Hi Guys-
Been a long time since I've been on here. Finally getting back to my projects.
Anyways, I have a beater 71 Charger 383 that I bought maybe 10 years ago. So I finally got it road worthy, installed new front end pieces.
New QA1 upper and lower control arms, QA1 tie rod ends, QA1 strut rods, new shocks, ball joints, etc. New Borgeson steering box and pitman arm/tie rod as well.... new tires wheels as well......
So I took it to the alignment shop and we gave it -.5 degrees per side of negative camber, and plus 3.5 of caster. 1/16th of toe in...
Drove nicely on a flat road near the alignment shop. Big thumbs up.
Trailered it home and wanted to show off to my daughter what a nice driving machine this was, and I hit some minor bumps and grooves on the road and this thing is all over!!!!!!!
MAJOR bump steer! The only thing that I question is when I installed everything, my center link was not parallel to the street anymore due to the pitman arm being lower than my idler arm. I didn't think anything at the time because I thought "no big deal, everything is on ball joints"
So, could this be causing bump steer???????
Thanks for any info!

The MP chassis book and many others I assume, detail how to measure the bumpsteer and what you need to do to fix it. And yes, your pitman/idler arm height issue can be the cause.

With both outer links I believe the goal is to remain as parallel as possible with the movement of the LCA. If you visually can see that is not the case, bump steer is a given.
That is achieved by both sides being mirror image, and same distance from and height of the control arms measured from the ball joints pivot center to the pivot point of the LCA, and at centered steering. the inside control arm pivot longitudinally in line with the inside LCA pivot point.

Make sense?

i agree with the above. ^^^^^^^^^^^^^^^^^^^^^^
bump steer is there in the stock suspension, like it or not.
it isn't too bad if factory alignment specs are used, but when you start to use useful specs out of the factory range [especially caster] bump steer shows up quickly.
the DC and MP chassis books show how to correct this, and if you have these books, read up on the procedures and fix your situation. it's relatively easy to do. [in my opinion]
if you don't have these books, or access to them, they are a worthwhile investment.[again, my opinion]

Just to point out, you can also get a form of bump steer from the rear suspension. Highly arched springs can change rear toe as you drive.

More like thrust angle changes and not toe.

2 ways to describe the same thing. In the illustration, rr is toe in, lr is toe out. With a solid axle, total toe doesn't change.

Hmmmm, the rear suspension is all original stuff yet, and I'm sure the bushings are garbage and I know for sure the leafs are sagged out. That is my next step for sure. Who knows, maybe that could be the issue as well, never thought about that.....
Thanks for all the help so far guys!

Toe is measured in reference to the other tire on the same axle and the vehicle centerline. If you have a toe issue on a solid axle you have a bent one.

There is a reason they are different terms and they are NOT describing the same thing.

None of that contradicts my statement.

Total toe is measured from tire to tire, this can be toe in or toe out. Each tire can be measured relative to the vehicle centerline, and again toe in or out.

If you have a total toe problem on a solid axle, then yes it's probably bent. Which is the collary to my original statement.

Let's say we're working on IRS, how do you correct a thrust angle problem? By adjusting the toe of the rear wheels.

Neither above comment addresses the previous identified rear axle bump steer.
Thrust angle is an alignment problem. Rear toe on a straight axle is a bent axle. Both problems are not bump related.
Rear bump steer on OEM leaf spring setups is an intentional or acceptable chassis design compromise solution, and only corrected if unneeded or undesired by modifying the suspension, and not changed by simple alignment changes.

Disagree. Total toe on a straight axle can be correct AND individual toe can be incorrect. Yes, this phenomenon is usually called thrust angle but it comes down to incorrect rear toe angles. It really doesn't matter if it's a solid axle or IRS.

As for leaf springs causing bump steer. Yes they can. The spring oves the axle in an arc as ride height changes, effectively lengthening or shortening the wheel base. How severe this is depends on how level the front half of the spring is. Ideally the front half of the spring should be parallel to the ground at normal ride height. This minimizes side to side changes in rear toe.

Actually, rear bump steer is caused by changes in the thrust angle as the axle moves up and down more on one side than the other. Nature of how a leaf sprung setup works. Longer front spring length mitigates it to a point, less fore and aft movement as the spring goes thru it's arc. This is where a properly setup 4 link can come into play.

That's what happens when you don't reference your toe readings to the vehicle center line, which you have to do.

I reiterate, if you have an "individual toe" issue, you have something bent. For the record here, because you like to wander off into what if land, we are talking about the OP's solid rear axle setup and nothing else.

To further clarify things, toe is an issue with the tires pointing in or out, thrust angle is how square the axle is to the chassis. Separate issues.

