Failed XS750 Final Drive (Images and video link)

[3Phase]

I finished the autopsy on the XS750 final drive this afternoon: the large right-side ring gear carrier bearing failed and the ring and pinion gears are worn and wiped out.

After the final drive autopsy I took apart the original XS1100 final drive that was on my bike when I bought it so I could compare the two final drives. I made a Youtube video of the bad and good final drive side bearings and of course I took a lot of pictures.

On to the video and pictures! Click any image and it will load the large version:-


A bad XS750 final drive side bearing and a good XS1100 side bearing




XS750 and XS1100 final drives.

750 on the left, 1100 on the right. The 750 final drive ring gear is not as sturdy as the XS1100 ring gear and it does not have an axle tube support.





XS750 and XS1100 ring and pinion gears.

The 750 is on the left, 1100 on the right. The teeth on the 750 ring and piniongears are a lot thinner than the teeth on the XS1100 gears.





XS750 and XS1100 ring gear. The 750 ring gear is much smaller and thinner than the XS1100 ring gear.





The inner side of the XS1100 ring gear.

The ring gear and its inner bearing hub are both larger than an XS750 ring gear.

The wear pattern on the inner hub race shows that it is still in good condiftion.




The inner side of the XS750 ring gear.

The ring gear and its inner bearing hub are both smaller than an XS1100 ring gear.

The wear pattern and score marks on the inner hub race show that it is worn out and has begun to fail.





Gear tooth face wear on the drive side of the XS750 ring gear.

The contact pattern shows smearing and heavy wear with some chattering.

The chattering was caused by a failed side carrier bearing.

The smearing and heavy wear is caused by the ring gear flexing away from the pinion gear under loads it was not designed to handle.





Gear tooth face wear on the drive side of the XS1100 ring gear.

The contact pattern shows almost no wear and normal contact between the ring and pinion gears.





Gear tooth face wear on the coast side of the XS1100 ring gear.

The contact pattern shows almost no wear and normal contact between the ring and pinion gears.





Gear tooth face wear on the coast side of the XS750 ring gear.

The contact pattern shows smearing and heavy wear with some chattering.

The chattering was caused by a failed side carrier bearing.

The smearing and heavy wear is caused by the ring gear flexing away from the pinion gear under loads it was not designed to handle.





XS1100 pinion shaft, inner pinion bearing race and the pinion gear.

The XS1100 pinion shaft has teeth to hold the inner pinion bearing race.





XS750 pinion shaft, inner pinion bearing race and the pinion gear.

The XS750 pinion shaft has no teeth to hold the inner pinion bearing race.





XS750 final drive housing.

Apparently the gear oil overheated and blackened the inside of the housing.





XS1100 final drive housing.

The final drive has not been overheated and the inside of the housing and the gear oil are normal.

[latexeses]

Would the diagnosis be that the ring flexed away from the pinion under load? "Fluttered" might be a definition?
Or do you think the bearing let the pinion pull away?

Or, Torque x less pinion material = flex...

The difference in the ring gear size is surprising. Bummer

Great pics. Sorry they're yours.

[latexeses]

just watched the video. Failure due to bad lube? Makes me want to rethink the 850 drive.
Looks like either of them should be able to handle the loads on the bike though.

[crazy steve]

Thanks Scott for the interesting post mortem...

Looking at that, I checked the bearing numbers between the 750/850 drives and the 1100. They share three of the five 'main' bearings, but the 'big' ring gear bearing that failed ain't one of them. It's pretty obvious (IMO) that the 750/850 FD is a lighter-duty piece compared to the 1100 FD. Maybe OK for a rider with a more sedate riding style, but not so good for more 'spirited' riding...

I guess the big question would be, which came first? The failed lighter bearing, which allowed the deflection? Or deflection caused by the less-beefy ring gear, which caused the bearing to fail...

[3Phase]

Failure due to bad lube?

Nah, failure due to XSessive abuse -- I've been trying to kill this thing since I put it on the bike.

The pinion assembly is way too tight again. I haven't taken apart the pinion assembly yet but it looks like there is a bunch of debris packed and baked into it. I'm pretty sure that made the pinion bearings overheat and that's what burned the oil. It just started doing this in the last 8,000 miles or so and I'm not sure yet why it happened but I will figure it out before I drop in another 750/850 final drive.

