What is a CV shaft and why do I want one?

What is a SYE/CV driveshaft and why do I want one?

So what is a CV driveshaft and why do you want/need one?
I'm going to break this up into two different categories since there is a lot of confusion around this - non-Rubicon and Rubicon TJ/LJ's. But right now, I'll tell you neither the Rubicon nor the standard TJ/LJ have CV shafts from the factory.

What is a CV?
CV stands for constant velocity. It's a category of drive shafts that allows the transfer of torque from one direction to another.

Typical CV-joints work like this:

These are very common on front wheel drive vehicles, in independent front suspension vehicles, and in a few other applications. They are typically stronger than comparably sized U-joint systems, but are also typically more difficult to service or replace than a U-joint due to IFS, FWD or other hardware. Materials and design does make a huge difference though.

What is a U-joint?
U-joint stands for Universal Joint. Its another way to transfer torque from one angle to another.

They work like this:

The U-joint is what is used in the front axles, pinions and driveshafts of all TJs.

The Cardan Shaft
This is what we all consider a drive shaft. This ultimately transfers power from your engine to your axles. They have U-joints on each end to allow for misalignment between the drive side and the driven side.

The Slip Shaft
Suspensions move up and down with terrain, and with axle flex there is even lateral movement. This means the distance between the yoke on the axle and the output of the transfer case are always changing. To accommodate this, engineers have developed a splined shaft inside the drive housing. This means the driveshaft actually changes length as the suspension moves. Its long when the axles droop low, and its short when the suspension is fully compressed. At ride height its somewhere in between.

Non Rubicon Models
All non Rubicon model TJ/LJ's have a NP231/NV231J transfer case with a Slip Yoke. This means the Slip Shaft is actually inside the transfer case. The driveshaft changes length during suspension travel by moving this shaft in and out of the transfer case.

How it works:

On a Jeep:

Rubicon Models
Rubicon models have the NV241J transfer case, with a fixed flange output. You can't really consider it a fixed yoke since the NV241J does not have a yoke mounted to its output shaft, there is a flange and harmonic balancer there - so I'll call it a fixed flange. The indication of fixed means the Slip Shaft is in the driveshaft. The flange on the back of the NV241J bolts to a Flange Style CV Socket Yoke, which is also part of the driveshaft.

Fixed Flange:

Fixed Yoke:

Lifting your Jeep
When you install a suspension lift and/or a high clearance transfer case skid plate, you are changing the relationship between the axles and the output of the transfer case.

Because U-joints can only allow a certain degree of angle difference between the drive side and the driven side they can bind. By changing the relation between the axles and the output shaft you increase (or decrease) this angle.

On a short wheel base TJ, this is extremely important since the drive shaft is so short. Vibrations from the driveshaft indicate you are destroying the bearings inside the transfer case and the U-joints. On a LJ (long wheelbase TJ), you still need to pay attention, the driveshaft is much longer, but you are still susceptible to the same issues as a SWB TJ. Rubicon or not!

NV231J equipped Jeep's will typically install a Slip Yoke Eliminator or transfer case drop. The SYE removes the Slip Yoke from the transfer case and replaces it with a fixed yoke, like the picture above. In conjunction with the SYE, the SINGLE cardan shaft is removed and replaced with a Double Cardan Shaft.

NV241J equipped Jeep's don't have a slip yoke, they are a fixed flange output, as shown above, so they don't require a SYE. The rear of every Rubicon has a SINGLE cardan shaft. This means you can get driveline vibrations where the U-joints bind, just like a NV231J equipped Jeep. This means you'll need a Double Cardan Shaft.

FYI: Rubicon's use 1330 u-joints, whereas standard TJ's use 1310 u-joints. To swap a NV231J for a NV241J (or vice versa), you'll need new driveshafts front and rear OR just a new rear driveshaft with 1310 u-joints, and a new front yoke on the transfer case. Spicer also sells a 1310/1330 conversion u-joint which may be used. Talk to Tom Woods for your specific application questions (linked below).

The Double Cardan Shaft
Finally, this is point we've been leading up to. A Double Cardan Shaft has TWO U-joints on the transfer case side - in an area called the Head Assembly. This allows significantly more angle variation between the drive side and the driven side. This means a Jeep with altered driveshaft geometry due to a lift or high clearance skid plate can install one of these and not have to worry about vibrations damaging drivetrain components. Also notice all TJ's have front Double Cardan driveshafts, but have Single Cardan Driveshafts in the rear from the factory.

The Rear Double Cardan Shaft can be installed on a NV231J with a SYE, or it can be installed on a stock NV241J. The Double Cardan Shaft will always have the Slip Shaft built into its long axis.

Because the Double Cardan Shaft moves along the length of its shaft section it requires the ability to change the pinion angle. This means adjustable control arms or control arm cam bolts. The axle pinion must point toward the output shaft of the transfer case. Otherwise the U-joints on the axle side will wear out very quickly.

But I thought this thread was about CV Shafts?

Well, I can't say it any better than this:

Jeepster83 said:
A lot of people call the Double Cardan drive shaft a CV shaft so when you bring up this subject most people will say they are the same. When you get to specifics they arent the same at all.

I know I'm guilty of calling it a CV shaft, even though I know better, mostly because its such common convention...but technically it should be DC shaft, or double cardan shaft.

Maybe its miss-wording, laziness, miss-communication, who knows! But as you can see, a CV shaft is NOT the same as a DC shaft...yet they are often used to define the same thing.

