[DeTomaso] Bump Steer

[GW]

Yes it was. I mocked this up some time ago to get everything within 
range so to speak. I was able to bring bump steer near zero in mockup 
but I needed to get to where I am now to finish it up and fine tune it.

The a-arms I will be building will be close to stock dimensions. After 
buying and playing with quite a few ball joints, I found none which were 
acceptable except the Pantera ball joints. Chrysler taper was close but 
not exact and they sit a little deep so hitting it with a reamer is not 
a good alternative. Some use upper Chrysler ball joints for lower and 
get away with it but it's really not a good idea either.

As some one had pointed out to me, the price of new Pantera ball joints 
are not as bad as they used to be so I am using new Pantera ball joints. 
The jig I am working on  will allow me to build the a-arms with more 
caster and with some adjustability. They will be similar to the 
originals but not the same.

Some one who makes aluminum a-arms accused me of copying their aluminum 
a-arms. These will be clean sheet of paper tubular a-arms and a copy of 
no ones a-arms.

Gary



Tomas Gunnarsson wrote:
> Very interesting Gary! You mention that you're going to be making new
> A-arms. Was this simulation done using the dimensions of the stock arms and
> spindles?
>
> Tomas
>
> -----Original Message-----
> From: detomaso-bounces at realbig.com
> [mailto:detomaso-bounces at realbig.com]On Behalf Of GW
> Sent: den 27 mars 2011 00:38
> To: detomaso
> Subject: [DeTomaso] Bump Steer
>
>
> I am getting to my front suspension. I am still waiting for my A-Arm
> Jigs to be laser cut. Business is first so my projects get done when we
> get time on the laser.
>
> This has been a long ongoing project on the side burner...till now.
>
> As you may know I am doing a power steering conversion. This is a rack
> which was used on small cars and sold as a conversion for an MGB. It
> uses an electric pump which can be turned on or off which means the rack
> can be manual or electric.
>
> One thing I have really worked on is getting the geometry correct. As
> many of you know shimming the rack for bump steer is not a fix but a
> patch. All it does is shift the parabolic arch to where it is less
> noticeable in bump; far worse in droop.
>
> The real problem in bump steer is the rack length it self; specifically
> the length between the tie rod pivot. I started by pulling in suspension
> dimensions. I bolted tubing into the suspension mounts and welded it all
> up in a frame. I pulled it off the car and used it to measure suspension
> geometry:
>
> http://www.rc-tech.net/pantera1/frontsus/jig.jpg
>
> I took the numbers and loaded the program Susp Calc the best I can. I
> played with rack length and height in the program. I tweaked one, then
> the other zeroing in on the best of both. Basically, rack height will
> determine the balance of bump steer, rack length will determine  how
> much bump steer. The two do work together. By working them into the
> funnel so to speak I came up with numbers of total variance of  .01
> degree or less in bump and .03 degree or less in droop. In my book 1/100
> of a degree is zero.
>
> Here are the numbers:
> http://www.rc-tech.net/pantera1/frontsus/bump.jpg
>
> The key number is 11.862.....x2 = 23.724". That is the width of the
> pivot point in the steering rack. The stock rack is about 28".
>
> Now garbage in is garbage out. How close is my measurements? ! could be
> within a 1/16", I could be within a 1/4". The rack may need to be 23",
> or it may need to be 24" or somewhere in between. The final tweak is on
> the car.
>
> To do this had to cut the power rack. The power rack was too short to
> begin with. I needed a way to change the length of the steering rack to
> make in car measurements. One issue is welding on the original ball
> joint melts the inner liner and is not re-build able. My solution was to
> use a ball joint off an MGB rack I had. It has the same threads on the
> tie-rod and the shortened length will be exactly what I need.
>
> What I did was cut down a threaded tractor implement pin and weld it on.
> The nut was welded to the ball joint end. To do so allows me to check
> travel at different lengths. I have about 3/4" travel. The idea is I am
> starting too long. I can check the travel at 2 different distances, then
> shorten the threaded shaft till I get in range. If I had 100% confidence
> in what  have transfered to the computer program I would cut it down to
> 23.724"....In fact I know better. I need proof of concept on the car.
> Here are some photos
>
> http://www.rc-tech.net/pantera1/frontsus/sus.htm
>
> Gary
>
>
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