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By Geza-aka-Zombo
#95331
Anyone following this? Brown seems to be convinced that the torsion stress in the driveshaft is opposite if the shaft is assembled in the TT backwards. Several people are trying to straighten him out, but he's too obtuse. As Lt. Col. Frank Slade would say: "We've got a moron here". I think a clear description of what's happening is as follows:

Picture a wrench on the engine side of the driveshaft applying torque in a CW direction (like the engine would), and a wrench on the trans side countering this torque in the CCW direction (when viewed from the engine side), like the transaxle would. Now, while the torque is still applied, move your vantage point to look at the driveshaft from the trans side. Bingo - swapping the side of the driveshaft which is on the engine side does not change the direction of the torque the driveshaft sees. So, the answer to his question is his driveshaft, if in good condition can be reused without issue.

He mentions torsion bars and driveshafts (half shafts) - That these should not be swapped side to side, which I can agree with. In both cases, swapping these does change the direction of the torsion, which impacts fatigue life. If you have a torsion bar, which has been deflecting from 0 to ~20 degrees CW it's entire life (as the suspension moves), then start deflecting it 0 to ~20 degrees CCW, because you moved it to the other side of the car, it's going to reach its fatigue life at a much accelerated rate. So, don't do that.

Summary: depending upon which side of the driveshaft is attached to the engine, the rotation direction of the shaft will be opposite (one side vs. the other), but the stress direction will be the same, either way.
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By Ed Scherer
#95615
I couldn't take it any more. I had to add a video to that thread. Hope the visualization will put an end to that.

Plus, it validates my decision to keep this little piece of scrap stranded aluminum wire from a project I did a few months ago. :smile:

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By worf
#96286
Geza-aka-Zombo wrote: Fri Aug 06, 2021 8:51 am Looks to be doubling down on this one....
I am so glad I haven’t been back to that thread since I posted the kitty.
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By Ed Scherer
#96300
It's really weird. I find it to be an interesting study in stubbornness. I guess the more you're convinced that you're right and have punctuated your analysis with a "Period.", the harder it becomes to reevaluate your position. Pride?

I thought I offered a repeatable, compelling, simple demonstration. I guess it wasn't good enough. :confused:
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By amdavid
#96327
Ed Scherer wrote: Fri Aug 06, 2021 10:21 am It's really weird. I find it to be an interesting study in stubbornness. I guess the more you're convinced that you're right and have punctuated your analysis with a "Period.", the harder it becomes to reevaluate your position. Pride?
I thought I offered a repeatable, compelling, simple demonstration. I guess it wasn't good enough
. :confused:
You did, even I understood. :grin: You were good enough, must remember......you can't fix stupid. The best thing for you, and the worst thing for him is to let it go and not respond or engage.
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By worf
#96337
Image
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By Sazerac
#96516
I think we have to keep in mind that GB is not an engineer, but he profits from a huge amount of empirical evidence of seeing torn down 928 cars. I have a lot of his stuff, and some is quite good. But, seeing him argue in those threads is sometimes exasperating.

Some people have to just do it, see the results over and over again, and then be confirmed. Of course, mankind would have never made it to the moon like that, but it's a legitimate way of learning in some places. And, even in engineering, testing is the final verification...
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By Geza-aka-Zombo
#96529
I suspect he is too intrenched in his thinking that the TT drive shaft is operating the same as a torsion bar or half shaft. He's correct on the later two as the torque is reversed when swapped side to side. Just not in the drive shaft case.
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By Ed Scherer
#96632
OK guys, I'm going to blame it on you.

Why, oh why, weren't you more insistent that I keep a safe distance from that thread? Those were rather wimpy warnings.

That said, I probably should have listened.

Sometimes, I just can't turn away from a challenge.

The analytical side of me can't help to try to figure out the way some people are wired and see if I can adapt and be constructive when interacting with somebody completely different than I am. I failed this time. :rolleyes:

I hope some good came of it. Other participants and other onlookers might have had some mental exercise. Entertainment value probably dropped to zero, too, and it's just gotten depressing.
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By Constantine
#96636
Well the thread was somewhat enjoyable for me since I was not the person Greg was locked into mortal combat with which let me focus on the great explanations given about the operation of the drive shaft when the ends are swapped. Although this task was a simple one, the pupil wasn't cooperating.

