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Unread 04-02-2002, 12:59 PM   #1
Dan Morera
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I think I did something wrong

OK I got my 2.5" lift plus 1" shackles, and I just put my 31" and they still rub when I brake hard or little flex, what can you tell me.

I dropped my tranny but I still habe vibs when I'm in 1st and 2nd gear. What's with the angles????

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Unread 04-02-2002, 02:00 PM   #2
osburn
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If you dropped the tcase, did you also shim the rear axle? You can't do one without the other. You'd likely find if you put the tcase back up where it belongs, you won't have any vibrations. Usually it takes more than 3" of lift to require dropping the tcase and shimming the rear axle.
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Unread 04-02-2002, 02:41 PM   #3
Dan Morera
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I'm from Costa Rica, so my english is not that good, what you mean by shiming the rear axle.

What does "shim" means?

Sorry
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Unread 04-02-2002, 04:25 PM   #4
4wheeler4CJ
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Osborn,

With all due respect, why can't you drop the TC without shimming the rear axle, or vice-versa? They are a "cumulative" effect, no?

Dropping the TC lessens your angles, as does shimming the rear-end. Whether you do one, the other, or a combination of both, anything will help the driveline angles.
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Unread 04-02-2002, 06:51 PM   #5
RCHavok797
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I have a 2 inch suspension lift on my YJ with 31s and have never had any problems with rubbing. There is still plenty of clearence. How do you have rubbing with more lift than I do and the same size tires?
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Unread 04-02-2002, 07:13 PM   #6
osburn
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</font><blockquote><font size="1" face="Verdana, Arial">quote:</font><hr /><font size="2" face="Verdana, Arial">Originally posted by 4wheeler4CJ:
<strong>Osborn,

With all due respect, why can't you drop the TC without shimming the rear axle, or vice-versa? They are a "cumulative" effect, no?

Dropping the TC lessens your angles, as does shimming the rear-end. Whether you do one, the other, or a combination of both, anything will help the driveline angles.</strong></font><hr /></blockquote><font size="2" face="Verdana, Arial">When you run a standard driveshaft (ie one u-joint at each end) the angle of these two u-joints must be parallel. By dropping the tcase, you're changing the angle of the upper joint. You must change the angle of the pinion joint to compensate for the change you made on the tcase output. So if you do one, you must do the other. That's why this fellow is suffering from vibrations. Based on his lift, I don't think he needs to drop his tcase. That's what I'd try first anyway. If he still has vibrations, then lower the tcase back down and shim the rear axle pinion up 2.5 degrees.

This rule does not apply when you run a CV driveshaft. The two joints in the CV counter each other so the joint on the pinion must be zero. Or actually, it's best in practice if you set the lower joint to be about 2 degrees shy of zero.
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Unread 04-02-2002, 07:15 PM   #7
osburn
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</font><blockquote><font size="1" face="Verdana, Arial">quote:</font><hr /><font size="2" face="Verdana, Arial">Originally posted by Dan CR:
<strong>I'm from Costa Rica, so my english is not that good, what you mean by shiming the rear axle.

What does "shim" means?

Sorry</strong></font><hr /></blockquote><font size="2" face="Verdana, Arial">Steve's gonna love this cause I hate calling shims this, but he always does. Shims are degree wedges. You put them between the perches and the springs to rotate the axle. You need 2.5 degree shims. Put them in between the springs and the perches so the front of the axle rotates upward. But really, I'd just put the tcase back up first. You probably don't need to do this.
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Unread 04-02-2002, 07:41 PM   #8
jeepinarkansas
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heres my two cents...I put a 3" suspension lift on my yj, rough country. When I first installed it I didn't put in the tc lowering brackets. That vibrated the hell outta me in 1-3 gears. I then put on the tc bracket that helped a bit, I was confused because I still had vibrations with the 6 degree wedge that came with the rear springs. So I added a 2.5 degree shim. That also helped the vibrations. I also adjusted my motor mounts and replaced my factory transmission mount. Those things also helped. It may be helpful to rebalance your driveshaft tube as well. Good luck. Finding the right solution can be tricky.
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Unread 04-02-2002, 10:11 PM   #9
rollinashortysahara
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haha i have 31x10.5 and they don't rub unless i'm wheeling or there is about 400lbs worth of somthing in the back
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Unread 04-02-2002, 10:12 PM   #10
rollinashortysahara
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p.s. no lift
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Unread 04-02-2002, 10:25 PM   #11
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Osburns right guys. I did a shackle lift( about 2") out back and ended up with a 1-2 gear vibration. I just lowered my t/case about 1/2" this eve. and vib all good. The shackle lift itself tilted the pinion enough to cause the vib.
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Unread 04-03-2002, 09:34 AM   #12
damonejeep
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I must agree with the masses. There should be no need ot perform both mods. 2.5" should not require both, TC lowering and shims.
You might want to check on the drive shaft or U-joints. They could be going bad.
Other wise I agree with OZ. DO one, then the other, and if needed, both of them. See which works best for your situation, and if all else fails, find a trusty 4x4 shop in your area and take it to them. The last thing you want to happen to is bust your driveshaft on or off the trails.
Good luck. Keep us posted on how it goes.
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Unread 04-03-2002, 10:50 AM   #13
4wheeler4CJ
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</font><blockquote><font size="1" face="Verdana, Arial">quote:</font><hr /><font size="2" face="Verdana, Arial">Originally posted by osburn:
<strong> </font><blockquote><font size="1" face="Verdana, Arial">quote:</font><hr /><font size="2" face="Verdana, Arial">Originally posted by 4wheeler4CJ:
<strong>Osborn,

With all due respect, why can't you drop the TC without shimming the rear axle, or vice-versa? They are a "cumulative" effect, no?

