Castor shims

Ontheboulder · Apr 12, 2026

This question has most likely been asked before. My 87 gmc v2500 has a 4 inch lift. Springs in front, blocks in the rear. The lift was done by previous owner. The truck is kinda darts left or right at times. Spring bushing are tight as well as tie rods and drag links.brand new Redhead steering box,pitman arm is tight. Question is does it need castor shims between the springs and the axle? Seems that I heard that the lift would throw the castor out a little bit. Any thoughts or ides? Thanks

idahovette · Apr 12, 2026

More positive caster makes it go straighter down the road. Have you checked the toe-in? 1/8 inch is good?

xm20k · Apr 12, 2026

Positive makes it less likely to want to wander with road imperfections or less twitchy with minute inputs also will want to straighten itself with no input other than forward movement. The more positive it is the faster it will try to return to straight but may make it feel heavier to steer.

Negative is just the opposite, makes it twitchy with less and less input and can cause the vehicle to want to wander due to cracks patches or other variations in the road. It can get to the point where you're fighting every input that can affect steering like brake balance, wind, etc.

Could probably use a few degrees positive based on your description.

bucket · Apr 12, 2026

I think it depends on the specific leaf springs.

My Suburban has 4" springs that came with angle shims installed in the pack (I don't recall the angle, I'd have to measure them again), presumably to help the pinion angle. It *should* have hurt the caster angle, but it steers and drives fantastic.

nvrenuf · Apr 12, 2026

Measure
Measure
Measure

Just because it has shims doesn’t mean it needs them or that they’re correct.

The correct answer requires research on your part.

Ricko1966 · Apr 12, 2026

0* toe or toe out will make a vehicle dart around. It's actually a racer trick to get quicker turn in.

Ontheboulder · Apr 12, 2026

well I am getting old and I found that it DOES have shims but I don't know what the degree is, thinking at this point just a trip to the alignment shop is in order. I am not sure which way the castor is - positive or negative at this point, shims are on the front with the thicker end forward

Shorty81 · Apr 12, 2026

Make sure the steering arm is correct for a 4" lift. My truck had one for a lifted truck. I have a one inch lift and it handled poorly and darted around until I installed factory arm.

Keith Seymore · Apr 12, 2026

Ontheboulder said:
well I am getting old and I found that it DOES have shims but I don't know what the degree is, thinking at this point just a trip to the alignment shop is in order. I am not sure which way the castor is - positive or negative at this point, shims are on the front with the thicker end forward

I think those are backwards. Thicker should go to the rear.

And we ran A LOT of caster. Like 10 or 12 degrees a lot.

K

nvrenuf · Apr 12, 2026

On the front axle, the thick end of the shim should be towards the front bumper.

xm20k · Apr 12, 2026

Positive would-be lower ball joint farther forward than the top so thick end at the front if axle on top, and at the rear if spring on top.

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bucket · Apr 12, 2026

nvrenuf said:
On the front axle, the thick end of the shim should be towards the front bumper.

Right.

I think I need to reiterate... shims on the front with lift springs are typically to help reduce pinion angle. NOT improve caster angle. Helping one will hurt the other, so the lesser of two evils will need to be selected.

idahovette · Apr 13, 2026

bucket said:
Right.

I think I need to reiterate... shims on the front with lift springs are typically to help reduce pinion angle. NOT improve caster angle. Helping one will hurt the other, so the lesser of two evils will need to be selected.

ONLY if you need to reduce the caster angle if lifted........not if you need better driving characteristics and NOT if it's lifted for appearance only

bucket · Apr 13, 2026

idahovette said:
ONLY if you need to reduce the caster angle if lifted........not if you need better driving characteristics and NOT if it's lifted for appearance only

My simple mind doesn't quite follow...

But common performance vehicle thought is that more caster is better. And common procedure for a lifted, solid axle 4x4 is to shim the pinion up for better driveline angle... which in turn goes against the performance car ways.

Using my Suburban as an example again, my Suburban drove fantastic when it was stock height. I put a lot of miles on it like that. Then I lifted it 4" and it still drove fantastic. I put a lot of miles on it like that too. The lift spings I bought came with shims already installed in the pack. I have always ASSumed that those shims rotated the pinion up and reduced caster angle, compared to stock. With that said, I never measured caster or pinion angle before or after the swap. So those shims may be there simply to keep the stock geometry with a different spring arch design.

Ontheboulder · Apr 13, 2026

Pinion angle is good,so no issues with that,it has room to move without binding. Drove it again yesterday and steering seems lethargic, sluggish response..the current shim is with the thicker end forward, no idea of what the degree angle is

Castor shims

Ontheboulder

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idahovette

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xm20k

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bucket

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nvrenuf

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Ricko1966

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Ontheboulder

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Shorty81

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Keith Seymore

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nvrenuf

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xm20k

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bucket

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