X-over steering (all the juicy details)

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by bad_bo_ti, May 16, 2001.

  1. bad_bo_ti

    bad_bo_ti 1/2 ton status

    Feb 17, 2000
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    This may not be the spot to to post this since it is for school, i am doing a report on the science behind bumpsteer (angles draglink lengths ect) and how x-over helps get rid of that. any information anybody can add will be helpful. this is my last project for school so i thought i would do it on something cool and something i can learn on too..... please help. thanks
  2. misfit

    misfit 1/2 ton status

    Jul 21, 2000
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    south central wa state
    how bout writing how a long drag link has less angle than a shorter one, way it affects how many turns of the wheel, maybe throw a little trigonometry in there to detail the whole angle thing.
  3. solowookie

    solowookie 1/2 ton status

    Mar 31, 2001
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    would like to see your report when you get it done... [​IMG]

    <font color=blue> Jeff </font color=blue>

  4. BadDog

    BadDog SOL Staff Member Super Moderator Author

    Feb 24, 2001
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    Phoenix, AZ
    It's all about radius and it is possible to figure out using only the Pythagorean theorem (right triangles where the hypotenuse is the radius).

    A short radius (i.e. stock arm) will have more horizontal movement as one end moves vertically. Since this movement describes a circle, the further the moving end moves off the horizontal vertically, the faster the horizontal delta will change up to a point where vertical travel is no longer possible. This limit occurs where that vertical travel is equal to the radius and the horizontal delta is also equal to the radius (i.e. the arm is now vertical). So, as the short drag link (short radius) travels up and down with a flexible suspension, it is actually also moving forward and back which in turn, turns the wheels. So, as an arm with radius R (which is in a fixed location on the steering box end) moves up and down by some vertical distance V, the attached steering arm on the knuckle will move backward/forward by some horizontal distance H. Hence, we have bump steer.

    If the initial location of the drag link is horizontal, then the formula to find H for a given V is the following:

    H = R – SQRT( R^2 - V^2 )
    1.60 = 12 – SQRT( 12^2 – 6^2 ) for a radius of 12 and a vertical movement of 6”
    0.38 = 48 – SQRT( 48^2 – 6^2 ) for a radius of 48 and a vertical movement of 6”

    So, we can see that by lengthening the radius (drag link) we reduce the amount of Horizontal deflection due to Vertical movement. This is why a x-over helps.

    Note that since the pivot point on the steering arm (on the knuckle) is always lower than the pivot point on the pitman arm, the problem is exaggerated since we are already on the lower side of the circle (which is why droop on the drivers side has more impact than compression). In reality, the following more generalized formula is required. Here we have added D which is the vertical distance of the steering arm mounting point below the pitman arm mounting point and B which is the bump steer displacement.

    Note: This was not simplified so that the derivation is more obvious.

    B = SQRT( R^2 – D^2 ) – SQRT( R^2 – (D+V)^2 )

    Using the same values we used above and starting with the pitman to steering drop (D) equal to 4” (not unusual with 8”+ lifts) we see the following results.
    4.6 = SQRT( 12^2 – 4^2 ) – SQRT( 12^2 – (4+6)^2 )
    0.8 = SQRT( 48^2 – 4^2 ) – SQRT( 48^2 – (4+6)^2 )

    This is why it is important with the stock steering to keep the drag link as close to level as possible. The bigger D is, the more bump-steer you will have up to a point where you will have no steering at all...

    Note: These formulas are not perfect and are somewhat over simplified but they should be adequate for the purpose of understanding why x-over works.

    Bad Dog

    85 K30 CUCV, 350 TBI, TH400, NP205, D60/C14, 4.56 Locked
    Soon: 4" lift, 40" tires, massive cutting, shorter wb and rear overhang.
  5. Ryeguy

    Ryeguy 1/2 ton status

    Oct 30, 2000
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    Victoria, B.C.
    Few more things to consider:

    The axle is also moving fore/aft as the suspension cycles, due to the change in the arc of the spring. If that fore/aft movement is the same as that exhibited in the drag link, then in theory there will be no bump steer.

    Some suspensions (like Jeep, Ford, etc.) run panhard (or track) bars and crossover steering. If the track bar is the same length and angle as the drag link, then the axle is "pulled" by the panhard to follow the arc defined by the drag link.

    Sometimes the lengths and angles don't have to be the same. What's important (and oulined in the above post) is the amount of change in the two arcs, one in the steering, the other by the knuckle.

    Inverted Y steering set-ups (like on the Jeep TJ, XJ, MJ, ZJ, XJ) some Ford trucks, when mixed with a solid front diff, complicate issues.


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