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Originally Posted by RustyPile
Your caster angle of 7* is a major contributing factor to bump steer.. I'll try to explain this without being too technical.. Normal caster angles are + 2* give or take a fraction. This measurement is in relation to the ground (road surface). When we lower our trucks more in the front than the rear, this causes the caster to be in a negative angle.. The caster has to be adjusted, in your case, an additional 5* to attain the target 2*.. In a perfect world, the tie rod should be kept as near parallel to the line between the lower ball joint and lower control arm shaft as possible.. This parallelism keeps bump steer to a minimum.. As caster is added, this parallelism is lost.. The worse it becomes, the worse the bump steer becomes.. Caster also has an effect on the steering to return to straight ahead or track in a straight line.. Not enough caster and the truck has a tendency to "wander".. Too much and you have bump steer... There is a good explanation of this in section 3 of the service manual...
There's not a good solution to your issue.. But, if possible, reducing the caster will reduce bump steer..
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Not criticizing here but there are a couple of things in this post that have me scratching my head. I don't know everything but these statements beg me to ask....
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Normal caster angles are + 2* give or take a fraction. This measurement is in relation to the ground (road surface). When we lower our trucks more in the front than the rear, this causes the caster to be in a negative angle.
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As far as I can recall w/the 'C' series trucks & after, the rear is typically higher vs. the front from the manufacturer.
This is why it's common for the extra rear drop (an attempt to level things out). Trucks are unlevel from the factory so they can have additional flexibility for heavy load capacity. I can see where lowering could possibly amplify the 'rake' but it's still there so I'm not understanding the relation suggested since drops typically account/compensate to minimize this factor.
Quote:
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In a perfect world, the tie rod should be kept as near parallel to the line between the lower ball joint and lower control arm shaft as possible.. This parallelism keeps bump steer to a minimum.. As caster is added, this parallelism is lost.. The worse it becomes, the worse the bump steer becomes.
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The Center-Link to Tie-Rod end should be parallel to the lower a-arm pivot/lower BJ center. But, I've read where many spindles locate the TRE @ a non-optimal point (usually too low). So adding caster would seem to help to some degree vs assumed to be bad.
From my perspective, saying 7° is "bad" w/o knowing plotting points of the specific suspension seems assumptive.
Thoughts?