Adjusting bumpsteer necessary on a lowered fox?

I would disagree that you want it paralell with the control arm at all ever. If you do, it's coincidental only. The ground isn't a perfect reference but it's better than the control arm.

The reason you get bumpsteer is the arc that the tie rod end swings at as it moves up and down. If you start at a perfectly horizontal position. 1 degree of motion up or down gives a slight toe-in. If you go 1 degree further you get even more toe in. The difference of toe in between 0-1 degrees is less than between 1-2 degrees.

If your control arm is already pitched up at 2 degrees, and you put your tie rod paralell with that, you're already starting with an increased rate of bumpsteer. You also add the effect of toe-out when the suspension is unloaded, which is very bad because the nose of most cars lift when they gain speed, and toe out at speed makes for a very unstable car.

If you start with a tie rod at zero bubble horizontally, up or down you get toe in, which makes your car more predictable. Also at speed when the nose lifts, you'll get toe in, which will increase high speed stability.

You really shouldn't be using the ground, or the control arm as the reference. If you understand how it works you'll understand how to properly set it.

If you don't understand how it works, use a flat ground surface as a reference for the lesser of two evils.

seijirou
We'll disagree.
If your control arm is already pitched up at 2 degrees, and you put your tie rod parallel with that, you're already starting with an increased rate of bump-steer.
HUH???? I'm sorry, I'm going to have to say that you're *very* much mistaken about the *cause* of bump-steer.

For ONE, you can NOT make "blanket" statements about bump-steer, etc. You *want* the tie-rod to perfectly follow the FCA. When the tie-rod does not perfectly follow the FCA, you get "bump steer" - the distance from the hard point at rack to the tie rod end does not follow the spindle as it moves up and down.

So, a LOT of it depends on where the rack is mounted, the k-frame, the FCAs, etc.

So, if you set the tie-rod parallel to the ground (as you suggested), but the FCA is at an angle (which it *will* be), then as the FCA moves up, the angle between the hard point on the rack and the spindle WILL change.

But, DEPENDING on a *specific* setup, the ideal spacing for the bump-steer will differ. Even an 87 is NOT the same as a 93.


From:
http://www.competitionengineering.com/catalog/images/c2418_inst.pdf

13. To eliminate bump-steer the tie rod assembly must be parallel with the lower control arm. Mount the rod end to the modified spindle using the supplied 5/8” x 5” bolt. The bolt should mount through the top of the spindle hole.

14. Raise the tie rod end until it is as close as possible to being parallel to the lower control arm. Use the spacers in the kit to shim the tie rod the correct distance. Lock everything in place using the supplied lock nut.

To get this setting completely accurate, you must use a bump-steer gauge. If you do not have one, reassemble the remaining front-end components (using caution when installing the front springs) and take the vehicle to a professional alignment shop for final adjustments. If you do have a bump-steer gauge, adjust the tie rod up or down using the supplied shim kit to get as close to zero toe change for the first 1-1/4” of bump travel. Try to get toe out if zero cannot be achieved. 15. Tighten all bolts loosened during installation and use caution when re-installing the front springs. You may also re-install the brake calipers at this time.

16. Have the front toe settings checked at an alignment shop. You may also check your own toe settings by using Competition Engineering’s Toe Plates P/N: C9600.

More info:
From Griggs
http://www.griggsracing.com/installation/bump.html
10) ... To eliminate bump steer, you must adjust the relationship between the tie rod and the control arm so that they remain in phase throughout their range of motion. This does not necessarily mean that the arms will be parallel to each other. Insert the 5/8" bolt supplied in the kit through the steering arm of the spindle from above. Raise the tie rod end until it is roughly parallel to the control arm. This will approximate the correct relationship between the arms at designed ride height. Insert the approximate number of spacers to retain this relationship between the steering arm and control arm and install supplied nylock nut below the spacers. If you do not have access to a bump-steer gauge reassemble the rest of the front suspension according to your service manual and re-torque all fasteners to factory spec.



From:
http://www.longacreracing.com/articles/art.asp?ARTID=13
In order to accomplish zero bump the tie rod must fall between an imaginary line that runs from the upper ball joint through the lower ball joint and an imaginary line that runs through the upper a-arm pivot and the lower control arm pivot. In addition, the centerline of the tie rod must intersect with the instant center created by the upper a-arm and the lower control arm (See diagram below).

The instant center is an imaginary point that is created by drawing a line from the upper a-arm ball joint through the a-arm pivot where it is intersected by an imaginary line that extends from the lower ball joint through the inner control arm pivot. Where the two imaginary lines intersect is the instant center.

Sounds complicated? Really it is very simple. To achieve zero bump the front end must be designed correctly. The tie rod must travel on the same arc as the suspension when the car goes through travel. Simply matching lengths and arcs to prevent any unwanted steering of the front tires.

To exaggerate, if the tie rod were only 10" long and the suspension were 20" long then when the suspension traveled the tie rod angle would shorten much quicker than the suspension arc. In this scenario the tie rod would shorten much quicker through travel than the suspension and the car would toe in drastically over bumps. The shorter arc of the tie rod would pull on the spindle and toe it in through travel.

I. Bump Simplified

When designing a car, if the centerline of the outer tie rod lines up with the centerline of the lower ball joint, and the inter tie rod lines up with the lower pivot point then the length and angle of the tie rod and suspension will be the same resulting in zero bump. Most car builders design their cars in this fashion.

stang&2Birds said:

seijirou
We'll disagree.

Click to expand...

Haha, wow. that's some old-school schooling.

I just got back on to ammend my statement before. I realized last night thinking about it at home, that I forgot that the wheel also moves inboard and outboard with the control arm, and will do so at the same rate on the same arc as the tierod's balljoint if the distance between the pivot points of both is the same.

Literally the wheel moves on the same arc as the tie rod will, and this is why you need it to be paralell with the control arm.

For some reason, I was thinking the wheel is literally going to move straight up and down longitudally, while the tie rod would follow the arc.

Not enough coffee I guess.

So, in short, you're right, I was wrong! I shall think three times instead of twice next time before deciding to disagree with ya.

Begizzle : Just eyeball it. You don't have a lot of options. You'll have maybe 4 options given the spacers. Do the best you can.


seijirou:
No big deal. *I* make MANY mistakes.

Also, if you just look at the *simple* locations of the components, it *seems* like you want the tie-rod angle between being parallel to the ground and parallel to the FCA. But, the FCA moves in a complex motion. Also, there are ?6? different FCA/K-frame/Strut-tower alignments between 84 and 93. Ford moved things around for better ride, less brake squat, better launch, because they felt like it, etc, etc, etc. The book by Kirschenbaum goes into a little detail.

But, as a VERY WELL know and respected Stang expert said, (paraphrased) "On most/many cars, the ideal place for the the rod will be at an angle a little more than the FCA" (so, closer to stock than closer to being parallel to the ground). That is *counter-intuitive* from just looking at the simple hard points. But, when considering the complex movement of the modified McPherson strut system as it moves, I can understand how that could be the case.