It takes more force at the tooth face to turn the 750 ring gear than it does to turn the 1100 ring gear. It's all about leverage so the ring and pinion gears wore out because the 750 ring and pinion is smaller, lighter, and thinner. It doesn't have the axle tube support brace so the ring gear moves away from the pinion gear under load. The gear teeth look like they were flexing, too, but at least they didn't break or chip.

The flexing wiped out the tops of the teeth on the ring and pinion and eventually took out the right side ball bearing, the left side needle bearing, and it left ripple marks on the tips of the gear teeth where there isn't normally supposed to be any contact. The teeth are too thin and there's not enough root.

Oh, the XS11 pinion gear will not fit the XS750 ring gear. The gears would have to be recut or made from scratch so there'll be no 3.2:1 hybrid final drive ratio for Columbo.

[3Phase]

I guess the big question would be, which came first? The failed lighter bearing, which allowed the deflection? Or deflection caused by the less-beefy ring gear, which caused the bearing to fail...

Steve, it looks like both.

The gears and teeth are too thin to handle the load and they flex.

The ball bearing and needle bearing make a sort Bermuda triangle. When the ball bearing at the base of the triangle starts to wear it makes the ring gear hub/race at the top of the triangle move around inside the needle bearing rollers. That's 100% fatal to the rollers and races because they're not made to take loads like that and parts of the final drive begin to disappear.

The same thing happens to the rear wheel bearings: right side ball bearing wears out and starts to allow the axle and left-hand inner bearing race to wobble around in the unobtanium left side bearing rollers. Next thing you know you need both rear wheel bearings.

[WMarshy]

Scott,

Any chance higher load rated bearing will fit in the 750/850 envelope? Whats your next step? Tooth wear and backlash will still remain a potential problem but Ive seen a lot worse; with stronger bearings it might last...another 8000 miles.

Problem is, once you figure out the winnning combination you could have probably bought a chain drive unit and chose any ratio you want and have the ability to run a larger tire and get rid of the backlash effect!...

OK, doubtfull... they are expensive. Thanks for the write up!

[BA80]

There is no difference in the way these gears work than the gears in an automotive differential. Just a smaller scale.

As far as the ring gear being thinner, It HAS to be thinner to accomodate the larger pinion with the taller gear. You can't make the housing WIDER.

The same is true of the thinner teeth on the ring gear. Since there will be MORE teeth on the larger pinion there has to be MORE teeth on the 750/850 ring gear of the same diameter as the 1100 ring gear.

A properly lubricated bearing will not fail. It looks to me that that bearing overheated and failed do to lack of, or improper lubrication. If that bearing failed due to stress, why are all the others (subjected to the same stresses I might add) appearing to be OK?

The galling patterns on that failed gear appears to be due to improper pinion depth adjustment. The heal toe contact of the teeth is ALL WRONG.

I believe your analisis is all wrong here Steve. I believe you will need a few more than 1 control subject before you start GUESSING at causes.

Nice pictoral though.

IMHO

[BA80]

Sorry, I meant Scott.....

[WMarshy]

It would be interesting to see a side by side comparrison of the load ratings between the two FD's bearings. No guessing necessary then, one would know exactly how much more load the XS11 FD can handle over the 750/850. Although I wont go to the same extreem Greg is, I do tend to agree with the intent of his post. No telling what kind of abuse your 750 FD was put through before you aquired it. Maybe the PO never changed the oil and contributed to its premature failure...

[James England]

That's an excellent pictorial presentation. The only thing is, what state were each of the drives in before the swap? To be able to to be sure it's due to "loads it was not designed to handle", I suppose one really ought to start out with two drives from new and compare them after X number of miles. It may possibly be the case that the 750FD was worn like that before.... or did you check it before putting it on the 1100? I bought an FD and haven't a clue how it's been treated in the past, including lube type and quantity, without stripping it first. I just put it on the bike. If it fails, I'd be wrong to say it's failed because of loads it's not meant to handle. It might be the case, but it could have been an abused FD before I bought it. See what I mean? Good as the presentation is, it's not definitive proof of much unless you checked the 750FD before installation. Did you?

[3Phase]

Marshy:

I'll have to pull the drives apart again to measure the ID/OD and width of the right side carrier bearings. I put everything back together to keep out dirt and to keep from stinking up the garage and the house with eau d' gear oil but I'm pretty sure the OD is the same on both bearings. I think I swapped the ring gears with their carrier bearings between the final drive housings yesterday but I don't remember now so I'll have to recheck that.

