Rolling Drivetrain Noise

[Jandrews]

Car is 1970 911E with '73 2.4S built to RS Specs. 1971 911/01 transmission with 7:31 R&P.

Car runs great...pulls like a train. About a month ago I noticed a "new" drivetrain noise one day after a spirited drive. It seems constant whether under load or not, and continues with or without the clutch in. I have checked torque on lug nuts, as well as axle shaft bolts for tightness and play/slop. None found.

The sound could probably be best described as a bowling ball rolling down a wooden bowling alley. It is not a whine or growl, no squeaking or chirping. Maybe a tad bit "gravelly", but more of the rolling bowling ball...and yeah, maybe a bit gravelly. It is most noticeable at speeds above say 35-40 mph, but never gets really obnoxious. Just a general background noise that sounds a bit out of place. My interior is stripped and I have no sound pad, along with my share of wind noise. The interior noise in this car is substantial, but this sound is new and doesn't sound quite right.

Anyone have any thoughts on what to check first? I know it is hard to diagnose this over the internet, but I tried to give as much detail in my description of the sound as possible. Thoughts?

Thanks,

JA

[Grady Clay]

John,

There are probably two likely choices. Easiest is a real wheel bearing. Does it seem to come more from one side than the other? Does it change at all when you corner left or right? If you have an old garden hose, you can cut it into lengths, zip tie them to the trailing arms with an open end as close to the outboard CV joint as possible. Run the hoses in the rear quarter windows. Have a passenger compare the two by listening. Do the same with long hard cornering to the right. Repeat cornering to the left.

The other likely source are one or both ball bearings in the intermediate plate of the transmission. Usually this failure is load sensitive but I have seen them constant noise. Drain the transmission and inspect the magnet. While a lot of steel “fuzz” isn’t conclusive, it can be an indication.

If you can’t define the problem, an occasional transmission refresh isn’t a bad idea. It will also let you determine if you have the “simplified differential” up-date. A 911/01 trans rebuild is a DIY project we can lead you through.

Best,
Grady

[Jandrews]

Great feedback, Grady. My initial suspicion was a rear wheel bearing, although it does not clearly align with one side or the other. That garden hose stethoscope is a great idea! I have never heard of that before, but I'll bet it would be very useful. My rear wheel bearings are just about the only drivetrain / suspension component that has not been refreshed, and I think it would be worth doing them anyway.

I put new syncros in the 911/01 when I had the motor out a year and half ago or so. I don't believe I have the reinforced intermediate plate, and I probably should have done that when I had it out. I am suspicious that this may be the problem, as the sound seemed to come on somewhat suddenly after/during that spirited drive. Hard acceleration in 1st and 2nd gear must really put the strain on that transmission. My motor is really strong, and I can see where it might be pushing a tired transmission to it's limits. Would this situation create any leaks if this were the problem?

Thanks,

JA

[Paulporsche]

I had what sounds like a similar noise. It turned out to be my ring and pinion gear.

This is an expensive repair, so I hope for your sake. it's a wheel bearing.

[Jandrews]

Thanks Paul. I guess that's certainly possible. When you say it was your Ring and Pinion gear, which one was it, and what did it look like when you pulled it out? This definitely doesn't sound like a R&P bearing, but I guess it could be a rough spot on the ring or pinion gear, I'm just not sure what would cause that. Curious what your specific problem was.


JA

[Paulporsche]

Ha! If you do a search, you will see that this is something very few mechanics will mess with. A couple of guys have been fooling with one recently and it seems like a real biotch.

I didn't open up the tranny. I had it replaced with one that had been recently rebuilt by a dealer for someone else who had passed away before he could use it. There is only one of these in the trans. A search will show you what it looks like, what it should look like, and how difficult it is to set one of these things correctly.

I don't know what causes this. My trans was a rebuild that I put in the car about 70,000 mi ago, so who knows? I always used Swepco in it, but these are old cars with usually multiple owners, so it's too difficult to know when a component like this fails.

