Musings on final drive concerns

I’ve done quite a bit of thinking and research on the matter of BMW final drive (FD) failure(s).  There is much fragmentary documentation and discussion already on the ‘net  . . . some is factual, researched and engineering pertinent.  A smaller amount is fanboy emotional and ignoring of evidence.  Let’s address some ‘knowns.’  First, what is a failure?

Yours will be in a failing state long before it looks like this. Look for flecks of hydraulic fluid on the rim and/or seepage of hydraulic fluid from the boot. You or your mechanic should be inspecting this at about every 6K or no worse than 12K miles.

I’ll define “failure” as a defect (either in parts manufacture or assembly) that compromises final drive operation in a time frame considerably shorter than its prescribed lifetime.  I’ll define the final drive unit as the rear ‘hub’ that your wheel attaches to.  If you own a Paralever BMW motorcycle, you have this type of hub.  Inside your hub, the ‘final drive’ is composed of gears, hydraulic fluid, bearings and seals.  Most final drive failures point to the rear drive unit (RDU) was assembled with too much preload or defective bearings.  Two indicators that the FD crown wheel bearings are going bad are shiny metal flakes in the rear drive hydraulic fluid and roughness felt during wheel rotation.   In some cases, some of our fellow beemer brethren, have had a replacement final drive fail a second time, due to the same improperly shimmed crown gear.  Meaning, there are some current BMW riders with 50K-80K miles on their bikes that are on their THIRD final drive.  Many riders can go 100K-150K without a final drive problem, yet there is a sizable enough population of owners who have either spent sizable coin or had warranty replacements of final drives.  BMW’s secretive data-gathering and non-committal as to the scope of the problem with US dealers, compounds issues too.

I think there is a separate but related maintenance issue.  A competent dealer (or independent) mechanic will wiggle your rear wheel at ‘3 and 9’ and ‘6 and 12’ (think of a clock) to determine how much wheel movement there is.  There is not much wiggle or deflection allowed.  In fact, more than a millimeter of side to side or up and down movement is cause for concern.  It means one’s bearings are out of operating spec and will likely and progressively get worse.  Depending on the bearings and if you’re lucky, they might be removed, readjusted and simply reinstalled.  If not, they will have to be replaced.  The mechanic will also loosen and retract the plastic boot.  And hopefully, no fluid leaks out.  In my case, a pool of hydraulic fluid spilled out, covering about a 2×4 inch area.  A seal was failing/had failed.  Seal leaks can come from the transmission output seal leaking gearbox oil down the inside of the swing arm and out into the boot. The FD can also leak oil out past its input seal, sometimes called the pinion seal.  Fixing either of these is relatively straight forward.  If, however, the FD is leaking oil past the “big bearing” seal located where the FD mates with the real wheel (under the rear disc brake), you may have a third seal replacement to contend with.

Regarding the ‘rear wheel wiggle issue,’ I’ve owned two R1100R bikes (not RTs or heavy GS’s) and both needed replacing of final drive related bearings.  In the most recent case, it was the pivot pin bearings.  (I can’t recall the prior R1100R from some years previous . . but would bet, pivot pin bearings also.)  In the earlier case, those bearings were shot by 18K miles (which I attributed to the crummy Washington DC area roads) and in the most recent case, at just under 39K miles, where the bike has been in FL for the last 20K or more miles.  On my R, those were road miles with no passenger hauling, nor off-roading or commuting on salty winter roads.  That’s not much mileage in either case to suffer bearing failure.

Those Paralever owners who start to see ‘flecks of hydraulic fluid’ on their rear wheel rim or notice a small inconsequential seepage under the plastic boot . . . that is your bike’s way of saying, “there are issues you need to deal with fairly soon.”  Lastly, if these signs are ignored, are you on the path to a final drive failure?

I think . . . probably.  In summation, two issues-  there’s a small but known amount of final drive units with improperly shimmed crown bearings that have found their way onto BMW Paralever motorcycles over the last 15 or more years.  I’ve heard “around 4%” of units fail. Secondly . . . improper or ignored maintenance can lead to a final drive failure too.  Most units make it into 6 digit amounts of miles . . . . but maybe replacing a set or two of bearings and seals along the way is par for the course.  Is the Paralever design an engineering weakness?

Hard to say . . . . our friends in Munich would say, (for the bike’s first 36K miles and maybe 48 months) you have bought unique and superior motorcycle engineering that is capable of 175K miles or more, but requires you to conduct diligent and ongoing maintenance all along the way.  In my case, I’ve paid a $1,000 on two bikes to replace FD bearings and seals.  (In neither case, would one say I had a FD failure.)  Would I have paid similar repair costs on a couple of Japanese bikes?  (Chains, sprockets?)  I doubt it.

To close, one thing I’ve yet to see, anywhere . . . is how rear suspension (strut/shock) functionality works with the Paralever suspension.  One would think a rear strut on its final legs, would not meet preferred Paralever functioning requirements.  But I’ll leave that issue for another day.

For further research:

Sept 2008 issue of Motorcycle Consumer News, Bill Shaw, writer, p. 32

http://www.largiader.com/paralever/

http://bmwra.org/otl/final-drives

http://www.bmwlt.com/forums/bike-talk/41805-final-drive-failure-bmw-nov-2008-a.html

http://advrider.com/index.php?threads/nhsta-opens-investigation-into-bmw-final-drive-failures.761531/

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