A Fixed Yoke style Double Cardan shaft

A Fixed Flange style Double Cardan Shaft

A Slip Yoke Eliminator (Fixed Yoke) and Double Cardan shaft for a NV231J

JK Single Cardan flange mount, Slip Shaft in center, and a Double Cardan Joint

Flange Type Multiple Cardan Shaft - for very severe driveline angles and competition.

Additional useful info from dippert, originally posted here: http://www.jeepforum.com/forum/f9/double-cardan-vs-cv-drive-shaft-754301/index2.html#post8020553

dippert said:
DC, Rzeppa (which i posted animations of), Tripod, Thompson couplings. They are all CV joints of varying sorts. A lot of car people think of Rzeppa styles when we say 'CV' joint. Is that a Rzeppa on the stock JK?
If you feel the need to jump on me about this, why don't you call them what they really are? Rzeppa and DC joints. Just calling it a CV might confuse somebody. Obviously the OP was confused about the difference.

Double Cardan Joint

Rzeppa Joint

Tripod Joint

Thompson Coupling
YouTube - Thompson Coupling

When a Rear Double Cardan Shaft is Typically Required

As many people say - Every Jeep is different!!!

Some SWB TJ's get vibes around 2" of lift, some vibe at 1.5", some make it until 3.5" of lift. Generally, the vibes start around 2-2.5" of lift and get worse with height increases. You can augment the driveline angle I've talked about above by installing a Motor Mount Lift (MML) or a Transfer Case Drop - both point the transfer case output more toward the rear axle pinion. They also worsen the angle on the front axle pinion - but remember, it is already a Double Carden Shaft, so it will mostly likely be fine (depending on severity of the angle). Similar to LWB TJ's all the same rules apply. LWB TJ's typically experience vibes starting above 4" of lift. Even Rubicon's will experience vibrations as lift increases - they still have a Single Cardan Shaft, but they can get away with slightly more lift without requiring a Double Cardan Shaft because the transfer case output is shorter - meaning the rear driveshaft is longer, and the driveshaft angle is less severe.

Example:
- A ~2" lifted SWB NV231J equipped TJ will most likely experience some vibrations. A small transfer case drop will fix this.
- A 2.5" lifted LJ (LWB TJ) with a NV231J will NOT experience vibrations. No SYE or transfer case drop will be necessary - the driveshaft is long enough the driveline angle is not as severe.
- A 2.5" lifted SWB or LWB NV241J equipped TJ/LJ will most likely NOT require a Double Cardan Shaft. The shorter transfer case allows for a longer driveshaft - less severe angles.
- A 4" lifted NV231J SWB TJ WILL require a SYE or large transfer case drop.
- A 4" lifted NV231J LJ will likely NOT require a SYE. Slight vibes may start at 4-4.5" - pay close attention!! 4-4.5" of lift is about the max you can get out the LJ's single cardan driveshaft without experiencing some vibes. You can most likely do a small transfer case drop (stack washers) to eliminate the vibes, or install a Double Cardan Shaft (SYE for NV231J and Flange style for NV241J)

Rear Driveshaft Angle induced vibrations show themselves when you are driving. Its not as simple as whether you have vibes or not. Severe driveshaft angles will result in constant vibrations, you can feel them through the seat and they do not effect steering or handling. They will change as you accelerate and decelerate, get on the throttle or off the throttle - they are obvious. Slight vibrations will manifest similarly, but sometimes less obviously. You may have to accelerate full throttle or 3/4 throttle to feel them. While severe vibes are clearly very bad, even slight vibrations will shorten the lifespan of your U-joints & t-case.

Rule of thumb:

Total Lift Seen by T-case = Suspension lift + Skid Plate Lift

Skid plate Lift = distance output shaft moves up from stock location (usually same as advertised clearance increase)
Body Lift doesn't count - it only makes clearance for moving the drivetrain up with a Tummy Tuck.

FYI: 97-02 TJ stock skid plates hang down 4" lower than the frame, 03-06 TJ/LJ stock skid plates hand down 4.65" lower than the frame.

These are only guidelines I've experienced or observed...remember - Every Jeep is different!!! The best way is to check your pinion angle and output shaft angle - see Post 7.

Vibes are not the only reason to get a new DC driveshaft!
You can pull the rear driveshaft completely off the t-case output shaft under the right circumstances of flex. It is more common on YJ's and XJ's due to the way the rear axle moves, but theres nothing stopping it from happening on a TJ/LJ either. It would seem more likely with an LJ since the driveshaft is longer, giving a false sense of no-vibe security, but even a SWB could be capable of this. Just because a t-case drop and MML will eliminate vibrations, they don't prevent this from happening.

Example 1: A destroyed T-case as a result of a slip yoke failure.

Example 2: Rear Driveshaft Yoke Pulled Out.

Example 3:

from: MJR - View Single Post - slip yok eliminator

Example 4:

from: http://www.jeepforum.com/forum/f9/rear-driveshaft-snapped-1448949/

Example 5:

from: http://www.jeepforum.com/forum/f9/temp-fix-rear-output-shaft-vibrations-4254026/#post39747762

What about pinion angle, MML's and transfer case drops?
Thats covered here- Post 7
If you are experiencing driveline vibrations, its a result of u-joints binding. The faster a u-joint turns the less severe an angle it can tolerate - this is why you usually feel driveline vibes worst at highway speeds.

Watch this video, specifically at 6:37. This is what your driveshaft is trying to do when you feel vibes.