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Unread 09-10-2009, 12:37 PM   #31
mouse
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Ok so with 2.5 lift and a 1.25 BL and a MML theres nothing to worry about since i'm not changing the angels on the drive shaft any further than i already have. on the flip side the MML should fix the angels that i have already changed. Is this the correct process of thought?

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Unread 09-10-2009, 12:50 PM   #32
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Unread 09-10-2009, 02:40 PM   #33
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Quote:
Originally Posted by jgorm View Post


I would figure this to mean that you would need one if you go over ~2.5"
not quite, LJ's can take about 4-4.5" of lift before needing a Double Cardan Shaft. I updated that section a bit to clarify.

Keep in mind the slip yoke itself is still the same length for a SWB TJ vs an LJ. This means the driveshaft can only extend its length so much, regardless of wheelbase. You don't want your driveshaft to fall out.

Quote:
Originally Posted by mouse View Post
Ok so with 2.5 lift and a 1.25 BL and a MML theres nothing to worry about since i'm not changing the angels on the drive shaft any further than i already have. on the flip side the MML should fix the angels that i have already changed. Is this the correct process of thought?
Well, keep in mind regardless of lift height and MML's, you are always changing the distance between the output shaft and the pinion. The higher the lift, the more distance between the output shaft and the axles. The MML only points the transfer case output shaft toward the rear axle, nothing else.

Similarly, a transfer case drop moves the output shaft down and points it toward the axle - BUT it LENGTHENS the driveshaft sightly. Why? Think about the hypotenuse of a triangle! The motor mount isn't moving back, but the transfer case is dropped, so the transfer case mount actually moves toward the front of the vehicle- meaning the driveshaft gets longer - via the slip yoke extended slightly. Its only like a 1/4" tho

Extreme Example: Even a long wheelbase crew cab pickup with a driveshaft as long as an entire TJ may need a new rear driveshaft IF the lift height is high enough to cause the slip yoke to pull completely out the transfer case during suspension travel.

Last edited by Unlimited04; 09-10-2009 at 02:51 PM..
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Unread 09-10-2009, 02:55 PM   #34
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Quote:
Originally Posted by Unlimited04 View Post
not quite, LJ's can take about 4-4.5" of lift before needing a Double Cardan Shaft. I updated that section a bit to clarify.

Keep in mind the slip yoke itself is still the same length for a SWB TJ vs an LJ. This means the driveshaft can only extend its length so much, regardless of wheelbase. You don't want your driveshaft to fall out.



Well, keep in mind regardless of lift height and MML's, you are always changing the distance between the output shaft and the pinion. The higher the lift, the more distance between the output shaft and the axles. The MML only points the transfer case output shaft toward the rear axle, nothing else.

Similarly, a transfer case drop moves the output shaft down and points it toward the axle - BUT it LENGTHENS the driveshaft sightly. Why? Think about the hypotenuse of a triangle! The motor mount isn't moving back, but the transfer case is dropped, so the transfer case mount actually moves toward the front of the vehicle- meaning the driveshaft gets longer - via the slip yoke extended slightly. Its only like a 1/4" tho

Extreme Example: Even a long wheelbase crew cab pickup with a driveshaft as long as an entire TJ may need a new rear driveshaft IF the lift height is high enough to cause the slip yoke to pull completely out the transfer case during suspension travel.

