Question on caster alignment

[jcnoble]

88 XJ Rustys 4.5" lift. I hae always had death wobble problem when hitting potholes. I set toe in and adjusted teackbar. I am using the caster alignment procedure found here. It reads 0 degrees without the 9 degrees it says to add. Does that mean i am +9 degrees or - 9 degrees. The uca Rusty furnished is 16", lca is 16.75. The chart on this site says uca s/b 15". To me that means i am + 9 degrees. Which means my axle is rotated in the wrong direction. Caster s/b at about -5 degees. Which i had a diagram to show u. If looking from drivers side shouldnt the axle be rotated clockwise about 5 degrees. Mine is rotated counter clockwise ~9 degrees according to chart found on this site. To me it seems i need a shorter uca.

 

[xjj33p3r]

With a 4.5" lift, you don't need to worry about your control arms, you need to worry about your allignment, and you need to set the toe at 0, not in or out. If you still have deathwobble, check your rod ends to make sure they are still tight and not loose at all. If they are loose, they need to be replaced.

 

[Eagle]

Caster s/b at about -5 degees. Which i had a diagram to show u. If looking from drivers side shouldnt the axle be rotated clockwise about 5 degrees. Mine is rotated counter clockwise ~9 degrees according to chart found on this site. To me it seems i need a shorter uca.

No, caster should be positive. Stock spec is 7.5 degrees preferred, acceptable range 7 to 8 degrees.

When you lift, the axle swings down relative to the body, around the body end control arm mounts as fulcrums. The geometry of the UCAs and LCAs is such that you lose caster as you lift. Longer (or adjustable) LCAs are a way of putting some of that caster back in.

The problem is that if you put it all back, to get the alignment within spec, it puts the front pinion u-joint at too big of an operating angle, so the alignment for a lifted vehicle is a compromise, aiming for as much caster as you can get without cooking the front u-joint.

To get back to stock caster and stock wheelbase at 4-1/2" of lift, you would need LCAs that are 16-3/8" long and UCAs that are 15-11/16" long. But ... that's to get back to the stock 7.5 degrees of caster, which may be too much for the u-joint. So the UCAs might be made a tad longer, or the lowers a tad shorter.

 

[jcnoble]

Confused here, if i follow the caster alignment procedure on this site i have too much caster 8-9 deg., just eyeballing it the springs look bowed back a little which means i would need more caster. I did add 1/4" to the lca with shims and it seemed to handle a little better (it added 1 degree, measured with angle finder and level) but still dw on the pothole. Also the uca and lca numbers you gave me dont jive with the control arm length table posted on this site. I think ill take it in to the shop get some caster numbers and go from there. Toe in is 1/16, new tie rod ends, Rustys adjustable track bar. Had my kid turn the steering wheel back and forth while looking from under , no slop.

 

[Eagle]

Also the uca and lca numbers you gave me dont jive with the control arm length table posted on this site.

I am fully aware of that. I didn't say my numbers were from anyone's table that anyone here (other than MaXJohnson) has seen.

There are several XJ control arms charts floating around Internetland, and it has been widely reported that many (if not all) of them are incorrect. So I set out to make up my own.

What I did was measure up a stock axle, locate the axle and and chassis end locations, and plot them in AutoCAD. Then I progressively raised the chassis end in 1" increments (with a stop at 4-1/2", since that's a popular lift height) and measured the resulting control arm lengths.

As I stated in my post, the numbers I provided are the lengths that will maintain the stock wheelbase and the stock caster angle. Because keeping 7.5 degrees of caster with a lift results in unacceptable front pinion u-joint angles, what actually works is a compromise between optimum alignment, and a modicum of u-joint longevity.

Depending on how your tires fit your bodywork (if there's any left), the lengths I cited could be adjusted to improve the u-joint angle by either shortening the LCA length slightly, or lengthening the UCA slightly. Either one will reduce caster angle and raise the pinion. Using a shorter LCA reduces wheelbase, while using a longer UCA lengthens wheelbase.

 

[jcnoble]

Couple of questions please. What caster angle should i have with the control arm lengths you provided and does that include the shims that come with the stock setup? That musta been fun fun playin with autocad to do that. Thanks for your help!

 

[corbinafly]

If looking from drivers side shouldnt the axle be rotated clockwise about 5 degrees. Mine is rotated counter clockwise ~9 degrees according to chart found on this site. To me it seems i need a shorter uca.

What?! When you say counter clockwise 9 degrees are you talking about the caster or the pinion angle? If you're talking about the caster, then there is something seriously wrong. It should definitely be rotated clockwise from vertical.

