UCAs/Castor/Help me!!!! :)

Ok, so here's the basics.....

After installing my supposedly wonderful curry adj. UCAs....I started getting this awful shimmy in my steering when hitting ANY sort of bump in the road.

To make a really long story short....we installed them a little too long for my amount of lift i guess.....about 16-16.5"......

So, while I was driving cross country, my Jeep found it's way to a mechanic who proceeded to mess it all up good.......He decided to make the driver's side UCA even LONGER....and didnt touch the pass. side UCA......explain that!!

So.....we realized this when we inspected it this past week....shortened the driver's side to 15.25" as Rusty instructed us to do.....but, the pass. side jam nut on the UCA is realllllllly tight and we couldnt break it loose with the tools available.

So....yeah....I have been riding around like a dumbass with one UCA 1" longer than the other....and, believe me, its been a battle on the road.

The mech. also replaced all of my steering ends.....which helped prevent that awful shimmy....but, I can still feel it wanting to happen under there.

I have an appt. with a reputable mechanic up here in Baltimore this Wednesday....to fix this problem as well as a few others.

My question is....what do I tell this guy?? (other than I am an idiot and he shouldnt work on my jeep!!!)......

Castor should be as close to zero as possible?? Any better suggestion on how to determine the correct length for these UCAs....? Anything else....??....Anyone in Baltimore wanna meet me in a dark parking lot to show me how to measure this stuff......kidding.....ugh.....bad mood, bad mood.

Thanks in advance......

Emily "XJGrl" Davis

p.s. did the rubicon and moab....not in my jeep....during our trip....great time!!....pics soon.

Caster should be between 7 - 8 degrees, preferably 7.5. This is on my '89 at least, I wouldn't think a '98 would be all that different...

Sequoia

Yep, 7* would be good and factory specs say 0* toe in. I have run 7*caster and 1-3* toe in from 2" lift to 9". Always been good on mine. The extra toe in could help with the tire shimmy you are talking about. Good luck

I just removed my 4.5" RE Super-Flex kit.The lowers are fixed at 16",the upper adjustables are still set at 15.5" and I had about 5.5* of castor.

RCP Phx said:

I just removed my 4.5" RE Super-Flex kit.The lowers are fixed at 16",the upper adjustables are still set at 15.5" and I had about 5.5* of castor.

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5.5° sounds like a good compromise.

can't imagine getting 7°+ without a disco axle or hubs.

YMMV

Don't know how much adjusting you did initially but have the shop set your wheelbase back to 101.5 with the lowers and then dial in the caster to around 7*. I am assuming you have a LP D30 underneath so with 4.5" you play the catser vs. pinion angle game, but somewhere around 7* sounds right, you can always fine tune by adjusting the uppers in or out once you have your baseline.

Pete

If you try to get back to the stock wheelbase with those tires youll probably take out the front of the flares.I was running 32x10.5's and I was barely touching with my numbers!

I am not sure if you can go to 7 degrees of caster angle at that lift. The driveshaft would complain, at least it did on my truck. The compromise I have at the moment is 6 degrees, zero toe-in. It drives great.

Rgds

I think Em's 98 would have a high-pinion 30, but...

Is there a linear relationship between pinion angle and caster (if so what is it?

Example if the pinion is neasured at zero degrees (perfectly horizontal) what would the caster be? (or would it only be 'linear' if the housing were rotated about the C/L?)

I am sure I have my setup all wrong...recently swapped "everything" in the front end (housing, TB, coils, fixed lower/adj upper CA, steering) One thing I 'wish' I hadn't done was to omit the OEM caster adjusters in the LCA mounts. Mine drives acceptably (barely) but we'll see how that holds once the new 3" offset wheels and 35" MTs go on.