I am positive Greg also get's it, but at this point he is too dug in with his prior posts on that thread to acquiesce his position.

I do give an A+ for all the efforts on Greg's behalf however.

Cheers.
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By Ed Scherer
#96638
He just tossed in a weird distraction that might be (but probably isn't; I wasn't prepared to make a snap decision) be significant. Probably just a deflection. I tried to be gracious in exploring it, but then one more insult and I'm just done. :grimacing:

I like periodically seeing just how patient I can be (I deal with lots of situations, including working with people with dementia, where patience is a real virtue), but I've got my limits. And time constraints.
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By Ed Scherer
#96745
The thread is now closed. The O.P. put it out of its misery. Just as well.

Fascinating experience for a multitude of reasons.

The final post is actually a fitting finale. Don't miss it!

I'll be wearing my bemused face for yet another evening.
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By worf
#96752
Ed Scherer wrote: Sun Aug 08, 2021 6:41 pm The final post is actually a fitting finale. Don't miss it!
I know I’m going to regret this…
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By Ed Scherer
#96765
rbrtmchl wrote:I posted a couple of pictures in that thread, hoping to shed some light. Darkness remained...

@Ed Scherer - I thought your posts were excellent and on point.
Thanks! And ditto. Your diagrams were great!

It became obvious that — while the request was made for input from engineers — all analysis and citations were being rejected without even being considered or reviewed.

There's so much revealed in that final post (it's actually a rather nice summary of misunderstood points, failed deflection, projection, and a couple of final slams of others for good measure) that I hope it's preserved for eternity. I especially liked the abandonment of his "more torsion near the drive end" assertion when that wasn't working out, declaring the point moot, and returning to the real issue he was asking about before that deflection. And a few posts earlier, "I get it...people who are wrong don't like to be wrong and will grasp at any excuse to 'stomp out of the meeting'," and then shortly thereafter closing the thread. Don't stomp out of the meeting, blow it up! Priceless. Just priceless.

I guess none of us got the Hints that he had been dropping. Everyone else (especially those damn engineers whose analysis he solicited) was too stupid or incompetent!



I wonder if there is a site that gathers links to "train wreck" threads so everyone can share in the carnage. Hmmm.

----
"There's enough material there for an entire conference!"
Dr. Abbott referring to Basil Fawlty in Fawlty Towers episode "The Psychiatrist"
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By Geza-aka-Zombo
#96774
@Ed Scherer
The analysis you and a few others provided was basically correct. Brown is pretty much wrong on all accounts, and rude to boot. He obviously has no knowledge of the principles involved, and I suspect his structural engineer contacts would set him straight if brought into the discussion.
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By Constantine
#96779
SeanR wrote: Sun Aug 08, 2021 1:41 pm Greg was asking for this pic, feel free to copy/paste it over there. Came out of my wifes '90.

image.png

image.png
Hi Sean,

At the time Greg asked for this picture he probably would not have liked it since the picture shows the transmission end of the 28mm drive shaft which does not support his statement about the 28mm shear problem always happening at the front.

However all this is now moot.

Cheers.
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By Ed Scherer
#96794
It was pretty damn funny when (see post #121) Kiln_Red discovered and posted a link to a thread I started back in 2001 with my TT shaft... shattered in front of the splines on the rear end.

Being somewhat of a gentleman on my better days (e.g., Sunday), I said nothing. :beerchug:

Hmmm... I wonder what I ever did with that shard of metal. It's around here somewhere.

Image
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By worf
#96796
I didn't follow that thread after I posted the kitty. I did read the last message. My feelings on the general topic (not limited to the TOS thread that is the subject of this thread) are summed in the 'needle point' post above.

That written...

I don't recall hearing, reading, and I've never seen with my own eyes, that 928 drive shafts have a propensity to break at the front. I have seen, all too often and with my own eyes, driveshafts in automatics breaking off at the rear coupler just like in Sean's picture above. I have yet to see a 5-speed snap a shaft.

