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Rack and Pinion options for narrow IFS

Discussion in 'The Hokey Ass Message Board' started by blowby, Jan 4, 2013.

  1. blowby
    Joined: Dec 27, 2012
    Posts: 7,959

    blowby
    Member
    from Nicasio Ca

    Thanks guys. The Intrepid would be the ticket if not for the power assist, and I don't see where they made it without. Darn, close.
     
  2. gatz
    Joined: Jun 2, 2011
    Posts: 1,595

    gatz
    Member


    This all makes sense now that I've read through it a dozen times.

    What I don't quite get is what Mopar was thinking back in the 40's. The geometry of their steering & tie-rods is WAY OFF from adhering to the general rule of having the inner pivot locations in-line with this imaginary line connecting the UCA & LCA pivots. The right side might've been OK, but the left side would have caused extreme movement of the spindle when encountering a "bump" as that tie-rod is very short.
    Bump steer must not have been much of a concern back then, or maybe this was a compromise and was all they could make work.

    As an aside, on the 40 Chrysler that I have, there's a short extra rubber-bushed forging that connects to the steering arm. That is the attachment point for the tie-rods. Its purpose must've been to reduce the amount of shock to the steering box/steering wheel to smooth it out.

    Some other-make cars from back in that era used an idler type center steering arm setup with a connector from the steering arm to this idler.......was the intent to help with the bump-steer problem? Even those didn't follow that rule about the tie-rods' inner pivot being in-line with the UCA & LCA pivots; although the Ackerman was probably great.

    Looking down from above onto the 40 Chrysler's layout, the UCA & LCA pivots are "splayed", i.e. the pivot lines are not parallel with the center-line of the car and the intersection of the pivot lines lies about midpoint, or a little farther back, of the wheelbase (guessing here)

    Can a R&P from a Cav or similar be modified to be like the one shown in post
    #20 by gimpy (and of course it would have to be rear mount) to satisfy the geometry requirements?

    If that would be the correct approach, would the Ackerman then be good?

    thanks for any additional input

    gatz
     
  3. blowby
    Joined: Dec 27, 2012
    Posts: 7,959

    blowby
    Member
    from Nicasio Ca

    I've got a rack here, made by Sweet, for circle track I guess. Looks like I can make it work. According to their web site it moves 2.5 inches per turn, that sound about right?

    [​IMG]
     
  4. Frankenstein57
    Joined: Jun 16, 2010
    Posts: 75

    Frankenstein57
    Member

    The cavalier racks are a popular swap kit into 57 and up fords, I have one fabbed up in my 57, haven't tried it yet. The 57 has extremely long tie rods, longer than the a arm arc. We addressed this and machined a block of aluminum to spread the tie rod anchor points farther than the stock tie rods. We couldn't get the same length as the a arm arc, but we are much closer. I think the rack had to be offset to clear the headers, which limited our bumpsteer correction. When I get time I want to do a bump steer simulation with a dial indicator, maybe video it. My buddy sells chassis upgrades for independent rear end mustangs, he has a video to prove his stuff works. The stock IRS has rear bump steer. Mark
     
  5. 50dodge4x4
    Joined: Aug 7, 2004
    Posts: 3,535

    50dodge4x4
    Member

    Gatz,
    The inner tie rod mounting points on a Cav rack are metric bolts that screw into the traveling part of the rack. The bolts are a couple inches apart. All you need to do is make a "center link" that bolts to the rack using the two tie rod bolts that has a tapered hole at each end that matches the inner tie rods ends you want to use. The spacing of those two tapered holes can be anywhere you want them to be. On the 40s Mopars, the tie rod location is on the same plane as the lower control arms, so if you maintain the same pivot alignment as the lower control arms, bump steer will be reduced to near nothing.
    Ackerman means you simply need to have the outer tie rod attaching point in a line between the lower control arm pivot point (lower ball joint or lower knockle) and the center of the rear axle. With as many new cars that have paid little attention to Ackerman, I suspect close will be good enough. Most front steer production stuff is not even close.

    You have to be aware that most rack & pinion steering has a larger turning radius then the box and tie rod assemblies had. Gene
     
  6. blowby
    Joined: Dec 27, 2012
    Posts: 7,959

    blowby
    Member
    from Nicasio Ca

    All very confusing, particularly for first timer trying to set one up. At first glance it seems front steer would have reverse Ackerman but I read something about the location of the drag link and the swing of the steering box/idler arm somehow influencing things. So how does changing to a rack play into that?

    And what's with all these longer tie rod than control arm OE applications? Also read that creating some increased toe-in on bump steer is not necessarily a bad thing. Seems on a rear steer this would be the case, but on a front steer wouldn't longer tie rods create toe-out?

    All this makes engine building seem much simpler. I guess that's why I've always just built engines.
     
  7. Ackerman is the change in TOE with steering angle. It has nothing to do with how the center link is operated. Your pitman arm and idler should be equal length and parallel in all dimensions.

    Just flipping rear steering arms to the front (or vise versa) will give you reverse ackerman. The SAI line and tie rod hole should form a line that points at the center of the rear axle housing. If the steering arm is low enough on the spindle, you can use the lower ball joint only to figure this out.

    The short answer is, the tie rod hole in the steering arm should be further out than the ball joint for front steer, and inboard of the ball joint for rear steer.
     
