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Front suspension modifications

Discussion in 'LS-427' started by cobrapilot, Apr 27, 2016.

  1. cobrapilot

    cobrapilot Member

    I finished this up in March, but thought I'd go ahead and post a writeup, as there seems to be a lot of discussion on the front control arms and such. Also, I talked to Dennis during the Cobra Meet, and suggested that they make these changes, so here is some detail.

    Everybody knows that there have been some front lower control arm failures on the original LSC lower control arms. LSC made a new set of lower arms that were supposed to be stronger, but for reasons I don't know, they were also an inch shorter, which made the track width in the front of my car too narrow. (I am not aware of any of these newer units failing) I prefer not to use wheel spacers, as the existing suspension geometry has a very small scrub radius, which I want to keep.

    After seeing what Brian (Luce) did with his lower control arms, I decided to fabricate my own. I made the arms an inch longer to set the track width back to the original length, and also used thicker materials at the point where breakage had occurred on the previous gen. 1 LSC arms.

    From several of the LSC Gen. 1 lower arms I have seen break, the pivot housing weld joint was the point of failure. In my opinion, the joint is subject to failure for 2 reasons:

    1. The big difference in size between the arm and the pivot housing. As the arm moves up and down with suspension travel, a torque moment is applied to the joint between the arm and the pivot housing. Over time, this stresses the joint, and could cause cracking. This problem may be magnified by the sway bar mount. In a hard corner, the sway bar pushes up or down on the arm section, while the pivot housing remains stationary.

    2. The rear mounting tab on LSC frames is frequently not parallel with the front mount. This causes a situation where the rear pivot housing will twist when the arm moves up and down. This likely also caused cracks.

    Here is a pic of the LSC second generation control arm.
    (I had welded the extra tab in the center for the sway bar.)

    Notice how the small tubing of the arm intersects the much larger tubing of the pivot housing.

    Here are the new arms I fabricated:

    There are several key differences here. First, they are built with 0.120 wall mild steel tubing. The square and round arms are 1.25" OD. Notice that because of the larger tubing, the weld area between the arm and the pivot housing is much larger and stronger. Also, the amount of weld between the ball joint ring and the arms is much larger, giving better rigidity and strength.

    Here is a closeup of the arm to pivot housing joint after welding. The delrin bushings have been replaced with oil impregnated bronze bushings. The inner sleeves are about 0.020" longer than the flange bushings. They are turned on the lathe to be perfectly square.

    Here is one of the arms after powder coating. Notice that there is now a rigid pivot joint at the front end only. The rear joint has been replaced with a threaded rod end. A 3/4" heim joint will be mounted here. The spherical Heim joint will accommodate the non-parallel rear bracket by allowing some rotation when the control arms moves up and down.

    So, with the new control arms done, we proceed to the frame mounting...
    Most people I have talked to (myself included) are not happy with the way the control arms mount to the frame on one side only. I have not known any LSC cobra to break anything because of it; it just seems to me that this configuration will put a large shear stress on the pivot bolt, and possibly allow some movement of the joint if the pivot bolts are not tight enough.

    My solution to this was to add brackets to the opposite side of the existing brackets. I made the new brackets out of 0.25" mild steel. The stress on these brackets will be perpendicular to the frame. Using a somewhat thinner bracket than the existing ones, also allows the bracket to bend a little when tightening the bolts. This eliminates any gaps between the washers and the bracket which may cause unwanted play.


    Here, you can see the rear Heim joint mounted on the control arm. I put the rubber heim joint seals from speedway motors on all heims to prevent squeaking. Cone spacers are also mounted on either side. The front bracket is further back to allow clearance for the steering linkage. I turned a set of aluminum spacers on the lathe to fill the gaps. Note that there are steel washers facing the bronze bushings. Threaded holes in the pivot housing allow a zerk fitting for lubrication.

    Aluminum spacers after being machined:

    A closer look at the frame brackets:

    Here's a shot of the setup being assembled. I have also replaced the upper arms with the SPC brand. Several people have broken the Speedway Motors units. I did have to change the ball joint plate to the 0 degree type as the 10 degree unit the upper arms came with gave too much angle.


    I also used the QA1 ball joints, Wilwood 2" drop pro spindles, and QA1 coilovers with 250# springs. The ball joint dust boots are poly performance, as are the tie rod boots.

    With the 2" drop spindles, I did have to extend the coilovers by 2" using extension pieces available from QA1. In this configuration, the lower control arm sits level when the car is on the ground. With the previous setup, the lower ball joint was about 2" higher than the pivot, which adversely affected the camber gain geometry of the suspension.

    A little spirited driving proves it out. Wider track width and very small scrub radius makes for nicer handling.

    Hope this was informative for anyone else considering this....
    Thanks to Luce for advice and suggestions.

    Rick (Cobrapilot)
    cgsincc likes this.
  2. Luce

    Luce Member

    Well documented as always.

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