PHILES’ FORUM

“He did not think. He knew.”

Hello, Bimmerphiles! I had an interesting diagnostic case in the shop last week; a case that reminded me of the so-called Scopes Monkey Trial.

For those of us who were not in Dayton, Tennessee in 1925 and missed the trial (and the books and movie [Inherit the Wind] it spawned), 24-year-old high school teacher John Scopes was charged with violating the Tennessee law that proscribed the teaching of evolution as a possible explanation for the origin of humankind. In that day, when women had only recently won (seized? Brava!) the right to vote, it was mankind. The so-called, “Trial of the Century” pitted devout former Congressman and orator William Jennings Bryan, for the prosecution, against famous defense attorney Clarence Darrow.

Despite growing evidence that evolution was at least a viable theory as to the origin of humankind, Bryan just knew that the Biblical representation of creation had to be literally true and that it was simply preposterous even to consider that humans could be the descendants of monkeys. What nerve, those evolutionists! Bryan was also quoted as stating something to the effect that he did not want to be confused by facts. This all made more sense to me when I learned that he had been a three-time candidate for President of the United States. I think it is safe to say that Darrow did not take Bryan as seriously as Bryan took himself. (Unfortunately, in the aftermath of the trial, Mr. Bryan passed away suddenly at the age of 65.)

Regardless, Scopes was found guilty and fined $100. He had, after all, broken the law – a fact stipulated by the defense at the outset. The conviction was later overturned by the Tennessee Supreme Court; some say because the justices did not want the case heard by the U.S. Supreme Court. I cannot say that I blame them…

Fast forward 95 years to the Philes’ Forum shop. I was trying to diagnose a low-speed shudder in an E30. (Before you say to yourself, “Self, he’s segueing into another dang E30 article!”, note that the following applies to just about any Bimmer.) The shudder would only manifest itself during low speed, high driveline-torque situations – in first and second gears. As soon as the car speed increased a bit, the shudder would subside. It was most noticeable when starting from rest. Under very-light-throttle conditions, there seemed to be no shudder at all. Having worked on BMWs for forty-something years, I just knew that the problem had to be something wrong with the driveshaft. (On any Bimmer, the first things to check are engine mounts, transmission mounts, differential mount and subframe bushings are all new or nearly new).

The majority of BMW automobiles use a two-piece rear driveshaft (see Photo #1) that connects the transmission to the differential. The driveshaft attaches to the transmission with a flexible coupling (AKA Guibo), and the driveshaft midpoint is suspended by a center bearing that attaches to the car’s unibodychassis. Typical failure points of a driveshaft are:

• the flexible coupling

• the center bearing or its “rubber” supporting membrane

• either of the two universal joints

A failing Guibo is associated with highway-speed vibrations, while the center bearing and universal joints are known for low-speed shuddering. There are of course exceptions to these guidelines.

So, despite the fact that the E30 in question had been fitted with a new OE driveshaft, Guibo, and center bearing from BMW only about 80,000 miles ago, I removed the driveshaft. I just knew that I would find the problem there.

Getting the driveshaft out took me about an hour: first I lowered the exhaust system and removed its heat shield to expose the driveshaft. Then, after removing the two bolts securing the center bearing to the car’s unibody, I removed the transmission support and lowered the back of the trans as far as it would go. The center bearing membrane looked just fine, thank you very much, as did the Guibo. So I just knew I had a bad universal joint. Seven more fasteners (3 Guibo, 4 rear flange) and the shaft would be out and I would be vindicated.

Well, that driveshaft had some nerve! After removing it I found the universal joints and center bearing to be absolutely perfect: no tight spots, no looseness, niente! I mean, they were perfetto! At this point I decided to call it a day and think about things.

The next day, after having dreamt about driveshafts all night, I reengaged the enemy with a rested pair of eyes and from a somewhat humbler perspective. With the parking brake firmly applied, I applied torque to the differential input via the pinion nut and checked carefully for any lash in the differential and the half-shafts’ CV joints, all of which are original on this 35-year-old Bimmer with about 170,000 miles. No problems found. The recently replaced wheel bearings seemed fine as well. Then, grasping at straws, I rechecked the torque of the wheel bolts. Again – NPF.

Finally, after thinking that I had checked every component between the transmission and the road, I realized that I had not carefully inspected the tires (other than checking their pressures). But wait, even considering that tires could cause a low-speed shudder (with no other symptoms) seemed preposterous.

Well, the left rear tire had a moderately cupped inboard tread (see Photo #2), but it was just inconceivable to me that this could be the problem. However, installing a known good pair of tires on the rear cured the shudder. And that fact even William Jennings Bryan, were he alive today, could not dispute.

“He did not think. He knew.” – Clarence Darrow, in describing Bryan.

That’s all for now, Bimmerphiles. See you next time!

Anyone wishing to contribute to Philes’ Forum can contact me at [email protected] I’m interested in tech tips, repair/maintenance questions, repair horror stories, emissions-inspection sagas, product evaluations, etc. Copyright 2021; V.M. Lucariello, P.E.

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