  8. THE FRENCHTOWN FLYER
    Joined: Jun 6, 2007
    Posts: 3,808

    THE FRENCHTOWN FLYER
    Member
    from FRENCHTOWN

    Flaming River lists a narrowed MII rack IFS for narrow cars.
     
  9. blowby
    Joined: Dec 27, 2012
    Posts: 7,959

    blowby
    Member
    from Nicasio Ca

    Found what I was referring to in regards to the drag link moving with the arcs of the idler arm and steering box, and how it influences toe. I pirated it from here:
    http://www.thirdgen.org/techboard/suspension-chassis/653009-people-racecrafts-spindles-2.html
    and explains it better than I could. Is he right? I don't know, but I do know that the front steering arms on my Corvair front end are 2-1/4" inboard of the ball joints, so unless there is something like this going on there is some serious reverse Ackerman. I don't see any way you could run front steering arms outboard of the ball joints without hitting the wheel.

    [​IMG]

    Not my words, from the link above:

    Bringing this back because I have learned some new information concerning our steering system that is important to consider.

    The assumption that our cars have anti-ackerman may be incorrect. I (and I think others) based this assumption upon the fact that our steering arm points are inboard of the ball joint pivot. Many web pages offer a simplified explanation to ackerman geometry stating that the angle of the arms relative to the balljoint is what determines ackerman.

    This is an oversimplified explanation and only really correct for a steering system with a single drag link connecting the two steering arms... something like this:

    Obviously our steering system is different, here is a visual:

    There is an important thing going on here that affects ackerman. As you turn, the pitman and the idler turn which moves the inner tie rod pivot points forward. At the same time, the outer tie rod pivot points move rearward (depending on position, one always moves rearward at least). Since we do not have a solid link between the two tie rod outer end pivots, the effective length between the two tie rod outer pivot points changes.
    If you started out with the tie rods without any angle as viewed from above, the distance between the two outer tie rod end points would have to shorten as the centerlink moves forward.

    In our situation though, the tie rods look like the image above. So what happens is the angle between the tie rods decreases. As this angle
    decreases the overall length between the two outer tie rod end points increases.

    This effect counter acts the angle between the balljoint pivot and the outer tie rod end pivot. To what degree would only be known if we measured everything. With the steering arms angled inward, the distance between the two outer end points has to increase if we want to maintain the same change in angle at the wheels.

    So you can see the angle of the tie rods looking from above is pretty important.

    Now what does this mean for shortened steering arms? Well when you shorten the arm, you are flattening the angle of those tie rods when looking from above. As you turn the wheel and the tie rod outer pivots move rearward and the inner pivots move forward. The gain in length for a given degree change in angle is not as great. There is more lateral travel at a steeper angle for a given degree change. This is the same idea why the angled steering arms change ackerman.

    Shortening the steering arm with no other changes will decrease ackerman. It can multiply if the arms end up angling rearward such that the inner pivots are ahead of the outer pivots.

    So then how do we improve the situation to gain ackerman? Well there are a couple of ways but they will yield different ackerman curves. You could most easily space the inner tie rod end rearward. In looking at my car it appears as though you may have a half inch or so to work with before hitting the K member.
    I believe this would end up giving you an ackerman curve with a greater rate gain from center but the rate would slow the further away you get from center. It may still be more than stock through the entire range. It would have to be measured or modeled to know for sure.

    The second way is to move the outer tie rod pivot outward. This will give you an increasing ackerman curve. I believe this is preferable to the decreasing gain ackerman curve of angling the tie rods more by spacing their inner pivot rearward.

    Since we can't move the outer tie rod point outward without shortening the effective length of the steering arm, the solution is to shorten the arm as little as possible to allow the tie rod end room so it doesn't hit the wheel. Any shortening of the arm will have to be accompanied by spacing the inner tie rod pivots rearward to maintain the same tie rod angle as viewed from above. Even doing so, you will not maintain a stock "angle curve" of the tie rod because you've changed where its end point is and where it moves.

    Ultimately, you'd have to model or measure the result of changes in order to know how well they worked. I think the first step in doing that is getting accurate measurements of our steering linkage, ball joint centerlines, and steering arm angle and length. (I believe I have accurate specs for the center link, pitman, and idler arm so far)
     
  10. This is the key point. The geometry pictured must be modeled as installed in a particular vehicle for correct ackerman action. It only works right if the components are placed correctly.

    Having the correct steering arm angles on the knuckle is much simpler and idiot-resistant in most cases; and yes it is possible to have them outboard of the ball joint. Millions of cars have been built that way. The steering arm may need to be raised, or different wheels fitted.
     
  11. blowby
    Joined: Dec 27, 2012
    Posts: 7,959

    blowby
    Member
    from Nicasio Ca

    OK, as installed, well not quite. I need to get everything a little better cinched down but I seem to be close to zero Ackerman. From what I've read that may be OK (street car).

    Bump steer - haven't measured yet. The upper A frame pivot to ball joint is 8", lowers are 13". The wood test tie rods in the photo are 10". How does that sound for potential bump steer? The outer tie rod ends are about level with the lower ball joints, should the inners be level with the lower A frame pivot? The alternative is to fab a drag link to move the inner tie rods closer together.
     

    Attached Files:

  12. Yes. That would be a good place to start.

    In the photo, the tie rods appear to be too short. The general idea is that the tie rod follows the same arc as the knuckle. In your case, with the outer tie rod mount that close (in height) to the lower ball joint, the tie rod arc should be very close to the arc of the lower a-arm.
     

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