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Technical Cross steering questions.

Discussion in 'Traditional Hot Rods' started by RocktimusPryme, Jan 24, 2020.

  1. RocktimusPryme
    Joined: Sep 22, 2013
    Posts: 89

    RocktimusPryme
    Member

    So I got the BBC mocked into the 52 with the headers. Now that I can see where the exhaust is, I can build the steering.

    Have the steering column in place with zip ties. I had originally bought a double u joint and a brace assuming I would have to use it. But now I think there is a chance that I can go straight shot and avoid that extra work.

    my question is what are the most important geometry factors in a straight axle cross steer setup? This is going in a race car that I eventually have delusions of going 120 mph in. So death wobble at 60mph won’t do.

    Most of the pictures you find are 4x4 stuff where the link from the pitman arm is way above the steering arm on the spindle. So how important is it that you sync the pitman arm end and the steering arm end vertically? How about front to back? I should be close there by default. But even in Speedways example it does have an angle. Is one of these more important than the other?

    The third variable is the pitman arm angle itself. If I mount the steering box how I intend to in the simplest position. The box and therefore the pitman arm with be angled. How important is it that the arm swing parallel to the ground?

    for reference this will be a heim joint system with the steering box mounted forward. The steering arm will be mounted forward on the passenger wheel with the drag link running behind the wheels.

    I can take some further pictures to clarify when I get home. I’ve got some stuff mocked up with tape.

    Edit: I added a couple rough doodles to illustrate the pitman arm angle question and my general proposed setup.
     

    Attached Files:

    Last edited: Jan 24, 2020
  2. David Coleman
    Joined: Oct 15, 2019
    Posts: 29

    David Coleman
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  3. David Coleman
    Joined: Oct 15, 2019
    Posts: 29

    David Coleman
    Member

    I usually get thrown off for being too wordy, but . . .
    Three things you should know: (1) a pitman arms swings through an arc, in neutral position, an imaginary line drawn through the pitman arm's center line (where it clamps to the steering box shaft) down to it joint end, and then out through the drag link, should be 90 degrees. If is not 90 degree when the car is going straight down the road, the car will turn quicker to the left than right or vice versa. (2) the shackles on your front spring are on the rear of the springs, so when the springs flex up or down (lets think down for now), the distance between the front and rear spring bushings gets longer, and the axle moves back towards the rear of the car (wheel base changes). In effect, you just pushed the steering arm forward, the same as if you turned the steering wheel. (3) Years ago, an engineer named Ackerman, worked out (using opposing doors in a hotel hallway) the theory we know as the Ackerman theory. It requires imagining a vertical line through the center (dropping a plumb bob) of the two rear wheels. Then imagining a (vertical) plane going through that first imaginal vertical rear axle C/L, up through (and off forward into infinity) the centerline of the king pins. You must mount your rack and pinion's rod ends (if rack & pinion), your horizontal swinging steering (like a 48 Ford), or your sideways swinging steering (like a 32 Ford) onto that imaginary plane or you will suffer what we call "bump steer" which is what Ackerman was tasked to fix.
     
  4. RocktimusPryme
    Joined: Sep 22, 2013
    Posts: 89

    RocktimusPryme
    Member

    Okay, point 1 I think I understand. Essentially the imaginary line through the pitman arm from hole to hole needs to be pointing directly straight down your line of travel. Which makes sense. Your swings will then be equal in either direction. Im still unsure of if I have an angle as I pictured is an issue. Im starting to think not so much. Except that the angle upwards obviously moves my pitman arm higher than the steering arm on the spindle. Which is not ideal, but Im still not sure how bad it is.

    I was wondering if there was a general "Go/No Go" for angles in the cross steer setup. Obviously if you just put everything even its the best, but not always practical.

    Point two, I understand, but that would just seem to be an issue with any straight axle setup. I don't know what you can do to combat it, really stiff shocks maybe. There is a reason vehicles aren't designed like that anymore.

    Two things I should add. From doing my research I have noted that a panhard bar is a big deal on straight axle setups, so I will add one of those. Easy enough. And a steering stabilizer strut, seems cheap and easy to combat bump steer.

