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Discussion Starter #1
So my 1968 mustang has an issue with Bump steer. So I have a bump steer gauge and been getting readings that show that it is toe in on both compression and rebound. I keep reading that "tie rod needs to be lengthen". What does that mean "tie rod needs to be lengthen"?
 

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What it means is a redesign of the front suspension and steering may be required. I think that phrase, "tie rod needs to be lengthen" is an over simplification. Because of the design of Mustang steering and suspension, bump at the extreme end of the suspension (droop) travel is almost impossible to eliminate, and the wheels will toe in. From your ride height, measure the compression stroke, then allow the same measurement for droop. Attempt to get the bump steer in that suspension travel measurement to 0 as much as possible. In the past, several people have tried limit straps tied between the lower control arm and the chassis to limit droop. Here is a link to just about everything you ever wanted to know about bump steer.
 

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So my 1968 mustang has an issue with Bump steer. So I have a bump steer gauge and been getting readings that show that it is toe in on both compression and rebound. I keep reading that "tie rod needs to be lengthen". What does that mean "tie rod needs to be lengthen"?
Understanding the cause is one step closer to figuring out how to mitigate the results...

When the suspension goes into compression, the location of the tie-rod attaching point on the spindle goes up AND because the upper control arm is shorter than the lower control arm, inward after an initial move outward as it swings through its arc. The same happens on extension, or the downward movement. The motion is sort of elliptical. There are only so many things you can do to change the location of that attachment point (not physically, but as a point in "space") such as changing caster angle (the tilt of the axis of a line drawn through the upper and lower ball joint studs forward or rearward) or SAI (steering axis inclination) by changing the length of the control arm or its mounting location to reduce the "swing" or frequency of the compression-extension arc... if that makes sense... To further complicate matters, since the geometry of the steering is ALSO controlled by the location of the INNER tie rod attachment point in relation to the OUTER.... the inner attaching point also moves in an arc that travels down as the steering gets closer to lock on either side since the Pitman and Idler arms don't swing parallel to the ground.

One of the things that "bump steer adapters" attempt to do is position the tie-rods (draw an imaginary line through the centers of the inner and outer tie rod ends...) parallel to the ground so that the arc traveled by the spindle's tie rod attaching point on the steering knuckle is more "balanced" during compression and extension. This works if compression and extension are, more or less, equal. Installing stiffer, longer springs that push the front end toward extension, while limiting compression due to increased spring rates, can significantly mess with bump steer.

The term "Tie-rod needs to be lengthened" is true, with respect to the steering geometry of a suspension with bump-steer at both ends of the spectrum, meaning that during the extension and compression cycle the deflection is LESS the longer the tie-rod, because the angles transcribed at the outer end are less with a long tie rod than a short one. This is fine if you have control over the design of the REST of your steering system since you'd have to compensate for the longer tie-rod by shortening the center link or moving the attachment points on the spindle further "outward".

Now, simply LENGTHENING the tie-rod, on a rear-steer suspension, is going to result in more toe-IN.

The following diagram will make you think about the arcs proscribed by the suspension and the effect they have on each other.
 

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Woodchuck gave a great explanation. A few years ago I did a bit of suspension work on my 66. I have a full Street or Track tubular control arms, lowered upper arms, camber kit and lowered. I've never really done a lot of work like this before or even alignment which I now do. So I am a good example of just about everything you can do to induce bumpsteer. Oh I forget to mention wrong geometry 72 spindles. On my own I now have my 66 driving without noticable bumpsteer. Not to say there isn't just that it drives without any funny business.

While you do want the tie rod assembly parallel to the lower arm so the lower arm and tie rod assembly operate as a parallelogram to not steer the spindle as the suspension goes up and down. The reality is you can't always do this. The next best thing is to mitigate this as much as possible in ranges where we drive the most. Mustangs have a ton of travel, perhaps too much. The characteristics of the Mustang are toe in when the suspension extends and toes out when compressed. This is important to keep in mind. What they are talking about with the tie rod length is the over all distance between the inner and outer tie rod stud distance. By lengthening it you're making a more gradual arc that more closely follows the arc of the lower arm inducing less bumpsteer. Things like a bumpsteer kit will lower the outer tie rod which will cause the over all length to increase while keeping toe the same. Another thing, add caster. As you add caster causes the spindle to lay down toward the firewall which cause the outer tie rod to lower closer to the road surface. Again making the tie rod assembly longer for a given toe setting. You can play with toe setting. Remember what I said about toe changing with travel? For argument sake let's say when the suspension is stock and in the middle of travel and toe is set. As the suspension travels either way there isn't a lot of travel and not a huge toe change either way. So we don't notice too many problems. When we lower the suspension we could have 3/4 of the suspension travel in the toe in range vs 1/4 of the toe out travel. Now when we set toe how we would in stock form, were now getting a huge amount of toe in which is the problem IMO. I feel understanding how travel and toe are related, what goes on with the tie rod assembly and experimenting with toe settings you'll be on your way to making it manageable.

I've added a bumpsteer kit, set my caster to 4° caster and played with my toe. I'm running about 1/16" toe in. I've even tried a little toe out but went back to toe in. This was because I was having unwanted input from the rear suspension which I have sorted out. I started to make a bumpsteer kit but gave up on that. I adjusted my bumpsteer by driving over the crest in the road in front of my neighbor's house until I had no more problems. Hope I gave you some of my insight.
 

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Discussion Starter #5
Wow! Thanks for all the info. How much bump steer did a 1968 mustang six-cylinder originally have?

So my issue is that the previous owner of the 68 Mustang put some incorrect parts on the car. The following were incorrectly installed on the car: inner and out tie rod, pitman arm, and center link. I have replaced these but not sure if the spindle nor the springs are correct. The car leans quite heavily to the left/drive side. So I decided to replace the front springs with coil-overs so that I can control the height of the car. The main reason is that I have found that if I drop the car 2" from the original ride height, along with a bump steer kit with 1/2" of shims, the bump steer is minimized. To show this see the attached pdf which contains data and graphs of the bump steer.
 

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