I always assumed that a loose pinch bolt was much more to blame than fatigue or the neck-down on the later shafts for the automatics breaking at the rear.
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By worf
#96797
Here's my "first" from 2010:

Image

Edit: That -^ was "extra-fatigued" from being driven around for thousands of miles with the vacuum to the shift modulator disconnected.
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By MattiasH
#96821
Hi guys,
The ending of the thread "over there" was so funny, so I had to register myself here.
Interesting that he still insist on that the twist gets reversed after more or less everyone telling him the opposite.
This explains the thread, Greg being the horse. :deadhorse:
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By SeanR
#96824
I've had 3 shafts snap at the rear and found it odd that he said it was only at the front. Never seen that befo. I generally don't go over to RList but found that thread to be entertaining.
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By Mrmerlin
#96833
Im gonna guess that the dynamic of the shaft shearing is that the torque converter places significant loading on the shaft morso than the drive side makes.
So the shaft sees the most deflection at the TC.
Think of a whip
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By Ed Scherer
#96854
(Note that much of this is probably just an expanded version of what Stan just wrote.)

Since I know there are some fellow engineers here If you don't mind a few more thoughts from "an armchair engineer" (admittedly, educated in EE and CS, not ME and about 40 years since my classes in statics and dynamics :smile:) and a few of you with obvious interest in shaft failures, I'd offer the following for your consideration.

I believe the drive shaft physics (and consequently, design and engineering challenges for implementation and failure analysis) is way more complicated than anything being discussed in that train wreck thread.

While static analysis at a very coarse level would indicate that swapping the ends of a shaft is perfectly OK (torsional twist being distributed linearly along the length of the shaft and any "memory-based" fatigue thus equally distributed), I suspect that that the state of the ends of a used shaft may very well be different if the ends have been seeing different shock loads. Remember that the shaft is a spring. If one end receives abrupt changes in torsional loads, those will propagate to the other end, but as a function of time. This becomes a much more complicated modeling problem involving time, frequency, etc. I'm tempted to dig into it, but I just don't have the time.

My hypothesis regarding the propensity for the shafts to break at the rear for AT cars is that the dynamic torsional loading changes more abruptly at that end than at the front. The shaft is a low-pass filter, and those shock loads will be reduced by the time they reach the other end of the shaft.

What's at the front? An engine that produces torque of slowly changing magnitude.

What's at the back? A TC and gearbox that (upon gear changes) produces rapid shock loads, i.e., changes in torque magnitude (although my initial guess would be that the TC would filter those out to a large extent).

This doesn't by any means support the claim (that all here seem to agree is incorrect) that (as MattiasH paraphrased it: "he still insists on that the twist gets reversed"), but I believe it does raise the possibility and indeed likelihood that the condition of torsion-induced fatigue present in a used shaft varies from end to end.

In summary, I'd expect that:
  • the end of the shaft that has seen the most dynamic changes in torsional loading changes would be most fatigued;
  • reversing the shaft would certainly not change the direction of most (i.e., there will still be periods of reversal of torsional loading if using engine braking) static loading;
  • upon reversing the shaft, you may very well extend its lifetime, as you've stressed one end and now have a less stressed end to attack :smile:;
  • engine braking probably contributes significantly to stress fatigue on shafts (IIRC, this has already been discussed on various occasions), as it it a complete reversal of torsional loading of the shaft
What do you think?

Contrary opinions are perfectly welcome and I assure you I won't be offended, question your competency, or close the thread if you point out any flawed reasoning. :smile:
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By Geza-aka-Zombo
#96863
Now that that is over, perhaps we can set the record straight and clear up some of the misinformation as relates to the 928 TT drive shaft and summarize the accurate findings:

• Swapping the TT drive shaft end to end does not change the direction of the torsion forces in the drive shaft. Since the forces are the same, so are the stresses. However, swapping the shaft end to end does reverse the direction of rotation of the shaft.

• In all cases I can envision, relocating a torsion bar or half shaft from side to side does change the direction of the torsion forces/stresses.

• The torsion stress within the constant cross section portion of a drive shaft is the same anywhere along the length. The stress in the shaft ends is higher due to factors such as reduced cross section, and stress risers, like the addition of splines and the shaft diameter neck down (28 to 25mm).

• The twist angle of the shaft is directly related to the shaft length. Half way down the shaft length, the twist angle is half the total. The non-fixed end (free end) of the shaft twists the most because it is an additive condition; if you break the shaft length up into segments, each segment twists and the subsequent segment twist angle get added to the previous segment(s) twist angle(s). The largest twist angle being at the free end does not equate to this end having greater stress – stress is the same across the length as described above.

