Phase
My 1993 GSPD has taken me to school on this issue.
About 20 months ago, preparing to ride to Alaska and Prudhoe Bay, I swapped the driveshaft with one that had fresh u-joints. I pulled the still seemingly good driveshaft out of bike when I installed the rebuilt unit. The bearings on the old shaft's u-joint felt nice and tight.
I made it through the last two summers without problems on the rebuilt shaft.
A few weeks ago, the bike had that "clunk," and I pulled the driveshaft out. The front u-joint had shed it's bearings.
I inspected my old driveshaft and the one I had just taken out of the bike. I put them each in a vise and checked to see if the the front and rear u-joints were "in phase," meaning that the u-joint's "U's" that are fastened to the shaft body/rubber damper assembly, are parallel. I put small straight-edged rulers on these pieces and a visual inspection showed BOTH shafts has their u-joints about 7 degrees out of phase.
U-joints, unlike CONSTANT VELOCITY joints, which have their input & output sides running at a constant velocity with each other regardless of angle (thus the name), have small accelerations/deaccelerations that increase with the angle of the bend. Unlike the u-joints you find on rear wheel drive cars, the Airhead Paralever u-joints run at a rather extreme angle, thus greater accelerations/deaccelerations.
Having the u-joints in phase, or in alignment, minimizes the affects of these accelerations/deaccelerations. Likewise, an out-of-phase shaft has the u-joints fighting each other, and beating that poor damper like a red-headed stepchild.
The natural rubber damper breaks down from the added stress and the shaft twists even more, pulling it more out of phase, which adds more of the hammering stress, which then pulls it even more out of phase...a cascade effect, mind you.
I am of the belief that simply replacing the u-joint bearings and not addressing the phase issue is throwing money after bad.
OPTIONS:
NEW DRIVESHAFT--expensive, but will give you a servicable shaft for, what, 20,000 to 40,000 miles? At this point, a chain drive begins to sound real good.
SOLID SHAFT--but if the BMW bean counters at the time were willing to risk catastrophic transmission failure by leaving out a $.85 circlip, why use the expensive rubber-dampened driveshaft assembly if a simple piece of metal rod would work as a drive shaft? That rubber damper has a purpose, perhaps to protect the final drive or transmission. Heck, my Honda 90 has a rubber "cush hub" between the rear sprocket and the hub. We're dealing with about 10 times the power with the Airhead driveline from my 90. I am skeptical of the long range utility of this solution. Anybody have any real data or experience on this?
REPLACE THE DAMPER--this is the solution I am pursuing. Guy Henderson (209-962-7500
guy@hendersenprecision.com) pulls out the worn natural rubber piece and replaces it with one made of urethane rubber of the same durometer, which, according to Guy, should stay in phase about 3 times longer than the stock, perhaps 90,000 miles? Cost, currently, is $175 US.
SIGH: BMW= Bring My Wallet
