ZERO "Bump Steer"

 

**MAIN PANTERA PAGE**

 

BUMP STEER is induced in a Pantera due to the steering rack. Some people shift the location to move the effect. This minimizes the effect of bump in compression, but it doesn't actually change the bump steer; in fact it worsens in droop.

The root of the problem is the distance between the tie rod ball joints; where they pivot is about 2" too far apart.  

The following is how I accomplished ZERO bump steer the right way; eliminating ALL bump steer in both bump and droop. 

 

Note 1: Zero bump steer as I am defining here as.....change in toe measured 8 feet in front of the front axle measured 1/16" or less. 1/16" equates to less than 4/100 of a degree which I would consider close enough to be zero. 

Note 2: While this was done to a power steering rack, the stock rack could be modified to do the same thing. 

Note 3: This may be a "How I did it" but it is more importantly "What needs to be done".

Note 4: NO modifications required modifying the upright! 

***Click on photo for larger image***

 

Dimensions from the jig were put into "Suspension Calc" (software). From there the ideal rack width and height was determined.
This jig was used to determine suspension dimensions. The tubes were bolted in each pivot point. The frame was welded to each tube. The frame removed from the car then the points could be measured. Since the results from the suspension software were subject to the accuracy of many measurements, I needed a way to fine tune both the width and height of the rack.
The power steering rack I chose was 6" shorter at tie rod pivot then Pantera rack.

Pantera rack a little too long anyway.

 

The end was cut off the power steering rack to make altercations. The end was originally press fit. A small hole still exist in area cut off.
Who said there are no tractor parts in the Pantera? Just need the threaded section of the pin.
Threaded section cut to fit in hole. Welding is hard on a ball joint. I used an MGB rod end. The entire ball joint can be dismantled so the components are not damaged in welding.
Ball joint is adjustable. Set up is a little too long to start with but this will allow me to make some measurements, make some changes and go a little at a time till I hit the right spot. With the Pantera rack installed, height and width are determined

 

The bump steer result will tell you if your rack is too high or too low. 

Once your rack is the correct height, your bump steer results will also tell you if your rack is to wide or narrow. 

First set height, then adjust width. 

The original rack measured 28 1/8" between tie rod pivots. 
The adjusted rack measures almost exactly 26" between tie rod pivots. That puts the Pantera rack about 2 1/8" too wide.
Threaded bolts allowed for fine tuning height. 

Now the location is made, a third mount will be added to support the rack from the other side. 

While not shown, once the width is set, the nuts were welded in place. Greasing the rack again and 2 boots added to the left side still need be done.
NOTE: With the custom a-arms I build, caster is around 7 degrees. This rotates the upright which ADDS about .39" to the ball joint mount.  This equates to dropping the steering rack .39" or ...you can think of it as requiring the steering rack to go up another .39"

 

MEASURING BUMP

 

To make these measurements, I used laser levels from Harbor Freight. 

They are laid against an aluminum channel strapped to the rim. 

IMPORTANT!:

The aluminum channel is laid against the rim; not the tire. The tire is not always symmetrical and is not a good point o reference.

Laser reference always measured with laser being  level. Any pitch can alter the measurement.
Distance is measured in front of the car (about 8 feet) and behind the car I used 16 feet for simplicity).  Google "triangle calculator" and you can easily determine degree of toe.
I first find the center point where the distance behind the car is the same as the distance in front of the car. This would be zero toe in.

You can measure the laser off the side/rear of the car to find center.

Using the triangle calculator determine where your target toe in setting is and mark a vertical line in front of the car.
With the laser level pointing at the reference toe mark, this becomes the target.
Bump measured by jacking on a-arm; not tire.
Droop measured as well. Laser must remain level.

 

RESULTS

 

The result is
Steering rack quite a bit higher then stock; about 6.75" above the bottom of the frame rail. Steering rack shorter between tie rod ball ends; 28.125 stock, vs 26" for correct width.
 

Bump steer:

 

ZERO!