• As long as the drive shaft stress remains below the Yield Point of the material (the point where the shaft would permanently (plastically) deform), the shaft will return back to its neutral position after the torque load is removed. It has no “memory” that it was deflected.

• Regarding the load on the splines: the engine load is applied to the left side of the splines on the drive shaft, with the reaction force applied to the right side of the splines on the driving collar. On the other end of the shaft (transaxle side), the spline load is on the right side, with the reaction being on the left side of the collar (all when viewed from the engine end of the shaft. Since swapping ends of the drive shaft reorients what is right and left, suffice it to say the same surface of the splines on the drive shaft and collars are always transmitting the drive load, regardless of the shaft orientation.

• During normal operation the torsional load in the drive shaft does change direction. When the engine is driving the wheels, the drive shaft is twisting in a CW orientation (viewed from the engine side of the drive shaft). When the throttle is closed and the drive wheels are back-driving the engine, the torsion load in the shaft reverses to a CCW direction. However, this CCW torque load is minimal, only having to rotate the engine.

I think this covers all the disputed points in that dumpster fire thread. Now for some related thoughts and information from a practicing (not arm chair) M.E.
Thoughts on drive shaft failure:

• In addition to the torque load, which is continuously varying and even reversing, there are also bending loads on the driveshaft for multiple reasons:

o Drive shaft bending/displacement due to vibration.

o Torque tube bending/displacement due to vibration, carrying the drive shaft with it (see white paper I did on the TT vibration damper and resonant frequency).

o Tolerance stack up of the various mating pieces (bell housing to block, crank to block, TT to bell housing, TT to TT end fittings (welded), TT to transaxle, TT bearing concentricity, etc.) These items and more all have a dimensional tolerance which has the ultimate effect of putting the drive shaft out of concentricity with the rotating parts of the engine and transaxle, which are the rigid lumps in the system. The driveshaft, being the most flexible component will take up these tolerances by slightly bending. I suspect over time, the compliant portions of the TT bearings help to reduce the drive shaft bending by deflecting and extending any tolerance mismatch over the entire drive shaft length, reducing localized bending.

• Interesting things happen when you bend a rotating element – the drive shaft. The bending stress continuously reverses as the shaft rotates. On a slightly deflected (bent) shaft, one side of the shaft surface is in tension and the opposite side is in compression. As the shaft rotates, so does the sides in tension/compression, continuously reversing the stress.

• Stress concentration factor due to the 28 to 25mm shaft taper.

• Loose drive shaft collars allows for increased shaft to collar displacement, which I suspect leads to more localized bending in the shaft. I’m thinking the bend which is distributed over the length of the shaft starts to get concentrated towards the ends as the collar loosens.

The combined effect of the varying torque load, reversing bending load and stress concentration adds up to increase material fatigue, which shortens life and causes failure. The side of the shaft that ultimately fails is determined by which combination of these issues fatigue the fastest based on the local conditions.

These are those white papers - Geza-aka-Zombo @ TT Vibration Damper, and Engine and Transaxle mount white papers
Last edited by Geza-aka-Zombo on Mon Aug 09, 2021 1:19 pm, edited 2 times in total.
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By Geza-aka-Zombo
#96877
Another thought. Though not a normal problem in my field, this shaft design is pretty standard M.E. stuff.

