No one said the balls had to stay in the same joint in the same location with the same outer race and the same star and the same cage….all in the same orientation.

Data point: I swapped my drive shafts/CV joints left to right at 100Kmi. Now at 124Kmi. For the first 100Kmi, it had the 1973.5T CIS motor in it, so that didn't put a lot of load on the drive shafts. Still the original CV joints and boots too. Not bad for 50 years.

Just because your axles have not broken does not mean it is a good idea or to be recommended. Odds are most people can get a way with swapping axles side to side. I doubt Porsche worried about it. But, it's clear that it is a bad idea from a mechanical stress perspective. So why do it?

Because it changes the wear on the CV joint by driving the balls and other parts in the opposite direction. That's good mechanical practice too. You rotate your tires for a similar reason. Maybe my axles will break eventually, but probably not. I'll probably die of old age first!

As noted earlier, axle shafts see torque in both directions anyway, but the vast majority of it on street cars is in the forward drive direction, so that's the direction the CV joints tend to wear.

Going back to the torsion bar example: When the relatively thick torsion bars are manufactured, they are twisted in the direction of intended torque until the outer part of the bar yields plastically, then relaxed. This process puts a pre-stress on the inside of the bar and a pre-stress in the opposite direction on the outside of the bar. Although these stresses are unequally distributed when relaxed, they are more equally distributed when being twisted in the intended direction, and the surface is under less stress when twisted than it would be if it were not pre-stressed. That improves fatigue resistance and reduces the tendency for the surface to crack and propagate through the bar.

I have never read that Porsche (or VW or other European makers with half-shafts) do this to those shafts. On the other hand, I have read that Strange Engineering, who makes drive axles for solid rear axles in American cars does do this treatment for certain of their racing applications. If so, then the axles are marked L or R.

The general idea is that once axles have been run on one side of the car, they need to stay there. There is a bunch more torque in the drive direction than when braking. This builds that stress you are talking about being done on purpose for torsion bars. Running the axles on the other side stresses the axle in the other direction which is not a good thing to do. Just because you can get away with it does not mean it is a good idea or something to be recommenced.

As far as CV joint wear...you are probably going to clean and inspect the CV joints so you can swap them from one axle to another. That will allow you use a different wear area and keep the axles on the same side of the car.

Clean and inspect CV joints? Nah, I run 'em until they make noise, then replace 'em. I'd rather be out driving.

After reading this for a few days it feels like there is a tension but general user views and those who race/compete. For the non-racer swapping sides with driveshafts seems like a low risk option that can actually get more life out of the assembly. And although there may be some historical competition view that swapping was good, there now seems a view it is not. Things change over time don’t they. I don’t race and swapped sides with my CV joints about 19 years ago and saw no negative issues.

Please, feel free to do whatever you want. It doesn't hurt me if you ever have a driveshaft fail.

My point is there are sound technical reasons to not swap driveshafts from one side to the other. Sway-A-Way sells driveshafts to street, DE, and racer folks. They say don't swap shafts side to side and they stamp an "L" and an "R" on the driveshafts they sell. Why? Because is not a good idea to swap shafts side to side. Even if the odds are that nothing bad will happen.

I know a guy that used a hand drill for 45 years never wearing safety glasses. Today he is blind in is right eye because a drill snapped off and destroyed his right pupil. The odds were in his favor. Just because the odds are in your favor does mean it is good idea to play them.....

To add to the discussion with a single data point....I have no clue which of my driveshafts is right or left, I just put them in, again, the theory being decelerative loads exist as much as accelerative.

Now, racing would be definitely a different matter, especially with very high horsepower engines, so all bets are off there.

This is different than torsion bars in my mind (and I have never kept track of those either by the way) as torsion bars seem to get far more compressive loads than rebound.

So, max horsepower I am speaking of is a 3.2 Kremer, so about 230 din on autobahn and driven aggressively all over Europe.

No issues with my approaches for over 30 years.

But do what makes you comfortable on the street is my advice.

Racers, you are in a different world, do your thing there as you see fit.

D.

Well, let's see if we can get some data.

Scott, or any of you pro's: Have you seen broken half shafts in street cars that didn't have significantly more power added, say, more than 300HP? How about race cars?

