Why do we not use a CV drive shaft to power the front axle?

someguysjeep

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why exactly do we not use a CV drive shaft to power the front axle (or rear)?

been trying to figure a few things out, my front axle has some vibes when get up into OD.
i'm still running the OE shaft,

OK so this double cardan we have up front..........i'm never gonna get a straight shot at the TC without totally wrecking out the castor. i just cannot get the joints to line up properly at this angle, the bottom end and they'll never be a straight shot into that pinion.
i found info that states the DC shaft has a limit around 30-35* b4 it'll bind, while a true CV would go 45*
now i also find info that the CV is just not as strong as the DC, but yet they are top shelf parts for axle shafts.

why is that? how do we trust then in a high torque application at the wheel, but not as a drive line connection?
Jeep put CV drive shafts on several models, are they just junk? is there a better made CV? why don't we use them? or am i missing something in how they function?

i'm aware this is not uncommon and the usual fix is dial the castor back (a little) to find the balance, if able.
 
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The double cardan joint is already plenty strong enough and flexes enough for the vast majority of lifted jeeps. In addition, the double cardan joint is significantly cheaper than a CV joint.

The front driveshaft is far longer than the rear, and extreme angles are rarely seen on the joints.

You are far more likely to see failures at the axle shaft u-joints due to the torque multiplication of the differential. That is why CV shafts (like RCV) are commonly used to replace the single cardan joint. In addition, the angles on the axle shafts are generally far higher than any angles you would see on the front driveshaft double cardan joint.

Some later Jeeps used Rzeppa joints in the front driveshaft instead of double cardan joints. Unlike the double cardan, the Rzeppa is a CV shaft, but the factory Rzeppa joint is often replaced with a double cardan shaft in aftermarket retrofits.
 
I would like to see a Rzeppa or other CV joint used in a rear driveshaft not because of angles or strength, but due to vibration concerns. The double cardan joint is not a CV joint because the center carrier must accelerate and decelerate twice in each rotation of the shaft. How much it must accelerate/decelerate is dependent upon the angle of the joint.

Meanwhile, a CV joint such as a Rzeppa does not require rotational acceleration of any of the components, and thus would not result in torsional vibrations.
 
the rear is not really hard to get a good line on the DC shaft set-up, or lets say, has not been for me.
the rear pinion angle is free of the restrictions our front axle sees. once lifted our front axles cannot be set to the correct line again without reconstruction or total disregard of the castor angle.
that top double joint balances it counterpart to nullify the rotational offset, but without being able to align the bottom to very close, it's just moving that issue to the bottom, since we cannot get that almost straight line, no?

they are all over JK Rubi's whether or not we like it Jeep did do many improvements to the JK, they just F'd it up with BS electronics and more computer dependent sys. the chassis has a lot of things improved and a few the dorked.

the Rubi is supposed to be the top end of the model and built for as close to offroad as they'll get ya. so i question the choice of that flanged pinion and CV shaft, and why it's being abandoned.
can we just not get a flange adapted to the front of our TC's?

and it dawns on me this could be totaly not a TJ issue, yall got a 6* head start on me to play with. may have to toss this1 to the JK crowd and see what comes up.
 
Are you referring to using the CV in place of the single cardan joint? I.e., at the axle end? Swapping the double cardan at the transfer case with a Rpezza or other CV wouldn’t eliminate the issue of pinion angle on the front differential.

You can already order a double double cardan shaft (a DC joint at each end) that will allow any driveshaft and pinion/caster angle you could ever want. They’re not popular for typical TJ lift heights, but do exist. You may need to swap out the pinion yoke for a flange.
 
the Woods site used to have all kinds of info on a lot of different shafts but seems to have been revamped and is specialized to locate certain things only, i could not find the whole gambit of shafts they make in 1 spot like it used to be.

your correct the angle off the TC won't change and that end is working as a DC it's not being able to straighten the lower joint enough to let it run inline with the shaft, it'll have the 1/2slow 1/2fast rotation at that end causing the vibes, no?
 