A major component in designing the shaft is to account for the reverse stress and shock load conditions, as it is a dynamic, not static situation. To do this, you don't apply the typical material properties of Ultimate Strength or Tensile Strength. You have to take these material properties and derate them to what is called the Endurance Limit. This lower limit of allowable stress (~50% of the Ultimate Strength) takes into consideration the reverse stress conditions for >10 million cycles - normally considered life. On top of this, the Endurance limit is further derated by other factors such as shock loading and even surface finish of the shaft (a rough shaft being more prone to fatigue than a smooth shaft).
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By Ed Scherer
#96878
Great post, Geza! I'm here to learn, and this has been the best post I've seen. A few questions:
  • Where you wrote, "In most cases, relocating a torsion bar or half shaft from side to side does change the direction of the torsion forces/stresses. In the case where the bar is completely symmetrical, it can be relocated but may also need to be swapped end to end to maintain the original torsion force/stress direction." Isn't the bolded part inconsistent with what we've all agreed on and described in your first bullet item?
  • Where you wrote, " As long as the drive shaft stress remains below the Yield Point of the material (the point where the shaft would permanently (plastically) deform), the shaft will return back to its neutral position after the torque load is removed. It has no 'memory' that it was deflected." is definitely something that I had wrong in my understanding. I thought I had seen it in some places, or perhaps that shafts did tend to yield during their operational lifetime. Is that also to say that as long as that yield point isn't exceeded, there's no reason to consider a shaft unfit for use with torsional forces predominantly in the opposite direction? That seems like an extremely important principle.
  • Given the previous item, is there any reason to even pursue the concepts I was curious about (dynamic analysis, modeling as spring, shaft as low pass filter, impulses, etc.)?
  • Given your summary, "The combined effect of the varying torque load, reversing bending load and stress concentration adds up to increased material fatigue, which shortens life and causes failure. The side of the shaft that ultimately fails is determined by which combination of these issues fatigue the fastest based on the local conditions." (which certainly sounds right to me), how does this all ultimately argue for or against a recommendation to swap ends of a drive shaft? (After all, that's what started this!)
I've got to say that it's a breath of fresh air to get a post like yours and learn things. This is what an educational thread should look like.
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By Geza-aka-Zombo
#96890
Ed Scherer wrote: Mon Aug 09, 2021 12:14 pm Great post, Geza! I'm here to learn, and this has been the best post I've seen. A few questions:
  • Where you wrote, "In most cases, relocating a torsion bar or half shaft from side to side does change the direction of the torsion forces/stresses. In the case where the bar is completely symmetrical, it can be relocated but may also need to be swapped end to end to maintain the original torsion force/stress direction." Isn't the bolded part inconsistent with what we've all agreed on and described in your first bullet item?
  • Where you wrote, " As long as the drive shaft stress remains below the Yield Point of the material (the point where the shaft would permanently (plastically) deform), the shaft will return back to its neutral position after the torque load is removed. It has no 'memory' that it was deflected." is definitely something that I had wrong in my understanding. I thought I had seen it in some places, or perhaps that shafts did tend to yield during their operational lifetime. Is that also to say that as long as that yield point isn't exceeded, there's no reason to consider a shaft unfit for use with torsional forces predominantly in the opposite direction? That seems like an extremely important principle.
  • Given the previous item, is there any reason to even pursue the concepts I was curious about (dynamic analysis, modeling as spring, shaft as low pass filter, impulses, etc.)?
  • Given your summary, "The combined effect of the varying torque load, reversing bending load and stress concentration adds up to increased material fatigue, which shortens life and causes failure. The side of the shaft that ultimately fails is determined by which combination of these issues fatigue the fastest based on the local conditions." (which certainly sounds right to me), how does this all ultimately argue for or against a recommendation to swap ends of a drive shaft? (After all, that's what started this!)
I've got to say that it's a breath of fresh air to get a post like yours and learn things. This is what an educational thread should look like.
First Bullet - Yes, I agree - my mistake. I will correct my write-up. Thanks! I was thinking about different configurations of bars and got lost.

Second Bullet - Operating at or above the yield point basically means failure, so we don't go there. My post above this one might shed some light into supporting the following regarding a torsion bar. In normal operation, lets say a torsion bar operates from zero stress to max stress in one direction (when suspension bottoms out). If we switch the location, it now operated from zero stress to minus max stress. So, we've effectively doubled the range of stress the bar operates in, which, you might expect, gets the bar to fatigue quicker. Fatigue analysis is somewhat complicated, but is related to cycles of stress on either side of a mean stress. So, with the torsion bar, the mean stress would be half the max stress, and the bar would cycle around this. If you swap the location of the bar, the mean stress is now zero, and it cycles the full max on either side. This speeds up fatigue, which is why swapping the torsion bars is not a good idea.

Third bullet - I'm not sure I understand this and what you are trying to accomplish.

