is it possible to understand "caster angle"

[Kilo19]

So searching the world wide web, I'm trying to understand caster angle and how to fix it. I've read about shims, and changing the angle, none of this looks easy, at least checking the angle.

I saw this post
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Check out this artical, it explains everything you need to know about lift kits & jeeps.

http://mysite.verizon.net/res108eps/...ion-basics.pdf

Read more: http://www.jeep-cj.com/forums/f19/caster-vs-driveline-angle-15702/#ixzz2Qpz4i36k

but if I'm looking at this right (the link) then the pumpkin needs to be aligned to the TC, but by doing that (i understand the caster angle goes down the taller you go) you loose your cast angle, so then i've read about putting in shims, but here's where i get hung up

where do the shims go, how does that change the pumpkin angle changing the caster angle? im not at this point in my build yet, but i'd like to understand how this works, of course i'd like to understand how transmissions work (on the inside).

thx
​

 

[Stodg73]

Changing the castor angle has nothing to do with the pumpkin. It has to do with the steering knuckles. These are welded in place and cannot be changed except by the use of shims. The shims are placed on the spring perch and above the springs(SUA application). This is to correct the angle of the knuckles, getting them straight up and down. At least this is how I understand castor and shims.

Others will chime in...

 

[Old Dog]

The factory should have the caster set with the pumpkin plumb. But if you have longer than stock shackles, it changes the angle. At that point you either need to cut and turn the C's or add shims. If you imagine a straight line through the ball joints there should be a 8* or so angle.
I believe Kane has a pic. of him setting the angle in his thread about putting a scout axle in a cj.
FYI a spring lift should not change the angle, longer shackles will.

 

[PaRenegade]

The easiest way to check it is with a $10 angle gauge laid across the castle nut of the top ball joint or top of the steering knuckle.


Here's a link to a pdf file on steering geometry. Go to page 3 for a good illustration and explanation of caster. Think shopping cart.
http://mysite.verizon.net/res108eps...ilderfiles/Steering-and-suspension-basics.pdf
edit: opps, I just noticed that's the same article you already posted Kilo.

old dog - It shouldn't matter where your pumpkin is at because the correct angle is measured FROM plumb (vertical). However the Jeep should be on level ground, not on a hill, because technically it's referenced from a line drawn thru the centers of the front and back wheels. That's the rolling plane.

 

[jerry1847]

So to clarify,

The angle measure should sit on top of the castle nut parallel with the springs, not the axle. I should be looking for ~6° of castor. your image looks like the wheel is off and is showing what looks like -4° ish. If I'm measuring from the other side without taking the wheels off, I should be positive correct? Then, when adding the shim, the fat side faces forward to add castor.

I have a 1/2 in shackle lift and steering.

Thanks!

 

[PaRenegade]

Basically castor angle is the relationship of an imaginary line drawn thru the top and bottom ball joints, to absolute vertical or plumb. You want the top of that imaginary line tilted back so that if the bottom joint were the center of a clock the hands should be at approx. 1:05. Your looking to get that angle at 4-6 degrees. I doubt that only having 1/2" of shackle lift would effect your steering to any drastic degree. I've had 1 1/4" inch of shackle lift with no noticeable issue. You may want to look into other issues such as ball joints, toe-in, tire bulges, tie rod ends, etc. How much "spring" lift do you have, if any, over stock?

As for the image. The reflection makes a reading hard to see and while yes the reading is in the negative quadrant, it's the line between zero (on top) and zero (on bottom) that is the actual angle that is your castor line. Remember, the base of the gauge is setting on the knuckle top which is always 90 degrees to the ball joint line. So it's rotated in a clockwise direction also (top isn't level per say).

Yes the fat side of the shim goes towards the front, which rotates you in a cw direction. The shim goes between the axle perch and the spring with the center bolt keeping all three pinned together at the same point. You may need to get another center bolt depending on the shim construction, and don't use aluminum ones, get steel.

 

[jerry1847]

Got it measured today and I'm right at 4-6° with the 1/2in shackle lift.

My steering issue was from a sloppy yj spring conversion. One of the PO's decided to not install the wider shackle hangers and just forced the shackles to fit and used a couple washers. I corrected the issue but have not driven since.