,500 miles of ownership. It was very difficult for me to pull the driveshaft out of the gear coupling, taking almost all my strength (admittedly not much). Still, it was almost as bad as trying to pull twowheelnut away from gerbils at a pet store.


So at the rear of the driveshaft are the splines, a rubber oil seal, and then a washer and circlip. The washer and circlip are there to hold the oil seal in place.

Here is a close-up of the washer and circlip, and then the rubber oil seal:


And here, I am pulling back the outer portion of the oil seal so you can see where the inner part of the oil seal is resting against the washer.


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Another contentious debate has centered on what lubricates the rear driveshaft and gear coupling splines. Unfortunately, the FJR service manual is, as usual – weak in this area and incorrect conclusions have been drawn. The debate has been whether the splines in question are lubricated by final drive oil, or whether they should be greased. Well, I am here to tell you they are lubricated by the final drive oil.

When I was finally able to pull my driveshaft out of the gear coupling, there was a big pool of oil inside the gear coupling that spilled everywhere. I wasn’t sure if this was normal, or if I had a failed oil seal somewhere. At the time, my gear coupling had been pointed in a downward orientation. When I raised the gear coupling to an upward orientation (to stop the damn spilling), the oil drained back into the final drive, or pumpkin. You see, there are two oil passages that allow the final drive oil to pass between the pumpkin and gear coupling, lubricating the splines with the gear oil.

Here is a photo looking directly into the gear coupling. Unfortunately I was having trouble with proper lighting for taking pictures, but you can barely make out one of the oil passages at the 11:30 position. It is semi-circular in shape.


Here is another photo from the same orientation. The photo is terribly overexposed, but it shows clearly the two semi-circular oil passages at the 11:30 and 5:30 positions. Again, it is these two passages where the final drive oil can flow from and return to the pumpkin, lubricating the driveshaft and gear coupling splines. I experimented, letting oil flow out of these to the splines, then draining back.


Another question I had answered during this exercise was, “where does the black nasty “splooge” that occasionally gets on the rear wheel rim come from?” Some have thought it was grease coming from the “driven” splines, or the splines that connect the rear wheel and final drive. That is not the case. As you can see below, there is no grease or residue coming from the final drive splines. The photo was taken after approximately 20,000+ miles where no maintenance or cleaning had been performed in this area.


If the the rear wheel “splooge” was coming from grease from these splines, there would be an obvious trail, especially at the bottom of the photo.

So where is it coming from? It is coming from small amounts of final drive oil that get past the oil seal on the driveshaft, and then finally escapes through the weephole at the bottom of the swingarm.

Here is a photo of the swingarm weephole on my FJR taken at around 5,000 miles. It is relatively clean in this area.


Here is a photo of the same area during my recent disassembly (64,500 miles). All kinds of muck! That’s from the final drive oil leaking past the seal over time. I am not talking a lot of oil. There has never been a drop on my garage floor, and were talking around 50,000 miles here, so it’s a slow process.


Here’s a photo of the inside exterior of the swingarm, above and close to where the weephole is located. More muck.



So the last part of the “great driveshaft debate” is whether one should pull the driveshaft out of the gear assembly to service it. Well, since we now know that we don’t have to grease it, and it’s all pretty well sealed, it’s not really necessary to mess with it, right?


Personally, I could see pulling the driveshaft out of the gear coupling to service the oil seal, not the splines. I am not going so far as to recommend it, but it might be prudent every great once and a while. As you can see from my above photos, the seal is getting pretty gunked up. Perhaps leading to more final drive oil seepage? I don’t think it would lead to enough escaping to cause problems with the final drive, (if so, it would be obvious by the puddle on the ground, or the HUGE mess everywhere), but enough to cause a minor mess as shown above. So I will leave that decision up to you, but I am guessing that occasionally servicing the oil seal will aid in its proper functioning. I will probably do it every 2nd or 3rd tire change. It’s a relatively simple and straightforward procedure. By the way, the service manual does call for greasing the oil seal. Not as a regular maintenance item, but when disassembling and reassembling.

Here’s what my parts looked like after cleaning and re-lubing.

Rear of driveshaft splines and oil seal all cleaned up. Note the strange uneven shape to the oil seal. It is not warped, or sitting funny. That is its natural shape. Weird. Wonder why?


Same piece all lubed and ready for re-assembly with Mobil1 synthetic grease:


Driveshaft re-inserted back into the gear coupling. It went in much easier than it came out. I guess now that everything was cleaned and had fresh lube. Twowheelnut and his gerbil friends would know about that.


And interior of swingarm cleaned of all the muck. It will be interesting to see how long it takes for it to come back.


Hope this was helpful and clarified things for you. It did for me.