I've seen one broken driveshaft at the track on sub 300HP car. I've seen one at an autocross too. I have no way of knowing if the driveshafts were ever swapped side to side.

Regardless, that really isn't the point, is it? I have already stipulated that the odds are that nothing will ever happen if the people here swap driveshafts side to side. My point is that there are solid mechanical engineering reasons explaining why it is not a good idea.

All of you do as you please...I am just pointing out the engineering side of it.

Well, if we are talking engineering, let's' be nit picky and precise, for the sake of accuracy. If the axles are not prestressed by twisting past the yield point, then there is no reason against swapping them. Stress in steel in either direction, within the fatigue limit, can occur an infinite number of times.

BTW, I misspoke in the long post above, and I corrected it. I wrote "yields elastically." That should have been "yields plastically, " or more simply "yields," since that is by definition plastic deformation.

Not poking at you Scott, this is just interesting stuff to me. SmileWavy

That's not how it works with axles. They are stressed by use. So, once an axle is used on one side, it should stay on that side. This is backed up by Porsche with their Cup Car manuals and by Sway-A-Way in their documentation.

Or do you think they say this stuff just for fun?

In Sway-Away's case, they mark the axles L and R. That strongly indicates to me that they have pre-stressed those axles, like I noted with Strange Axles. Likewise for the Cup cars (which crank out substantially more HP than the regular cars). Therefore, one should not swap them.

Do the stock axles deform plastically in street use on stock cars (and thus become stressed in a particular direction)? Without measuring them, I can't rule it out. However, it seems to me that would be a very poor design by Porsche, because that virtually means the axles would be stressed beyond their fatigue limit in normal use, and would fail eventually. IIRC, the fatigue limit for most steels is in the range of 60-70% of the elastic limit.

If stock axles break on street driven cars, it's a very rare occurrence, as we hear so little about it. Yes, I know that doesn't prove the negative case. But many people driving high-mileage Porsches have the original axles still in them. That indicates the axles are not operating at their fatigue limit, let alone their yield stress. Exceeding the yield stress is required to stress them in a particular direction.

Sway-A-Way does not say anything about pre-stressing the axles yet they do say that about the torsion bars. That tells me they don't pre-stress them.

Porsche does not mark the Cup Car axles yet says once they are used they should not be swapped side to side. That tells me they are not pre-stressed.

The simple fact is that you increase the likelihood of an axle failing by swapping it side to side. Does that mean it will fail or is likely to fail? No. It's kind of like using a highly stressed bolt in single shear. If you make it big enough it won't fail. It would still be better to design the joint to be double shear.

If it were me, I would not swap axles side to side because they are not significantly larger diameter than they need to be. You and everyone else here are free to do whatever you want.

Again,

Yield stress = plastic limit? Fatigue stress = elastic limit? Or the other way around? Or are the terms for twisting forces different than those for tensile forces like fasteners?

Lots of us have taken the Caroll Smith fastener (Screw to win) class by reading his books, and learned that steel, stressed below its plastic limit, can be stressed indefinitely absent some issue (like rust as a stress riser or something), unlike the aluminum in aircraft wings (which I ponder when flying, but trust the replacement specs). And that a bolt stretched beyond its plastic limit isn't necessarily no longer useful, but its UTS certainly is lower and the safety factor is reduced, and we'd never reuse a rod bolt which didn't meet the manufacturer's allowance of a few thousandths longer than when new.

In another book, he states what most of us recall being told - axles take a slight set, and should not
have their direction of rotation reversed.

It does seem to me that, all else being equal, that if you took a right side axle and put it on the left side, with the stamped R on the transmission side, you would not be reversing the rotation? Maybe if you only had one spare and it was the "wrong" side?

Walt: First, recognize that Scott and I are going down a trivial rabbit hole regarding whether to swap axles, because, well, it's fun! Both of us agree it probably doesn't make a difference in street cars with stock or nearly stock horsepower. IN race cars with substantially more power, it is not advisable, depending on your application.