JW had a test for this. I'm not 100% sure, if I'm remembering correctly, but I think he said to drive the car @ low speed in a high gear; then accelerate and see if the stick moves forward . If it does, then it's likely the R&P. If you search my name, his name, and R&P you will probably find his response.

The symptoms were a rrrrrrrrrrrrrrrrrrrrrrrrrrr sound at all speeds and gears heard above the normal noises.

[porsche930dude]

sounds like a wheel bearing to me. Sometimes its very hard to tell which side its coming from. Try jacking the car up safely and running it up to the speed that it is normally at and have someone stand outside and listen for the sound. Its really easy to locate it like that

[Jandrews]

That's a good suggestion 930dude... Thanks.


JA

[Grady Clay]

Guys,

The difficulty with diagnosing a failed/failing rear wheel bearing is the lack of load on the bearing. A ’69 and later rear wheel bearing may seem like a simple 2-row ball bearing. It is actually a very special bearing and is HIGHLY loaded.

The vertical loading on the tire (weight of the car) and the side loads (cornering) generate a torque around the center of the bearing. That torque generates reaction forces in the bearing. Since the size of the bearing is small compared to the tire, the forces are proportionally greater.

The bearing is built where the two inner races just touch when the axle is torqued. This (and the ball size) makes a pre-loaded bearing. In failure, the balls and races either have worn where there is no longer pre-load and the wheel can wobble or the hardened race has locally failed (I think this is called “brinelling”) and the bearing gets noisy. In this failure mode the hardened surface of the race starts to pit by the surface-hardened part falling off. This is also the failure mode of the 901/911 transmission intermediate plate ball bearings.



In this post about axle angularity you can see the difference in the bearing spacing between SWB (which almost never fail) and LWB (which regularly fail) and have resulted in a progression of larger bearings. A 930 Turbo even went to the extreme of having two wide-spaced tapered roller bearings.

“Randy,

They did run straight with SWB cars. That is why they could use
such short axles. With the LWB they kept everything in place
except moved the rear wheels back. In order to keep the angle
as low as possible they dramatically lengthened the axle to get
the CVs far apart.



This SWB axle is ‘late ’66-’68 Lobro and the LWB is a 100 mm
friction welded CV from a 911SC. Axles complements of
Eurosport, Ltd.

This, in turn, necessitated narrowing the axle flanges at the
transmission and changing from the well supported SWB
hub-bearing arrangement to the LWB relatively thin 4-point
bearing at the outboard end.

Here is the SWB cross-section. The distance between the
ball bearing and the roller bearing is about 11 cm.
"
"
© 1965 Dr. Ing. h.c. F. Porsche A.G.

Note the universal joint of the Nadella axle.


Here is the LWB cross-section. The distance between the
two rows of ball bearings is only about 2 cm.
“
"
© 1969 Dr. Ing. h.c. F. Porsche A.G.

You almost never see a SWB rear wheel bearing failure other
than lack of grease. On the other hand the ’69-’73 rear wheel
bearing needs to be replaced most often of any. This was the
reason for the change from the 75 mm bearing to the larger
80 mm bearing in ’74 (along with the alu trailing arm.)

It was the ’75 930 Turbo that got the real strength back with
widely spaced dual tapered roller bearings. Porsche could fit
them because of the dramatically wider rear track and thereby
keep the long axle. Another advantage of the Turbo and Turbo
Look 911s.

I think one of the fun technical aspects of the 911 is to watch
the progression as Porsche engineers learned. Of course at
the same time the car became heavier, more powerful and
had better grip through tires and suspension design. Some
times they were ahead of the curve and some times behind.”


Back to the issue at hand.

I have tried unsuccessfully to diagnose rear wheel bearing noise by running the car supported on a lift and the rear suspension supported on jack stands. I’m sure if a bearing is bad enough you can discern the L-R difference with a stethoscope.

Best,
Grady

[Grady Clay]

Here are some better images.

"
"
© Dr. Ing. h.c. F. Porsche K.G.

