[DeTomaso] Bump Steer

[GW]

LOL,

I wouldn't live long enough to get much else done!

Bjoern Flesland wrote:
> You should open a Pantera restoration shop! 
> Cheers Bjoern
>
>
> Den 26. mars 2011 kl. 23:37 skrev GW <gow2 at rc-tech.net>:
>
>   
>> 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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