"Just like the lower control arm bushings in the Mustang the upper control arm can have a significant impact on the vehicle’s ride, comfort, handling, acceleration, noise and vibration. When the car leans (i.e., rolls) in a turn, one side of the chassis moves upward relative to the rear axle, the other side moves downward, and the upper control arm must twist (rotate) to allow for the axle to articulate through its suspension travel. This rotation causes the control arm bushing to bind. If this bind becomes excessive, it will raise the rear wheel rate and produce sudden, uncontrolled, undesirable changes in handling (e.g., snap oversteer).
Ford minimizes this suspension bind by using compliant rubber bushings in the upper control arm. This very “soft” bushings help accommodate the necessary motion of the control arms during body roll. However, the rubber bushing does not provide much in the way of forward and aft support, which can cause wheel hop during hard acceleration and braking. It has become common practice to replace the stock rubber control arm bushings with solid or two piece polyurethane bushings to resolve the shortcomings of the soft rubber bushings. Hard polyurethane bushings eliminate wheel hop, reduce axle deflection, and improves straight line grip. However, the downside of common aftermarket bushings such as delrin, steel, stiffer rubber, solid or two piece polyurethane bushings is they prevent the necessary movement of the control arm during body roll, which in turn produces significant binding in the suspension when the vehicle is cornering. The polyurethane bushings also place unnecessary high stresses on the upper torque box mount, which is the attachment point for the control arm to the chassis. Standard aftermarket control arms do not allow for rotation of the control arm during cornering because of the stiffness of the bushings."
The stock uppers/or Steeda stock uppers with a plate welded have the stock rubber bushing on the chassis side. It is pretty mushy so the control arm can twist or rotate. When you put in a aftermarket arm with a 2 piece poly bushing the arm will not twist or articulate without binding.
Look at my post on the J&M arms:
"The 1/2" grade 8 bolt twisted in half at 9.2 degrees which was 210 foot/pounds of torque."
The J&M arm with a 3 piecs poly ball bushings "10 degrees of total rotation = 41.7 foot/pounds of torque"
So 42ftlbs of torque vs twisting a bolt in half!
I have had one car where I tried to put a aftermarket arm on the upper with a 2 piece poly bushing and couldn't get it to line up with the rear end....the stock arms if they are on the chassis side you can just pull them over to go on the rear end. They can move all around because the rubber is soft compared to a poly bushing where the arms don't hardly move. This is why they say aftermarket arms bind.
So with the Edelbrock that were shown they don't have poly on the chassis side they can move and won't bind....I wanted no "Harshness" and the only aftermarket arm with a poly bushing that I found that wouldn't bind was the J&M arms. The center bushing is a "Ball" shape and the 2 outer cups which are flat on the outside allow the arm to rotate without binding.
This is why they sell the spherical bushing kits for the rear end because of the binding that occurs with aftermarket arms with 2 piece poly bushings.
Ok..hold on.. not making sense. maybe explain in simpler terms... I don't understand how a poly bushing is going to make a car handle WORSE?
The 2 piece poly will bind because it will not let the arm rotate like a stock soft rubber bushing. Aftermarket 2 piece make the car feel much "Tighter" handling, but you are putting alot of stress on the torque box's ( which is why the tear) and when the rear does bind it induces snap oversteer.
Will you notice this in everyday driving around on the street not really pushing it hard in corners??? Probably not,.... but if you start to push it hard or out on the track yes it will bind.
If they didn't bind then why even go to the trouble of making 3 piece bushings, or MM saying not to use 2 piece bushings?
Here is a good photo on the 3 piece poly bushings that J&M uses, it happens to be a link for Chevy arms but you will get the idea. You can see how the arm will still be able to"rotate"
CamaroZ28.Com - J&M Products Releases New F-Body Lower Control Arm Bushing
On #58 I went with the only poly arm bushing that I know of that is a 3 piece and won't bind on the chassis side. I didn't use a spherical solid bushing because I didn't want more harshness. So I used 3 piece Poly on the chassis side for both the upper and lower arms.
I used stock Ford rubber replacements for the upper rear arms (they are pressed into the axle), and a spherical bushing on the lower arm to the axle......with Bilstein HD's the ride is very very nice. Not harsh at all.
Spherical bearings on the control arms won't make the ride as harsh as you think. I have them at one end of all four control arms and I can't tell any difference in ride from when I had the stock control arms. The only thing I did notice was a little more road noise transmitted into the cabin but that's a more than acceptable trade-off for the transformation in handling.