In the above, there is side force (green arrow) from hard cornering


"
"
© Dr. Ing. h.c. F. Porsche K.G.

In the above, the red arrows indicate the reaction forces at the
bearing. The cornering force (green arrow in the top image)
tries to twist (torque) The bearing (green circular arrow) around
the center of the bearing (green dot).

This tends to unload the outer (left) row of bearings at the
bottom and highly load them at the top. Just the reverse
at the inner (right) row of balls.

If the bearing races and balls are sufficiently worn, the pre-load
is no longer there. In this case an unloaded bearing (no weight
or cornering) is free to spin without making audible noise. It is
only when you have the combined forces of the weight and
cornering does the noise appear.


If the rear wheel bearings are over 100K mi., I would replace
them on general principals. That would also eliminate them
as a noise source.

Best,
Grady

[Jandrews]

That is a VERY comprehensive post, Grady, and sure highlights the inherent weakness in the '69-'73 LWB hub/bearing design. Your detailed post also made me think of something else that would certainly be relevant here, that I unintentionally forgot to mention in my initial post. This is an RSR bodied car, and I am running 17x9's with 275's in the rear. That is almost CERTAIN to exacerbate and accelerate the wear/load on those bearings. I don't know the exact age/mileage of the current bearings, but I have had the car for 9 years and about 20,000 miles, and I have not changed them. Sounds like a no brainer to go ahead with that piece of work. Then, as Grady suggests, I can at least eliminate the real wheel bearings as a noise contributor, and if the sound is still present, move ahead with the 911/01 intermediate plate.

Thank you for that, Grady. Now, time to do some searching for rear wheel bearing replacement threads. Is there anything specific about the '70 chassis/suspension that I need to look for when searching? How about the 911/01? Can I assume if I find a thread talking about changing rear wheel bearings on any car that has the Lobro shafts that mine will be the same?

Thanks again!!!


JA

[Grady Clay]

John,

This might be the time to consider converting to the aluminum trailing arms. They have significantly larger bearings and are lighter. There has been a lot posted on this easy swap.

Best,
Grady

[Jandrews]

Great point! I have considered that before, and now would be the time to do it. Which years will fit?


Thanks,

JA

[911s55]

John; Kind of a Thump Thump noise? 901 trans has 1st gear outside the intermediate plate, depending on how much torque your motor has, and how hard you were standing on it in 1st gear, it could be pulling off the shaft, creating an alignment issue or ? I experienced similar irritating sound last spring, checked axles, wheel play, and it disappeared temporarily after the car was back on the ground. My car is a 71 targa with a 2.7 W/webers and close to 180 ft lbs torque, trans is a 902/16. Hauntingly, sound returned, back into garage an do planned suspension bushing replacements, torsion bars, changed trans fluid, reassembled car, been about 9 months and a couple thousand miles. No bowling ball! Go figure. I also don't accelerate hard from stop anymore.
David

[Jandrews]

Thanks for those thoughts David. Not really any thump, thump going on...more of that harmonic rolling sound. You are right about the 1st gear though, and yes, my car will easily spin those 275's when accelerating hard while rolling in 1st gear. That can't be good for that outboard 1st gear.

Either way, it could be one or both of these things, but I will probably start with the wheel bearings, as they are a maintenance item anyway. That will rule them out if nothing else. Does anyone have a pic of the bearing that goes bad in the intermediate plate in the 901?


JA

[Roger 911]

I'm looking to replace the rear wheel bearings in my '72. This thread has some really useful info. However, it also raises a question for me.

I recently swapped in a '84 3.2 along with the matching 915. I also swapped the entire axle assemblies as well (CV joints, axles, and stub axles) given the difference in the CV joint/output flanges between the '72 and '84. It certainly appeared as though the '84 stub axles were a perfect fit into the '72 hubs. However, after reading this post about the '69-'73 hubs using a 75mm bearing, and the later cars using an 80mm bearing, I'm wondering if I missed something here.

So my question is; Are '84 stub axles compatible with '72 hubs?

Thanks.
Roger