Fourth bullet - Since the direction of the torsional load cycles don't change, swapping the shaft should not impact the fatigue life at all. In some respects, if the bending stresses are worse on say the Transaxle side of the shaft (due to geometric constraints, such as length of shaft to the first support bearing). I suspect it's possible that swapping the shaft might actually improve life by distributing the stresses (and the fatigue they cause) of the bending to both ends of the shaft, instead of concentrating them on one end. Interesting.

I added the link to those white papers I was referring to in my post above.
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By worf
#96900
Ed Scherer wrote: Mon Aug 09, 2021 11:07 amWhat's at the back? A TC and gearbox that (upon gear changes) produces rapid shock loads, i.e., changes in torque magnitude (although my initial guess would be that the TC would filter those out to a large extent).
It is logical to think that the TC might filter shock load from the gearbox to the driveshaft.

However, if you drive around with the manifold vacuum line disconnected you’ll know otherwise.

I suspect that ATF is a non-Newtonian fluid. At least a bit. If not then the two above sentences must have a different explanation.
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By Ed Scherer
#96919
Geza-aka-Zombo wrote: Mon Aug 09, 2021 12:59 pmThird bullet - I'm not sure I understand this and what you are trying to accomplish.
I'm probably best served by educating myself on my own first; this is just my first pass at trying to form a mental model based on lack of deeper knowledge. I was just starting to think about how an impulse torsional force on the end of a shaft would propagate through through the shaft (deflection and bending) and whether this is a case of wave propagation. For example, if one considers what might be a linear equivalent: a conventional coil spring, if you apply an impulse force at one end, do you not get to see the force propagate through the spring as a wave? As I said, these are poorly-developed initial "gut" instinct thoughts, but enough to keep me interested in the topic.

Your comments after "Now for some related thoughts and information from a practicing (not arm chair) M.E.
Thoughts on drive shaft failure:" show that the topic is indeed nontrivial, but well understood and modeled.



Your answers to my other questions all make good sense.

Thanks once again for taking the time to provide such helpful information. I'll take a look at your whitepapers soon when I've got the time to dedicate to it; I'm sure they deserve some time to absorb.
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By Charlie
#97073
I stepped away from 928land for a while due to a big personal backlog but decided to rejoin this morning. Happy to see nothing has changed, but sorry I missed the live train wreck. Geza and Ed, thanks for keeping that conversation going and on the engineering rails. Worf, the kitteh is the best post.

Just in a further attempt to confuse everything, here is my TT stub (from the rear) along with the replaced stock front clamp/coupler. A great paperweight!

Is there no end? Yes, there is is, sitting in the coupler.

Image
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By Constantine
#97100
Image

The 28mm drive shaft also shears at the front, but not anywhere as often as the rear in my experiences.

Picture was sent to us by an owner looking for options after this event.

What is interesting is the rusty part of what looks like the inner drive shaft material which makes it seem as if the drive shaft had been suffering a fracture/delamination for a long time before finally shearing.
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By Geza-aka-Zombo
#97142
Thought I'd run some quick hand calculations on the 25mm shaft, making a few assumptions on the shaft length (60 inches - guestimate) and diameter reduction near the splines (22mm - I scaled it off a picture). Again, this is not my area of expertise, and it may have some nuances that I'm not accounting for, but it is basic Mechanical Engineering.

Results:

For each 50 lb-ft of torque, the shaft twists ~0.97 degrees. For the 28mm shaft (calculation not shown), this number would be 0.62 deg. for 50 lb-ft torque.

From a stress perspective, both the 25mm and 28mm are basically the same because they both neck down to the same configuration near the splines (this is highest stress area). However, depending upon the details of radius near the splines and material properties, one could be better than the other. Either way, the stress numbers I'm getting require very strong steel. Not knowing the specifics, it appears shaft failure is certainly feasible, as supported by actual evidence. It's not like there is a huge margin of safety. IMO, Porsche made a mistake carrying over the same spline diameter configuration as the engines got larger and more torquey. They should have upgraded the shaft/coupling design.

Why did they increase the diameter from 25mm to 28mm knowing that it wasn't stronger? Well, it is stiffer in both torsion and bending (both by ~57%). I don't know the history, but is this when they went from 3 to 2 TT bearings?