I'm sitting at 6" with the RE 5.5 kit on my 97. my caster is 6.2* which I'm told is well within specs. With what you may be paying a mechanic. You might consider the RE control arm drop brackets I don't know how they would work with 4.5" but at 6" I didn't need to adjust my caster angle. My UCA and LCA are set to stock lengths and the ride is very good. Good luck.
Barger

Em and Woody,

I think Em has a LP D30 in her 98....but anyway. If the pinion angle is zero on a LP D30 you have 12* positive caster, if its on a HP D30 (like mine from a '94) you would have 9*. My advice is still to set the factory wheelbase, please unless you plan on relocating the coil seats on the "uniframe" or don't mind your front coils approaching the look of a slinky Use the uppers to set your pinion angle (and therfore your caster) they are linked. You are really working on the u-joint operating angle at the axle pinion yoke...you have a CV at the t-case. Rotate the pinion "up" to gain positive caster, down to lose. The angle you want at the pinion for a LP D30 is 4-6 degrees I'd think, resulting in 6-8* positive caster. If you have all 4 adjustables you don't need to pay anyone to do this for ya, trust me on that. In case you have the HP you would want around 1-2* degrees at the pinion yeilding about 6-8*caster. I have had a few cocktails this evening so I hope this isn't SPOBI fodder!!!! But it sounds right. Next time you are in Richmond stop by and we'll get it fixed up.

Pete

The '98 will have a High Pinion D30 unless she swapped it out.

If it is a 98' it is a HP not a LP unless it was swapped in for some reason.

Woody there isn't exactly a linear relationship, but there is, kinda. Make sense. If you adjust the pinion angle up 1* it doesn't change your castor 1* It is more like .5* the steeper the angle of the pinion the less it affects the castor. basically it is because you are NOT rotating the axle on a plane. You are only rotating the top of the axle cause you don't move the lower arms. See what i am saying?

If you rotate the pinion up you LOSE positive castor not gain. The ball joints are rotated toward the back of the Jeep.

Ok I send before i meant to.

You want to get your pinion angle as close to 0* from the driveshaft as you can. Sometimes with large lifts you need to set it then drive it. Then change it to suit your driving needs. I have about 5-1/2 to 6" of lift and got the pinion to 1* below the driveshaft and it drives perfect. Castor is about 5* if I recall correctly. I can check on that when I get home.

Here's the table I used for the JKS arms... no DW yet...

Lift...........Lower Control Arms/Upper Control Arms
********Decimal**Fraction**Decimal**Fraction
Stock........15.75.......15 ¾........15.00.......15
2”.............15.92.......16............14.88.......15
3”.............16.09.......16............14.92.......15
4”.............16.33.......16 3/8......15.03.......15
4.5”..........16.47.......16 ½........15.11.......15
5”.............16.62.......16 5/8......15.21.......15 ¼
6”.............16.97.......17............15.44.......15 ½
7”.............17.36.......17 3/8......15.74.......15 ¾
8”.............17.81.......17 ¾........16.09.......16
9”.............18.30.......18 ¼........16.50.......16 ½
10”...........18.82.......18 ¾........16.96.......17

Matt99XJ said:

If you adjust the pinion angle up 1* it doesn't change your castor 1* It is more like .5* the steeper the angle of the pinion the less it affects the castor. basically it is because you are NOT rotating the axle on a plane.

Click to expand...

Changing the pinion angle by 1 degree DOES change the caster by 1 degree. It has to. The diff housing, the axle tubes, and the steering knuckles are one solid assembly. However much you rotate the diff is how much you rotate the knuckles.

The relationship between castor and pinion angle is inversely proportional. If you increase pinion angle by 1°, you will decrease positive castor by 1°. Increasing castor by 5° will decrease pinion angle by 5°, etc.

Drop brackets are great for reducing control arm angles, resulting in an improved ride. They do nothing to ease the problem of castor vs pinion angle on a lifted Jeep.

To achieve 5 1/2° castor, adjust pinion angle to ~4° on a high pinion D-30 or ~7° for a low pinion.

Everyone with a 10" lift running 18 3/4" lower arms, raise your hands.

What's this? No Hands?

robs, throw your chart out. The higher you lift, the more worthless it becomes.

Thats a big 10-4 on ditching that chart.Ive got a LP30 so compromise is definately a must.

Sorry I wasn't explaining myself correctly. Now that i read it it doesn't make sense.

What I was trying to say is.

When you change the pinion/driveshaft angle 1* closer to each other you are only changing the actual pinion angle by .5* Because the pinion moves up .5* and the driveshaft comes down .5*

I guess I should get more sleep before I post technical stuff huh. Sorry for the confusion.

Thats an excellent point that hasnt been mentioned in quite a while !!!