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Old 03-04-2011, 08:54 AM
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Originally Posted by Jerry Bransford
Yep that joint with the two u-joints is a constant-velocity joint aka CV joint, aka double-cardin joint. That's an aftermarket driveshaft so it has three u-joints total and that is what you typically run with an SYE. The factory driveshaft only has two u-joints total, one at both ends.
So how can I tell what type of driveshaft do I have? I haven't seen any markings on so far. Does any of this explain why I'm killing u joints? Maybe I just bought some garbage ones... Maybe I won't have anymore problems. I don't drive my jeep hard at all. I don't mud, I don't crawl, it's a dd with maybe a small trail here and there. So is it a good thing to have this aftermarket driveshaft and sye? I'm assuming it is. I have a 6 inch suspension lift so I assume I have to have a sye, right? I'm finding so much stuff out about my jeep that I never even knew was aftermarket. I guess that's a good thing...
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Old 03-04-2011, 08:59 AM   #32
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Often it's a simple thing that causes premature wear. Many shops don't know the "trick" and very few backyard guys do.

After installing the U-joint, notice how it is tight. It doesn't just "flop" around like it should. They are bearings, it should move very freely.

Take a hammer and "smack" each ear next to the bearing one at a time. You'll notice it "flops" easy now.

What happens is when you press the bearing in enough to get the snap ring on, it puts the bearing in a bind, but it doesn't pop back out. It's just a matter of a few thousandths, "Smacking" them after you've finished allows them to seat properly against the snap ring. Hit the yoke's ears, not the bearing caps themselves.

You'd think it would move enough to stop binding by driving it - sometimes it might, but sometimes it doesn't, causing early failure.

Since they are so new on yours try going under and smacking each ear now - it may work. It's much easier than replacing them.

No matter what kind or quality U-joints you use, they all need to be "stress relieved" that way.

Sometimes that binding is the cause of vibration too.

It's no wonder the country is falling apart - stupidity abounds!
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Old 03-04-2011, 09:09 AM   #33
Knows a couple things...

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That your rear DS is an aftermarket CV driveshaft is not why you had u-joints go bad. What can cause u-joints to go bad is if the rear pinion angle is incorrect which causes the u-joints to vibrate which will ruin them in short order. Or lack of lubrication from being old if they are sealed, or lack of lubrication if they have grease fittiings on them and they aren't greased regularly. Personally I run sealed u-joints in my driveshafts and prefer them over greasable u-joints for the type of wheeling I do.

Yes it's a very good thing to have a CV driveshaft and a SYE installed which is pretty much required once you have a suspension lift that is taller than 4" as your 6" is. The SYE shortens the transfer case length which allows for a longer driveshaft which decreases its operating angle which decreases the angle at the u-joints which is a good thing.

With a two u-joint driveshaft, like the factory installs in the rear , the two u-joints divide the angle as you see below in the first illustration. The problem is that that angle gets very steep with a tall suspension lift and the u-joints can't cope with such steep angles so they vibrate with tall suspension lifts.

With a three u-joint (CV) driveshaft as shown in the second illustration below, the rear u-joint sees no angle at all. At the front CV joint, the two u-joints divide the angle between them so neither u-joint sees much of an angle. It's a very cool design. Incidentally, the factory installs a CV (three u-joints) driveshaft in the front of all of our TJs.
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