Topic: 1966 Upper Control Arm Mechanics

[bryancobb]

I need to discuss and do a deep dive in the design of the upper control arms and how they are put together and how they are supposex to work.  I'm hoping to be educated here.  Let's li it the discussion to the OEM/Assembly-line control arms.  I don't have one in my hand.  I'm working from memory and pictures.  Thanks in advance for any help.

The shaft ends have very coarse threads, approximately 8 TPI.  The big hex-cups are internally threaded with those same threads.  If I remember correctly, both cups screw onto the shaft in the "rightie-tightie" clockwise direction.  As the car is driven, and the control arm pivots upward and downward about the shaft when the suspension does its job, the entire control arm moves fore/aft along the shaft's axis since one big hex-cap will be screwing ON and its brother on the other end of the shaft will be screwing OFF.  Then, on the other side of the car, its control arm is also migrating fore/aft depending on whether that side's suspension is compressing or extending. 

You can see that caster on the left side and caster on the right side are being varied in ways that are rarely symmetrical, as the control arms shift for/aft !  The control arm vertical travel is probably only 45 degrees so the fore/aft movement is not a lot, but but caster only has to be off-target a tiny amount to make the car handle poorly.

I'd like to hear everyone's thoughts on this cheesy way of attaching the suspension to the car.  The sway-bar tries to prevent the car from leaning but it still does.  See the next post I make pertaining to how to install the shaft in the control arm.

[bryancobb]

In the picture above, when opened full size, you can clearly see one hex-cap has been screwed deeper into the hole in the control arm, than the other hex-cap is.  There's several more threads showing on one side vs. the other.  Now these external threads on both hex-caps are FINE.  Probably 18-20 TPI.  If I remember right, the big hex-caps are supposed to screw all the way into the female threaded holes in the steel arm.  They bottom-out and are torqued tightly.  What this has caused in this picture, is the shaft is not centered between the ears of the control arm. 

I cannot see how the hex-cap can be simultaneously screwed onto the coarse-threaded shaft and into the fine-threaded control arm. ?  Please help me understand this.

[Bossbill]

There are a lot of odd design choices made on this Short/Long Arm (SLA) configuration.

As for the thread issue, as the fine thread screws in you can back off on the pivot as the fine thread screws in since it's essentially a thread ramp.

But you are right on with the caster. As the suspension goes up and down the caster will change a bit. A leftover from the Falcon.
Back in the sixties a way to make steering lightweight was to have almost zero caster. This has two bad results. There is almost no self steering as the car goes down the highway and there is not any return to center after a turn. If you wonder why the idler is a bushing with teeth in it it's to force the drag link back to center. The bushing is bonded so as to twist radially and will then want to return back to center. A radial rubber band?
The same with the lower spring pivot. Why they did this is unknown since the Fairlane had bushings. A side effect is that it imparts additional "spring" value into the coil spring equation that is non-linear.

The reason for the Shelby (Arning) drop is to fix the upper arm's static angle since it is angled down. When the front suspension goes up (or jounce) using the  stock configuration the upper arm must still finish its arc to horizontal for the first inch or two of suspension travel. This means there is caster loss for the first portion of suspension travel. That's exactly backwards for handling. The Arning drop lowers the inner pivot so the arm is near horizontal statically. Camber gain under jounce! I can explain this better using the clock hand analogy, but this is already long-winded.

Also note that if you draw a plan view line forward through the 65-66 upper control arm bolt holes you'll find those lines non-parallel and slightly pigeon toed.

As a historical aside, before ball joints, some front suspensions used a screw-like the upper Mustang pivot but vertically. Right hand thread on the top, left on the bottom. This allowed the spindle to pivot on the screw threads. It also resulted in some up/down motion while turning!
Look at any year MG/MGA/MGB for this 1930s style front suspension.

[67350#1242]

I would think jounce would cause caster gain on one side and caster loss on the other since the UCA's are same and interchangeable.   There is a lot of WTF's in the front suspension design.

[Bossbill]

Exactly.
And the gain is on the outside of the turn tire and the loss on the inside! Backwards!

One of the reasons for the extremely high spring rates on track cars is to keep the spindle within a very narrow range so it never gets into the wonky range.

On my modified car the only stock front suspension parts are the drag link and a few Ford tie rods.