Dropping the TC lessens your angles, as does shimming the rear-end. Whether you do one, the other, or a combination of both, anything will help the driveline angles.</strong></font><hr /></blockquote><font size="2" face="Verdana, Arial">When you run a standard driveshaft (ie one u-joint at each end) the angle of these two u-joints must be parallel. By dropping the tcase, you're changing the angle of the upper joint. You must change the angle of the pinion joint to compensate for the change you made on the tcase output. So if you do one, you must do the other. That's why this fellow is suffering from vibrations. Based on his lift, I don't think he needs to drop his tcase. That's what I'd try first anyway. If he still has vibrations, then lower the tcase back down and shim the rear axle pinion up 2.5 degrees.

This rule does not apply when you run a CV driveshaft. The two joints in the CV counter each other so the joint on the pinion must be zero. Or actually, it's best in practice if you set the lower joint to be about 2 degrees shy of zero.</strong></font><hr /></blockquote><font size="2" face="Verdana, Arial">Where the heck did you get that info? I did a 2.5" suspension lift, did not shim my rear axle, then lowered my TC a good inch or more, and never had vibes. The U-Joints do not require to be at the same exact angle on each end, there's no way to justify that reasoning.

A TC case drop will better your vibes. If it doesn't solve it, then you have to look elsewhere, but that doesn't mean that dropping the TC case caused it or worsened it, LOL!
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Unread 04-03-2002, 02:08 PM   #14
osburn
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Ok, I looked for some reference to explain the issue here. What you must understand is that while a u-joint is rotating, it causes a change in speed or velocity. The amount of change in speed is directly related to the angle it operates at. In other words, the more steep the angle, the more the u-joint changes velocity as it rotates. This is the cause of vibrations generally, and not necessary the angle in and of itself.

Here's a small excert from one site explaining how a CV joint works in relation to a standard joint:

&lt;quote&gt;
CV' stands for 'constant velocity,' and the joints are called that because, unlike old-fashioned, cross-type U-joints, they're able to transmit power through fairly steep angles without constantly gaining and losing speed. Standard universal joints tend to 'whiplash' at sharp angles.

Why does a standard U-joint vary the speed of the shaft being driven? Because the tips of the cross attached to the driven shaft don't stay in the same plane as the driving tips (the ones attached to the driving shaft). By their very nature, they're out of phase if the joint has to turn any kind of corner. If paired and phased properly to another cross-type joint, and if the working angles of both joints are relatively shallow and matched to each other, most vibration is self-cancelling and virtually a non-issue.
&lt;unquote&gt;

Here's more info. I couldn't find anything that was really simple in it's explanation.
I've included the URLs where I found this info in case you'd like to look into it further. The second part of what I've copied here is actually much clearer in its explanation, but you need the first part to know what they're talking about in the first place.

Here they're talking about a standard driveshaft. The first sentence is talking about the two yokes at one end of a driveshaft containing the u-joint.

<a href="http://www.autotune.com/cprotest/cpc12c.html" target="_blank">http://www.autotune.com/cprotest/cpc12c.html</a>
&lt;quote&gt;
Remember, both yokes travel at the same speed of rotation, taken as a unit. That is, 2,000 RPM in the drive yoke will produce 2,000 RPM in the driven yoke. Unfortunately, this 2,000 RPM in the driven yoke is not delivered at a constant speed. Rather, because the cross-and-bearing oscillates, the output RPM is delivered at an oscillating speed. It gets faster and slower then faster then slower as it turns. From the horizontal plane or rotation, you have to accelerate the mass of the yoke (and its cross and bearing units) from rest, to 1/4" off plane, then come to a complete stop (when the tip is at maximum wobble point) and accelerate it back to zero again repeating the acceleration/stop/acceleration cycle for the yoke tip below the plane of rotation. At just a couple of RPMs, this variation in speed, or "velocity" (a non-constant velocity) doesn't make all that much of a difference. Also, at very low angles of variance, the difference in velocity is very small.
&lt;unquote&gt;

Now they go on to explain how to deal with the changing velocity in a driveline.

<a href="http://www.autotune.com/cprotest/cpc12d.html" target="_blank">http://www.autotune.com/cprotest/cpc12d.html</a>

&lt;quote&gt;
This variation in velocity would normally be felt as a rather annoying vibration in the driveline of the car, except for one design trick: if you put another universal joint assembly at the other end of the driven shaft and connect it to another driven member, then the wobbles at either end of the driveshaft "cancel out", so long as the angles expressed at either end of the shaft are very close to identical. In order to keep them identical, it is necessary to keep the output angle of the transmission and the input angle of the rear axle essentially parallel. ("Perfection" is not required. Minor variations in parallelism are acceptable.)
&lt;unquote&gt;

So it all boils down to the u-joint constantly "wobbling" in velocity. It may be hard to imagine, but you have to believe it's true. It is the reason your rear pinion is not just point up at a zero angle to tcase from the factory. If what you said was true, why wouldn't they just take all the angle out at the pinion end? The point of all this is that you must keep these joints parallel. That's just the way it works. Remember, a CV is a totally different story though.
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Unread 04-03-2002, 02:18 PM   #15
osburn
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The top of this page seems to cut to the chase on this subject.

<a href="http://www.sixstates.com/service/drivelines/learn.html" target="_blank">http://www.sixstates.com/service/drivelines/learn.html</a>
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