Ok so that kind of brings me back to the same question. How high can i go before i need a SLE or a DC and keep the drive train vibes under control?
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Unread 09-10-2009, 03:00 PM   #35
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Quote:
Originally Posted by mouse View Post
Ok so that kind of brings me back to the same question. How high can i go before i need a SLE or a DC and keep the drive train vibes under control?
re-read the first sentence of what you just quoted.
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Unread 09-10-2009, 03:12 PM   #36
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Quote:
Originally Posted by Unlimited04 View Post
re-read the first sentence of what you just quoted.
Sorry you posted back up before I could do and edit of my last post.
Ok since I only plan on running 33's on my rig and going no bigger due to having to regear and all that fun stuff. Having a 2.5 kit already installed I can do 3/4 spacers and still be ok with the addition of a BL and MML and extra hardware such as a TC bracket ECT. So I don't rub under full flex. Now that would give me 4.5 inches of lift for tire clearance not 4.5 inches of true lift that I would get from a full out kit. This also keeps me from that border line of having to install the SYE and DC. Correct? Or would it just be better to do the BL and MML and not worry about any further lift in the suspension. My understanding with the BL is if you don't do the MML then you have to trim the fan shroud.
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Unread 09-10-2009, 03:25 PM   #37
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Quote:
Originally Posted by mouse View Post
Sorry you posted back up before I could do and edit of my last post.
Ok since I only plan on running 33's on my rig and going no bigger due to having to regear and all that fun stuff. Having a 2.5 kit already installed I can do 3/4 spacers and still be ok with the addition of a BL and MML and extra hardware such as a TC bracket ECT. So I don't rub under full flex. Now that would give me 4.5 inches of lift for tire clearance not 4.5 inches of true lift that I would get from a full out kit. This also keeps me from that border line of having to install the SYE and DC. Correct? Or would it just be better to do the BL and MML and not worry about any further lift in the suspension. My understanding with the BL is if you don't do the MML then you have to trim the fan shroud.
Ok tire rub has nothing to do with transfer case/SYE or lift height. Thats an entirely different subject.

With a 2.5" lift and a 2-2.5" Tummy Tuck (high clearance transfer case skid plate), you are right on the border of needing a Double Cardan Shaft - thats 4-5" total lift. I have a 1" BL, 1" MML, 2.5" OME lift and a 2.25" TT - I had slight vibrations from the driveline, so I installed a 1" transfer case drop. This pointed the output shaft toward the axle enough to eliminate the vibes. All this is in My Build Thread, linked in my sig - yes, I have a TT with a t-case drop. I've seen this same lift on a Rubicon LJ, one which had slight vibrations from the drivetrain - the double cardan shaft was required.


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 TT.

When you install a High Clearance t-case skid, called a Tummy Tuck AND you install a MML, you are basically retaining a close to stock driveline angle from the crackshaft to the output shaft. This doesn't mean you can subtract 1" of lift from the above equation - it just means the output shaft isn't pointing upwards on a more severe angle than stock.

Last edited by Unlimited04; 09-10-2009 at 04:08 PM..
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Unread 09-10-2009, 03:47 PM   #38
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A picture is worth a thousand words. PLUS you had a thousand words. This is a great explanation! I'm sure this thread will be linked many times.
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Unread 09-10-2009, 09:16 PM   #39
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Quote:
Originally Posted by Unlimited04 View Post
so as I described in post #1, CV's are Constant Velocity joints. A true CV axle would be an axle shaft with a CV joint in it.

Like this front wheel drive application:


Remember they work like this:


Notice there is a boot covering the mechanism. Remember, CV axles are stronger for their given size than a U-joint system, which is why they are used so frequently where space is premium - like Front Wheel Drive sedans and Independent Suspension systems.



Strength is always about two things - material choice and how much force you put across how much area of that material. CV's are stronger by design - they have more degrees of movement and more surface area to spread torque across. U-joints have two points of contact - each roational axis of the 4-pointed U-joint. Constant Velocity joints have the little ball bearings inside that spread the same force across a much greater area - think of it almost like a multiple-pointed U-joint.

The major downside to CV axles in a TJ is they will most likely require some grinding or knuckle modifications and they are EXPENSIVE.

But since we're talking about drive shafts here, read this thread and start another if you feel the need -
http://www.jeepforum.com/forum/f9/fr...t-some-649714/

First, great write up! Second, and maybe I missed it, but does anyone make a true CV driveshaft? If they're smoother and stronger, what's the drawback? Is it just too expensive to justify when the DC will perform almost as well?
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Quote:
Originally Posted by Imped View Post
Your shaft isn't long enough to handle that kind of droop...
My build thread: http://www.jeepforum.com/forum/f9/ru...thread-792423/
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Unread 09-10-2009, 09:29 PM   #40
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Quote:
Originally Posted by RubiconRazorbac View Post
First, great write up! Second, and maybe I missed it, but does anyone make a true CV driveshaft? If they're smoother and stronger, what's the drawback? Is it just too expensive to justify when the DC will perform almost as well?
I know of no company that makes a true CV driveshaft, but I've never really looked. I don't know why you'd need one. When's the last time you even heard of a broken driveshaft or U-joint from torque alone - not smashing it into something? I wouldn't think it would be all that hard to fabricate one if you had the right stuff, but it would likely follow the same trend as other CV applications - expensive.

