Rack & Pinion Setup Geometry in Custom Apps

Discussion in 'Suspension and Brakes: Repairs and Modifications' started by 2Loose, Mar 24, 2013.

  1. 2Loose

    2Loose reliable source of info

    Looked around and didn't find much. I would think the alignment of the R&P components is critical. Also dimensions of the rack and the end links. Have a friendly discussion going right now in our group. Want to get it correct.
    Aloha,
    Willy
     
  2. grumpyvette

    grumpyvette Administrator Staff Member

  3. 87vette81big

    87vette81big Guest

    Bump Steer becomes an issue on some custom rack and pinion steering
    Installations. Avoiding any bump steering issues is a main concern.

    Brian
     
  4. 2Loose

    2Loose reliable source of info

    The concern I have is this: A buddy is installing a Corvette R&P (don't know what year) in his mid 80's big block (540 ci) hot rodded Camaro. He has located it pretty far forward to clear a lot of other things and to get a pretty straight shot at it from the steering column to clear the front cross member.

    This results in the end links on the R&P angled back to the steering arms at a pretty severe angle, in my opinion. The arcs of the links at the ends of the rack look like they would pull the steering arms inward on a bump, causing the wheels to toe in pretty severely on a bump. He claims that it was setup that way on the wrecked Corvette he pulled it out of, that he took measurements and set it up the same way in the Camaro.

    I never saw the original Corvette he pulled it out of, but the geometry just does not look right to me. I think he is going to have tire wear and handling problems with it the way it is setup. I'll try to take a pix. Of course now that he has it installed, he does not want to hear this, if I am right it would be a lot of work to modify the install.
    Willy
     
  5. grumpyvette

    grumpyvette Administrator Staff Member

    I would think that if all else failed he could take the car down to a good alignment shop and get at least some good advise or he might be able to get custom built end links fabricated, this is one area where having a good two post lift is very helpful as you can very easily have a buddy up in the car moving the steering wheel, moving the suspension from lock to lock, left to right and back and actually see whats going on while your under the car.

    [​IMG]
     
  6. 2Loose

    2Loose reliable source of info

    What ya really need to do while up on the lift, is prep by removing the springs and shocks, and articulate the suspension up and down through it's movement with the rack connected and see if the spindles move inward a significant amount, enough to affect handling and tire wear. I could actually map it out on autocad if I had good measurements as to the amount of toe-in increase that could happen from a significant bump or a normal bump just driving down the street, and then someone who knows more then I do would have to tell me if this is dangerous or not.
    All I am saying is that I do not know, but looking at what my buddy is doing alarmed me, and I hope I am wrong, but wanted to discuss it somewhere where I could get good answers, and this is my favorite place for good answers....
    Aloha,
    Willy
     
  7. 2Loose

    2Loose reliable source of info

    I went through each article Grumpy referenced above, and found these three references to bump steer and geometry:
    (The underlining and italics are by me....)

    http://www.hotrod.com/techarticles/hrdp ... ewall.html
    "As the front suspension moves up (compresses) and down (extends), the lower control arm moves in an arc around its pivot point on the chassis. The tie rod, connected to the lower control arm through the steering arms, also moves in an arc. If these two arcs are different, the wheel will turn slightly either in or out as the suspension moves through its travel. The result is a change in toe-in, which makes the car dart one way or another with no driver input. In extreme cases, it can be very dangerous."

    "The two main things that affect bumpsteer are the angles and the lengths of the tie-rod ends and lower control arms. Ideally, they should be the same length and perfectly parallel to each other, but that's not always practical. Most factory suspensions are pretty good in regard to bumpsteer (since it's a safety and therefore a liability item for the OEs). Because an aftermarket rack-and-pinion swap obviously changes the location and length of the tie rods, there is a good chance that it will adversely affect bumpsteer if not adjusted properly, especially on lowered cars."

    http://www.cachassisworks.com/Attachmen ... DS_WEB.pdf
    "A common issue with many custom steering and suspension conversions is
    achieving correct steering geometry using the limited selection of OEM and
    aftermarket rack and pinions. To solve this problem the g-Machine front-steer,
    manual rack and pinion can be sized from 15.5” to 29.5” (measured from inner tierod
    pivots), in one-inch increments, without affecting rack travel, steering effort, or
    reliability. This enables correct steering geometry
    for vehicles ranging from compacts
    to full-size pickups and allows the assembly to serve as a direct-replacement
    upgrade for 24.5” Mustang II racks."

    http://www.steeroids.com/html/technical_info.html
    "TIE ROD ENDS
    Bump Steer Adjustment
    Bump Steer (or Bump Steering) is a term used to describe when a vehicle hits
    uneven or rough surfaces and causes the car to wander or become darty. What
    causes that? Basically, if the tie rods and the lower control arms are not the same
    length and parallel to each other when the suspension travels up and down they
    will be traveling at a different radius from each other
    . This causes the alignment to
    toe in and toe out as the suspension moves, causing the car to wander and dart
    while on rough surfaces – which is annoying and dangerous!
    Every Steeroids Rack & Pinion kit comes with adjustable tie rod ends that include
    a series of three spacers which allow you to fine tune the angle of your tie rods to
    match the angle of the lower control arms
    – helping eliminate bump steer.
    Correct Adjustment of Tie Rod Ends to Eliminate Bump Steer
    Add and remove spacers provided to move the outer tie rod end up and down. The
    goal is to get the steering tie rod assembly parallel with the lower control arm
    pivot points. NOTE – you are NOT trying to get the tie rod assembly parallel with
    the ground
    . Be sure to make adjustments when the vehicle is resting on its own
    weight."

    There is a possible minor error in the statements quoted above. On many of the suspension systems I have looked at, the steering arms are located on their own individual arc path that is somewhere between the longer arc path of the lower control arms and the shorter arc path of the upper control arms. It seems to me that the arc path and angularity of the R&P end links needs to parallel and match the length of the actual arc path of the steering arm connection point, the point where the R&P end links join the steering arms. In all cases that I have actually looked closely at it, the arc and angularity of the steering arm connection point is actually slightly shorter then the arc of the lower control arm, and sometimes the upper control arm has a slightly different angularity than the lower control arm does, so the angularity of the arc for the steering arm control points is also somewhere between those of the two control arms, but closer to the lower control arm.

    I'm putting 2" dropped spindles in my '55 project, with aftermarket tubular control arms and coil-overs. The control arms have additional castor built into them. However the steering arms are in their original stock location on the spindles so the stock geometry should apply. Am looking at R&P options but haven't decided on a specific direction as yet.

    The stock steering center link and outer tie rod links are still there, so I can take measurements and try to find something that matches the dimensions of the original. Or if a supplier can convince me that a variation from the original will give better results, I'm willing to consider that also.
    Aloha,
    Willy
     
  8. grumpyvette

    grumpyvette Administrator Staff Member

    thats BOTH INFORMATIVE AND A WELL LAID OUT POST thanks!
     

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