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Unread 11-01-2013, 08:51 AM   #1
dirtyb
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Degree Shim to Fix Caster and Pinion Angle

Front:

So my Jeep CJ-8 came from the PO with ridiculously long shackles (6 1/2" bolt to bolt) on the front and rear. The steering was seriously dangerous. I didn't bother measuring the caster at this point, because I knew it was wrong. I removed the shackles and put stock on. With the stock shackles, the front caster was 4 degrees and the steering was fine. I did some quick trigonometry, and figured that a 6 degree shim along with the 6 1/2" shackles would put me back at 4 - 5 degree caster. I added the shims, but now find my caster angle is only 2 1/2 degrees. Steering is not quite the way I'd like it. It's manageable, but could be better. My plan is to replace the front shackles with 5".

How did my math get messed up? I measured the length of the springs to be 43 inches. If I added 3 1/2 inches of shackle length, then

Tan (theta)= opp/adj = 3.5/43

theta = 4.7 degrees

Caster angle = (what you had stock) + (what the shim should add) - (change induced by longer shackles)

Caster angle = 4+6-4.7 = 5.3

But that's not what happened. I should have a caster of 5.3, but I'm measuring 2.5.

Did I put this stupid thing on correctly anyway?

img_20131101_092157-2.jpg

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Unread 11-01-2013, 08:59 AM   #2
jeepwhore
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The massive picture makes it a bit hard to see, but you have the shim installed in the wrong place. Shims go between the leaf pack and axle perch and typically have the thick end towards the front. Where you have it will do nothing except change the angle of the u-bolt plate.
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'84 CJ-7: Yota axles (spartan locked front, e-locked rear) 4" BDS, 1" Daystar & 3/8" shackle lifts, 35" X-Terrains on steelies, YJ Tub & family roll bar, heater blower upgrade, 4.2L w/MC2100 & Team Rush, Warn 8274. Rebuilt 4.2/4.0 hybrid, AX15 and twin sticks/clocking ring waiting to go in.

The advertised position for a topless, dirty jeep girl has been filled......by my wife. Thanks to all that applied.
<------------and yes, that is her.
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Unread 11-01-2013, 09:17 AM   #3
dirtyb
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Not to sound like a smart aleck, but are you sure it goes on top of the springs? I thought about it, and didn't install it that way because the shim would start to cover up the round head of the bolt if I did it that way. Am I making sense? If it's installed at the bottom, the way I did it, the round head of the bolt fits into the circle under the perch and there's full engagement. If I put the wedge on the top, the wedge will cover up some of the head of the bolt. Maybe it's not enough to matter?

On the other hand, it makes perfect sense that I'm only rolling the plate.
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Unread 11-01-2013, 09:22 AM   #4
CSP
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x2 on the shim location. It goes between the spring and the axle.
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Unread 11-01-2013, 09:24 AM   #5
jeepwhore
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Positive. Think of the springs as a static part of the frame when the jeep is at rest. The caster was rotated by altering the spring orientation via the shackles. The only way to adjust the caster back is by altering the point of contact between the springs and axle perches.

That said, use quality steel shims that are meant to be bolted to the leaf pack.
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'84 CJ-7: Yota axles (spartan locked front, e-locked rear) 4" BDS, 1" Daystar & 3/8" shackle lifts, 35" X-Terrains on steelies, YJ Tub & family roll bar, heater blower upgrade, 4.2L w/MC2100 & Team Rush, Warn 8274. Rebuilt 4.2/4.0 hybrid, AX15 and twin sticks/clocking ring waiting to go in.

The advertised position for a topless, dirty jeep girl has been filled......by my wife. Thanks to all that applied.
<------------and yes, that is her.
:-D
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Unread 11-01-2013, 09:27 AM   #6
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Quote:
Originally Posted by jeepwhore View Post
Positive. The caster was rotated by altering the spring orientation via the shackles. The only way to adjust the caster back is by altering the point of contact between the springs and axle perches.

That said, use quality steel shims that are meant to be bolted to the leaf pack.
What jeepwhore said.

Solid advice that could save your life and your Jeep too.
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Unread 11-01-2013, 09:32 AM   #7
dirtyb
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Quote:
Originally Posted by Texian View Post
What jeepwhore said.

Solid advice that could save your life and your Jeep too.
Well, I got that part right.