You’re correct there. The lower joint is often the cause of vibrations, and why compromises have to be made between the caster and u-joint angle. Increase caster, and you increase the difference in speeds between the two sides of the joint.

The ideal solution would be to use CV joints at both the transfer case and the pinion. In practice, this is avoided due to the cost and complexity. Double cardan joints are significantly less expensive than true CV joints and generally quite durable, and that is why they are used at the transfer case end instead of a CV. The angle at the transfer case isn’t usually large enough to warrant a CV over a double cardan.

As to why a double cardan is rarely used at the differential pinion, it is simply a matter of “good enough”. Generally you can compromise with the pinion angle to get somewhat adequate caster and driveshaft angle to the point that neither is causing any significant issues. For example, a caster angle at 4.5 degrees and a u-joint angle at 2.5 degrees is a common compromise. Not ideal, but it works in most situations.

Theoretically, if the pinion and transfer case angles are roughly parallel, you could switch to a double single cardan shaft, or a shaft with a single u-joint at each end. This would work for minor angles, but at steeper angles, it runs into the same issue as the double cardan joint: even though the ends of the shaft always rotate at the same speed, the intermediate part of the shaft must accelerate and decelerate, and that alternating moment must come from either upstream or downstream, causing torsional vibrations.

That intermediate acceleration and deceleration is exactly the reason why a double cardan joint is not classified as a constant velocity joint. Even though the ends of the joint do not move at different speeds, the center carrier does, and thus it must transfer some amount of fully reversing rotational momentum twice every revolution. It usually doesn’t cause nearly as much issues as the single cardan joint, which is not rotationally conservative end-to-end, but it can still cause issues, especially at steep angles, high RPMs, and when the intermediate carrier has high rotational inertia.

If you wanted a truly smooth shaft, the only solution would be to put true CV joints at both ends. Then you could run any angle at both the pinion and the transfer case without any torsional vibrations, as the shaft and all it’s intermediates are rotating at the same speed throughout the range of travel.

A double double cardan shaft would closely approximate a double CV shaft, and likely be indistinguishable at low angles. However, as the angles increase, the intermediate carriers in the double cardan assemblies begin to cause issues. You might be able to clock out the issues by rotating the SC joints 90 degrees from one another, but that would depend upon the mass and stiffness of the middle tubing. In a worst case, you could develop torsional resonance, leading to very bad vibrations.

However, I would go on a limb and say that TJ front driveshaft angles would not be large enough to cause noticeable vibrations from a double double cardan shaft. The rear would be a different story.

Practically, this is how I would apply the above:
1. Start with a factory style one ended double cardan shaft. Work out a compromise between caster and angle at the single cardan joint.
2. If a compromise between caster and driveshaft angle cannot be made, switch to a double double cardan shaft. Set the caster at whatever angle you like.
3. If that results in torsional vibrations (unlikely), switch to a double CV shaft using Rzeppa joints or similar.
 
Another major reason CVs aren’t used in place of double cardan joints is that the driveshaft moves significantly faster than the average CV joint can survive, especially when geared to some low ratio like 5.38+. As CV angle increases, life decreases. As CV speed increases, life decreases. Jeep owners tend to increase both significantly over stock.

That’s one of the reasons JK owners often ditch the Rzeppa for a double cardan.
 
You’re correct there. The lower joint is often the cause of vibrations, and why compromises have to be made between the caster and u-joint angle. Increase caster, and you increase the difference in speeds between the two sides of the joint.

The ideal solution would be to use CV joints at both the transfer case and the pinion. In practice, this is avoided due to the cost and complexity. Double cardan joints are significantly less expensive than true CV joints and generally quite durable, and that is why they are used at the transfer case end instead of a CV. The angle at the transfer case isn’t usually large enough to warrant a CV over a double cardan.

As to why a double cardan is rarely used at the differential pinion, it is simply a matter of “good enough”. Generally you can compromise with the pinion angle to get somewhat adequate caster and driveshaft angle to the point that neither is causing any significant issues. For example, a caster angle at 4.5 degrees and a u-joint angle at 2.5 degrees is a common compromise. Not ideal, but it works in most situations.