Basic strength of materials info for steel alloys:
Two types of forces on a sample of steel: linear, or tension/compression; and rotation or torque. Steels can deform in both of these ways, more or less independently for small deformations.
Stress is the force applied to a steel sample divided by the cross section of the sample. Engineers express this as pounds per square inch, "psi" or in thousands of pounds/sq.in., "ksi." In metric units, it is megapascals, "MPa."
Strain is the measured elongation of a sample under a given stress.
Elastic limit or strength. All steels will stretch or compress a certain amount and spring back to their original length. Likewise, they will twist a certain amount and spring back to their original position. This is how torsion bars work. As long as the sample returns to its original shape, we say it deformed elastically.
Plastic deformation: At some level of stress, the steel will be pushed far enough that it deforms plastically, which means its shape has been changed. When you bend a bolt or coat hanger or wire enough to deform it permanently, it has deformed plastically. Steels will deform plastically in torsion too.
Yield point: the stress level at which the steel begins to deform plastically. The "yield strength" is similar, but typically refers to a particular part. It is the force that causes the part to begin to deform plastically.
Ultimate tensile strength (UTS): The maximum stress before the breaking point of a steel sample. After the steel deforms plastically under increasing force, it continues to stretch or compress or twist until it breaks.
Fatigue limit: typically expressed as stress. Below that stress (which is always below the yield point) the steel can be subjected to cyclic stress (on and off, or bending one way and then the other, or twisting one way and then the other) indefinitely, and it will not crack and break. Example: Valve springs operate at a level of maximum stress on the surface of their wire coils that is safely below the fatigue limit, thus they can operate "forever."
Fatigue failure: when a part fails due to metal fatigue, cracks have started and propagated through the part, weakening it until it breaks. The higher the stress is above the fatigue limit, the fewer cycles it will take to break the part. Bending a part into plastic deformation repeatedly very quickly breaks the part. Think about bending a wire, like a coat hanger, back and forth until it breaks. Further, any notches or small cracks in the material of the part will concentrate stress at that defect and greatly lower the fatigue strength of the part. Cracks due to corrosion are very common and have a special name, "corrosion stress cracking."

Now to get to some of your questions. The fatigue limit is not just below the yield point. It is significantly below the yield point. Depending on the alloy, heat treatment, work hardening, etc., it is commonly about 50% of UTS, or about 60-70% of the yield stress.

Bolts and screws, however, should not be subjected to cyclic stress. If they are, that's a poor design or poor application, or they were incorrectly installed or insufficiently tightened. Instead, bolts should clamp the parts together so tightly that they do not move at all. Example: the conrod bolts clamp the rod caps to the rods so tightly that they do not move. If they do not move, then they receive no cyclic stresses. They only experience the constant stress of being tightened to a specific torque/stretch. This leads to a practical rule about screws and bolts in general: If it does not fail during tightening, it will not fail in service.

Axles: Axles (really drive shafts) are torsion bars too, as Scott pointed out above. If the axles are twisted hard enough to cause plastic deformation, then they "take a set" for that side of the car. If they are not twisted hard enough to deform plastically in torsion, then they do not take a set. So that's the key question: do they deform plastically in use?

Remember what I said about fatigue above. If the forces of the transmission twisting against the resistance of the tires (until they spin) are sufficient to plastically twist the axle, then they also certainly are operating above the fatigue limit for those axles (recall the 60-70% ratio above). That would be poor engineering. If so, then we should expect to see failures of axles due to fatigue in high mileage cars. But those failures are pretty rare in street cars. Race cars are a different matter.

Now, you will note I talked only about steels. The same terms and properties also apply to aluminum and most other metals, and many other materials, except for fatigue limit. Titanium and some other metals also have fatigue limits, below which they have infinite durability. Aluminum and many other metals do not have this property, and instead specifications commonly use the term "fatigue strength" which means the metal will not fail in fatigue in a specified number of cycles, often 500 million. So don't worry about the airplane wings, as they won't see 500,000,000 cycles of stress, whereas the parts inside your engine will.

Last, your question about the direction of torque on the axles if you swap them from one side to the other. I don't know how to produce an animated video that demonstrates the torque direction on both sides of the car, so you'll have to hold an axle, or pencil, or something to simulate one in your hands and visualize which direction the transmission output flange rotates the axle on one side versus the other. You will see that they are in opposite directions. Whereas, simply flipping the axle on one side does not change the direction of torque on it.