    Additional question about the steering stabilizer. Is it more effective when mounted to the drag link, or steering link? Or would either have the same effect?
     
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  5. nochop
    Joined: Nov 13, 2005
    Posts: 1,448

    nochop
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    from norcal

  6. RocktimusPryme
    Joined: Sep 22, 2013
    Posts: 89

    RocktimusPryme
    Member

    That didn't help my specific questions here but I saved the resource, it has some good infor on the joint phasing and stuff. Thank you. It did say its better to tilt the box itself like I was planning. But many OEMs when they do this use an angled pitman arm that may put the end back in parallel with the ground.

    I did find some helpful info and pictures in this old thread
    https://www.jalopyjournal.com/forum...nk-not-parallel-to-axle-looking-down.1065453/

    I don't think I can mount my box for rear steer easily. I think the short steering shaft would have to joggle too far right, but I will re-evaluate it when I get home. Because that just seems like the easy button if it works.

    But given info from that thread. Including pictures where others have their pitman arm at a heavy angle. It looks like everything Im saying will work, I can angle the box upward to make the steering shaft angle the easiest. And then the best thing for me would be to try and get the pitman arm end as near as even vertically (height from the ground) to the steering arm end.

    I cant help but think Im over-thinking this. But I would kinda like to do it once and get it right.
     
    Last edited: Jan 24, 2020
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  7. nochop
    Joined: Nov 13, 2005
    Posts: 1,448

    nochop
    Member
    from norcal

    Maybe mock it up with dowel rods?
     
  8. Go to a tire store, they usually have a model on the counter that demonstrates camber, akerman, toe and caster.
     
  9. RocktimusPryme
    Joined: Sep 22, 2013
    Posts: 89

    RocktimusPryme
    Member

    I do plan on doing that, but that really only shows me that it clears. It won’t really tell me how it functions or if it’s doing to induce death wobble.
     
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  10. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,203

    seb fontana
    Member
    from ct

    Ackerman had nothing to do with bump steer; he figured out how to make the front wheels track at the proper angle in turns..Steering box in front of axle like pic 1 is fine, make Pittman arm shaft as vertical as possible and arm as level to ground as possible to not waste any motion..End result is you want the drag link [from pittman arm to steering arm right front spindle] to end up being level, under load..By adjusting the height of the steering box and the height of the spindle steering you can get the drag link level.. Be sure box is placed so that at full left the pittman will clear the left front spring..You will find that if you clock the pittman arm two splines from strait back to the left you will end up with equal steering wheel revolutions right and left..As in the 1st pic looking down onto the drag link slight rearward angle, about an inch will help to not loose motion..One more thing, you can use a box that is reverse of the one in your pic so that pittman arm will point forward and box can be mounted farther back..
     
  11. RocktimusPryme
    Joined: Sep 22, 2013
    Posts: 89

    RocktimusPryme
    Member

    Honestly after seeing how simple and easy this looks, Im going to spend some time tomorrow seeing if I feel like I can make this work. But with a u joint on either end instead of a double in the middle. This gives you an angle on the arm itself, but the linkages are pretty parallel.

    I might have to send my 30” column back in for a 28”.

    And I think my pitman arm has 4 big master splines, I dont think you can mount mount it two teeth to the left. I think it only has 4 positions possible.
     

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    Last edited: Jan 24, 2020
  12. Baron
    Joined: Aug 13, 2004
    Posts: 3,375

    Baron
    ALLIANCE MEMBER

    What you have set up here looks fine. I like to keep the drag link parallel to the tie rod. I usually have the drag link about 1 inch higher at the Pitman arm side. My 55 Chevy is set up like this as is my 57 Corvette. Both cars have been well over 100 mph ,no bump steer and no death wobble. One thing I'd like to add is I prefer using tie rod ends over heim jts. Last longer, not noisy and more preload than heim jts. Good luck.
    Sent from my SM-G950U using The H.A.M.B. mobile app
     
    Last edited: Jan 24, 2020
  13. RocktimusPryme
    Joined: Sep 22, 2013
    Posts: 89

    RocktimusPryme
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    your saying yours is set up like that picture I found? The rear steer one?
     
