what are shims, and how can they fix castor?

MD11Fr8Dog

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[quote author=Jackson link=board=1;threadid=5629;start=msg44512#msg44512 date=1064432022]
I was reading the bump steer thread and decided I should probably know what a shim is. I'm envisioning the wood shims I use to level things out... I suspect it's the same idea.
[/quote]

287-82b.gif


Figure 1. Shows the stock caster angle of the steering arms.
Figure 2. Shows how that angle is affected by a shackle lift. As the axle housing is rotated forward, the angle of steering axis is no longer correct in relation to the road.
Figure 3. Shows the axle housing rotated back to its stock position using a spring shim.

Installation Note: When installing the Spring Shims, the larger of "fatter" end will always go towards the front of the vehicle . This rule applies when installing shims on either the front or rear axle.

springshim_front.jpg

springshim_rear.jpg
 
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What is the easiest way to find out what degree shim you need.

I am going to be installing extended shackles on my mini and will need to know..


Thanks,
Nick
 
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NAPA has .5' up to 4' in alum or bronze. The bronze is much better as you can get it to conform to the inside arc of the spring and it will not break. A bit issue is that the spring ctr pins must be long enough to pass through the shim and still seat into the axel housing. Almost anything over 4' will require pins with longer heads.
 
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valentine said:
What is the easiest way to find out what degree shim you need.

I am going to be installing extended shackles on my mini and will need to know..


Thanks,
Nick

You can calculate how much the longer shackle will change the angle.

Example: 1 inch longer shackle and 40 inch eye to eye springs.

1) Divide 1/40 = .025

2) then using the calculator that comes as an accessory in Windows (or any other scientific calculator), enter .025 and check the Inv box and then click the Tan button.

InvTan .025 = 1.4 degrees. So lengthening the shackle by 1 inch on a 40 inch spring will change th caster by 1.4 degrees.

HTH
 
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bad_religion_au said:
so how do you assess what size shim you need?
Here's a chart from the SOR site

Shackle Shim
1"-1 1/2" longer than stock = 2.5 degree
2"-2 1/2" longer than stock = 4.0 degree
3" & longer = 6.0 degree
 
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I've seen offset knuckle bearings in some old Aussie 4x4 mags. Never seen these in the US though.

Dave
 
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Pin_Head said:
You can calculate how much the longer shackle will change the angle.

Example: 1 inch longer shackle and 40 inch eye to eye springs.

1) Divide 1/40 = .025

2) then using the calculator that comes as an accessory in Windows (or any other scientific calculator), enter .025 and check the Inv box and then click the Tan button.

InvTan .025 = 1.4 degrees. So lengthening the shackle by 1 inch on a 40 inch spring will change th caster by 1.4 degrees.

HTH

ok, how about stock 40 front springs that have been extended 1.5 inches with stock shackle?
 
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bad_religion_au said:
ok, how about stock 40 front springs that have been extended 1.5 inches with stock shackle?

You mean moving the stock spring hanger forward 1.5 inches? If so, there is no way of calculating it easily. Measure the spring mounting pad angle before and after and compare the angles to determine if the caster changed.
 

lovetoski

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bad_religion_au said:
so how do you assess what size shim you need?

One way is to get an alignment shop to measure your castor, compare to spec, and tell you what you need.

I like PinHead's method better though - that man remembers his trig!
 
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no i mean by having a custom main leaf made up, the main leaf is the same lenght solid hanger to the spring pin but longer from the spring pin to the shackle end...... can't measure before angle because the springs i used are now the "extended springs"...

an alignment shop won't touch it... they won't touch anything with modified suspension in my part of australia, due to litigation reasons...
 

tornadoalleycruiser

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Roger Brown will make custom shims out of steel google him or he's the Toyota truck editor in the trails.. And I just bought steel shims from Tom Woods in Moab last week. they had about any degree you wanted in stock.. he's an advertiser with the trails as well. I would only use steel shims. You'll need a new center pin in your springs as well.. Please use a c-clamp on the spring pack when removing the old center pin we don't want any dead iH8Muders..
 

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