Lowered and Bump Steer issues
 

on my 67 I have late 80's 3" drop spindles. Also have drop coils, forget how much 2 or 3". My steering box has some play so I'm going to address that but I noticed when the truck was on the alignment machine that when I get in or out of the truck the toe on the left side changes 1.5 degrees. I have stock arms, does anyone else encounter this?

Caster = 7°
Camber = -0.7°
Toe - set at 0.7° in per side but it will toe out when I jump in

Re: Lowered and Bump Steer issues
 

These trucks & most of GM's stuff from the 60's-80's had bumpsteer. A majority percentage of people never even realize it's there. Of the small percentage that do notice, some it bothers, others not so much.

Re: Lowered and Bump Steer issues
 

If it really bothers you, ask if you can sit in the truck while the alignment tech does his thing. That way it will be set correctly with driver weight in place. ;)

Re: Lowered and Bump Steer issues
 

Most alignment guys would prob be ok w/tossing the equivalent of weight bench weights in there.

Re: Lowered and Bump Steer issues
 

I always have my vehicle done with weights in the drivers seat.

Re: Lowered and Bump Steer issues
 

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..

Re: Lowered and Bump Steer issues
 

How did you achieve the 7 degrees of caster?

Re: Lowered and Bump Steer issues
 

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?

Re: Lowered and Bump Steer issues
 

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...

Re: Lowered and Bump Steer issues
 

Correct. I didn't mean to imply that you did. Thanks for the input.

Re: Lowered and Bump Steer issues
 

I have some questions as to what exactly the the OP is meaning in his post. "on my 67 I have late 80's 3" drop spindles." Does mean they were purchased in the late 80's to fit the 1967 stock suspension, or has the front crossmember been swapped out with a late 80's truck with drop spindles?
As previously mentioned photos would help alot here
He doesn't indicate whether the truck has any issues when driving?

Re: Lowered and Bump Steer issues
 

To answer some questions:

I am the alignment tech, usually. I've done all my own alignments for 20 years and build tube chassis off road racers on the side. I did not expect this level of tech discussion on this forum, but I'm glad we are having it.

However, this last alignment was done by one of our techs and it did not drive well after, hence why I put it on the machine myself. The truck drives fine until you get over 60mph or then it wants to dart around. Not wandering softly, it goes left or right abruptly. Everything in the front end is new (replaced by me) except the steering box, only because I don't really know what it is. I thought 67 year trucks didn't have power boxes or the frame indention to have one but this one does making me wonder what I have from the previous owner.

The truck is a 1967 GMC, but I truly have no idea what was done to it before I got it. Assuming it had factory 1967 front suspension. The control arms are not rubber bushed so its earlier.

I put 3" drop spindles for a late 80's C10 with a 1" wide rotor and have Camaro rotors to get 5x4.75" bolt pattern.

Ball joints were changed to match the spindles and tie-rods were changed to match the spindles. I used a Tapered reamer to open up the holes in the center link to fit the larger tie-rods from the 80's trucks.

Rear leaf springs were disassembled and rebuilt with all new bushings. Rear axle is a Ford 8.8 with 4.10 gears and a Posi mounted on top the springs. 15x10 slot mags with 295/50R15 tires. Front wheels are 15x8.5" and I forgot tire size. I'm actively trying to change to 7" front wheel if I can find 2 more slot mags.

It drives like it has bump steer, confirmed by the alignment machine. I prefer to run more caster than most but I usually stay in the 4-6deg range. Both sides were at 7deg from my tech that did it last, I did not change anything this go round just gathered data. Now that I'm thinking about it I wonder if the issue has to do with differences in the 80's center link vs the 67. I did not put them side by side to verify they were the same but I don't recall people changing them when they put the later crossmember assembly in.

The truck has a supercharged LSA engine but I'm scared to take it to the track until I sort this steering issue out. That and it will be much more enjoyable if I can just relax and cruise down the highway. I've been looking into Rack conversions but I'm hesitant because I have never ran into this issue before on any of my other C10's so trying to figure out why this one is doing this.

My thoughts are that the upper control arm is at too much of an angle and the camber change is causing the track width to narrow resulting in the bump steer.

So, I will follow up tonight or tomorrow with pictures if I can snake my phone in there and I will take angle readings on both arms and the tie-rod.

Re: Lowered and Bump Steer issues
 

Just checking. Is your center link installed like this one? If so, it is backwards and the vehicle will dart about at speed as you described.

Re: Lowered and Bump Steer issues
 

^^^This

Re: Lowered and Bump Steer issues
 

Common mistake.

Also take a picture of your upper control arms.

Re: Lowered and Bump Steer issues
 

Unless you find something like the center link position, a bad/loose wheel bearing, or some new part that is bad, I am suspicious of your spindles.
I ran my Burban with similar alignment specifications on air bags and stock spindles with no scary tendencies, (even with the bags very low) for more than 20k miles.
However I do have my lower control arm shafts re-indexed for more caster.
After completion of this alignment we checked the toe at full drop and at full height of the bags and came up with about 2.75 degrees of toe change over the full range of wheel movement. (About 10" of shock travel)
All this leads me to believe that something about the geometry of your new spindles doesn't work with the center link or its position as dictated by the Pitman arm and drag link.

I recommend double checking all the new parts and posting photographs of the suspension.
You also might check to see that the ball joints are not out of travel and bottoming out.

Good luck and keep us posted.

Re: Lowered and Bump Steer issues
 

I would have felt really silly if the center link was backwards but it is not. It appears to me these drop spindles have the tie rod position too high. I'm taking the truck to LST this weekend so when I return I will pull a wheel and compare the spindles to stock.

I'm trying to upload a picture