Bump steer geometry question

[bryant]

On my Maverick, I am now chasing bump steer. I put Heights gm pro G drop spindles on. I used A body steering arms shortened 1.5". I put a mustang II bump steer kit on the arms. Today I made a bumpsteer gauge and have found that I am out of adjustment range on the spindle end. Its all the way down. So I need to move the inner tie rod up. Im looking at how ridetech does it with their true turn system by making a plate that raises the inner location. It also looks like they move the mounting holes out as they move up. As best as I can determine, they are moving the tie rod up along the angle between the lower and upper control arm inner pivots. Is this the correct way/ direction to raise the inner tie rod to achieve 0 bump steer?

[David Pozzi]

You are correct, the books all say to place the inner tie rod pivot along a line from upper inner pivot to lower inner pivot of the A arms. In reality you may need to alter this a bit inward or outward to get what you want. number one, you may be assuming the stock location was ideal but that may not be the case, Number two, the spindle swap may have changed geometry, Three, lowering changes things as the bump steer stock may be pretty good at stock ride height but not good when lowered. Tall ball joints can cause bump steer issues.

The tie rod needs to point to the instant center of lines projected from the A arms and this IC moves up and down and also inward outward as the ride height changes. With more severe geometry to get a lot of neg camber gain, the IC moves faster and more inward to close to the lower ball joint of the opposite side at full bump travel.
I suggest making a 3/16" x 1.25" steel plate, with two holes in it, one for the tie rod to hold it bolted in. The other end make a slot so you can slide it up and down and pivot left / right. then use it to hold the tie rod in the elevated position and you can swing it left / right or up/down to test for bump steer. I'd like to see some toe out in bump, maybe .060" per side at tread, depending on how much Ackerman you achieve. If Ackerman is low, then use more bump toe out to compensate.

It's best to graph the toe curve, it will show you which way you need to go and what's happening. A lot of curve means your tie rod length is wrong.

[astroracer]

I always point to these articles on Longacres site to help explain what is going on with bumpsteer.
This one explains the causes.
http://www.longacreracing.com/techni...icle=Bumpsteer Tech - Back to Basics
And this one helps design it out.
http://www.longacreracing.com/techni...2&article=Bump Steer
Hope this helps!
Mark

[bryant]

I'd like to see some toe out in bump, maybe .060" per side at tread, depending on how much Ackerman you achieve. If Ackerman is low, then use more bump toe out to compensate.

Thank you for the reply!
I am not sure how much ackerman I have yet. Im still working on grasping how to measure it accurately. So when you say .060" per side at tread, Does this mean the measurement is taken at the tires tread distance from center? Is it .060" between the front and rear tread edge or from the wheel center to the tread edge?

[David Pozzi]

Difference between front tread to rear tread.