Front subframe sitting on bump stop ?
This isn’t actually true. What is not true? I understand that there are different optimal settings for radial vs bias ply tires, but this person is running his suspension at the bottom of the available travel and actually on the bump stops. That is not good for several reasons, which don't involve the difference between the settings for radials versus bias ply tire settings.
Using a 2 inch drop spindle, and adjusting the torsion bars up so it is closer to the middle of the available travel, is going to allow for be much better ride quality among other benefits than lowering the suspension onto the lower bump stop.
You mention that the geometry is altered with drop spindles due to the movement of the spindle location in relation to the ball joint positioning. I learned long ago that you are very knowledgeable in regard to suspension tech, and have enjoyed many of your posts but, I'm still convinced a drop spindle is the way to go if a person wants such a dramatic lowering of the front of the vehicle. I personally don't like my cars that low, but to each his own.
Are you saying that this isn't true? Please elaborate.
First, I agree that for the OP the 2" drop spindles are probably his best option. That's entirely because of the cars use for drag racing and the OP's desire not to upgrade his torsion bars.
The part that isn't true is about the optimum geometry not being at the bottom of the available stock travel- that's
exactly where it is.
If you look at the OP's pictures you can see that his LCA's are in fact very close to parallel to the ground. The control arms sitting parallel to the ground is in fact where the best camber curve for radial tires occurs. With the stock ride height the control arms sit at a decent downward angle from the frame. Under compression what that means is that the camber gain is actually positive for part of the initial compression (until the control arms hit parallel to the ground). For a bias ply tire that's ok, because bias ply tires don't do well with negative camber. But the opposite is true for radial tires. For radials to have the best handling performance the static camber setting is best being a little negative, and the camber gain should also be negative to maximize the contact patch on the outside tire, which is getting most of the cornering forces.
So then comes the suspension travel problem. Obviously sitting on the bump stops is not ideal and will not give you good performance or ride quality. But that problem can be solved. Larger torsion bars don't need as much suspension travel. Factory torsion bars had wheel rates in the 90 to 110 lb/in range. That's terrible even for a grocery getter with radial tires. By contrast, the 1.12" torsion bars I run have a 300 lb/in rate (which is much closer to modern performance cars). Clearly with a 300 lb/in wheel rate you don't need as much travel as you do with a 100 lb/in wheel rate. So the car can be lowered substantially from the factory settings. To lower that much you need to modify the bump stops to re-center the available suspension travel. The factory lower bump stops are 1-3/8" tall, the ones I run are 3/8". So you go from touching the top of the factory bump stop to having a full 1" of travel by swapping those out. Now, the factory made use of the bump stop for suspension travel, that's why the lower bump stop is triangular (it's progressive). So you can't just swap out to a 3/8" tall lower bump stop and keep the factory torsion bars, you have to upgrade the bars so you don't use the bump stop very frequently because a 3/8" tall bump stop is not progressive at all. And it turns out the factory must have planned on using that lower bump stop pretty frequently.
So you upgrade the torsion bars to something like 1.08" to 1.12" or better, replace the lower bump stop with a 3/8" tall piece, and replace the upper bump stop with something around 2 to 2-1/4" tall. The taller upper bump stop keeps pressure on the torsion bar adjusters at full extension of the suspension, because the larger bars won't always tolerate having that much down travel without unloading the adjusters. The suspension travel is re-centered around the new lowered ride height and the total amount of travel remains similar to factory (5" to 5.5" ish)
Now, about the 2" drop. So you have a 2" drop spindle, which lowers the ride height of the car 2". Then you have the bump stop deal, that only gets you 1" of clearance at the frame. But you have to do the math, because a 2" drop in ride height (or at the spindle) does not equal a 2" change at the frame. In fact, a 1" change at the part of the LCA where the bumpstop sits works out to about 1.67" of drop at the spindle. You can do the trig on the triangle created by the length of the LCA, the location of the bump stop on the LCA, and the a-b number in the FSM for the lower ball joint height compared to the pivot height (1 7/8" for most A-bodies). The drop at the frame to bumpstop is about 40% less than the drop at the spindle. So even just the smaller bump stop paired with larger torsion bars get the LCA pretty close to parallel to the ground.
There are other tricks out there, I run tubular LCA's on my Duster. The early QA1 LCA's didn't have the bump stop provision so they actually gave about a 1" increase in suspension travel because of their design. I run a bump stop on my frame horn, but the end result is my car is actually lowered about 2" from factory without the use of drop spindles.
The bottom line is that if your goal is best handling performance, you don't need or want drop spindles. You just run larger torsion bars, which you want for handling anyway, and you modify the bump stops and lower the car. And you don't have to go as big as 1.12" bars like I did because most folks probably don't want to use a full 2" drop, so you can trade some of that lowering distance for less torsion bar. Now, if you want to drag race or are lowering the car for purely cosmetic reasons, then drop spindles are fine. They do add bump steer, and they do raise your roll center. And, if you don't lower the car a full 2" you actually make those effects worse, because the angle of control arms is actually worse than stock if you don't drop the car a full 2" with those spindles.