Theoretically, if the pinion and transfer case angles are roughly parallel, you could switch to a double single cardan shaft, or a shaft with a single u-joint at each end. This would work for minor angles, but at steeper angles, it runs into the same issue as the double cardan joint: even though the ends of the shaft always rotate at the same speed, the intermediate part of the shaft must accelerate and decelerate, and that alternating moment must come from either upstream or downstream, causing torsional vibrations.

That intermediate acceleration and deceleration is exactly the reason why a double cardan joint is not classified as a constant velocity joint. Even though the ends of the joint do not move at different speeds, the center carrier does, and thus it must transfer some amount of fully reversing rotational momentum twice every revolution. It usually doesn’t cause nearly as much issues as the single cardan joint, which is not rotationally conservative end-to-end, but it can still cause issues, especially at steep angles, high RPMs, and when the intermediate carrier has high rotational inertia.

If you wanted a truly smooth shaft, the only solution would be to put true CV joints at both ends. Then you could run any angle at both the pinion and the transfer case without any torsional vibrations, as the shaft and all it’s intermediates are rotating at the same speed throughout the range of travel.

A double double cardan shaft would closely approximate a double CV shaft, and likely be indistinguishable at low angles. However, as the angles increase, the intermediate carriers in the double cardan assemblies begin to cause issues. You might be able to clock out the issues by rotating the SC joints 90 degrees from one another, but that would depend upon the mass and stiffness of the middle tubing. In a worst case, you could develop torsional resonance, leading to very bad vibrations.

However, I would go on a limb and say that TJ front driveshaft angles would not be large enough to cause noticeable vibrations from a double double cardan shaft. The rear would be a different story.

Practically, this is how I would apply the above:
1. Start with a factory style one ended double cardan shaft. Work out a compromise between caster and angle at the single cardan joint.
2. If a compromise between caster and driveshaft angle cannot be made, switch to a double double cardan shaft. Set the caster at whatever angle you like.
3. If that results in torsional vibrations (unlikely), switch to a double CV shaft using Rzeppa joints or similar.

i get that , thanks.
now is there a compromise or strength issue with a true CV. it is not out of the question that i could score a front shaft from a jk if i have the length for it.................or are they junk?

my exact deal is i'm running JK rubi axle HP it 's 6* to castor and 0* to the pinion at 4" of lift
 
I would also argue that the Rzeppa style CV joint is probably the better option (barring cost) compared to the single cardan on the axle shafts.

The speeds on the axle shafts are much lower than the driveshafts, and the angles at high speeds are minimal, and the speeds at high angles are minimal. (You’re not going to turn the wheel to full lock at 80 mph.)

Since the CV joint takes the place of the single cardan joint, it results in a smoother transition of power from the differential to the wheel, without any of the surging feeling caused by u-joints at high angles.

Theoretically, a double cardan axle shaft would perform just as well as a CV (the speeds are low enough that the variations in intermediate carrier speeds aren’t enough to matter); however, practically there isn’t the space to fit a double cardan joint at the wheel hub.
 
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I would also argue that the Rzeppa style CV joint is probably the better option (barring cost) compared to the single cardan on the axle shafts.

The speeds on the axle shafts are much lower than the driveshafts, and the angles at high speeds are minimal, and the speeds at high angles are minimal. (You’re not going to turn the wheel to full lock at 80 mph.)

Since the CV joint takes the place of the single cardan joint, it results in a smoother transition of power from the differential to the wheel, without any of the surging feeling caused by u-joints at high angles.

Theoretically, a double cardan axle shaft would perform just as well as a CV (the speeds are low enough that the variations in intermediate carrier speeds aren’t enough to matter); however, practically there isn’t the space to fit a double cardan joint at the wheel hub.
Another thing I’ve considered an advantage to front axle shafts having CV joints is that the torque load is consistent from the inner shaft to the wheel in sharp turns, whereas with a u-joint, the torque load on the inner shafts has peaks and troughs to maintain the same even torque out at the wheel. This should lower the peak torque seen by diff components and everything else upstream.
 
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