  14. blue 49
    Joined: Dec 24, 2006
    Posts: 1,217

    blue 49
    Member
    from Iowa

    steering box.JPG 114.JPG
    My set up is like you have planned, although I have since moved the tie rod up front with the steering arm and drag link. Works fine up to 80 MPH (no problems, just haven't pushed it any faster yet). Ackerman geometry is to minimize tire scrub in turns, shouldn't affect you going straight a 1/4 mile at a time. You can also see my panhard bar and that I tried to keep it parallel to the ground.

    Gary
     
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  15. Baron
    Joined: Aug 13, 2004
    Posts: 3,375

    Baron
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    Here is a picture of the Corvette. This was before I replaced the straight axle with a 2 inch dropped axle. But the steering geometry is still the same. The 55 Chevy is front steer, but the same basic principles apply. 20150417_171154 (2).jpg 55 Chevy steering finished.JPG
     
  16. flatheadgary
    Joined: Jul 17, 2007
    Posts: 719

    flatheadgary
    Member
    from boron,ca

    correct me if wrong. i have always built my axle setups with the box mounted behind the axle then the solid spring hanger is mounted on that end. if the box is mounted in front of the axle the shackles are mounted in the back and the spring hanger is mounted in the front with the box.
     
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  17. Baron
    Joined: Aug 13, 2004
    Posts: 3,375

    Baron
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    That makes no difference with cross steering. Now if you had the draglink running parallel to the frame rail, that would make a difference where the spring hangers and shackles mounts were located ( just as you stated).
     
    Last edited: Jan 24, 2020
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  18. RocktimusPryme
    Joined: Sep 22, 2013
    Posts: 89

    RocktimusPryme
    Member

    Okay. Got it all mocked up. The rear steer just wasn’t going to work. Angles too extreme. But I’m pretty happy with this. Unfortunately as good as that pitman arm sits it doesn’t fit this box. It’s a power steering arm I had laying. So now I need to buy a pitman arm for a 525 box with an end for a heim joint that also has about a 2” drop.

    I also may have to dent that one header tube but that’s not a big deal.
     

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  19. Baron
    Joined: Aug 13, 2004
    Posts: 3,375

    Baron
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    Don't be afraid to angle the box up a bit more in the front. I used one of Speedway Motors Universal Street Rod Pitman arms (GM 525/605 Box) on my 55 Chevy. This car has been together since 2008.
    Speedway Universal Street Rod Pitman Arm for GM 525/605 Steering Box
     

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  20. Desmodromic
    Joined: Sep 25, 2010
    Posts: 513

    Desmodromic
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    Annotation 2020-01-27 202111.jpg Characteristics for "ideal" cross steering:

    Let me preface this with some comments on semi-elliptic parallel leaf springs, which are an inferior (but traditional!) method to locate a front axle, and preclude any hope of achieving ideal steering system geometry. This is aggravated where the springs have substantial radius, in order to elevate the body, for weight transfer.

    Assuming the shackle location to be on the back end of the springs, compression of the springs will result in the axle moving back, with accompanying changes in the geometry of steering links. This effect is more pronounced if the car is leaning, as might be the case when landing after lifting the front wheels; one end of the axle might move back further than the other, resulting in bump steer. This effect can be minimized by keeping the front half of the springs horizontal, and possibly stiffer than the back half, to minimize braking torque rotating the axle.

    Sketch A, below, indicates some of the suggested features to optimize steering design. (Sorry, the order of the sketches was inadvertently inverted.) Not indicated but very important are:


    If a Panhard rod is used (recommended), it should be attached to the chassis on the steering box side.

    The cross link and Panhard rod should the same length, and be parallel to the ground.

    Theoretically, the steering box output shaft and right kingpin centerlines should be parallel. (I doubt that anyone does this.)

    The arrangement shown will provide "symmetrical" steering, i.e., rotating the steering wheel X degrees in either direction will result in the wheel turning the same toward the right or the left. I don't think this is terribly important, unless you're hiring Lewis Hamilton to run your car in Formula 1.

    Sketch B is my understanding of your arrangement. Symmetrical steering has been lost, but would come closest to symmetrical if the Pitman arm axis were parallel to that of the steering arm, and of the same length.