DOG GONNIT!
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Unread 11-01-2013, 09:40 AM   #8
jeepwhore
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Quote:
Originally Posted by dirtyb View Post
Well, I got that part right.

DOG GONNIT!


__________________
'84 CJ-7: Yota axles (spartan locked front, e-locked rear) 4" BDS, 1" Daystar & 3/8" shackle lifts, 35" X-Terrains on steelies, YJ Tub & family roll bar, heater blower upgrade, 4.2L w/MC2100 & Team Rush, Warn 8274. Rebuilt 4.2/4.0 hybrid, AX15 and twin sticks/clocking ring waiting to go in.

The advertised position for a topless, dirty jeep girl has been filled......by my wife. Thanks to all that applied.
<------------and yes, that is her.
:-D
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Unread 11-01-2013, 09:42 AM   #9
Spieg8
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Agree with the above. Here's a pic of how shim should be installed;
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Unread 11-01-2013, 09:45 AM   #10
Texian
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Quote:
Originally Posted by dirtyb View Post
Well, I got that part right.

DOG GONNIT!
Glad to see you could get part of it figured out anyway.
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Unread 11-01-2013, 10:56 AM   #11
JeepHammer
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The first thing I see is...

Spring Shims correct pinion angle.

Caster is corrected with ball joint offset bushings (Up to 1.5 Degrees) or grinding loose the outer axle tube 'C' and repositioning it for correct caster angle...

To try and correct BOTH pinion angle and caster with wedges at the springs probably won't get you where you are going, especially if you have taller tires than stock...
Taller tires require more caster to center correctly, so that multiplies the issues...
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Unread 11-01-2013, 11:31 AM   #12
CSP
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People use shims all the time to correct caster. It may not be the most correct method, but the reality is that most go this route for DIY caster correction.

Pinion angle will suffer, but caster takes precedence over pinion angle, especially on a front axle that doesn't have the front driveshaft spinning at highway speeds most of the time anyhow.
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Unread 11-01-2013, 01:58 PM   #13
dirtyb
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Quote:
Originally Posted by CSP View Post
People use shims all the time to correct caster. It may not be the most correct method, but the reality is that most go this route for DIY caster correction.

Pinion angle will suffer, but caster takes precedence over pinion angle, especially on a front axle that doesn't have the front driveshaft spinning at highway speeds most of the time anyhow.
That's the way I look at it. The pinion angle for the front drive shaft doesn't need to be perfect. I'm in 4x4 mode maybe 5% of the time, and then never at high speeds etc. If this was a professional mud riddin' buggy, I'd change my answer.

Also, I read somewhere that taller tires accentuate whatever type of caster (either positive or negative) that you have. If the world was perfect and tires didn't deform, then tire size wouldn't affect the "feel" of caster. However, since a bigger tire leaves more of an instantaneous footprint, if your caster is in the right direction, a bigger tire makes it "feel" like you get even more caster. If your caster is in the wrong direction, a bigger tire makes it "feel" like you even worse off than before. If I understand correctly....
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Unread 11-01-2013, 02:00 PM   #14
CSP
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You understand the concept of caster and bigger tires correctly. The moment arm becomes longer as the diameter of the tires increases.
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Unread 11-01-2013, 02:14 PM   #15
JeepHammer
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CSP, you are correct that 'Most People' would use just shims...

I'm not 'Most People' and I hate the wear/vibration that comes from the front drive shaft/U joints when the pinion angle doesn't match the angle out of the transfer case.
Once I get rid of that vibration, wear and potential failure point,
Then I move to caster and correct it, since I like to go down the highway at 55 or 60 without taking my life into my hands or hitting a bump and having bump steer make involuntary lane changes on me!

The *FIRST TIME* I did weld grinding to fix caster I was scared to death of it...
Now it's a snap and my oversize tires/lift kit go down the road without a white knuckle grip on the steering wheel!
Wish I would have done it MUCH sooner.

Offset ball joints, caster shims *Might* get your caster back once you shim pinion angle to kill vibration, but it won't do a thing for steering geometry that lift shackles screwed up... Or in my case, too much spring lift and tall tires and it's habit of bump steering...

Lowered pitman arm, reasonable caster made all the difference...

YUP!! I did EVERYTHING WRONG at least three times before I gave in and did it 'Correctly'... Which scared the crap out of me but really wasn't that hard...
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