Just not gonna let it go, are ya? :D Actually, since that seal turns with the shaft, it isn't gonna much resemble a common oil seal such as we're used to. It just needs to flex a bit depending on driveshaft angle. Nice write up though, see, it was worth it, wasn't it? ;)

My favorite line?

It went in much easier than it came out.
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Quit using your FJR for dirt road excursions and you won't have that crud back there. :D

Good right up Greg, might consider changing the subject to "SkooterG's Rear End Explained" :dribble:

also noticed a minute amount of rust on the shaft in one of the pre cleaning piccis, did you also apply a very thin coating of grease to the shaft?

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Good right up Greg, might consider changing the subject to "SkooterG's Rear End Explained" :dribble:
Great Scott! Now why the hell didn't I think of that?

also noticed a minute amount of rust on the shaft in one of the pre cleaning piccis, did you also apply a very thin coating of grease to the shaft?
I always lube my shaft! :D

Yes, after wire brushing any corrosion off, I lubed with a light coat of Mobil 1 grease, as I always do.

I always lube my shaft! :D
Yes, after wire brushing any corrosion off, I lubed with a light coat of Mobil 1 grease, as I always do every 40,000 miles or so.
Repair completed. :D

Note the strange uneven shape to the oil seal. It is not warped, or sitting funny. That is its natural shape. Weird. Wonder why?
Shape relates to the fluid dynamics of the moving oil about the gear's teeth. Spinning gear would create a higher constant pressure on straight cut seal, whereas the bevel cut seal kicks the oil back, lessoning the pressure, reducing the chance of oil loss past the seal.

Otherwise, nice write up, Junior.

- Prof. John Bush, MIT Department of Fluid Dynamics and other Egghead Shit ;)

What a thorough write up, SkooterG & good pixs too. Almost tempted to to print out a copy. Guess I did the right thing by leaving my shaft in when lubeing my swing arm bearings & U joint. Also good explaination as to the crooked seal, Twowheelnut, makes sense to me, sorta like a cheep oil pump. By the way SkooterG, I finally changed my F/tire on my ’04 FJR at 20,850 mi. Later,, De :rolleyes:

I'm impressed.

But, I was impressed with the Monica Lewinsky cigar trick, so what the heck do I know?

Good write-up, should post somewhere as a single article with 2wheelnut adds, and minus gerbil stuff....(although funny) -- perhaps on :clap:

tanx! :agent:

Good right up Greg, might consider changing the subject to "SkooterG's Rear End Explained" :dribble:
Great Scott! Now why the hell didn't I think of that?
.....resisting urge to abuse my Admin priveledges and change the title as suggested :hyper:

Very good. A definite "maybe" on servicing the rear splines. Seems everyone was right. Servicing then is a good idea, but they are lubed by the rear pumpkin fluid, so it wouldn't be needed as often as the front and driven splines.

I just bought my first FJR last weekend. (Yes, I love it!) However, it is also my first bike with a shaft drive, so I have a question.

Yesterday I washed my new baby including the rear rim which had some oily/greasy gunk on it. I assumed it was from the original assembly at the dealership. Afterwards, I went for a 10 mile cruise around the neighborhood. When I get home there was new oily/greasy gunk all around the rear rim on the left (driveshaft) side. Since the bike only has 250 miles, is this simply oil/grease from the original spline assembly that will eventually stop flinging off, or does it always do that? I though shaft drives, especially new ones, were supposed to be spotless and clean?

I got a little carried away with the Honda Moly my last tire change, and have been slinging the shit off ever since. Thin little tiger stripes around the left side of the rim. Fast, long, hot weather rides bring it out.

You tell us. Is the "gunk" the consistency of gear oil or of grease?
Hard to tell. Neither? I just went to the garage and checked it. I wiped some off and was certainly wasn't oily. Maybe a little greasy, but there wasn't that much (it's only been 10 miles since I washed it) and it seemed dry due to dust? I'll ride it another 100 miles or so and report back.

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Great technical article and pictures!

Cleared up all my confusion on this subject. And I'm taking the stance of if it ain't broken [or leaking] don't fix it. The whole thing looks pretty failsafe, really, with the internal oiling.

Any of you high mileage guys had a u-joint failure, or changed it as preventative maintenance? That I could see failing; little bearing cups and no realistic way to lube it.

Edit: this article and most of the replies are from April. I wonder how I missed it; musta been still hibernating. Anyhow, a good read.

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Great write up! Wish you had done a year ago. My final drive was leaking from the drain hole on the swing arm. Would leave drops on the garage floor and the wheel was a mess after a ride. I finally had enough and changed the seal you pictured. Since then no more leak and wheel is clean. But you are right seal is hard to pull out. I ended up cutting on the old one to get out. But this bike is designed to be easily worked on which is a plus in my book.