K

 

[jcnoble]

I'm just trying to figure out the caster alignment procedure I got from this site. It says to measure the front axle rotation with a angle finder and level, then subtract fron 9 degrees for my 88XJ. The numbers i got dont make sense so im revisiting my geometry class and tryin to make sense of this.

 

[Eagle]

Couple of questions please. What caster angle should i have with the control arm lengths you provided and does that include the shims that come with the stock setup? That musta been fun fun playin with autocad to do that. Thanks for your help!

The shims are intended to fine-tune the stock control arms to provide 7.5 degrees of POSITIVE caster. The purpose of my AutoCAD exercise was to determine the control arm lengths required to maintain that same caster angle while lifting the chassis. I didn't move or change the axle ends when I did this. In each case I just raised the mounting points for the chassis ends, and measured the resulting distance between centers. So the numbers I have would result in the same 7.5 degrees of caster, with no changes to the shims.

My next step will be to add the pinion u-joint and front drive shaft to the picture, determine what the maximum acceptable operating angle should be, and then ratchet the chassis up in one inch increments to determine (a) the amount of lift tolerable before the pinion angle becomes a problem, and (b) how much the caster has to be reduced at each height in order to keep the u-joint angle within an acceptable angle.

 

[jcnoble]

I took it to the shop to get caster numbers, haha. The first thing the guy said was "we dont do alignment on lifted vehicles". Someone musta sued them, CA go figure. Frkin CA! Anyway i explained to him I only need the caster and toe numbers, I will do the rest, OK? So there it sits for 2 hours and then i get a dandy printout for $60. And they dont even have to adjust anything , $60 bucks wow. Anyway, once i get some numbers I can proceed from there.

 

[jcnoble]

Caster is +6.5 degrees, toe .4 in, about 1/16 on my tape measure.

 

[jdougn]

Correct me if I'm wrong, (lol) but doesn't the pinion coming out of the differential point towards the t-case and not towards the ground? (Talking about clockwise vs counter clockwise has got me all screwed up.)

 

[jcnoble]

Ya im screwed up too, haha. What i was tryin to say i think was that as the caster changes the axle rotates. Positive caster i believe is the axle rotating clockwise as view from the drivers side. Thus the need for longer lca's to correct with a lift. Of course I could be mistaken.

 

[TheNerd]

As I stated in my post, the numbers I provided are the lengths that will maintain the stock wheelbase and the stock caster angle.

I would think you would want to reduce your wheelbase (slightly) when adding a lift, such that, when your wheels are fully stuffed, they fit into the center of your wheel well. If you keep the stock wheelbase at lifted height, then flex the front your wheels will hit the front of your wheel well, no?.... of corse, this is assuming that you are using the stock control arm mounting locations.... if you get the drop brackest, then all this crap goes out the window....

TheNerd

 

[jcnoble]

I'm just tryin to use what came with the lift. I think I would be better off with adjustable controm arms, but by the time I spend more money on those (with drop brackets) I could almost have a longarm unit. The wheels are in the center of the wheelwell at rest so yes it seems they would ram into the front of the wheelwell under flex. It seems my UCA s/b 1/2" shorter than what i have.

 

[jdougn]

...To get back to stock caster and stock wheelbase at 4-1/2" of lift, you would need LCAs that are 16-3/8" long and UCAs that are 15-11/16" long. But ... that's to get back to the stock 7.5 degrees of caster, which may be too much for the u-joint. So the UCAs might be made a tad longer, or the lowers a tad shorter.

Eagle, I like reading your responses (more than just here too) because you make sense with what you're saying.

On the control arm lengths, could it also be expressed as the difference in length between the uca & lca. In other words, using the example you gave, could you say that the lca's are 11/16" longer than the uca's? I know my control arms are not the length you gave although I'm setting real close to 4.5" lift.

 

[langer1]

 

[jcnoble]

Yes, Eagle was right on the lengths. If my UCA was a tiny bit shorter that 16" , about 15 11/16" would give me 7.5 degrees caster. I estimate that every ~1/4" change will give 1 degree caster.

 

[ang31-0f-d347h]

ok... well since we are on the subject.

How do you adjust your toe and make sure that your alignment is right (i realized I cant trust my mechanic and need to do things myself)

 

[jcnoble]

Here ya go.

http://go.jeep-xj.info/HowtoAlignment.htm

The toe in procedure works good, I am still tying to figure out the caster measurement.