Quote:
Originally Posted by SCOTI
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....
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.
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?
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First off, I didn't say 7* caster is bad.. I said it contributes to bump steer.. The OP didn't provide enough information about the spindle design, only that they are dropped by some 3 inches, so we don't know exactly where the outer tie rod end is located in relation to the lower ball joint.. He only guessed at the amount of "drop" in the springs.. Something in the neighborhood of 6" total lowering.. Those shorter springs destroy the lower control arm to tie rod geometry..
Some of what you say is truth, but some of it is conjecture.. No matter the design "layout" of the spindle, adding any amount of caster will lower the outer tie rod end in relation to the lower ball joint.. This changes the geometry and the parallelism between the lower control arm in relation to the tie rod's full length.. It's a known fact if the tie rod is not parallel to the line formed by the lower ball joint and the lower control arm shaft, there will be bump steer.. The OP stated the truck does in fact have bump steer.. I doubt very seriously that ADDING caster will eliminate some or any of the bump steer.. That's why I suggested trying a lower caster setting..
Something I didn't cover in my previous post is camber change and it's relationship to changes in the upper control arm as it moves through normal suspension articulation. Those caster changes are neutralized somewhat via good engineering and design.. When a vehicle is lowered via "dropped" springs, the ride height angle of the upper control arm is changed.. As the suspension articulates normally, the camber angle changes beyond the designed amount and causes the toe angle to also change.. When weight, be it a human being or weights from a gym, is added or subtracted and the toe angle changes, it's because of the camber angle change..
In the 12 years I spend designing and building race cars, I learned a lot about steering and suspension geometry.. I know exactly how changes in one angle, be it caster, camber, or toe, effects the other two. Some people don't even know there is a proper sequence to setting those three angles..
This discussion between you and I is the reason we always ask for pictures.. Pictures taken from the right camera angle will show the parallelism between the tie rod and lower control arm.. Hopefully the upper control angle will be in one of those pictures...