Control Arm length... need 3 degrees of castor.

I just got my adjustable uppers and I'm at 7.5* of castor and would like to get it down to 5*. How much longer should I make the Upper Control Arm from what it is to get 2.5 more degrees?

In case you are not aware, pinion angle ALWAYS takes precedent over caster angle. With 5" of lift, getting castor back to 5* is not gonna happen feasibly.

hmm, well I'm at 7.5* now and only get slight vibe, so I was going to try 5* and see how that did for the pinion angle...

So I measured the front of the diff and it was about 3 degrees, with the driveshaft at about 9 degrees. Therefore I need about 7 degrees of castor back. Considering I'm at about 7.5 degress of castor now, how will that put my steering and highway driveability? It's not my jeep anymore but my brother's and I need to get it so it has 4wd and is still DDable.

And if it's fine to put my castor at 0* how much longer do my uppers need to be to get 7*?

ty20404 said:

So I measured the front of the diff and it was about 3 degrees, with the driveshaft at about 9 degrees. Therefore I need about 7 degrees of castor back. Considering I'm at about 7.5 degress of castor now, how will that put my steering and highway driveability? It's not my jeep anymore but my brother's and I need to get it so it has 4wd and is still DDable.

And if it's fine to put my castor at 0* how much longer do my uppers need to be to get 7*?

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How are you determining your caster ?
You 2000 XJ is low pinion right? Unfortunately, the low pinion D30 in the 2000-2001 XJ are more prone to driveline vibes.

Place an anglefinder vertically across the diff cover bolts above and below the axle. Then the formula for a low pinion is: 12 - diff angle = caster.

So if you have 3 degrees at your diff, then; 12 - 3 = 9 degrees caster. You can go as little as 4 degrees caster and you should still be ok. We are talking positive caster, so the the inner axle knuckles are leaning back toward the rear of the Jeep. So lengthen the UCAs until you have 7 degrees at the diff and try it out. A complete write-up is at: http://www.go.jeep-xj.info/HowtoAlignment.htm

As for how much you need to lengthen the UCAs, I think that is a matter of adjust, then measure the angle.

BTW - A low pinion can result in driveline vibes. A bad caster angle can result in loss of return to center in the steering, very poor handling and can be a safety issue. You decide which is more important. The statement that pinion angle is always more important than caster is not true.

I understand everything you said, fortunately I'm not a newb so I'm not very gullible when i comes to this shit...

It is actually a HIGH PINION, i swapped it out a couple months ago so you might have to throw me some new measurements. My caster angle was just the angle that Firestone gave me on the printout during the last alignment. The measurement on the diff bolts in front was in fact right between 2 and 3 degrees, and I was sure that going lower than 5 degrees is going to F with steering, so right now I'm looking to just get it back down to 5 degrees of castor. Just need to figure (about) how much longer I need to make the Uppers to get that much caster back (about 3 degrees). Thanks.

ty20404 said:

I understand everything you said, fortunately I'm not a newb so I'm not very gullible when i comes to this shit...

It is actually a HIGH PINION, i swapped it out a couple months ago so you might have to throw me some new measurements. My caster angle was just the angle that Firestone gave me on the printout during the last alignment. The measurement on the diff bolts in front was in fact right between 2 and 3 degrees, and I was sure that going lower than 5 degrees is going to F with steering, so right now I'm looking to just get it back down to 5 degrees of castor. Just need to figure (about) how much longer I need to make the Uppers to get that much caster back (about 3 degrees). Thanks.

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Yeah, nice friendly reply there.
You could have stated that you had a high pinion and saved us both some wasted time. No one said you were gullible, and if your not a newb then you shouldn't have to ask the question about length as the only way to do it is to adjust and measure. I won't bother telling you the formula for your high pinion, since you're not a newb.

Sierra Drifter said:

Yeah, nice friendly reply there.
You could have stated that you had a high pinion and saved us both some wasted time. No one said you were gullible, and if your not a newb then you shouldn't have to ask the question about length as the only way to do it is to adjust and measure. I won't bother telling you the formula for your high pinion, since you're not a newb.

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The newbiness wasn't actually aimed at you, it was aimed more at the reply prior to yours. Yours are actually what I use this forum for. Sorry for that confusion. Thanks.

Chris @ GATR said:

In case you are not aware, pinion angle ALWAYS takes precedent over caster angle. With 5" of lift, getting castor back to 5* is not gonna happen feasibly.

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:scared:
Please dont believe that.

Take it from someone who had bad caster on a SOA CJ - incorrect caster settings can lead to you upside down in a ditch on the way to the trail.

Alright, I did some trig and I'll have to extend my upper by about 3/4" to get the castor to about 5*. Does that sound about right?

Hopefully that will reduce front end driveline vibes to an amount that will hardly be noticeable or have too high of a detrimental effect on anything. I'm thinking that if you can hardly hear it, it will still cause premature wear on the U-Joints but not enough to make them go out to quickly....

Id say no way,maybe about a 1/4" at the max.

really? I mean, I found the lower arm to be approximately 17" and the upper to be approximately 15". The distance between the upper eye and lower eye is about 8" (these were all quick measurements, so if someone has exact please let me know). This make a right triangle with the angle at the upper corner being 90*. I don't have the math anymore (left it at home, now back at school), but I used the law of cosines to figure out the length that the upper arm had to be for the original lower angle plus 4*... I know the math was right, but I'm sure my measurements were not exact.