This is what I'm saying.

With the same correction. Go back to your illustration, remove the green arrow and draw in the vehicle centerline. Now measure the toe of each wheel. The actual mechanics of correction depends on the specific suspension design.

We agree, that is why it's called thrust angle, and if that toe you note is symmetrically opposite on a straight axle, the axle itself is usually considered straight, and it's corrected by improving the alignment of the entire axle or thrust angle, and not by reducing the toe of each wheel individually.

There is almost no consideration or concern nor downside to a I suspect rather rarely found slightly bent straight axle has that exact opposite toe angles.

Note, that begs the question, do alignment machines measure each sides wheelbase, to ascertain if rear axle toe measurements are exactly opposite, giving zero thrust or do they just measure angles of toe on rear axle ?

Depending on machine, some can measure wheelbase differences. But mostly the concern is the angles.

Note that I've been trying to emphasize that thrust angle can be off no matter what rear suspension design you have. If it's a 71 Charger, you shim a spring hanger. If it's a Camry, you fiddle with toe adjusters. In either case you want the wheels parallel to the vehicle centerline.

I'm not sure where anyone got the idea that I suggested that a straight axle has individually adjusted toe. Sure, some fwd cars had bolt on spindles that could be shimmed but that's unusual.

IMO, it was not really worth mentioning in the first place on this particular thread, and really just made for a side trip into the weeds, with no real harm.

The original point was you can get funny handling from the rear too.

it's amazing what heating a spot on the rear axle tube and water quenching can do to "change the toe" on a solid axle.
try it some time.

Or Camber.

this as well.
as a machinist, it is amazing to me just how much slop is stacked up when axle, bearing, bearing retainer/adjuster, bearing receiver/axle tube end, axle tube, axle splines, and side drive gear are all considered.

Nascar guys figured this out a long time ago. Go find a straight axle housing from them.

Your idler arm is not a ball joint. It's a plain bearing that only rotates around one axis. If your centerlink is angled, the idler has some binding. May not be that apparent in resistance, but will wear things wrong and change the bumpsteer because the inner tie rod is moved up or down from stock.

You have the wrong pitman or idler in your car. I did that once putting an E-body pitman I had laying around in an A-body. The centerlink was angled. OR your Borgeson steering box is tilting your pitman the wrong way OR the splines on the output shaft are moving you pitman into the wrong place (was a common issue with the 1st units)

Well yes and no IMO. Most bump steer measurements are taken I believe with wheels pointed straight, and if both links are not parallel with the LCA and located inline with ball joints and pivot points, bump steer is present. I suspect its possible one side could be relatively bump steer free and the other side not to be, if any of the three conditions you note are present. I believe once straight-ahead bump steer on both sides is corrected/improved, any steering issues regarding binding, accelerated wear, etc will have already been resolved.
What I can't wrap my head around yet is if both sides could have exactly opposite bump steer problems, effectively canceling out normal observed bump steer issues while driving.

If I follow what you're saying, then yes it's possible that total toe wouldn't change (one side toes out, the other toes in) but there would be some side effect. For example, if the vehicle goes straight, the steering wheel would rotate one way or the other, conversely, if the steering wheel doesn't rotate, the car will change direction.

This is often seen on lifted straight axle trucks. More often on one's with push pull drag links that don't follow the spring arc.

Toe changes that would be less noticeable would be if both wheels toe in or out equal amounts. The car would still go straight and the steering wheel wouldn't rotate. This would be noticeable however on roads with uneven traction. Say a patch of ice on an otherwise dry road. The tire with better grip will steer the car as the other slips.

Yes we agree. that likely would circle back to a thrust angle issue?, but still not a bump steer issue.

Is it possible you have a '70 K-frame? Hard to tell with the Schumacher steering box. The stock box should be in a straight line with the steering column. The box is mounted at a different angle for '70 B-body compared to E-body and your car. Jumping around hitting bumps isn't the definition of bump steer. Either move the wheel up and down, and measure the toe change, or try to drive the car in an open area turning, and hit the brakes. The wheel jounce will turn the car in or out of the turn if there's bump steer. If parts are all correct, you can correct bump steer by shimming the steering box to move the pitman arm up or down, or slot the idler arm mount, and weld washers to locate it once you adjust all the bump steer out. Yes, it'll take time, but looking into the K-frame would be my first step. When I get home, I'll look at my stuff and tell you which is which. There's an indentention on the steering box mount for one body that's not on the other, but can't remember which.

Since the '70 K-frame also contains the idler mount and LCA pick up points, swapping K-members will not change bump steer. Most of the critical things that effect bump steer are being changed as a whole.

It will change the box to steering column angle. But that doesn't effect bump steer.