Front prop shaft .. dont see logic for double cardon

[pwcan]

defender 90.. front prop shaft - vibration with 2" lifted truck.  Lets start by sharing principles .. please see https://youtu.be/Idk3BVDVHq4. Unlike most manufacturers LR don't set the drive line angles symmetrical at both ends. If they did then it would be in balance and could be run in phase. On Defender the front and rear UJs operate at slightly different angles so to correct that LR slide the front section in so the UJs front and rear are out of phase one spline I believe..

With around 2" lift some aftermarket companies sell a double cardan prop shaft to eliminate vibration with the increased and changed UJ operating angles. The double cardan cancels out the rotational difference between the connected flange and shaft so they rotate at uniform speed. The other end of the shaft has just 1 cardan still though !! - WHICH ON A DEFENDER is operating at any angle so will now introduce elliptical rotation at the connected flange again. How can double cardan shaft help reduce vibration in a defender ? logic suggests this should not work. Appreciate any expert opinion on this ? .. I've contacted some vendors of the LR double cardan shafts.. not too helpful generally. Seems the phone staff a little light on the principles.

I'm assuming the disco2 had very shallow or no angle at the single cardan end to work.. I had one but cant remember..

[pwcan]

For the Defender application I can see a shaft with two double cardans working or a shaft with CV joints.. OR change angle of axle/flange at the front diff so near inline with the shaft... ?

[miketomcat]

I can help on the tech side however my tomcat had a discovery front prop on the rear (re-phased). I suffered horrific vibration on over run so tried a re-phased disco 2 (double carden) front prop on the rear. It completely cured the vibration and I ran it like that for about 5 years. 

Mike

[lo-fi]

The key thing to remember is that the (proper) setups using DC have the diff angled to match the prop. The slight angle this changes by as the suspension cycles can more or less be ignored - the non linear effect of a UJ is negligible at shallow angles, and it sits pretty close to parallel at normal ride height. The other end, having the prop at an angle to the transfer output, needs the constant velocity nature of the DC to eliminate what would be quite a serious non linear relationship that isn't canceled the other end.

Simples, really excellent explanations here:

http://www.4xshaft.com/driveline101.asp

I should add that a defender lifted without castor corrected radius arms angles the diff up like this, so DC could be used to try and help if you're getting prop induced wobble. 

Edited March 30, 2018 by lo-fi
Extra thoughts

[pwcan]

Thanks very much for the excellent link and replies. When I get back in a few days I will take a fresh look at the shaft angle to the axle. May be my recollection not accurate. Seems like the DC is the proven way to go..

Cheers..

[Ex Member]

Keep in mind that DC shafts themselves have well known reliability problems.  The centering balls wear out pretty much 100% of the time and are costly to fix versus u-joints.  So...don't do it...unless you actually have vibration issues that can't be solved with a normal propshaft.

[pwcan]

Had a d2. The dc failed but i thought that was due to fact it was not greasable and ran close to hot exhaust.. not the case with defender(?). I thought the dc was just 2 ganged ujs. Does the fact it is dc and additional rotating weight create more stress?

[Ex Member]

Between the two u-joints is a centering ball.  That is what tends to fail.

[sonoronos]

I'll try to answer your question: "How can double cardan shaft help reduce vibration in a defender ?"

First, your question is a logical conflation of two totally different things: Why do driveshafts vibrate and What are double cardan shafts. These are two totally different questions and conflation of these two concepts will lead to confusion.

Let's answer your two totall different questions independently:

What are DC Shafts:

1. In total fact,  Double Cardan Shafts have one and only one purpose: They exist only to reduce the U-Joint operating angles in a driveshaft.

2. By reducing the U-Joint operating angles, you reduce the issues related to non-zero U-Joint operating angles.

Why do driveshafts vibrate:

1. Imbalance

2. Out of phase operation

3. U-Joint angle.

Imagine, for a moment, a perfectly balanced, in-phase driveshaft operating at a 0 degree U-Joint angle. That is, the driveshaft is perfectly inline, straight, with no bends. Imagine that the driveshaft is accelerated to 6000rpm. How do you feel about the vibrations of that system?

Now Imagine, for a moment, that this driveshaft spinning at 6000rpm is aligned so that the U-Joints are now operating at a hypothetical 45 degrees. How do you feel, by gut instinct, about the smoothness of the operation of the 45 degree driveshaft at that speed?

The truth is, that driveshaft operating at 45 degrees is experiencing incredible rotational acceleration forces upon each rotation. One every rotation, each joint is constantly accelerating and decelerating, applying a torque to the U-joint and the driveshaft (think of cracking a whip.) The driveshaft and U-Joints have to take these torques and then transfer them to the next U-Joint. These forces are massive, the very material of the driveshaft will experience twisting and bending. Given these huge forces, which are enough to destroy the U-Joints and driveshaft itself given enough strain, how do you feel about the total vibration of the system, despite the fact that the driveshaft was balanced and perfectly in phase? Do you feel there would be some vibration?

Now, imagine reducing that U-Joint angle to 27 degrees. What do you think happens to the rotational acceleration forces of that U-Joint? Are they reduced? If they are reduced, do you feel it may contribute to a reduction in vibration?

I hope in this example you have found an answer.

Edited April 5, 2018 by sonoronos

[Ex Member]

A double cardon joint is two u-joints ina  row at the same angles.  A single u-joint varies rotational speed in a sinusoidal pattern.  2 joints in a row at the same angles causes the two speeds to cancel out, so you get constant rotational speed.  This makes it a constant velocity joint.

The point made earlier is that if you then have a single joint on the other end and it is at an angle, you will get a speed variation.  It thus needs to be close to no joint angle.  It should be fairly small at the diff end.

Edited April 5, 2018 by Red90

[Ex Member]

Watch this video.

 

[sonoronos]

Thanks John

The only other thing I want to add is that despite the combination of two U-joints and centering ball performing similarly to a constant velocity joint, the two U-joints in the Double-Cardan joint still experience sinusoidal acceleration individually. Therefore, there is still a U-Joint angle dependent rotational torque generated by and through the double-cardan joint. This rotational torque is one of the main components of wear in the centering ball (which John mentioned earlier.)

In practice, of course, the idealized CV-Joint created by the double-cardan will become less ideal with wear of its components and also the U-Joint angle. That is, the sinusoidal cancellation of the double-cardan joint is a mathematical idealization.

In practice, the Rzeppa-style joint (as in a steering CV joint) is closer to ideal, although it has its own problems (namely reduced strength relative to the operating angle, and of course usually less maximum operating angle than the double/dual cardan.) But I do want to caution that I am not describing one as better than the others. These are all tools to fix problems, some may be appropriate in some cases more so than others.

That video you posted is nice but I think can be misunderstood to imply that the sinusoidal cancellation somehow completely nullifies the U-Joint angle problem. The computer graphics is calm, but the reality of the "fluctuating output" can be quite energetic depending on the U-Joint angle.

Edited April 5, 2018 by sonoronos

[sonoronos]

Sorry, I just understood what John was getting at. When I wrote "the sole purpose of the DC joint is to reduce U-Joint angles", I was incorrect, the DC joint's primary purpose is as a Constant Velocity joint - the reduction in u-joint angles is important, but incidental.

Edited April 6, 2018 by sonoronos