I did clean up the pinion assembly this morning. It turns easily again by hand and the pinion bearing preload is still okay. Nothing changed significantly, physically or mechanically, in the pinion assembly after I set it 40,000 miles ago and that is a real load off of my mind. The pinion assembly was just gummed up so it was hard to turn until I cleaned and oiled it.

Greg, James, et al:

The XS1100 final drive parts in the pictures are from the original final drive that was on my bike for ~39,000 miles before the 750 mod.

Yes, I did make, like, maybe, one or two hard launches and more than a few double, even triple, downshifts with full bags and camping gear to pass cars and have fun on twisty mountain roads. I believe the 750 final drive actually held up rather well considering everything I did to try and make it fail!

When I first got it I quite literally had to go over every part of the 750 drive with calipers, a dial indicator, a bright light and a large magnifying glass looking for bearing and gear damage or runout.

The small amount of oil that was left in the 750 final drive housing after shipping looked good. There was a normal amount of fine metal on the magnetic drain plug but very few flakes and sparkles in the oil or inside the housing. The only problem was that the pinion bearing preload was too tight when I got the drive; way too tight and the coupler almost could not be turned by hand. The original crush washer was more or less crushed so it had to be replaced but the drive had not been ridden like that or I'd have seen the damage when I inspected and set up the drive.

I bought a new crush washer from Yamaha and reset the pinion bearing preload but I did (very) briefly consider having the parts magnafluxed and replacing the bearings, races and seals. That would have been total overkill as it would have cost more than just buying two or three more used 750/850 final drives.

After inspection I painted the gears and made sure the drive was set up correctly: ring and pinion depth/contact pattern, lash, and all bearing preloads were within factory spec. The final drive was in great shape or it would never have been put on my bike. Fullstop.

To make sure it was still running correctly I always checked the final drive every time I changed the rear tire when I greased the splines. Remember: I was trying to destroy the 750 final drive so I was watching closely for any sign of failure.

For the soft Lasertec tires I was using for a while that was about every 5,000 miles or less and I change the oil in the final and middle drives every other tire change so they both got fresh oil about every 8,000 to 10,000 miles. I also changed the oil before and after any long ride like the one I did last summer up to Canada and over to Colorado; this summer was over to Colorado then up to Washington. Both trips were a little over 4,000 miles each with about 1,000 miles of rain during the trip last summer but nowhere near as much rain this summer.

[James England]

When I first got it I quite literally had to go over every part of the 750 drive with calipers, a dial indicator, a bright light and a large magnifying glass looking for bearing and gear damage or runout.

And, presumably, it wasn't like it is in the pix at that point. or you'd never have fitted it. Looking at the 750FD, that looks like a hell of a lot of wear. I bit disturbing........ How many miles has the 750FD done?

[crazy steve]

...I believe your analysis is all wrong here Steve. I believe you will need a few more than 1 control subject before you start GUESSING at causes.

Thanks for correctly assigning 'credit'....

But I have to admit that I agree with Scott. Yamaha went to the trouble and expense of redesigning the 750 drive for the 1100, putting in stronger bearings in the two places they would be needed to prevent this sort of issue and beefing up some other areas as well. This would make the rumor that Yamaha originally was going to use the 750 drive plausible, but the change IMO was made not so much in search of 1/4 miles times but due to durability issues found in pre-production testing. After all, this was their flagship performance model and you know they figured that many owners would beat the hell out of the bike. Get even a few reports of failed FDs, and the bike's reputation would have gone in the toilet...

Does that mean that every swap will fail? No, of course not. But I'd be willing to bet that the factory found that a certain percentage would fail, leading to the 'upgraded' unit that ended up in the production bikes.

[DAVINCI]

The final drive was in great shape or it would never have been put on my bike.

Remember: I was trying to destroy the 750 final drive so I was watching closely for any sign of failure.......

........and I change the oil in the final and middle drives every other tire change so they both got fresh oil about every 8,000 to 10,000 miles.

You're not making sense. What's the point of risking your life trying to destroy parts? It's either bull**** or sheer stupidity. Then you contradict yourself. If you WERE trying to destroy it you wouldn't have treated it so well.