I'm positive there are performance applications where stronger CV driveshafts would be a benefit - especially drag racing applications. Lots of traction and lots of torque there - enough to shear lug nuts and wheelie....even in mud and sand events, not just on pavement.

Heck Mythbusters took an old 70s Plymouth Fury and literally cut 50%-75% of the material of the yokes out with a grinder, and they still couldn't get the driveshaft to break by driving the car at 35mph in Drive and slamming it into Reverse.
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Unread 09-11-2009, 07:31 AM   #41
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Originally Posted by kellerumps View Post
To Quote Spicoli from Fast Times at Ridgemont High..."Awesome....Totally Awesome". I need to buy you a beer......




Great thread very informative. The animation is a great idea, did you do it yourself or find it somewhere. That helps a lot!

Good Job!!
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Unread 09-11-2009, 07:40 AM   #42
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Quote:
Originally Posted by Doogie1022 View Post
Great thread very informative. The animation is a great idea, did you do it yourself or find it somewhere. That helps a lot!

Good Job!!
The only image that is my own is the Fixed Flange. Its the NV241J I bought from Gerald at Savvy Off-Road.

Everything else came from these:

Sources:
Constant-velocity joint - Wikipedia, the free encyclopedia
Universal joint - Wikipedia, the free encyclopedia
Drive shaft - Wikipedia, the free encyclopedia
Pirate4x4.Com - Extreme Four Wheel Drive
http://www.jeepforum.com/forum/f11/o...l-read-426483/
http://www.jeepforum.com/forum/f9/do...-shaft-754301/
Tom Wood's Custom Drive Shafts - Tom Woods Custom Drive Shafts Custom Driveshafts Specialist

Google Images found these: http://www.motorera.com/dictionary/pics/S/slip_yoke.jpg, http://www.oconeeoffroad.com/catalog/18676_60.jpg, http://image.4wheeloffroad.com/f/171...inator_kit.jpg, http://jeep.off-road.com/jeep/data/a...4/tomwoods.JPG, http://www.quadratec.com/Assets/Imag...9/84109-md.jpg
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Unread 09-12-2009, 02:39 PM   #43
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Originally Posted by maaghimself View Post
Anyone ever wonder why they put the fixed yoke with the double cardan shaft on the front with the longer (and less steep) driveshaft angle, and put the slip yoke on the rear of the transfercase and coupled it with a ridiculously short drive shaft?
It's all about angles. To put the double cardan on the rear and a single cardan on the front, they would need to point the front output shaft downwards so that the pinion angle and the output shaft angle were equal. This would require tilting the whole drivetrain down, which would put the oil pan of the engine about 2 inches off the ground . By tilting the whole drivetrain upwards instead, they get the oil pan further off the ground, and can set up the rear upper control arms to keep the pinion angle pretty much constant in relation to the output shaft as the suspension travels. With a double cardan setup on the stock short arms geometry, the pinion angle at full compression is *not* pointing at the transfer case output shaft the way it should be -- it's pointing somewhere below the transfer case output shaft. And on a short driveshaft it's even further out of spec than with the long driveshaft on the front. We really don't care too much for offroad purposes because the suspension will be compressed that much only at very slow speeds, but the OEM has to make sure that under normal conditions the drive train will last out the warranty period, so a setup that automatically is out of spec at full compression simply isn't going to be put on there stock.

So anyhow, that's why Chisler did what they did. It wasn't about being cheap (well, sort of), it was about keeping angles such that stock Jeeps with stock suspension's U-joints would stay in spec over full travel so that they'd last out the warranty period. Since we check out our rides regularly and change out the U-joints as needed rather than require them to last out a 100K mile powertrain warranty, we can ignore those requirements to a certain extent, but Chrysler can't.
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Unread 09-12-2009, 04:25 PM   #44
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Great thread.
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Unread 09-12-2009, 04:58 PM   #45
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Unlimited,

Thanks for the writeup.

I feel like dolt, but I'm not understanding why the pinion has to point right at the t-case output? Isn't the purpose of the u-joint to deal with an angle difference? I know I'm missing something simple.

Thanks!
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