I never really bought the idea that twisting of the drive shaft causes it to pull out of the front collar. When driving 300 lb-ft of torque, the shaft only twists ~5.8 degrees (0.97 x 300/50). Doesn't seem like a lot? At one point I was thinking about differences is coefficients of expansion, but never dove into it.

Image

Image

Image
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By worf
#97156
Geza-aka-Zombo wrote: Tue Aug 10, 2021 4:32 pm For each 50 lb-ft of torque, the shaft twists ~0.97 degrees. For the 28mm shaft (calculation not shown), this number would be 0.62 deg. for 50 lb-ft torque.
Is that the worst case model where the rear of the shaft can’t rotate?
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By maddog2020
#97175
Here is mine from my 90 model. I nailed the gas and it snapped from the downshift. Don’t remember if it was front or rear of the driveshaft. I haven’t seen and auto torque tube in person for at least 15 years. I do remember that the driveshaft was twisted and shredded. The cracks had rust in them.
Image
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By Geza-aka-Zombo
#97177
worf wrote: Tue Aug 10, 2021 5:02 pm
Geza-aka-Zombo wrote: Tue Aug 10, 2021 4:32 pm For each 50 lb-ft of torque, the shaft twists ~0.97 degrees. For the 28mm shaft (calculation not shown), this number would be 0.62 deg. for 50 lb-ft torque.
Is that the worst case model where the rear of the shaft can’t rotate?
No, not a worst case model. This would be the amount of twist in the shaft, whether static torque/fixed end or rotating torque/rotating end, while transferring 50 lb-ft of torque to the transaxle.

For anyone following my crackulations, I wanted to point out something. For the stress calculation, the J (polar moment of inertia - .055) is that of the necked down section where the stress is being calculated. For the twist angle, I based J (.092) on the actual shaft diameter, since this larger diameter (25mm) is maintained across almost all the length.
Last edited by Geza-aka-Zombo on Tue Aug 10, 2021 6:15 pm, edited 1 time in total.
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By SeanR
#97180
maddog2020 wrote: Tue Aug 10, 2021 5:48 pm Here is mine from my 90 model. I nailed the gas and it snapped from the downshift. Don’t remember if it was front or rear of the driveshaft. I haven’t seen and auto torque tube in person for at least 15 years. I do remember that the driveshaft was twisted and shredded. The cracks had rust in them.
Image
Seems to happen on the '90's more than others, those are the ones I've had to replace.
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By Geza-aka-Zombo
#97182
Does anyone know how the change in the shaft dia and number of TT bearings relates to the year '90?

My analysis above was only addressing torsion stress, and did not consider bending stress, which makes the conditions even worse. Bending could be impacted by the TT bearing configuration.
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By worf
#97742
Do “we” actually know the identity of the alloy used in the shaft?
Last edited by worf on Fri Aug 13, 2021 9:06 pm, edited 1 time in total.
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By Geza-aka-Zombo
#97772
worf wrote: Fri Aug 13, 2021 4:30 pm Do “we” actually know the identity of the allow used in the shaft?
I don't - best thing would be to get the Porsche drawing for the shaft, which would specify the steel type and any other requirements such as heat treatment, surface finish, hardness and other mechanical properties.
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By Charlie
#97842
Apologies to Geza and anyone else who actually knows what they're doing. My driveshaft was replaced with the Precision Motorwerx 300M version (yes, Greg Brown's). This thread made me wonder if it was actually any better than stock.

According to makeitfrom.com, the alloy referred to in the Porsche book excerpt as 42CrMO4 is 4130 or standard "CroMo". It's a strong and reasonably priced alloy. Medium price bicycles use CrMO tubing, for example.

300M is a modified version of 4340 called 4340M. 4340 is much better at everything than 4130, and 4340M (300M) is better at a few things than 4340. 300M as I understand it has just under twice the stiffness, strength, and resistance to stretching vs. CrMo. The properties also depend on the finishing for the steel - annealed, normalized etc. I got lost at that point.

So I got that going for me.
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By Geza-aka-Zombo
#97880
The material property that determines stiffness and the amount of stretch (strain) is the modulus of elasticity, aka Young's modulus. All steels have about the same value, about 30 million psi, so there is no real advantage with Brown's shaft material regarding stiffness/stretch. Strength is a different matter, it may be much better, but I haven't looked at the numbers.

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Panorama September 2021

great representation ~!