    Sketch C is to clarify a misconception on pitman arm shape. The three steering box locations shown, and the shape of the Pitman arms, has no impact on steering system characteristics, as long as dimensions "L" and "H", and angle "a", are equal.
     
    Last edited: Jan 27, 2020
  21. RocktimusPryme
    Joined: Sep 22, 2013
    Posts: 89

    RocktimusPryme
    Member

    Thank you, I appreciate the visuals. And I think Im actually Sketch A, save for the pitman arm will be facing backwards. Other than that, that's how my system will be. Steering link forward of the axle, with a tie rod rear of the axle.

    I bought two types of steering arms, one drop set that goes rear of the axle with the passenger side having two holes for rear steer. And one single holt set without the drop. Im going to end up using both, with one facing forward. The rearward hole in the link setup for rear steer will not be used.

    Question on the Panhard bar. I had been picturing it as attached to the axle somewhere in the center, and then on the frame (steering box side). But you are saying making it the same length as the steering link is advantageous. Just find a spot on the axle all the way out near the spindle to fabricate it onto one way or another? My axle is a 49 or so Chevy truck axle. I wonder if there is one pre made for it. To negate any drilling or welding.
     
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  22. Baron
    Joined: Aug 13, 2004
    Posts: 3,375

    Baron
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    With parallel leaf springs, a Panhard bar isn't necessary. If you do decide to run one, it should be as long as possible.
     
  23. Desmodromic
    Joined: Sep 25, 2010
    Posts: 513

    Desmodromic
    Member

    If the Panhard rod is the same length as the cross-tie rod, it will cause the axle to swing through the same arc as the wheel end of the cross tie rod, with chassis vertical movement. If you picture a very short panhard rod, it will pull the axle to the left more than the cross tie is pulling the steering arm, due to it smaller arc radius, resulting in some degree of bump steer. The amount of bump steer depends on the magnitude of the vertical bump. If the springs are pretty stiff and you don't lift the front end much, the effect will be less pronounced.

    Secondarily, the springs want the axle to move upwards vertically, whereas the Panhard rod insists that the vertical axle displacement be in an arc, which must be accommodated by lateral flexibility of the spring. The shorter the rod, and stiffer the lateral compliance of the spring, the greater the "disagreement" will be.

    I have no idea if an off-the-shelf bracket is available for a Panhard rod to Chevy axle connection, but it should be simple to fabricate something that attaches to the axle/spring bolting, with a ball joint connection further outboard, to keep the Panhard rod as long as feasible.
     
    Last edited: Jan 29, 2020
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  24. Desmodromic
    Joined: Sep 25, 2010
    Posts: 513

    Desmodromic
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    I agree, if the springs are pretty stiff laterally. If the spring leaves are narrow, the spring bushings are soft, and especially if the springs have a high arch (as seems fashionable in many gassers), a Panhard may be desirable. If in doubt, I would want to engineer a Panhard into my design, but try the car without and see how it goes.
     
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  25. RocktimusPryme
    Joined: Sep 22, 2013
    Posts: 89

    RocktimusPryme
    Member

    They are a bit skinny. Its a 40's Gm truck setup so it has 1.75" wide leafs. Ill put it all together and then post back and see where a panhard bar would fit. Honestly, in the scheme of things it seems cheap and easy, so I don't see why not.
     
    Last edited: Jan 29, 2020
  26. ..doubt you'd need a panhard bar,..don't really see many parallel fronts using one,..
     
  27. blue 49
    Joined: Dec 24, 2006
    Posts: 1,217

    blue 49
    Member
    from Iowa

    Past discussions on here have said the panhard bar prevents side loading the shackles with cross steering. Also, shackles and steering should not be on the same side of the axle, which I didn't do, so I made up the panhard bar.

    Gary
     
  28. RocktimusPryme
    Joined: Sep 22, 2013
    Posts: 89

    RocktimusPryme
    Member

    Okay, my shackles are in the back and my steering link is in the front. So maybe I will just put it together as is and see how it goes. I need to order a steering dampner strut still too.
     

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