[DeTomaso] Bump Steer

[GW]

Thanks John, but I don't think I hold a candle to Goran.

Gary

John Taphorn wrote:
> Gary
>
> Great work.  I really appreciate that you are sharing this project 
> with us. I am impressed when fabricators, like you and Goran, work out 
> the mathematical calculations to optimize the opportunity and figure 
> out a way to construct a solution.
>
> I'd like to do the same if I had the tools; although, I admit it would 
> be a knowledge stretch for me as well.
>
> Keep inspiring us
>
> JT
> ----- Original Message ----- From: "GW" <gow2 at rc-tech.net>
> To: "detomaso" <detomaso at realbig.com>
> Sent: Saturday, March 26, 2011 6:37 PM
> 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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